EmRR

Re: the 1954 Limiter!!
« Reply #60 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
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


Re: the 1954 Limiter!!
« Reply #61 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.

EmRR

Re: the 1954 Limiter!!
« Reply #62 on: February 13, 2019, 09:43:33 PM »
Awesome
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

opacheco

Re: the 1954 Limiter!!
« Reply #63 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.






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)



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



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).





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....



Can you upload a schematic and some pic again please?

Thanks
Opacheco
I love the Vaccum Tubes Sound!!!

Re: the 1954 Limiter!!
« Reply #64 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.

scott2000

Re: the 1954 Limiter!!
« Reply #65 on: February 17, 2019, 05:26:00 AM »
Sweet!

Would the 250k pots be more of a threshold?

Re: the 1954 Limiter!!
« Reply #66 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.

EmRR

Re: the 1954 Limiter!!
« Reply #67 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. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

PRR

Re: the 1954 Limiter!!
« Reply #68 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.

DaveP

Re: the 1954 Limiter!!
« Reply #69 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
Soundcloud: Delayed Action.


Re: the 1954 Limiter!!
« Reply #70 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).

scott2000

Re: the 1954 Limiter!!
« Reply #71 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???
« Last Edit: February 18, 2019, 01:42:58 AM by scott2000 »

Re: the 1954 Limiter!!
« Reply #72 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.
« Last Edit: February 18, 2019, 02:32:02 AM by davemascera »

DaveP

Re: the 1954 Limiter!!
« Reply #73 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
Soundcloud: Delayed Action.

Re: the 1954 Limiter!!
« Reply #74 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!

Re: the 1954 Limiter!!
« Reply #75 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
« Last Edit: February 18, 2019, 11:50:24 PM by davemascera »

DaveP

Re: the 1954 Limiter!!
« Reply #76 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
Soundcloud: Delayed Action.

Re: the 1954 Limiter!!
« Reply #77 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.
« Last Edit: February 20, 2019, 04:17:01 AM by davemascera »

DaveP

Re: the 1954 Limiter!!
« Reply #78 on: February 20, 2019, 05:06:57 AM »
Good voice!

Noise sounded like the mic stand moving.

All sounded good to me.

DaveP
Soundcloud: Delayed Action.

scott2000

Re: the 1954 Limiter!!
« Reply #79 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



 

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