Poor Man's Tube Compressor

[ruffrecords]

You need a lot of negative grid bias to really turn them off.... up to -70V.

Nice pics and useful work.

For a compressor I think it is not necessary to turn them off completely. All we really need to be able to do is vary gm over a 10:1 range for 20dB of gain reduction and that you can usually do with a 10:1 or less variation in plate current and a correspondingly modest range of grid bias.

Cheers

Ian

 

[PRR]

> a long tailed pair

Distortion at large input is much less with "short tail". Under 100 ohms. This aint a feedback amp.

 

[MeToo2]

> a long tailed pair

Distortion at large input is much less with "short tail". Under 100 ohms. This aint a feedback amp.

Fairchild 660 used ±2.05K  (2* 1.8K fixed + 2* 500 ohm pot) cathode resistors shared across 8 tubes.

I've tried (and I guess failed) to fully understand the TBar mod on the Manley Vari Mu (TM) to see how they wired up their 6ba6's (Yes it seems that Manley successfully used a pair of triode-wired pentodes). There seems to be 3 off 1K resistors and 3 off 3K32ohm resistors plus what I guess is a balancing pot on a published photo. I can't make sense of that odd number of resistors or quite see the color codes on their published documentation. http://www.manleylabs.com/techpage/TBAR.html  http://www.manleylabs.com/techpage/t-bar%20mod%202-22-10.pdf

The version 4 mod board seems to make more sense: 4 1K (?) resistors and 2 332R (?).  But the photos are not very clear, and not consistent either (the position of the balance pot seems to move sides even within the one document from page 3 to page 4).

Anyone any better info on this board and the resistances used? Could be a good starting point for tinkering.

 

[ruffrecords]

I now have a selection of remote cut off pentodes and triodes to try some experiments with. I'll report back when I have some results.

Cheers

Ian

 

[ruffrecords]

And now, just to make things interesting I have 'discovered' Vactrols - neat little LED/LDR combinations in a light tight box. Already used on an LA-4 project here and only a couple of GBP from Farnell. I had always shied away from using opto because of the high price of the electro-luminescent based parts used in the LA-2 but if we can get suitable opto for just a couple of GBP then the poor man's opto compressor becomes a distinct possibility.

Cheers

Ian

 

[baadc0de]

A tube "LA-4A" could be even more interesting for me than a new vari-mu (with Altecs here, BA6A in the near future and a 670 in far).

 

[shabtek]

for 'tube' and 'vactrol' see DAOC; a popular project around here

 

[baadc0de]

for 'tube' and 'vactrol' see DAOC; a popular project around here

True, I didn't think of that one..

 

[gemini86]

Fred Forssell has a design for an SS sidechain for an opto, even with provisions for metering.

 

[shabtek]

=1 forssell diy design

fits the poor part, works well.

 

[Joechris]

A side chain could be made with a quad opamp, but then the powersupply gets complex. Thats what made the Fairchild 663 transistor schem look interesting since it runs of 6,3 Volt.
Mabye a tube like 6SQ7 could be used to make that led "blink"

 

[gemini86]

the entire side-chain of the federal am864/u is the 6sq7, one tube doin all the work. As usual, the most expensive part of a (good) tube build is the iron. Whether it be power or audio... So no matter what design you go with, vari-mu or opto, it doesn't get much more poor man than the PRR-vari-mu, or a forssell side chain on whatever tube amp you like.

 

[ruffrecords]

for 'tube' and 'vactrol' see DAOC; a popular project around here

Yup, that is pretty much what I had in  mind - three tubes and optional iron - can't get much poorer than that, although, reading some of the correspondence, it does seem to have some strange quirks.

Cheers

Ian

 

[analag]

Need some ideas, boys?

 

[ruffrecords]

Need some ideas, boys?

Not at the prices you charge 

Cheers

Ian

 

[analag]

Need some ideas, boys?

Not at the prices you charge 

Cheers

Ian

I actually designed a compressor with EL84 in the CV section as an upgrade to the PM660 but I never bothered to post it.
Maybe I will.

 

[analag]

Here's one I was playing around with a few years ago. I have a large quantity of 5687's

 

[Matthew Jacobs]

Analog, that looks nice...

If I where to try and build this P2P, what kind of Transformers would work?

Is that a 10K:10K on input? 15k:600 on output and side chain?

 

[analag]

Analog, that looks nice...

If I where to try and build this P2P, what kind of Transformers would work?

Is that a 10K:10K on input? 15k:600 on output and side chain?

600:10k it is.

 

[Matthew Jacobs]

Thanks.  ;D

I may very well try that one... as I missed out on the PM660