gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« on: January 26, 2007, 08:48:56 AM »
Would love to learn more about this interesting limiter I think I remember PRR posting a while back but I cant find it now tho.

I found this

http://sujan.hallikainen.org/BroadcastHistory/index.php/GeneralElectric

My electronics understanding isnt great but I want to learn....so please be gentle.

Heres my guesses

I can see a little pre at the begining,below that the 6J5 splitts phase {like a tran with centre tap ??]

Is the 6Y6 section a control amp ???? says its a cathode follower in that web page ....looks push pull to me ....but im not sure

Then does that feed the 6X5 rectifiers ?????? if so why two ???????

whats the 6SN7 DC amp ???? does that amp the control voltage  ??? for some reason

The pair of pull push 6J7s and the 1621s is the output amp ???with 6J5s providing feedback instead of usual capacitors ????
if so I take it that is a good way to do it then ??? expensive

The 1612s are the remote cut off valves ...yes????
I cant see where the control voltage comes in ?????

Well thats my stab at it prob completly wrong so if any1 can help me here I would like to see how it really works

Does any1 know how these sound too ???

Thanks for reading......................Gary O.


gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #1 on: January 26, 2007, 09:21:48 AM »
Hmmm a 1621 is power output tube i just read...well that makes more sense in the circuit.
But cant see a remote cut off tube anywhere.....
So this is not Vari Mu then  :shock: .....how exiting.


Gary O.

EmRR

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #2 on: January 26, 2007, 10:29:08 AM »
I only have about three seconds to look this morning, but at a glance this appears to be a variable feedback limiting circuit, with the 6J5 pair around the amp being the active feedback path.  Everything on the bottom appears to be control voltage.  

I may have the manual.  I think I have an article that goes into detail about it.  I have no time.....
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

plexi

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #3 on: January 26, 2007, 01:50:20 PM »
Hi, I also have interest in this limiter because I own one.  Bought it for $25 from my hamfest buddy.  No power suplly, I need to build one and get this up and running....long on the list of projects.

The first 6J7 and 1621 are line amp.  Then it is picked off for the sidechain and sent to the 6J5 phase splitter, then drives the 6V6's then rectifies thru the 6X5's.  There is some threshold voltage inserted at this point as well.  The filtered (attack release) signal then to the 6SN7, half is meter driver, the other half is maybe a buffer amp to drive the variable feedback 6J5's in the upper line amp.  The control in the cathode circuit is labelled "slope" .

The thing that throws me off is the time delay circuit in front of the line amp?  Feed froward design?  Delay in the signal path to reduce attack time?  I'm not so clear on this....

The variable gain is not a vari-mu style in the traditional sense applying voltage to the grids thru a balancing transformer,   instead it accomplished vari-mu by varying the neg feedback from the 1621's thru the 6J5's back to the cathode circuits of the first 6J7's.

More feedback at heavy limiting will tend to reduce distortions, I have not seen this on other limiters....don't know how this thing sounds because I still have to build a power supply.

+250V  and -250V  is required, the sidechain has to start at a negative voltage (6J5 control tubes cut off) and swing in a positive direction to turn them on and increase the neg-feedback.   AHA, the 6SN7 inverts the polarity of the control signal in order to acheive this.....

If I were to ever get off my ass and build the supply, re cap the limiter, I could give some real data, but for now,........only theorize.  

I bet it sounds just as good as any of those BIG vari-mu's out there.

Anyone have one up and running?

gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #4 on: January 26, 2007, 08:25:51 PM »
Hey thanks guys Im quite pleased with my self for getting a lot a that right ..im learning ...and made much clearer by you guys thanks once again. Wow plei yr a lucky boy what a find $25 im soooo green with envy, I have heard these things swallow up loads of gain with ease, get that PSU made...best of luck.


Cheers .............Gary O.

NewYorkDave

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #5 on: January 26, 2007, 09:12:19 PM »


gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #6 on: January 26, 2007, 09:46:32 PM »
Coool, thanks Dave what a great circuit are there other limiters like this one ?? how does L11A L11B work ??? what are they kinda like  chokes?transformers ????
can they be made now days ??? is this whole thing DIY able Id love to DIY one or make the bais and delay sections to control another amp perhaps.sorry for all the questions but its got me thinking.


gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #8 on: January 26, 2007, 10:16:47 PM »
My brain hertz ....hands up.. its too much for me but still interesting maybe il grasp some of it if I keep reading.......I wonder if mr Sowter can make those , must be important part of this animal tho.

gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #9 on: January 26, 2007, 10:36:00 PM »
What if I using some previous projects I use a LA2A as a line amp followed by the delay inductors feeding a RCA ba21a push pull mic pre slightly adjusted for line level build the 6J5 feedback tubes into the BA21A line amp build the half 6SN7 and full wave rect tube circuits and feed them with my BA6A as the bias amp....could that be made to work

Im kinda thinking out loud here havent thought about this properly for insance I not sure if the BA21A is suitable as only back end is push pull I think, am I in the ball park with this idea or several miles up the road.


gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #10 on: January 26, 2007, 10:41:00 PM »
Hmmm how to get such high control voltage tho ...I think the ba6a s 6H6 only produces around 60volts or so...is the why GE has 2 rects full wave...ah then theres the DC amp 6SN7 ..........have I just answered my own question.

gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #11 on: January 27, 2007, 09:56:37 AM »
Im wondering could these Inductors work for that delay section

http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1191519

Dont no much about this sort of thing so prob miles off here.

Or maybe make something using Steffens great page


http://www.diygallery.de/DIYsites/inductor.html


Any thoughts any1.

Cheers .......................Gary O.

CJ

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #12 on: January 27, 2007, 06:26:48 PM »
Did anybody shrink that reduculous 8,000,000 by 6,000,000 pixel circuit diagram yet?
I would love to read the schematic, but I spen 4 hours unlocking my computer after trying to laod it ten different ways, with ten different softwaRE PROGRAMS?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

ciminosound

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #13 on: January 27, 2007, 06:58:43 PM »

gary o

GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #14 on: January 27, 2007, 07:11:31 PM »
his is a clearer schematic...it a link within my link in my first post ..sorry shoulda pointed it out, its 1.2 Meg

http://sujan.hallikainen.org/BroadcastHistory/uploads/GeBA-5ASchematic.jpg

EmRR

Re: GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #15 on: October 25, 2017, 12:49:18 AM »
Ok.....almost 11 years since the last post. 

I've belatedly (sorry) finished going through a pair of these, and they are pretty cool sounding. 

No slouches in General Electric Engineering dept, I can't think of anything else in tube audio electronics that called for a bipolar 250V PSU, looking at tubes with their plates connected to ground and -250VDC fed through resistors to cathodes will  mess with your head.   Massive external dual tube regulated PSU feeding an amp with 14 tubes, and only a single electrolytic in the amp on the +250VDC side, none at all on the -250VDC side.    23 total tubes for mono, in 13RU and around 80 lbs. 

The super comprehensive DC current meter (30 microamps) will check B+ and B-, most of the tubes, balance match of the 6J5 feedback pair, and gain reduction. 

Using it along with the VU, which can look at signal before and after the gain reduction amp, you can get comparative gain reduction readings on 2 different scales, which confirms you've aligned it correctly.  If it's not aligned correctly, the GR meter will tell you it's doing something when it's not doing much at all. 

These things are damn flat for 1945, both examples are within a dB or so from 20-30K, audio going through 4 audio transformers and one all-pass filter. 

Thordarson transformers too, so frequently maligned as inferior.  No, they definitely aren't.  1940's Collins Radio gear with Thordarsons also measures at higher bandwidth than many competitors. 

That input transformer for the gain reduction amp:  also rarely seen, a 600:600 feeding PP tube grids.

Really meaty and hi-fi sounding.  None of the smush of a vari-mu....since it isn't one.   Reminds me of the other feedback tube limiters I've heard, Collins 26-C being one.   Differs with the all pass filter creating instantaneous limiting, and the multiple time constant means it really doesn't pump like a vari-mu.  I tried altering the time constant to see what would happen, and it was nothing good. 

The input control is more precise than most tube limiters, (30) 1dB steps. 

The output control is interesting, these are meant to hit a very specific +12dBm output level with limiting calibrated as designed, so the range is 9dB total in 0.2dB steps.  I haven't seen a Daven control so fine in detail anywhere else.   It does not affect compression in any way, so as the manual states, effectively a vernier control for max output. 

There's an interesting story I tend to believe, that at least one NBC owned high power station used this, with all other equipment being RCA ( mandated, naturally), though the GE was repainted and rebranded RCA to avoid scrutiny from visiting corporate types. 
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

rackmonkey

Re: GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #16 on: October 25, 2017, 12:41:18 PM »
Thanks for the write up, Doug. Very, very interesting. I've had an interest in feedback comps/limiters for a while. I have a Thordarson feedback transformer that was specifically designed for a  Thordarson designed feedback comp/expander that they would sell you the plans for for $15 or thereabouts back in the 40's.  Look in the 1947 Thordarson catalog that's floating around on BAMA and elsewhere and you'll see a little box about it with a functional diagram. Have not been able to locate that schematic, hard as I've tried.  Strong performance claims over Vari-mu designs, including faster response and lack of distortion when compressing.

By chance, is there a visible model number on that feedback transformer?

And as for Thordarson bashing, I've used their Bantam and Major series Tru-Fidelity transformers in 2 different projects, and really love them.  I wouldn't hesitate to use them again.

Thanks again,

BT
Whether you think you can or you think you can't, you're probably right.

EmRR

Re: GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #17 on: October 26, 2017, 10:28:41 AM »
In this case it's not a feedback transformer, it's a 6SN7 stage from the side chain driving a pair of 6J5 grids which conduct, or don't, to varying degrees to change feedback in the output amp. 

I have that Thordarson feedback transformer, and the provided drawings.  I'll get it copied.  Memory says the plans are more around PA type amps.  I'm not sure what's really different about it from many other interstage transformers. 
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

rackmonkey

Re: GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #18 on: October 26, 2017, 12:22:44 PM »
Thanks, would love to see the drawings. Kind of you to offer to dig those up. Don't go to too much trouble, though.

Understanding the details of how the BA-5A works just from the schematic was beyond my circuit analysis abilities, but I ran across some comments on it elsewhere that aren't clearly stated (at least to me) in the theory of operation NYD posted in the original thread.

One explanation of the circuit theorized that the original frequencies of the incoming audio signal are multiplied to above-sonic levels. Compression is then applied to this (frequency-multiplied? modulated? - what's the right term?) signal, then passed through a high pass filter which cuts out the subsonic stuff (and thus the thumping) before it's converted back to the original frequencies. 

Another guy who said he'd used and loved these working in the broadcast industry in the 40s and 50s gave a different explanation.  He said that "the network of inductors and capacitors in the main audio channel is an all-pass filter circuit that rolls the phase of the higher frequencies 180 degrees. This helps to even out the positive and negative peaks , especially of the human voice. ( think Khan semetri-peak) In this case the low frequencies remain phase normal as the higher ones are spinning around the axis. The control voltage is developed ahead and parallel to the main audio, then is applied to the feedback limiter tubes in real time to vary the amount of gain. This limiter has instantaneous attack time and when working properly has a transparent sound quality."

The comments were on a post about the unit over on Preservation Sound's blog.  The latter theory seems to align better to what I can see in the circuit, but curious if that's fully accurate.

Oh, and the latter guy stated that these things weight 75lbs? I know there's a lot of iron in there, but holy smoke. Is that an exaggeration, or do these things really weigh that much?

Thanks,
BT
Whether you think you can or you think you can't, you're probably right.

EmRR

Re: GE BA 5A Vari Mu limiter guided tour needed any info ?????
« Reply #19 on: October 26, 2017, 01:02:45 PM »
The one explanation is wrong, no frequency shifting, that's the BA-7 which is vari-mu, but has a side chain which put audio onto an RF carrier frequency for processing, then strips it back out.   (loosely)  2nd description is right, not sure what phase rotation exists....actually I do have a phase plot....more later.....

Yeah, 13 RU total, steel chassis, 7 transformers, 2 of which are identical 150mA DC/8A fil power.  Really is that heavy.   Just a small part of the overall weight of a transmitter chain.   WE 110A lists at 68 lbs, RCA 96-A is similar separate regulated PSU, 14RU total, 97 lbs.   Your average Gates SA-39/Collins 26U/everything else all fall around 50 lbs. 
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


 

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