Vari-Mu Compressor Question ( Thump )

[buildafriend]

> I found this in the old RCA book

That's a clipper. When diode overcomes battery, gain goes way low, output does not increase so much. There's no envelope filter-- "hard" compression (not brick-wall because the diodes are soft).

This is a talk-limiter for communications radio. Speech may splatt but is highly intelligible due to improved loudness and vowel-clipping ("F" above).

Cool thanks PRR!

 

[rackmonkey]

> I found this in the old RCA book

That's a clipper. When diode overcomes battery, gain goes way low, output does not increase so much. There's no envelope filter-- "hard" compression (not brick-wall because the diodes are soft).

This is a talk-limiter for communications radio. Speech may splatt but is highly intelligible due to improved loudness and vowel-clipping ("F" above).

I’ve got an old Electro Voice model 1000 speech clipper for ham radio comms in the closet somewhere. Uses a 6H6 dual diode as well, with a single 6SC7. I’ve thought on and off about what to do with it, and keep coming back to playing around with the circuit to work as a tube fuzz or distortion box for guitar. Tube clipper waveforms aren’t far from Ge distortion on a scope, with the soft knee as you reference.

Not sure if it’s worth even messing with, given how easy it is to build Ge fuzzes/distortions.  Maybe it’s got enough unique potential to spend time on though. I’d be interested in thoughts either way, though it’s only loosely related to the topic.

BT

 

[buildafriend]

Hey rackmonkey, that sounds like a cool piece to have laying around and I hope you find a use for it. I have so many parts for fuzz pedals laying around that I'm making Ge ToneBenders just to clean out the shop. The valve wizard's basic triode literature has some pictures of hot and cold biased 12AX7s which might be nice for you if you ever want to show someone how sharply a valve clips a nice sine wave by moving the cathode bias around.

Moving forward I'm looking for literature on Vari-Mu compressor design and I wonder what the sources are for these vintage electronics magazines that show cool circuits like the one Rob Flinn posted.

General consensus seems to be stick with the "good" topologies like that of STA level, 436C,  BA6A, 660/670... I'll keep gawking at those and RDH4 for now.

 

[mjrippe]

I’ve got an old Electro Voice model 1000 speech clipper for ham radio comms in the closet somewhere. Uses a 6H6 dual diode as well, with a single 6SC7. I’ve thought on and off about what to do with it, and keep coming back to playing around with the circuit to work as a tube fuzz or distortion box for guitar. Tube clipper waveforms aren’t far from Ge distortion on a scope, with the soft knee as you reference.

Not sure if it’s worth even messing with, given how easy it is to build Ge fuzzes/distortions.  Maybe it’s got enough unique potential to spend time on though. I’d be interested in thoughts either way, though it’s only loosely related to the topic.

BT

I have a pair of those on the shelf as well.  I did hook one up to power and plugged in a guitar and it was a nice tube fuzz.  I'm not a player though, so hard to judge how "good" it distorts.  It would take up a lot of space on a pedalboard once you add a PSU.

 

[rackmonkey]

Nice to know, Mike, and thanks guys for indulging the side topic. Think i’ll At least put it on the list to play with once i whittle down the queue of projects a bit.

Buildafriend, there are several repositories online with lots of those old publications you asked about. Check out http://www.americanradiohistory.com and look at the “Technical” section as a starting point. Gotta run right now to do some stuff for my daughter’s birthday, but i’ll Post a full list later today if others don’t beat me to it.

BT

 

[bluebird]

Ive built this clipper in stereo for program material and it is really cool. Kind of like a decapitator plug in. Its soft on the high end so you can dig into it pretty good without it getting too spitty. I added an input transformer and output driving circuitry. At the end it was a lot of work and a lot of parts just for some "good" clipping. Might not be worth it for most.

 

[Matt C]

I added an input transformer and output driving circuitry.

I know this is getting off topic, but what was the output circuitry you added/what problem was it solving?  I'm always looking for more flavors of distortion so I'm interested in this thing

 

[rackmonkey]

We should probably start a thread on clipper circuits rather than totally hijacking this one. Just my $0.02. Happy to do that if that’s the consensus.

Buildafriend, another source for ancient thermionic wisdom is Audio Engineering Magazine. Ran from the early 50s to the early 60s, I think. It’s where I found that Vari-mu patent I posted about in this thread. I think they have all issues scanned here:

http://www.vintagevacuumaudio.com/magazines-1952-1962/

But like the American Radio History archives, be prepared to have to hunt if you’re looking for something specific. It’s a lot of material, and a lot of it may or may not be of great interest. But there are gems peppered throughout if you’re prepared to spend the time.

The Meta has most of the links to specific, commonly sought manufacturers/models, of course.

Hope this helps a little.

BT

 

[brujo]

Ive built this clipper in stereo for program material and it is really cool. Kind of like a decapitator plug in. Its soft on the high end so you can dig into it pretty good without it getting too spitty. I added an input transformer and output driving circuitry. At the end it was a lot of work and a lot of parts just for some "good" clipping. Might not be worth it for most.

Attachment is down, any chance to reupload? thanks!

 

[scott2000]

Attachment is down, any chance to reupload? thanks!

Looks fixed.....

Here's a solid state version too....

 

[duffharrisnyc]

One thing I keep toying with is the idea to make a BA-7-A with solid state oscillator and balanced modulator/synchronous detector for single sideband modulation. I don’t really know all of this modulation stuff as well though so reading about it now. It’s a pretty cool circuit though. Has both feedback and feed forward options, and single ended vari-mu. From what I understand, it works by adding an IF carrier frequency and whatever “thump” comes about gets demodulated out of the signal once it passes through.

 

[abbey road d enfer]

One thing I keep toying with is the idea to make a BA-7-A with solid state oscillator and balanced modulator/synchronous detector for single sideband modulation.

Do you have a schemo for this unit? Can't find it on the web.

 

[duffharrisnyc]

Do you have a schemo for this unit? Can't find it on the web.

http://bee.mif.pg.gda.pl/ciasteczko...ic_ge-ba-7a_audiomatik_limiting_amplifier.pdf

Id imagine one could get it down to 5-6 tubes (instead of 17!) if the oscillator and RF modulation/demodulation were all done with op amps and solid state diodes. The part that throws me is all the variable inductors. Doesn’t seem to be a lot of info on them even here. Would be a cool way to use up single remote cutoff tubes with no match!

 

[abbey road d enfer]

Thank you for posting this document. However, I couldn't make much sense of it. What do all these oblique junctions mean?
For example, try to trace the output of L9. It's the output of the local oscillator.
One leg goes to the plates of V5 via a 100k resistor, the other goes to the same via a 47pF. It doesn't make sense to me.

Regarding "variable" inductors, they used to be pretty common, now supersede by ceramic filters. However there are still some possible sources.
https://www.ebay.com/itm/1460462055...9+QPjpJDMH3enyDBYl3s+iZvw=|tkp:Bk9SR8TumdLLZA

or
https://talonelectronics.com/shop/item.aspx?itemid=9110

member rogs, uses them in his RF condenser micophone.
https://www.amx.jp137.com/index-parts.html

 

[duffharrisnyc]

It’s super confusing and poorly drawn. I had to redraw each section as I could understand it. Will upload pics later tonight after a gig.

 

[duffharrisnyc]

It also seems that the “B- Bus” is an artificial ground around 100V I think.

 

[duffharrisnyc]

It does say “slant lines indicate the direction to the nearest connection point.”

Thank you for posting this document. However, I couldn't make much sense of it. What do all these oblique junctions mean?
For example, try to trace the output of L9. It's the output of the local oscillator.
One leg goes to the plates of V5 via a 100k resistor, the other goes to the same via a 47pF. It doesn't make sense to me.

Regarding "variable" inductors, they used to be pretty common, now supersede by ceramic filters. However there are still some possible sources.
https://www.ebay.com/itm/146046205561?_skw=10.7+mhz+if+transformer&itmmeta=01J9EJ6EWJEWQ452P64GQQ4V0Q&hash=item2201083e79:g:j0QAAOSwBgFm8FV1&itmprp=enc:AQAJAAAA0HoV3kP08IDx+KZ9MfhVJKk/pRmmxCypaenCpydcpBGznrS2HXv/1G07xDnSzuzXwM8WqD7fSrE8CyW9ywqCHSPwuPu0mNsJ3Xch9AHXwccSN6OFaIO39xgg3lHAZCAIIvagbcRwY2GJNJU0oo3sakOl6miM3DniOXFP2kFbnCIFSnzypW2K2IF5OU8GyB2PMbIdUTzeDFNdvheCaUpQd7MvX8L4R6mSNfVGq2gRs+3nw8WEiXj1gn7u+Hcjsn9+QPjpJDMH3enyDBYl3s+iZvw=|tkp:Bk9SR8TumdLLZA

or
https://talonelectronics.com/shop/item.aspx?itemid=9110

member rogs, uses them in his RF condenser micophone.
https://www.amx.jp137.com/index-parts.html

One side goes to ground, not a 100k resistor. It’s just confusing to look at.

 

[CJ]

food for thought, don't ask, >

 

[Murdock]

food for thought, don't ask, >

I know you said don’t ask, but do you know which output transformer is used in that circuit? Is that a separate feedback winding?
Also, the use of a FET in the SC reminds me of the TAB Varicom.
Schematic is on the bottom of the pdf.
I asked myself before if the varicom could be used with other Vari mu compressors. I know they were only used with U73 units.

 

[abbey road d enfer]

It does say “slant lines indicate the direction to the nearest connection point.”

Which makes no sense at all to me.

One side goes to ground, not a 100k resistor. It’s just confusing to look at.

To me, all these points seem to be connected together.
If not how to untangle it?
I give up.