Tube Compressor

[lowell]

So I heard Come Together on the radio the other day... Beatles original version. It reminded me that I absolutely love the sound of the lead guitar. It sounds "clean" but quite compressed and "tubey." Has anyone played around with diy tube compressors for guitar? I'm now on a mission to build a guitar amp with built-in tube compression, but without "distortion." I know, technically compression is distortion but ya know what I mean. A cleanish sound, but compressed. I've done some surface research on google. Seems there are two main types of tube compressors: Variable-mu and opto-coupled. I think for now I'd like to stay as simple as I can with this, but not without getting the right sound. It has to have that spongy feel, or it's all for naught. I certainly don't want a hard limiter feel. I think a custom built 5E3 modded to be loud and clean would be my ideal amp circuit to incorporate this into. So I guess my question is does anyone have any schematics they would share? Or any thoughts on this.

[es345]

Search for the schematic of the Trace Elliot V8, there you will find a tube compressor inside the preamp.

[Diablo]

Could you use a large value screen resistor on the output tubes to give you that compression? Or maybe use a rheostat instead of the screen resistors?

[printer2]

Google gates sta level schematic, Teletronix LA2A, Fairchild 670, Analogue Tube AT101

try gearslutz.com or groupdiy.com

MAXSON CA-1589 Tube Limiter and Tubes

[Merlinb]

Seems there are two main types of tube compressors: Variable-mu and opto-coupled.

All valve compressors are variable-mu (short for variable mutual conductance, or variable-gm).
Opto compressors are quite separate, and built from light-dependent resistors. You can bolt a valve preamp or side chain onto an opto compressor, but it's a bit grandiose to call the end result a "tube compressor". For example, the LA-2A is just an opto compressor with some valves in it, not a valve compressor. Same goes for the EH blackfinger.

You could get the same effect from an SS opto compressor (or indeed any SS compressor) and a separate valve preamp.

[lowell]

Could you use a large value screen resistor on the output tubes to give you that compression? Or maybe use a rheostat instead of the screen resistors?

While a largish Rs provides some squish it's definitely not a "compressor." There is not envelope, attack etc.. controls. Also, as Rs goes up, bias voltage needs to go more positive, thus reducing the maximum clean signal one can apply to the grid. Same goes for a self-biased amp and Vgk. That all being said one could still run this setup "clean" and have some gain makeup afterwards. It's something to consider.

[lowell]

Thanks Merlin so I'm thinking I'll stay away from an opto-coupled compressor. That seems to me like "faking it." It's akin to those low voltage "tube" stompboxes.

[Steve Conner]

While a largish Rs provides some squish it's definitely not a "compressor." There is not envelope, attack etc.. controls.

If you use a large shared screen resistor with a capacitor, then you have a time constant that gives a more "compressory" feel.

[lowell]

If you use a large shared screen resistor with a capacitor, then you have a time constant that gives a more "compressory" feel.

A capacitor in parallel with Rscreen?

[Merlinb]

All valve compressors are variable-mu (short for variable mutual conductance, or variable-gm).

Actually it just occurred to me that there must be some diode-bridge compressors around, using valve diodes. Those would also qualify as proper valve compressors!

[lowell]

Ok so that's also something I've seen. A diode rectifier on the OT secondary, which feeds back to the input tube's grid/s.

[printer2]

I would think adding a couple of tube diodes in the side chain would not impart too much tubeyness to the dc signal.

[lowell]

would NOT? Hmmm... not sure but I'd initially think the internal resistance of a tube rectifier might play a part here. However, that is only the case when current demand is high so in this particular setup it wouldn't impart "sag". correct?

[lowell]

Found something online "suggesting" that the Fairchild 670 may have been used on the recording. It's an interesting schematic with all that transformer coupling going on. There are said to be some mistakes on this forum:

http://recording.org/diy-pro-audio-forum/40934-fairchild-670-readable-schematic.html

http://www.vintagedesign.halmstad.ne...s/670schem.pdf

[loudthud]

Did anybody notice that the bridge rectifiers are drawn wrong?