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General Discussions => The Lab => Topic started by: cannikin on April 09, 2008, 08:10:39 PM

Title: the 1954 Limiter!!
Post by: cannikin on April 09, 2008, 08:10:39 PM
Finally here is the fruit of my labors for the past 2 years (mostly searching for parts... especially the XFRMRs).  I got the circuit from a 1954 RadioCraft magazine, hence the name "1954 Limiter".  

I wanted this project/unit to be grand in size -- big chassis, big meters, big knobs -- just like the old days before everything was optimized to be small and compact -- With each unit in a seven space rack -- I think I achieved that goal.

Bear with me as I try to describe what's going on in the circuit.. still learning:
The limiter uses UTC A-12 (600:80K) input transformer and 2 6SK7s.  The thing I liked is that it implements 250K pot to adjust the screen voltage on only one of the 6sk7 to get the 2 6sk7s screen voltage in balance.  The bias is controlled by a 500K pot (input attenuator) and the grids even have a 500 ohm pot.  The 12AT7 amplifier is used ahead of the 6AL5 rectifier to get a rapid attack - the 6AL5's cathodes are biased positive with respect to their plates and will not conduct until the output signal is fed thru the 12AT7 to have a limit level.  Control of both bias if the 12AT7 and the screens of 6sk7s is obtained with the OD3 voltage control tube.  Adjusting the 25K pot on the OD3 changes the limit level. the attack and release time is fast (for tube limiter).  I really love the sound of it, it uses a UTC A-27 (100K:600) for the output -- sh*t with these transformers how could it sound bad?  The switch is a 3 position switch, limiter "on" "off" and "Balance"-- balance makes is the unit a linear amplifier with no limiting.

(http://homepage.mac.com/cannikin/.Public/1954limiter/1.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/2.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/5.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/6.jpg)

The 2 6SK7s use DC heater power (I got the DC heater circuit from a Marshall amp that uses DC heaters on the input tubes), the other heaters are AC and have a hum balance pot -- just in case I'm getting some 60Hz in them.  (everything the orange cap and couple of resistors to the right on the picture is part of the limiter on/off/balance switch)

(http://homepage.mac.com/cannikin/.Public/1954limiter/heater.jpg)

I used a bus ground scheme and the only ground point is near the input... seemed to have served me well.

(http://homepage.mac.com/cannikin/.Public/1954limiter/turret.jpg)

The meter circuit just toggles between input and output, I tried to implement a compression meter circuit (like the la2a or federal) but the result was a disaster - I burned 2 nice meters in which the needles on the meter melted and now are bent and always point down ;-)  oh well I learned not to do that........

The Dual B+ power supply is 250V (for each channel) which I put in its own 2 space rack chassis to hopefully have ZERO AC hum issues (which worked!! in addition to the ac heater balance pot and dc heaters on the input tubes.  60Hz/120Hz hum has been my nemesis since my first project).  The power connects to the units via a barrier strip =B+/GRND/Heater1/Heater2.   I used a 6AX5 rectifier and since it eats 1.2amps, I had to add a triad filament transformer to power everything else.  The first two filter caps are grounded to the center tap of the power transformer and chassis, the last Filter cap is grounded at the buss bar of the limiter circuit.  This gave me a good ground point (zero volts).

(http://homepage.mac.com/cannikin/.Public/1954limiter/3.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/4.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/7.jpg)

The unit doesn't have a ton of gain like a federal limiter but the limiter starts at -30dbm on the input (input attenuator at max) when the output reaches -20dbm.  I might have to build a line amp to put on the backside of the unit  but I will test it the studio before I start another project....

(http://homepage.mac.com/cannikin/.Public/1954limiter/8.jpg)
Title: the 1954 Limiter!!
Post by: mattnj on April 09, 2008, 08:24:49 PM
That looks great! 9U for 2 limiters.  Excellent, I like your style. :-)

Is this schematic readily available? I'd like to see what it looks like.  

I look forward to hearing how it sounds in action.
Title: the 1954 Limiter!!
Post by: cannikin on April 09, 2008, 08:27:18 PM
you mean 16 rack spaces for 2 limiters (including power supply) :grin:  :shock:

my scanner is dead, I'll try to get the it scanned at work
Title: the 1954 Limiter!!
Post by: ruairioflaherty on April 09, 2008, 08:27:30 PM
Let me be the first to congratulate you - they look fantastic.  I admire the effort that goes into pulling a project like this together.

I haven't even got the time to assemble kits at the moment so clocking in at the end of the day is as close as I'll get to DIY for now.  It's 1.22 am and I'm just finishing work - I started at 9am :?

Would love to hear a sound sample.  Usually I have no interest in random audio samples but I'd love to get a sense of the character of these monsters.  A before and after vocal or drum kit?

Stay well,
Ruairi
Title: the 1954 Limiter!!
Post by: ruairioflaherty on April 09, 2008, 08:29:15 PM
I type too slowly  :grin: Perhaps it's the glasses of Vino Nobile di Montepulciano that accompanied the last hour of my work day... :thumb:

Cheers,
Ruairi
Title: the 1954 Limiter!!
Post by: Rob Flinn on April 09, 2008, 08:39:15 PM
David, They look great.  Do we get to see the circuit diagram ???
Title: the 1954 Limiter!!
Post by: EmRR on April 09, 2008, 08:44:26 PM
finally!  Another DIY he-man equipment, destined to break toes and rodents when dropped.  Plenty of room for additions and mods too!  Looks great!
Title: whoa
Post by: shabtek on April 09, 2008, 08:46:08 PM
that is sick
those screw term strips: is that for filament, B+ etc.? and is there a cover that protects that?
Title: Re: whoa
Post by: cannikin on April 09, 2008, 08:58:03 PM
Quote from: "shabtek"
that is sick
those screw term strips: is that for filament, B+ etc.? and is there a cover that protects that?


yup it is I have some plexi-glass cut to put over top of it.
Title: the 1954 Limiter!!
Post by: gevermil on April 09, 2008, 10:01:22 PM
I was wondering what you were up to .
Fantastic  :thumb:
Title: the 1954 Limiter!!
Post by: CroatianSensation on April 09, 2008, 10:23:12 PM
:shock::!:
Holy crap!!
.......it's beautiful.
I am not worthy.
Bet it sounds as good as it looks!
Title: the 1954 Limiter!!
Post by: PRR on April 09, 2008, 11:36:11 PM
> balance makes is the unit a linear amplifier with no limiting.

The original "Balance" position puts 60Hz on both 6SK7, which "should" cancel to the output.

The cathode trim will have most effect at high gain, your static trim. The screen trim will work over the full range, will trim the residual heavy limiting thump.

> the output reaches -20dbm. I might have to build a line amp

Indeed. The max clean output near 20dB GR is around 0.5V in 600 ohms. You don't get "real line level" from a couple low-current tubes. You either go big, or lots, or add an output stage. Narma liked massively simple. Most others felt that an output stage was the best path.

It is a fine low-level limiter. It isn't going to drive 10 miles of phone line to a transmitter. May be plenty ample for many studio jobs, if they are set-up for this unit.

The intended GR monitoring is a HIGH impedance voltmeter at the 6AL5 plates. As a $10 10-second hack, put your DVM on 19.9VDC. But DVMs don't dance properly. You could use a VTVM plan, without all the range-switch junk, but that really wants a dedicated floating 100V supply, or at least a good negative supply.

You could try a current meter in the 6SK7 cathodes returns, but it will be 4mA to 6mA, NOT 50uA. Use a 1mA meter with 10 ohms across its terminals, that will read low at idle, raise to 15 or 22 ohms. You could do this with a uA meter (or VU meter) but as you know, any screw-up is costly.

Attack time is ~~3mS. First release time is 330mS. A second time constant is around 1 Second.
Title: the 1954 Limiter!!
Post by: cannikin on April 09, 2008, 11:53:02 PM
HI PRR

I was so hoping you would give your expertise on this circuit.  you are brilliant truly brilliant!  "Attack time is ~~3mS. First release time is 330mS. A second time constant is around 1 Second."  I can't believe you figured that out just by looking at the schematic.... wow! thats what it said on the article of RadioCraft.

Quote "By use of a Dual R-C circuit, attack time is rapid, while release time is prolonged.  The parallel R1-C1 combination of 3M3 and 0.1uf charges very quickly; in the order of 0.00006 second, and discharges in about 1/3 second. By adding R2-C2 of 3M3 and .3uf in series across the R1-C1 combination, R2-C2 will charge slowly; in a little less than 1 second, and discharge even more slowly, in something like 2 seconds.  Thus on the very short duration peaks only R1-C1 will be effective, since R2-C2 does not have time to charge, and recovery time is short.  On Continued peaks, however, R2-C2 charges, and the recovery time is lengthened.  This Prevents the "thumping" and "breathing" that occurs on some limiters and yet does not "dig a hole" in the program as would a quick attack long release type limiter"  End Quote

this was designed as a limiter for the consumer Television set to knock down the volume of TV commercials in 1954

I will give your meter suggestion a try....

Thanks
Title: the 1954 Limiter!!
Post by: Kamel on April 10, 2008, 02:45:21 AM
an electric point

as i saw on the PSU photo, you have direct access to the high voltage socket, to me, it seems a little bit dangerous.

did you plan to cover the HV socket?
Title: the 1954 Limiter!!
Post by: MartyMart on April 10, 2008, 03:43:38 AM
And there's me trying to cut down on rack space !!
Brilliant - HUGE and lot's o fun  :thumb:

MM.
Title: the 1954 Limiter!!
Post by: EmRR on April 10, 2008, 10:33:08 AM
Almost EVERY piece of similar manufactured vintage gear has similar exposed electrical connections.  Entire broadcast facilities were wired up in this fashion.  It requires one to not be an idiot and keep ones digits in place, tongue in mouth, etc.  It is quite a challenge to grab a terminal block like that and short anything; it can be done but the dividers are tall enough to prevent most accidental shorting.
Title: the 1954 Limiter!!
Post by: cannikin on April 10, 2008, 11:03:29 AM
I got that idea from looking at a lot of older gear,  its going to be in my studio in a closed rack, I know not to reach behind there when the units are powered on, and they will hardwired to my patchbay so I'm not really worried about it.  I do have plexiglass plates that I intend to put over the top of the strips away.  Its fine.
Title: the 1954 Limiter!!
Post by: gary o on April 10, 2008, 12:24:49 PM
Wow david that looks great & thanks for showing the schematic too looks like a cut down BA6A the attack release daul constenant set up seems the same as BA6 from memory.... I bet she sounds sweet ...who cares about exposed wires love it  :grin:

What resistors do u have ...I love the old carbons not sure about the mojo ...know about the noise, but I love the look of them

Cheers
Title: the 1954 Limiter!!
Post by: gary o on April 10, 2008, 12:38:04 PM
I see ( I think) makes sense to take output from first stage & next tube for side chain amp to feed the 6al5 for control voltage...the actual audio is just 12sk7s then...cool...no feedback for nice tubey sound & short signal path & output tran doesnt need to take HT......sorry if Im just repeating whats been said its just Im starting to understand these vari mu s more now...what a nice circuit
Title: the 1954 Limiter!!
Post by: PRR on April 10, 2008, 02:52:36 PM
> I can't believe you figured that out just by looking at the schematic....

It is basic.

(http://headfonz.rutgers.edu/cantikin-2.gif)

C1 charges-up from 12AT7 10K and 6AL5. I guess I estimated 12AT7+6AL5 plate resistance around 20K, so 30K total. 30K*0.1uFd is 0.003 seconds.

"The parallel R1-C1 combination of 3M3 and 0.1uf charges very quickly; in the order of 0.00006 second"

I believe they are wrong. The 3M3 has negligible effect. To get 63% control voltage in 0.00006 seconds, the charging source must be 600 ohms. Yet the 6AL5 must be several hundred Kohms, there is an explicit 10K resistor in the path, and the impedance at 12AT7 plates is dozens of ohms. 0.00006 seconds in their dreams.... they copied that number from a better sidechain, did not stop to think.

> knock down the volume of TV commercials

And 0.00006 seconds, 0.06mS, 60uS, is too short for this purpose. System overload is a non-issue, the only judge is the listener's ear. It will take many mS for the ear to cry "loud!". We don't want it ducking on every ignition pulse, only on "HELLo there, LET me Tell YOU aBOUT the NEW....."  Seveal mS response is good. 50mS would be fine.

There is another detail which makes it slower. In addition to ~~30K resistance, there is a 0.03uFd cap in the charging path. When the 12AT7 plate rises, charge divides between the 0.03uFd and the 0.1uFd. So it won't come close to filling on the first half cycle. Actual response time will be many mS.

The main decay is simple. 0.1uFd holds the control voltage, 3.3Meg bleeds it away. 63% will be gone in 0.1uFd*3M3= 0.33 seconds or 330mS. 63% control voltage is not the audible "release" point. Depends on your definition of "release" and on the 6K7's gain/voltage curve. But in that ballpark.

There is another R and C, the C is 3 times bigger, so something else happens 3 times slower, around 1 Second.

This dual-stage plan was floating around, but I suspect someone saw a Fairchild. Which does have 50uS attack, but from a much beefier sidechain than one 12AT7.

The interfacing is odd for consumer TV. It is approximately the level between volume control and first audio stage, but the ratio detector and volume control will not drive the 600 ohm input well.
Title: the 1954 Limiter!!
Post by: gary o on April 10, 2008, 03:25:50 PM
The input section looks very similar to BA6A the RC network look the same .0006 sec attack 0.33 discharge release for RCA.. could the 00006 extra 0 be a typeo... theres also a audio engineering 1950 artilcle here somewhere about the new BA6a with an explanation of the single and daul attack & release of the Ba6 it sounded very much like the Radio craft explanation...could they have been reworking the BA6A circuit for 1954 & as the RC network was similar thought attack release times would be the same so re wrte the audio eng piece... Im shootin in the dark a bit..if I understand correct the 1954 would be slower attack due to less power to drive rectifier ??....
id be very interested to play with this circuit

bottom line is tho 1954 sounds good as well as looks great.
Title: the 1954 Limiter!!
Post by: Viitalahde on April 10, 2008, 03:50:03 PM
That's a beautiful thing.  :thumb:
Title: the 1954 Limiter!!
Post by: ioaudio on April 10, 2008, 05:12:05 PM
great build  :thumb:

great analysis, prr  :thumb:
Title: the 1954 Limiter!!
Post by: skipwave on April 10, 2008, 07:35:14 PM
Quote from: "PRR"
This dual-stage plan was floating around, but I suspect someone saw a Fairchild. Which does have 50uS attack, but from a much beefier sidechain than one 12AT7.


This got me thinking; Since the 12AT7 doesn't have the poop to drive the 6AL5 as we'd like, and we might not want to add a beefy sidechain or output amp, like 6V6 pair in Collins, et al., why not use a 6SQ7 in the sidechain as in Federal Limiter? It looks like the 6SK7 plate drives the 6SQ7 grid just fine, or is that only possible with the 12k plate resistors, as opposed to 47k. Would we load the 6SK7 plate too much?

If that would work you'd shed 2 tubes and have a 3 tube limiter. Then it could fit in less than 7RU box, too.  :green: Of course, I think cannikin's look positively smashing and magisterial in their big boxes.  :thumb:

My other immediate thought was that a lesser stepdown in the output transformer could yield higher levels into common higher Z inputs. Even though we like our tube gear to drive lowZ input passive EQs, a healthy level into modern higher Z inputs would be plenty useful. Maybe an interstage transformer for plate to P.P. grids used backwards as ~100k:10k?

Just sittin' here thinkin'.
Title: the 1954 Limiter!!
Post by: PRR on April 11, 2008, 01:36:54 AM
> why not use a 6SQ7 in the sidechain

Makes things worse.

6SQ7 (half a 12AX7 only lamer) has higher output impedance than 12AT7. Actually 5 times higher.

That means C1 will charge slower.

> as in Federal Limiter?

The Federal is also slow. Even slower than the 1954. It was never made to be great audio.

That's not bad. I was taught to avoid clipping more than a few mS. Some systems are hyper-sensitive and need 50uS. But clipping is widely used for "sound". You can't put drumset in a mix without violent peak reduction, which pretty much means clipping, and often quite a lot of it. Fortunately a ADC accepts overdrive without telling you about the lost bits, no rail-bounce/stick such as some analog channels do.

If you want "FAST" audio limiting, with the usual hold cap values, you need under 1K source impedance. And since GR voltage needs to be larger than peak signal voltage on the vari-tube grid, you want, 5, 10, 20, even 40 Volts behind that 1K. You want a 0.1 Watt to 1 Watt Power Amplifier.

Several of the Classics used the existing 0.1 Watt line output stage to also drive the rectifier. This makes some control difficult; also tends to load-down the peaks. Both flaws are very acceptable, either in safety limiting or limiting for effect.

Sand-state devices dump current tons better than tubes: faster attack. Gain is cheap too: flatter limiting. If you wanna "make it better", easy: go sand. Heck, put a DSP in the sidechain and compute any dynamic response you like. (Also program it to self-test and compensate tube unbalance.)

lesser stepdown in the output transformer

To a point. 10K instead of 600 would give 4 times the level, except many of our inputs really are 10K so we could lose half of that, get only double the level.

The real question is: what is the output power? To get 20dB compression (surely more than TV commercials merit) from the operating point they seem to have selected, 6SK7 current has to fall to about 0.2mA. Depending on the load, that's not much over a milliWatt, well short of traditional Line Levels which reference 1mW and go up from there. No doubt this beast can jam a ADC to max, but a lot of engineers are not happy unless levels are HOT, even though they end up padding-down.

Getting a limiter to just "work good", control audio with minimal damage, is harder than about any other "basic" audio design. Doing it well in tubes is just awful hard. Fortunately most modern music needs "color" more than accuracy.
Title: the 1954 Limiter!!
Post by: cannikin on April 18, 2008, 06:41:12 PM
Guys the more I play around with this limiter the more I LOVE it.
if you can you should put this on your next build list.

good sh*t!
Title: the 1954 Limiter!!
Post by: tommypiper on April 19, 2008, 02:15:26 AM
David, great.  I love how you go into new tube projects on your own.  How about some audio samples?
Title: the 1954 Limiter!!
Post by: zebra50 on April 19, 2008, 03:14:33 AM
Looks amazing - great!

But the schemo has gone from page one. Any chance you could post it back up, please, or email me a copy?


EDIT - Just found it here
http://www.groupdiy.com/index.php?topic=27372

 :thumb:
Title: the 1954 Limiter!!
Post by: cannikin on April 19, 2008, 03:26:08 AM
I will definitely post some soon
Title: Re: the 1954 Limiter!!
Post by: templatezero on February 20, 2009, 01:37:45 PM
Hi all! Recently, I've started breadboarding the 1954 Limiter with a 6SN7 in push-pull as make-up amp. So far, the audio amp is completed and it passed audio without much problem. I did some quick voltage measurements on the unit and was quite surprised that the DC voltage on the 6SK7 cathode is 83V!! ??? Could this high voltage be due to the use of a high value resistor in the cathode circuit (it looks like 24K in the schematic)? Am I way off from the original value?

Many thanks to cannikin for posting the schematic. This is a great forum and I have learnt a lot of things here. Thank you guys!!!  :)

Cheers!
Gavin
Title: Re: the 1954 Limiter!!
Post by: alexanderperfilyev on March 27, 2009, 05:43:41 PM
the photos are gone? and scheme too...please, reupload it!
thanks beforehand!
Title: Re: the 1954 Limiter!!
Post by: EmRR on August 04, 2012, 11:35:04 AM
Looking back at this, it's pretty much an update of an AGC that NBC used around 1940, and the adjustable bits looking like BA-6 or 86 have hallmarks of RCA. 

(http://www.groupdiy.com/index.php?action=dlattach;topic=49415.0;attach=14564;image)


Title: Re: the 1954 Limiter!!
Post by: davemascera on January 12, 2019, 03:52:10 PM
Feel bad resurrecting this but doing it anyway cause I'm genuinely interested and I've been told multiple times to ignore that red notice up there...

So what exactly was the problem with this thing again? It doesn't put out enough juice while significantly reducing gain?

I was thinking about building this.... mostly because it seems extraordinarily easy compared to a lot of the tube limiter circuits and I have all the parts except the voltage regulator tube and the transformers. But if it can really manage only half a volt while reducing gain by 20 db... maybe it isn't worth it??

Anyone have any ideas on modifying it to account for that in a relatively simple/elegant way?

Is it also really necessary to build in the 6.3 VAC part that allows you to bias the screen grid via phase cancellation?? Just seems a bit weird to build a biasing tool into an interface....

Also, anyone have any idea to what degree the pot near the voltage regulator tube will actually allow you to control the limit threshold?
Title: Re: the 1954 Limiter!!
Post by: EmRR on January 12, 2019, 04:02:13 PM
Feel bad resurrecting this but doing it anyway cause I'm genuinely interested and I've been told multiple times to ignore that red notice up there...

GOOD ON YOU!  No reason for a new thread. 

So what exactly was the problem with this thing again? It doesn't put out enough juice while significantly reducing gain?

PRR covered it on page one.  It's basically the first stage of a limiter, so it needs additional make-up gain to be useful at line levels. 

Anyone have any ideas on modifying it to account for that in a relatively simple/elegant way?

put a 990 SS amp after it.

Is it also really necessary to build in the 6.3 VAC part that allows you to bias the screen grid via phase cancellation?? Just seems a bit weird to build a biasing tool into an interface....

Tubes change parameters over time and many of these controls on vintage limiters get tweaked frequently. 

Also, anyone have any idea to what degree the pot near the voltage regulator tube will actually allow you to control the limit threshold?

That's what it does, it's limited to a low compression range.  Make that R string a 25K for higher ratio/threshold. 
Title: Re: the 1954 Limiter!!
Post by: davemascera on January 13, 2019, 05:20:28 PM
GOOD ON YOU!  No reason for a new thread. 

PRR covered it on page one.  It's basically the first stage of a limiter, so it needs additional make-up gain to be useful at line levels. 

put a 990 SS amp after it.

Tubes change parameters over time and many of these controls on vintage limiters get tweaked frequently. 

That's what it does, it's limited to a low compression range.  Make that R string a 25K for higher ratio/threshold.

Hmm I guess it makes sense if they're to be set that often....

The 990 is not a bad idea at all, but since I do have a 6SN7 right in front of me, is there any reason I can't just plop the output stage of the federal right in there?
Title: Re: the 1954 Limiter!!
Post by: scott2000 on January 13, 2019, 05:39:46 PM
Hmm I guess it makes sense if they're to be set that often....

The 990 is not a bad idea at all, but since I do have a 6SN7 right in front of me, is there any reason I can't just plop the output stage of the federal right in there?

lassoharp did a similar build......

https://groupdiy.com/index.php?topic=44011.msg549861#msg549861


BTW, if you don't mind me asking. what substitute transformers did you use for your BA-2 build?

Title: Re: the 1954 Limiter!!
Post by: davemascera on January 13, 2019, 08:16:55 PM
lassoharp did a similar build......

https://groupdiy.com/index.php?topic=44011.msg549861#msg549861


BTW, if you don't mind me asking. what substitute transformers did you use for your BA-2 build?

CMQEE-3440A and CM-27101

I was having problems on the low end (-10 at 20 Hz)... talked with Dave at Cinemag and it was that the CM-27101 wasn't beefy enough.... I loaded it with a 600 ohm resistor on the secondary and then I was only about -3 at 20 Hz, nearly flat at 30. Left it that way and use it almost every day. Bought the originals from Doug to add in a second preamp and ended up reselling them.

Hmm interesting. So he's basically just using the output stage from the federal combined with a lot of elements from the BA6A...
Title: Re: the 1954 Limiter!!
Post by: scott2000 on January 13, 2019, 11:06:09 PM
Awesome! Thanks for that!
Title: Re: the 1954 Limiter!!
Post by: davemascera on January 25, 2019, 12:53:53 AM
Hmm so I'm doing this build now... gonna try to put a 6SN7 in and attempt to implement the cathode return meter... might try to use an interstage transformer like an A-18 for sh*ts instead of some coupling caps (course I know the 12at7 will still need-em).. not sure..

Does anyone know if there is any downside at all really to increasing the value of C1 if one wants a slower attack?? What about making R1 a pot to ground in case someone wants to make the release faster? So far as I can tell, I don't really see the harm in it... seems like

Was thinking of making a selector switch with a bunch of increasing cap values on it, maybe maxing out at ~30ms....
Title: Re: the 1954 Limiter!!
Post by: DaveP on January 27, 2019, 09:11:55 AM
Quote
Does anyone know if there is any downside at all really to increasing the value of C1 if one wants a slower attack?? What about making R1 a pot to ground in case someone wants to make the release faster? So far as I can tell, I don't really see the harm in it...

Most of the old compressors were designed for radio transmission, so the timings were fixed  (they were playing mastered recordings after all).  For recording purposes, you need a selection of attack and release times.  The normal way to do this is to keep the cap value constant but switch in various series resistors to feed it for attack, then a similar switch in parallel for release.  The advantage of switches is that you can make a record of the settings for the track, not really possibly with just a pot.

0.1uF can allow a slight ripple which on some feedback compressors can turn into motorboating,  1uF is used on the Fairchild and others, but many other comps get by with 0.25 or 0.5uF.  Resistors that add with each position are mainly used to avoid switching noises, changing caps will probably cause some  sudden charging noise.

DaveP
Title: Re: the 1954 Limiter!!
Post by: davemascera on January 27, 2019, 11:34:24 PM

Most of the old compressors were designed for radio transmission, so the timings were fixed  (they were playing mastered recordings after all).  For recording purposes, you need a selection of attack and release times.  The normal way to do this is to keep the cap value constant but switch in various series resistors to feed it for attack, then a similar switch in parallel for release.  The advantage of switches is that you can make a record of the settings for the track, not really possibly with just a pot.

0.1uF can allow a slight ripple which on some feedback compressors can turn into motorboating,  1uF is used on the Fairchild and others, but many other comps get by with 0.25 or 0.5uF.  Resistors that add with each position are mainly used to avoid switching noises, changing caps will probably cause some  sudden charging noise.

DaveP

Hmmmm interesting... so if I'm understanding this, throwing in a 300k pot will probably make my attack around (if fully engaged) 33 ms, as opposed to a fastest-case (and apparently not plausible) 3ms. That's if the first time constant cap remains at .1 uF, which you're saying may create motor-boating, or rather does in some circuits.

It might be a good idea to omit the second time constant altogether. I get the feeling that this was also created with television advertisement broadcasts in mind.

So if I then threw a 2.5 Meg pot in front of an 800kohm resistor, that would supposedly range from about 80 ms to 330 ms on the release...

And all of this is ideal and likely won't really perfectly reflect the reality of the time constants.

Lots of experimenting to be done I suppose. I don't care for switches (though I'm already using one of those Chinese dual 24-step attenuators on the input). This will likely be used on vocals and I compress before it hits the tape.
Title: Re: the 1954 Limiter!!
Post by: DaveP on January 28, 2019, 05:34:24 AM
Quote
Hmmmm interesting... so if I'm understanding this, throwing in a 300k pot will probably make my attack around (if fully engaged) 33 ms, as opposed to a fastest-case (and apparently not plausible) 3ms. That's if the first time constant cap remains at .1 uF, which you're saying may create motor-boating, or rather does in some circuits.

It might be a good idea to omit the second time constant altogether. I get the feeling that this was also created with television advertisement broadcasts in mind.

So if I then threw a 2.5 Meg pot in front of an 800kohm resistor, that would supposedly range from about 80 ms to 330 ms on the release...

And all of this is ideal and likely won't really perfectly reflect the reality of the time constants.

Lots of experimenting to be done I suppose. I don't care for switches (though I'm already using one of those Chinese dual 24-step attenuators on the input). This will likely be used on vocals and I compress before it hits the tape.
You may actually get 3mS.  The fastest attack speed relies on the source resistance of the driving circuit.  A Fairchild for example has an output resistance of 100 ohms before its diodes, which enables its speed to get down to uS.  The 12AT7 is run fairly cool so it is probably around 25k, then there are a couple of 10k resistors as well.  I would try losing the 10k's and putting in two cathode followers from another tube to drive the 6AL5, this would take the source resistance down to around 250 ohms.

Have fun :)

DaveP
Title: Re: the 1954 Limiter!!
Post by: EmRR on January 28, 2019, 07:27:33 AM
It might be a good idea to omit the second time constant altogether. I get the feeling that this was also created with television advertisement broadcasts in mind.


Mmmm, no, nothing to do with advertising.  The dual time constant can be found in many things, BA-6A's, Stalevels, etc. The early NBC vari-mu AGC's were meant to be driven at an average gain reduction of 15dB, and the slower dual time constant sets a baseline for the faster to work off of.  It reduces the amount of pumping and also extends the useful range of compression.  It sort of makes for a virtual threshold when you are into heavy amounts of compression.  You can also put it on a switch, like a Stalevel. 
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 10, 2019, 04:16:08 AM
So far so good... build is basically done... only issue thus far is that beyond about 25 ms of additional attack delay, the circuit does indeed begin motorboating, but I also haven't 'balanced' the unit yet. Fortunately, that's a fine parameter for me anyway. The sucky part is getting a new pot.

B+ is a tad cold on the 6SK7's and a tad hot on the 6SN7... not horribly worried about it.. but might f**k around with the dropping resistors... gonna check plate dissipation soon.

Gonna check for hum next shot I get (used all DC heaters and about 45000 uf of filtering so hopefully that bit pays off despite the low hum risk of this circuit anyway... I had a filament transformer sitting around and a lot of room on the chassis), then freq response and give it a good listening. If anyone cares I'll post a picture and some sound samples when its totally finished. Then if someone cares EVEN more (something makes me doubt they will) I'll post a schematic.

Sadly I got one of those 'rugged' OD3's that isn't quite as gaudy as the rest  :-[. Project was built into an old Magnavox Radio chassis and power xformer.
Title: Re: the 1954 Limiter!!
Post by: DaveP on February 10, 2019, 05:26:53 AM
Well done :)

DaveP
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 10, 2019, 07:13:01 AM
The sucky part is getting a new pot.


??? Why...... Are you having a hard time locating one??? Or just in general because of not having it on hand? What one??

Title: Re: the 1954 Limiter!!
Post by: davemascera on February 11, 2019, 02:43:59 AM
??? Why...... Are you having a hard time locating one??? Or just in general because of not having it on hand? What one??

https://i.ebayimg.com/images/g/JwoAAOSwz2lXCB7S/s-l1600.jpg

Gaudier

No, there just aren't many electronics stores around me, so it's a matter of ordering from the internet and paying for shipping... but I got the motorboating to stop.... the B+ was actually way more off than I thought it was, and the regulator tube was getting REAL hot and the foam bit around it began to melt (or the glue holding it did at least). Realized this baby wants 250V unloaded coming in, not for the node to READ 250V. After that and balancing, motorboating ceased and this thing seems like it's functioning as intended.

So I haven't gotten to plug this in at the studio yet and feed it with a real input and hear what it really sounds like, but it seemingly works. Provides more than enough output gain. Meter strapped to the cathode shows 'limiting' for sure, but it's a 1 ma meter and only drops about .2 ma at maximum gain reduction, so that's a bit ineffectual.. I tried 10 ohms initially. Brought the needle up to nothing. 100 ohms on the pot lets the needle hit about .8 ma.. no signs of melting or anything terrible... considering getting a smaller meter, but stinks that I already have the chassis cut for this one. Maybe having a full, balanced signal will make me change my mind.

If I remember correctly, a signal coming in at about 600mv with no attenuation outputs at about 2.4V... if I throw on the gain reduction, I can cut that to around or below the signal level.

But all of this led me to check what the idling plate current is on the 6sk7's... ~1 ma... so something feels horribly wrong on that front. Either that or I don't get the circuit as well as I thought. Probably the latter, especially (now that I think of it) considering that any normal amount of current coming through those plate resistors would require a rather large B+. Did try out the 'balance' feature using the cathode pot and the screen pot... pretty cool feature and a bit of a lifesaver with that whole motorboating thing.
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 11, 2019, 09:22:42 AM
Scary about the regulator.... I was thinking about how those ss octal replacements I linked are.......I guess not cool looking though compared to the gas...........

What do you mean by it wants 250v unloaded???  B+ should be 250??


Also, The original 1954 circuit is a bit vague on the metering and I haven't grasped how it is supposed to be done...

Do you have a drawing of yours yet???

Did you implement the one balancing circuit using the 6.3vac as well???


Excited for your progress...... I thought the original poster of this thread loved the circuit but I came across a thread in the black market , posted some time after, where he actually ended up selling it because he couldn't get it to work???? So I'm confused if anyone ever actually tried it......



Title: Re: the 1954 Limiter!!
Post by: EmRR on February 11, 2019, 10:32:50 AM
I thought the original poster of this thread loved the circuit but I came across a thread in the black market , posted some time after, where he actually ended up selling it because he couldn't get it to work???? So I'm confused if anyone ever actually tried it......

The OP got out of audio altogether, I recall he had them working fine. 
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 11, 2019, 10:51:44 AM
What do you mean by it wants 250v unloaded???  B+ should be 250??

Also, The original 1954 circuit is a bit vague on the metering and I haven't grasped how it is supposed to be done...

It should work with 250V B+ measured at the node when connected, that is what the drawing means.  Layout could cause motor boating, making the coupling caps too big could cause it, you can always throw more filter capacitance at those two points and see if it clears it up at the higher voltage.  How far down is B+ for it to work?

1mA idle is totally believable given the resistances. 

Look at the BA-6A metering, which is the type PRR referred to back on page one.  Same as many other limiters.  Look at SA-39.  Notice all the resistances here are scaled way up from the BA-6A, so current will be lower, will need a more sensitive meter. 
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 11, 2019, 11:09:45 AM

Look at the BA-6A metering, which is the type PRR referred to back on page one.  Same as many other limiters.  Look at SA-39.  Notice all the resistances here are scaled way up from the BA-6A, so current will be lower, will need a more sensitive meter. 

Thanks! I'll look over it again.  Both the op and davemascera seem to have had issues with metering and it would be nice to "see" what worked best.....at least in the 1954 build.........Lassoharp never really mentioned any issues in his design so, maybe it works in this???

I recall he had them working fine. 

I thought so too and have been interested in this build since.......I was just confused when he sold it...??

https://groupdiy.com/index.php?topic=59153.0
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 11, 2019, 05:06:55 PM
Scary about the regulator.... I was thinking about how those ss octal replacements I linked are.......I guess not cool looking though compared to the gas...........

What do you mean by it wants 250v unloaded???  B+ should be 250??


Also, The original 1954 circuit is a bit vague on the metering and I haven't grasped how it is supposed to be done...

Do you have a drawing of yours yet???

Did you implement the one balancing circuit using the 6.3vac as well???


Excited for your progress...... I thought the original poster of this thread loved the circuit but I came across a thread in the black market , posted some time after, where he actually ended up selling it because he couldn't get it to work???? So I'm confused if anyone ever actually tried it......





Well, I turned off the regulator before actual damage was done, but the glue had turned to liquid for sure atop the foam. The weird thing is that it wasn't drawing anywhere near maximum current according to the datasheet. It was at about 33 ma. So it's probably just a matter of aging parts. I should probably send a message to the ebay dealer if emrr is right and get my money back.

The only drawing I have I did with pen on a piece of computer paper and it's really just for me to understand... a lot of the PS values have changed, etc. I just put a 1 ma meter beyond the 22 kohm resistor to ground and put a 100Ω pot across it. It definitely functions correctly, but really doesn't dip enough for my liking. I have a switch for it in case it causes distortion. Also made a switch for the second time constant, and of course, created adjustable attack and release on the first one. Weird part is that adjusting the attack should also affect the release. Not a problem for me though.

I used DC heaters like I said from a seperate filament transformer. I hit a bridge rectifier, 45000 uf with .33 in between 15000 and 30000... got about 19mv of ripple. Thing sucks in about 3.2 amps of AC from the transformer, which is well below rating. So in order to implement the balance circuit, I took the 6.3 ac line off of the power transformer itself, let it hit a pilot light on the front panel (which has since burned out as it was likely about 80 years old) and let that hit the balance circuit.

I'm unsure cannikin ever got his working. I wouldn't be surprised if he did not considering that he didn't post audio. But maybe he did and we can no longer see it here.

It should work with 250V B+ measured at the node when connected, that is what the drawing means.  Layout could cause motor boating, making the coupling caps too big could cause it, you can always throw more filter capacitance at those two points and see if it clears it up at the higher voltage.  How far down is B+ for it to work?

1mA idle is totally believable given the resistances. 

Look at the BA-6A metering, which is the type PRR referred to back on page one.  Same as many other limiters.  Look at SA-39.  Notice all the resistances here are scaled way up from the BA-6A, so current will be lower, will need a more sensitive meter. 

B+ is about 196V now at that node. Regulator tube is warm, but drawing only 23 ma. I have definitely not overloaded on filtering (choke-20-30-30). This project was a lot of using crap I already had (aside from the interstage) so I didn't want to create too much sag  cause I was using a 5Y3.

However, I discovered a grounding problem in the sidechain.  Fixed it, might go back to the way it was for some more tests. Still, shouldn't result in the regulator smelling like that. That's what I get for picking the rugged one instead of the bare tube, I guess.

And yeah, you're right.  I'm not positive what kind of meter I'm going to need if I want to see a proper resolution. Will have to do some tests with a rather beefy signal.
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 12, 2019, 12:05:53 AM
So I plugged it into my technics stereo... still haven't taken it down to the studio...

I figured out that my 'motorboating' is not 'motorboating.' It's a static-y sound that gets made when the sidechain hits my dual ganged pot that is supposed to delay the attack. Very strange. When the sidechain is as intended, the circuit works. The release adjustment seems to work just fine though, and so does turning off the second time constant. Also, why did it go away when I took the B+ down? Maybe just cause the 12AT7 wasn't getting the voltage it wanted.

The big problem I have with the circuit is that it's light on the compression. If I had to guesstimate I'd say that if I put in a signal as loud as it can go without getting a large amount of clipping from  presumably the 6SN7, I can attenuate it maybe 4 decibels maximum using the threshold knob. Once it's attenuated it doesn't clip though. Basically a full volt of signal coming in will almost clip it if I turn the limiter off or relieve the threshold a certain amount.

I know emrr left some instructions for making the threshold a bit more extreme. May try and implement.

So the last three mysteries I have are can I increase the plate voltage on the 6SN7 to get myself more headroom, why does my dual ganged attack pot create a heavily distorted 'static-ish' sound whenever attenuation occurs, and how can I make the compression effect more extreme. Maybe the A-18 is handing the 6SN7 a hotter signal than it wants.

As an aside, the only way I could get the OD3 to have less than 40 mA on the cathode was to hit the first stage with about 230 volts. Weird. And I tried two different tubes, and beefed up my resistors considerably. Maybe therein lies the clue to the subtlety of the effect.

Here are some pics of this grungy thing. Gonna make a plate to cover that gap up front. It's not too terrible with hum.

(https://lh3.googleusercontent.com/4-uV7adz3Ykx0YN5oOPdqwK54ygrcVF6x3T5x50EbObEW87JVFspEBg0RFdjeFcKOSq0hgVm110Gt8PKeTVs1SktQ1zqRv5WGwGjd2oDjWYExwUMZ99KcH3ouUCdE4bO3Co_QNE_Wv4cWgiiiI8_A3p2gbSondnrbgOQ9NcMztWDIxUNJeo2K5cpCJESwuiItUkXBLaLR0Mq1hZOJvoaNDCV8iLxTJRXbfjECfHc30k-I6ERvfFuEHd43QKRQlECmZGQHYGHY--FIQgqNZqU2nhRCIW3mmhAoArkiYGmsthsmTWp5gk1wSQo2GvMAe9gwXu4y4Kbs8bm1bzbafGN1MaCfZsM6-lDWMwUYEDrC_jqUujvAoNsUQx-DZcL4dZhXOYcICngIVktfzkJMLVxBHgX6xN_C5CMvoQPW3yCGnuxwk31W8eTBB2VW_L5qhGV5vgXWGqMC4goX1nhNoUSAVehaAtD7nnXO0vHS8liyBPG3WC9-LlauZppk5YqbbJiVJT5oRFTVKhU22IX--MK6rgDJ9FpD-QpZk9G6tpQMyVfEiGZsBB8GnsXMPyx1YP7dt4bNqJME-nLX2bArT0n6fD971QT_uu7cHNOQcZUbWuPG723pfmsLmGQzeUrYa3SaqQra5M9pwH7rxerAV0i5c8Kl8NRuPo=w441-h588-no)

(https://lh3.googleusercontent.com/Ybh25Au9vkt31DUva522i7VeoC-REkG0kX2LbZtzD92Jf4sExnzJyrwSJYj7_Q7CX99GtTGccDh-Ut1EOM8uSbwAeKPk65U8AQyP0W6dfjzkgRsPOKgNMsjooWRKDN3CAx66gSsRPN-vUjw8BIih9iTuXr5pUwxnRg07ItXDO_QII-U4CkIkEgEV4MgIMKZZYdootLm9Emj3IHiN4EezklEIzhPji2aktDvDbnGLAiGxtghkbaqPGdN6kzDutPvRWqMKJwoE2liJqV6DbSwaMzqVAUodUviIn88xxJyF8SmPmyto69nKTNtQvH5K8BYGfWcZ6vTprUFWIQy1miWlIt48pshTjKHmZxVFEoGMZ9OMUTTcnqvSN6gbbk3aURU1wo4zZqQ-rO3szk4citlQPPV3tgmhkihxxKQRlDkNzGy4EGWZl5H-iASeuMIVr-yc9cKB0hGhvnJVkZmBFbz9mZuR6nQjh_PWnPk6jfVdk3D4QBxyj69IcXW2-CbYv3n6A8WGkE8E31oM0KVqNz5MHqs39ZlCEMZ1iGSLu50UmMAnWZ5d6jiQniRF2GqxpmbRbS3fqEzJ18Rb8Z6SUWnC_tJPJjvx5Af79PAb1A-4OAMRCln8bR2BGReBcRWqE8S3bVJNJjNo-Q-F45wxC47BjnxLRveymBc=w441-h588-no)
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 12, 2019, 01:11:06 AM
Cool!

I like the repurpose thing....

Hard to follow along as easy for me without a drawing.... You have an interstage too?

What are you using for input and output transformers?

On that note, what is the transformer output called in this 1954 ? The B+ is confusing to me???

Title: Re: the 1954 Limiter!!
Post by: davemascera on February 12, 2019, 01:52:52 AM
Cool!

I like the repurpose thing....

Hard to follow along as easy for me without a drawing.... You have an interstage too?

What are you using for input and output transformers?

On that note, what is the transformer output called in this 1954 ? The B+ is confusing to me???

I mean trust me my drawing wouldn't help you. It's basically mostly scribble. I was gonna make something totally new if this works out.

Basically it's the 1954 schematic, the only things changed on THAT section are I took the single 3.3 MΩ resistor (not the one paired in series with the capacitor... that pairing is on a switch so I can turn it on and off) and turned it into a 1.3MΩ resistor and a 2 MΩ potentiometer (to change release time).  I put in a dual ganged 300kΩ pot on each leg of the cathodes of the 6AL5. Also, instead of 250 volts hitting the circuit, I have 230, because the OD3 was outputting 42 ma with 246V, which is above its rated maximum.

I put in a meter between the 24 kΩ resistor that hits the cathode resistors of the 6SK7's and ground and put a 100 ohm pot across its terminals to allow current to escape and not melt it. This is also on a switch so I can bypass it.

Instead of an output transformer, I have this hitting an interstage (A-18). The primary center tap is just grounded like in the 1954 limiter scheme. On the other side of the interstage is a 6SN7 with the output stage and biasing resistor values from the Federal AM-864 implemented. I have about 200 volts hitting it, where on the federal, 185 volts is called for. Not really a large difference. The B+ to this tube goes through the actual output transformer just like the Federal.

The power supply is a 5Y3 hitting an 11 henry choke, hits 20 mfd to ground, 1500 ohms in series, connection to B+ (230V) of 1954 limiter stage and 30 mfd to ground, 1500 ohms in series, 30 mfd to ground and about 200V to the 6SN7.

Input and output transformers are edcor WSM 600/15000's I had from another project I didn't care about.
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 12, 2019, 05:56:22 AM
https://www.youtube.com/watch?v=cFr0QON3x8s

Here's a sh*tty video in case anyone wants to hear. It really doesn't get loud enough to be useful until about 40 seconds in. The other thing is that I show the balance circuit at the end and the frequencies probably didn't get picked up by the phone mic. I'm a failure.
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 12, 2019, 06:48:27 AM
Nice drum choice in the video.

Cannikin's story seems to have changed between OP and sale.  Who knows. 

OK, if I'm reading this right, the 12AT7 feed is still coming from the same place.  Take it from the output of the 6SN7,  a place with more signal level.  It may need more B+, from lower value plate resistors, like down to 33K, which'll draw a lot more current and give greater drive power, consider the cathode resistor again.  It may need voltage divider attenuation on it's input if the other places are too hot, could be a separate threshold pot. 

What orientation is the A-18?  I'd run it with the higher Z side as primary.  Consider the original circuit calls for a A-27 with 100K primary.  The load affects compression range. 

Attack control, not sure what you're describing, doesn't sound right.  It should be series resistance between the 6AL5 and the other time constant parts.  Look at the Altec 436C.    You're wiring a rheostat, so connect wiper to one side for securities sake. 

Where is 6SN7 B+ tapped from?  Should be right off the output of the PSU, straight to the transformer primary CT, to the 6SN7. 

The series resistor into the 0D3 can be larger to lower current.    The implementation strikes me as odd.  Many times a regulator is used to control absolute threshold reference, and/or GR stage plate voltage, here it appears to be primarily controlling screen voltage, though admittedly with a 2K series resistor it's gonna mostly control everything.   Screen as 'most controlled' looks odd to me.   I'm not even sure why this needs a regulator, it's not a broadcast limiter meant for a transmitter, and plenty of comps like this don't have one at all. 

October 1954 Radio-Electronics, page 77

https://www.americanradiohistory.com/Archive-Radio-Electronics/50s/1954/Radio-Electronics-1954-10.pdf

Hilton Remley as author is a straight up ringer for RCA, this is their circuit.   
Title: Re: the 1954 Limiter!!
Post by: PRR on February 12, 2019, 09:56:24 PM
On a pentode, the effects of G1 are proportional to Vg2, hardly at all to plate voltage. So it makes sense to regulate only Vg2 (and Vk).

It still seems odd and lame. The idle current in the 6SK7 must be very small, maybe less than 1mA. The current in limiting must be even smaller, so the limited output can not be large. The graph shows -17dBm, 110mV in 600 Ohms.

The stated purpose is to trim "several dB" in commercials. By that goal, it is not intended to limit uncontrolled music.

FWIW: despite his assertion, *everybody* had limiter amplifiers by 1954. RCA was pushing them onto stations just as hard as GE and Gates. A good limiter would double your audience area and could nearly double your ad revenue. The FCC was attentive to repeat overmodulation. (The deal with commercials is that they are mixed hot and often pre-compressed to be SURE they cut through.)
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 12, 2019, 10:31:37 PM
FWIW: despite his assertion, *everybody* had limiter amplifiers by 1954. RCA was pushing them onto stations just as hard as GE and Gates. A good limiter would double your audience area and could nearly double your ad revenue. The FCC was attentive to repeat overmodulation. (The deal with commercials is that they are mixed hot and often pre-compressed to be SURE they cut through.)

Oh yeah, everyone had them by 1940.  It was suicide NOT to have one by then.   WE, RCA, Collins, Gates, lots of home made. 
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 12, 2019, 11:53:19 PM
Nice drum choice in the video.

Cannikin's story seems to have changed between OP and sale.  Who knows. 

OK, if I'm reading this right, the 12AT7 feed is still coming from the same place.  Take it from the output of the 6SN7,  a place with more signal level.  It may need more B+, from lower value plate resistors, like down to 33K, which'll draw a lot more current and give greater drive power, consider the cathode resistor again.  It may need voltage divider attenuation on it's input if the other places are too hot, could be a separate threshold pot. 

What orientation is the A-18?  I'd run it with the higher Z side as primary.  Consider the original circuit calls for a A-27 with 100K primary.  The load affects compression range. 

Attack control, not sure what you're describing, doesn't sound right.  It should be series resistance between the 6AL5 and the other time constant parts.  Look at the Altec 436C.    You're wiring a rheostat, so connect wiper to one side for securities sake. 

Where is 6SN7 B+ tapped from?  Should be right off the output of the PSU, straight to the transformer primary CT, to the 6SN7. 

The series resistor into the 0D3 can be larger to lower current.    The implementation strikes me as odd.  Many times a regulator is used to control absolute threshold reference, and/or GR stage plate voltage, here it appears to be primarily controlling screen voltage, though admittedly with a 2K series resistor it's gonna mostly control everything.   Screen as 'most controlled' looks odd to me.   I'm not even sure why this needs a regulator, it's not a broadcast limiter meant for a transmitter, and plenty of comps like this don't have one at all. 

October 1954 Radio-Electronics, page 77

https://www.americanradiohistory.com/Archive-Radio-Electronics/50s/1954/Radio-Electronics-1954-10.pdf

Hilton Remley as author is a straight up ringer for RCA, this is their circuit.

So I just said f**k it and upped the voltage back to 246... hitting the 6SN7 with 230 now too. I don't really care if the OD3 is a bit off. It isn't getting that hot with the non-rugged one. Cathode is ~45 mA now once I let it settle down. Whatever. And there was a loose solder point on the attack pot.

I don't know if this makes sense, but the gain reduction and clipping has been functioning much better since I did that (as if following the circuit as written might be a good thing...).  So I'm gonna do some tests on some actual tracks and see what happens before I touch the circuit further.

I am using the A-18 'backwards' meaning the primary on the circuit is 80k and the secondary is 10k. The B+ for the 6SN7 is indeed coming through the Edcor output.

The attack pot had a loose solder point. It works now, but also reduces gain reduction in total. It's not like the average modern compressor in that sense. I'll try the way you described and is on the Altec and see what happens.

I've been tempted to try the cathode follower since I accidentally drilled an extra hole in the chassis, but I'm a bit burnt out at this point. If I am dissatisfied with the level I'm getting I'll give this idea a shot.

I have 120 Hz hum in this build. Likely because the output wire passes directly over both rectifiers. Too lazy to change it cause it's far too quiet to hear over any reasonable signal level.
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 13, 2019, 10:24:42 AM
Slower attack reducing gain reduction is to be expected in a simple to medium complexity vari/mu
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 13, 2019, 08:23:09 PM
Ok just tracked through this thing. Makes the 1176 sound like I'm singing through a plastic cool whip tub.

I can get maybe 15 DB of GR out of it from the ba-2 clone, well above what I ever plan on using. Meter looks a bit more appropriate now too.  If you're the kinda person who likes 20 decibels of reduction or a hard ceiling, it might not be for you. Totally exactly what I needed though and at a very good price.

It's difficult not to clip the second stage. Not positive on the best remedy for that at the moment but so far as I'm concerned it's better than anything I have right now.

Replacing the studio computer soon but I'll get a couple a-b's up.
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 13, 2019, 09:43:33 PM
Awesome
Title: Re: the 1954 Limiter!!
Post by: opacheco on February 16, 2019, 05:56:52 PM
Finally here is the fruit of my labors for the past 2 years (mostly searching for parts... especially the XFRMRs).  I got the circuit from a 1954 RadioCraft magazine, hence the name "1954 Limiter". 

I wanted this project/unit to be grand in size -- big chassis, big meters, big knobs -- just like the old days before everything was optimized to be small and compact -- With each unit in a seven space rack -- I think I achieved that goal.

Bear with me as I try to describe what's going on in the circuit.. still learning:
The limiter uses UTC A-12 (600:80K) input transformer and 2 6SK7s.  The thing I liked is that it implements 250K pot to adjust the screen voltage on only one of the 6sk7 to get the 2 6sk7s screen voltage in balance.  The bias is controlled by a 500K pot (input attenuator) and the grids even have a 500 ohm pot.  The 12AT7 amplifier is used ahead of the 6AL5 rectifier to get a rapid attack - the 6AL5's cathodes are biased positive with respect to their plates and will not conduct until the output signal is fed thru the 12AT7 to have a limit level.  Control of both bias if the 12AT7 and the screens of 6sk7s is obtained with the OD3 voltage control tube.  Adjusting the 25K pot on the OD3 changes the limit level. the attack and release time is fast (for tube limiter).  I really love the sound of it, it uses a UTC A-27 (100K:600) for the output -- sh*t with these transformers how could it sound bad?  The switch is a 3 position switch, limiter "on" "off" and "Balance"-- balance makes is the unit a linear amplifier with no limiting.

(http://homepage.mac.com/cannikin/.Public/1954limiter/1.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/2.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/5.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/6.jpg)

The 2 6SK7s use DC heater power (I got the DC heater circuit from a Marshall amp that uses DC heaters on the input tubes), the other heaters are AC and have a hum balance pot -- just in case I'm getting some 60Hz in them.  (everything the orange cap and couple of resistors to the right on the picture is part of the limiter on/off/balance switch)

(http://homepage.mac.com/cannikin/.Public/1954limiter/heater.jpg)

I used a bus ground scheme and the only ground point is near the input... seemed to have served me well.

(http://homepage.mac.com/cannikin/.Public/1954limiter/turret.jpg)

The meter circuit just toggles between input and output, I tried to implement a compression meter circuit (like the la2a or federal) but the result was a disaster - I burned 2 nice meters in which the needles on the meter melted and now are bent and always point down ;-)  oh well I learned not to do that........

The Dual B+ power supply is 250V (for each channel) which I put in its own 2 space rack chassis to hopefully have ZERO AC hum issues (which worked!! in addition to the ac heater balance pot and dc heaters on the input tubes.  60Hz/120Hz hum has been my nemesis since my first project).  The power connects to the units via a barrier strip =B+/GRND/Heater1/Heater2.   I used a 6AX5 rectifier and since it eats 1.2amps, I had to add a triad filament transformer to power everything else.  The first two filter caps are grounded to the center tap of the power transformer and chassis, the last Filter cap is grounded at the buss bar of the limiter circuit.  This gave me a good ground point (zero volts).

(http://homepage.mac.com/cannikin/.Public/1954limiter/3.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/4.jpg)
(http://homepage.mac.com/cannikin/.Public/1954limiter/7.jpg)

The unit doesn't have a ton of gain like a federal limiter but the limiter starts at -30dbm on the input (input attenuator at max) when the output reaches -20dbm.  I might have to build a line amp to put on the backside of the unit  but I will test it the studio before I start another project....

(http://homepage.mac.com/cannikin/.Public/1954limiter/8.jpg)

Can you upload a schematic and some pic again please?

Thanks
Opacheco
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 17, 2019, 03:41:12 AM
Here's a scheme. I drew it like a kindergartener because that's about where my drawing ability is at.

The scan cut off at the bottom so I had to photoshop in a chassis ground symbol. I also erased some of my mistakes.

Some things to note.. I don't know what the nominal secondary of my power transformer is... I'm guessing it's 300-and-something volts to each side.

Also, I haven't actually tested a .5 ma meter so keep that in mind. It just seems pretty likely it will work.

Last thing is that it's been brought up that my 'attack' adjustment doesn't make sense. It's the 250k dual gang pot near the 6AL5. It definitely does something but I have yet to verify that it actually slows the attack whilst allowing the eventual gain reduction to be expectedly sizable. It definitely reduces gain reduction at least a little. I know that much. I'll do some tests and figure it out.

I've been sick so I haven't gotten the chance to go back into the studio. But I'll get some A-B's in. If anyone has a preferred way to do it let me know. I'll probably just do what's easiest otherwise even if that means singing separate takes. I'm rather good at repeating.
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 17, 2019, 05:26:00 AM
Sweet!

Would the 250k pots be more of a threshold?
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 17, 2019, 06:45:27 AM
Sweet!

Would the 250k pots be more of a threshold?

Somehow, the effect they have really isn't gigantic on how far down the needle will go, but I haven't played around with it much since I swapped out the 100k dual for the 250k dual.

I think it does affect the attack greatly, but I really really would like to be sure before I assert it here.
Title: Re: the 1954 Limiter!!
Post by: EmRR on February 17, 2019, 10:01:03 AM
Nah, you're drawing is great, at least you know how to draw something.  Organic is great, we don't need robot overlords dictating our aesthetic.  It's not draftsman quality, but what is?  A drawing from a draftsman who went to school for that 50 years ago?   They don't even teach it anymore, not with pencils, I don't think. 

You definitely want to change the attack control approach.   

I would swap the power tap points.  You want the output section on the 1st and higher tap. 

A cathode current balance control on the 6SN7 could prove useful, especially with the Edcor output which isn't spec'd for DC current. 

The GR meter could be a 200 microamp for more range possibility, just changes the resistor. 
Title: Re: the 1954 Limiter!!
Post by: PRR on February 17, 2019, 02:16:22 PM
> It's not draftsman quality, but what is?

Way past kindergarten. For publication in a manual, I'd pay him to do one more re-draw, easing the cramped areas and condensing the simple output stage, maybe(?) use the Leroy lettering guide.
Title: Re: the 1954 Limiter!!
Post by: DaveP on February 17, 2019, 03:29:55 PM
Technical drawing was one of the best lessons and the most useful.  It helped me design chemical plant and layouts in industry and design in general.

Now we leave it all to the Chinese I guess.............big mistake.

Having said that, I like drawing circuits in MS Paint these days ::)

DaveP
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 17, 2019, 08:11:49 PM
Nah, you're drawing is great, at least you know how to draw something.  Organic is great, we don't need robot overlords dictating our aesthetic.  It's not draftsman quality, but what is?  A drawing from a draftsman who went to school for that 50 years ago?   They don't even teach it anymore, not with pencils, I don't think. 

You definitely want to change the attack control approach.   

I would swap the power tap points.  You want the output section on the 1st and higher tap. 

A cathode current balance control on the 6SN7 could prove useful, especially with the Edcor output which isn't spec'd for DC current. 

The GR meter could be a 200 microamp for more range possibility, just changes the resistor.

Oh, great idea on the cathodes for the 6SN7. I think I lucked out with this specific transformer and tube, but definitely could use to throw that in there. Granted, the choke was hitting the output with 120 Hz til I moved it to the side of the chassis. That should've been cancelling if the bias was truly spot on, right?

I want to swap to the PSU tap points as well, but first stage seems extremely sensitive to changes in B+. It really has to be on the mark to get great gain reduction without overwhelming the regulator. I could change to 500 ohm dropping resistors and have the first stage hit the 6SN7... do you think it could handle 260 volts or so (I know the tube definitely can, but what about the way it's biased... I'm unaware of the constraints of the whole grid and cathode biasing together thing and have always been too lazy to seek out a clear resource on it)? That would likely put it at around 250 again for the second stage.

I've been meaning to try your other attack idea. Just haven't gotten around to it. Caught some sort of flu earlier in the week and now I'm waking up at like 6 pm.

And damn, glad you guys like the drawing. There weren't any pencils in my apartment and I grabbed the biggest piece of paper I could find. Tried as hard as I could to keep it neat with a pen. I was really just worried that it was going to be difficult to follow more than anything, with a lot of crooked lines, small penmanship, scribbles, inconsistent component sizes, etc. Maybe I'll do it again when I finalize it.

I'm personally gonna leave my meter as is (don't really have the money to go experimenting with what's best).
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 18, 2019, 01:31:31 AM


I want to swap to the PSU tap points as well, but first stage seems extremely sensitive to changes in B+. It really has to be on the mark to get great gain reduction without overwhelming the regulator.



I wonder how much voltage could be gained with a cap before the choke???

or are you saying the od3 needs a steadier power???
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 18, 2019, 02:25:03 AM
I wonder how much voltage could be gained with a cap before the choke???

or are you saying the od3 needs a steadier power???

No, I was just mentioning how the first stage is pretty sensitive. Too much voltage and it fries the OD3, too little and it doesn't reduce gain like you want it. I mean, I could be wrong about that second part, but that definitely seemed to be the case when I tested it. I'm very mistake prone though so it's possible I messed something else up when I did that. Or maybe the loose attack pot solder is what caused it. If you made me guess I'd say the lowered B+ is what did it though cause I think I fixed the attack pot before I switched it.

Anyway, the PSU will get up way too high if the rectifier sees a cap. That's half the reason I have it seeing a choke. I think I just gotta halve the dropping resistors and switch taps. That's if the 6SN7 can handle ~260 in this circuit. I'm not clear on why it would or wouldn't. Might solve the clipping issue I'm having if it works?

EDIT: Just remembered the cathode balance pot. Maybe I'll just keep dissipation down with that when operating at the higher voltage. Maybe I want more dissipation though and I should just divvy up the cathode resistor. Too much crap to think about. Still gotta try to throw in that pot between the diode and sidechain.
Title: Re: the 1954 Limiter!!
Post by: DaveP on February 18, 2019, 03:22:55 AM
If you look up on Franks site,

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/0/0D3W.pdf

Tung Sol 0D3W, on page 4 it gives you the formula for calculating the correct dropping resistance for your application,

Its quite straightforward.

Good luck

DaveP
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 18, 2019, 04:40:50 AM
If you look up on Franks site,

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/0/0D3W.pdf

Tung Sol 0D3W, on page 4 it gives you the formula for calculating the correct dropping resistance for your application,

Its quite straightforward.

Good luck

DaveP

If I use the screen current from the 6SK7 datasheet, it comes out to 2020 ohms.

Using the info I have (42 ma, 246, 153) ..that means that my idling load current, in reality, is about 4.5 ma. Fine. So necessary resistor to make 40 ma at 250 is 2157. That's presuming that the 4.5 ma stays the same or around there. Obviously messing with the resistor is gonna raise the unregulated voltage though. Seems pretty clear that Hilton Remley wanted us maxing out the regulator current in any case though. I can put 200 more ohms on there and see what happens.

I guess your point was that I now have a degree of flexibility with the voltage I can use. Thanks!
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 18, 2019, 11:45:23 PM
Ok, so changed the dropping resistors to 500's.

6SN7GTB (didn't realize that the suffix is an important detail) is getting 275 volts now, I added an under-chassis cathode balance. 500 ohm pot, one side needed 260, other 340. Each plate is sitting at 2.5 W dissipation. I'm fine with that, but will test and see how this sounds and if the clipping issue is still there.

EDIT: Can the Edcor handle this or do I need a new transformer?

Regular circuit now getting a flat 250, I'm proud to announce. The OD3 is giving me 147 volts, so the series resistor actually had to be 2475. I used 500. Cathode current sits at 38 ma now. Great.

Removed 250k dual pot, took the 6AL5 output, hooked it to the slow time constant and the release resistor chain, put 300k 'attack' pot in series from that to .1 uf cap to ground and center tap of input tranny secondary just like some versions of the altec (I found one scheme that wasn't adjustable at all, actually). All seemingly working perfectly.

Unfortunately, I stripped the threads of the pot... they move around the rod with the hex screw somehow, so gotta pick another one up.

Gotta test more extensively. Still waiting on a new computer in the studio to upload something for you guys, but sounds pretty damn good coming out of the monitors. I think I'm close to done. I'll make another schematic at some point but I really gotta get some samples up
Title: Re: the 1954 Limiter!!
Post by: DaveP on February 19, 2019, 03:42:16 AM
Quote
EDIT: Can the Edcor handle this or do I need a new transformer?
Edcor make a transformer for this job, I think its called an  XPP1-15k:600, (pp for push-pull).  Its rated 1W which is about all you will get from that tube. (audio current)
The balance pot on the 6SN7 will help the frequency response in the bass.  This gives current cancellation in the primary and maximises the low frequencies.   Even a "matching"  Edcor will perform if the current is balanced,  You will have to check the balance from time to time as the tubes age.

DaveP
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 20, 2019, 04:09:44 AM
Here's a sample... not sure what that weird clicky noise is.... never heard that before from the limiter and last I checked the hum was just a tad quieter than the hum from the preamp... could've just been the tape machine idling... gotta look into it either way... i have a feeling it's the new computer cause the old one gave us tons of noise problems....

Excuse the song, I did this quickly and sang the first thing I could think of with loud and soft parts hahah. Since I didn't have much time, I dialed in the threshold, and left the attack/release as is on the schematic.

First is not compressed, second is compressed. It's with an RE-20 into the BA-2 clone through a whole bunch of things I didn't feel like unplugging (tape machine monitor, board tape input, 2-track tape machine monitor, duet). I'll send more stuff as I record it. I think there's about 6-8 db of gain reduction in the loud part here.

https://drive.google.com/file/d/1QyqhGJmdzwJOPNCH8OOMkwpu5w6bWn-W/view?usp=sharing

Tested freq response... it definitely passes 20-20.... seemingly more in both directions, actually... didn't try with compression on... we were in a rush to get out of there.
Title: Re: the 1954 Limiter!!
Post by: DaveP on February 20, 2019, 05:06:57 AM
Good voice!

Noise sounded like the mic stand moving.

All sounded good to me.

DaveP
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 20, 2019, 07:35:52 AM
Good voice!



All sounded good to me.

DaveP

+1

we were in a rush to get out of there.

Who's we? ;D

Title: Re: the 1954 Limiter!!
Post by: davemascera on February 21, 2019, 12:46:26 AM
+1

Who's we? ;D

Thanks guys... turned out the sound was from the preamp (it is very quiet sorta had to jack the volume to hear it). Thought the input tube was on it's way out but when I looked under it was this solder point that's been plaguing me for the past year. For some reason the coax cable I used for the grid caps doesn't wanna accept solder. But I scraped it up a bit this time, hit it with some solder that has more flux and lead and it's more solid than the other times so hopefully it stays.

We consists of my bassist and I lol.
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 21, 2019, 06:31:33 AM
You're not the Dave from M>D> are you???
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 21, 2019, 07:06:07 AM
You're not the Dave from M>D> are you???

Umm I mean my band is called Moa Desert. I'm not sure if that's what you mean. We have like 20 fans or something and haven't released anything since 2015 and are going to call it quits after this upcoming album.
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 21, 2019, 07:10:00 AM
I thought so....
You guy's are crazy good!

That Seal cover is insane
Title: Re: the 1954 Limiter!!
Post by: davemascera on February 23, 2019, 08:34:44 PM
I thought so....
You guy's are crazy good!

That Seal cover is insane

thanks man... that drummer isn't the same one still in our band

hope you like our new stuff cause it's the last we're doing... gonna sell all the equipment we have after this too lol.

recorded a bit with this limiter to a track and it really couldn't fit better.. i'll post a clip next week.
Title: Re: the 1954 Limiter!!
Post by: scott2000 on February 24, 2019, 03:29:31 PM
Can't say I've ever heard a plan  like that ... a cut to the chase....head it off at the pass.......?? lol

Excited to hear more.....

Got a cool sound......even reminds me of some Incubus in ways...... very cool....

Title: Re: the 1954 Limiter!!
Post by: opacheco on February 24, 2019, 07:00:26 PM
......A Fairchild for example has an output resistance of 100 ohms before its diodes, which enables its speed to get down to uS........

Dave, How I can get this figure from the Fairchild Schematics??

I would try losing the 10k's and putting in two cathode followers from another tube to drive the 6AL5, this would take the source resistance down to around 250 ohms.

I thought the same, two catodes followers and a I Solid State Diodes Bridge instance the 6LA5 will give us a very low charging times value like the Fairchild configuration!!

Opacheco.