Favorite Ways to Control Compression

[Tobyk]

I always wonder if I chase the same kind of “compression” as others.

Is the goal to limit output peaks? And/or, raise the volume of quiet passages?

Is the goal to have output volume stay the similar as you control of amount of “gain” with the guitar volume?

Is it to make the amp feel less strident? Less ice picky?

Does a clean tone need different set of variables then a dirty tone?

I have a whole bunch of other questions…
However, I can’t really offer suggestions or solutions to any of these mostly rhetorical questions.

Though, I am curious if anyone is amused that the dilemmas we seek as guitar players are often in pursuit of diametrically opposed goals?
Bright and fat?
Warm and crunchy?
Compressed and dynamic?
To me, it seems the magic happens surfing the lines between these attributes searching for conditions that lead simultaneous goals. There are always trade offs.

That’s s very good question. For me it’s the chase of the compression found in many vintage amps, coming mostly from worn, undersized output transformers. Which in turn gives you that creamy, even distortion without harshness or spikes.
To your other dilemmas – it’s true those things are each others opposites, so a happy medium is the goal I guess!

[bepone]

Thanks, have you tried this tube yourself?
Also, what does ’2H’ mean please – 2nd harmonic?

i have used it many times, mostly in triode connection for easier assebly, but also pentode is nice. this tube has linear and compression area, with biasing you can adjust desired effect. i have used also others too, like EF183, EF85.
2H yes second harmonic,

[Tobyk]

Thanks, have you tried this tube yourself?
Also, what does ’2H’ mean please – 2nd harmonic?

i have used it many times, mostly in triode connection for easier assebly, but also pentode is nice. this tube has linear and compression area, with biasing you can adjust desired effect. i have used also others too, like EF183, EF85.
2H yes second harmonic,

Interesting, you don’t have any schematics to share by any chance?
I’ve also heard of the ECC189 being used to create non-linearity in amps – is it only vari-mu pentodes that can generate compression, or vari-mu triodes as well?

[bepone]

Interesting, you don’t have any schematics to share by any chance?
I’ve also heard of the ECC189 being used to create non-linearity in amps – is it only vari-mu pentodes that can generate compression, or vari-mu triodes as well?

vari mu is vari mu, both tubes working the same, squashing one half of the sinusoide
you can do it from the head, take datasheet , see some recommended working point or what you consider ok for your task, calculate anode and cathode resistors to bias the tube in desired operating point. start with the testing and trim resistors to the taste. testing trimming in repeat in various circuits.
pentodes are more fun as you can change "in live" characteristics , by changing g2 voltage (you can adjust gain-LO-HI , distortion, compression)

[Tobyk]

Interesting, you don’t have any schematics to share by any chance?
I’ve also heard of the ECC189 being used to create non-linearity in amps – is it only vari-mu pentodes that can generate compression, or vari-mu triodes as well?

vari mu is vari mu, both tubes working the same, squashing one half of the sinusoide
you can do it from the head, take datasheet , see some recommended working point or what you consider ok for your task, calculate anode and cathode resistors to bias the tube in desired operating point. start with the testing and trim resistors to the taste. testing trimming in repeat in various circuits.
pentodes are more fun as you can change "in live" characteristics , by changing g2 voltage (you can adjust gain-LO-HI , distortion, compression)

Thanks, good info! But how does one know which sinus part is being clipped in the vari-mu pentode? I’m thinking if the pentode is preceeded by an asymmetric gainstage – if they are both hot or cold they’ll cancel each other..?

[bepone]

Thanks, good info! But how does one know which sinus part is being clipped in the vari-mu pentode? I’m thinking if the pentode is preceeded by an asymmetric gainstage – if they are both hot or cold they’ll cancel each other..?

it is the same like all valve stages with common cathode, if signal is taken from anode, then it is inverted from input grid signal, and checked with oscilloscope it is clear which part is supressed (for normal amp stage then is on input grid signal positive part of sine )
your second conclusion is correct, put 2 stages in sequence and it is how i wanted to design all tube compressor with vari mu tubes long time ago , but never realised.. but who knows

[Tobyk]

Thanks, good info! But how does one know which sinus part is being clipped in the vari-mu pentode? I’m thinking if the pentode is preceeded by an asymmetric gainstage – if they are both hot or cold they’ll cancel each other..?

it is the same like all valve stages with common cathode, if signal is taken from anode, then it is inverted from input grid signal, and checked with oscilloscope it is clear which part is supressed (for normal amp stage then is on input grid signal positive part of sine )
your second conclusion is correct, put 2 stages in sequence and it is how i wanted to design all tube compressor with vari mu tubes long time ago , but never realised.. but who knows

Ok, so the vari-mu tube inverts the signal like a normal gainstage, just more asymmetrical. Got it, thank you!

[Tobyk]

Thought I’d add some to this subject, which hasn’t been discussed much I believe. I prefer compression being built in to the amp, no pedals needed. Here are some ways that I know of to create some nice compression:

1. Undersized output transformer.
This often gives very good results. The downside being the OT might get hot. Way too small OT won’t work for long, though.

2. Big output tube screen resistors. Something like 10K–47K will add compression, mostly noticably when amp is cranked.

3. Preamp pentode screen bypass cap resistor or pot. This has greater effect than no.2, especially if the pentode is placed in a late gainstage. Try something like 220K–470K between screen bypass cap and ground. Or use a pot.

4. Have the signal from the OT go via a lightbulb before the speaker. A normal jewel light bulb will do. Swart Night Light attenuator uses this approach.

5. Put a 470 ohm resistor between rectifier tube and first filter cap. This will add sag and compression.

These are the five ways I know of. Feel free to share your insight!

6. Cathode follower with low Rk, say 47K with 100K Rp.
7. Vari-mu tube in a later gain stage, will add asymmetry and compression

[Tobyk]

Question: would imbalancing a cathodyne inverter (by lowering Ck) yield the same results (compression, 2H) as in a cathode follower stage?
People often say a little output imbalance is a good thing in amps, but you never see that in cathodyne inverters..

[pdf64]

No, unbalancing a phase splitter should give the same degree of asymmetry over the whole dynamic range, up to clipping, whereas we’re looking for high signal levels to be affected differently to low signal levels.

[Tobyk]

Interesting, you don’t have any schematics to share by any chance?
I’ve also heard of the ECC189 being used to create non-linearity in amps – is it only vari-mu pentodes that can generate compression, or vari-mu triodes as well?

vari mu is vari mu, both tubes working the same, squashing one half of the sinusoide
you can do it from the head, take datasheet , see some recommended working point or what you consider ok for your task, calculate anode and cathode resistors to bias the tube in desired operating point. start with the testing and trim resistors to the taste. testing trimming in repeat in various circuits.
pentodes are more fun as you can change "in live" characteristics , by changing g2 voltage (you can adjust gain-LO-HI , distortion, compression)

@bepone You seem very knowledgable – what would be your favourite way of generating compression in an amp? Vari-mu tube, power sag resisor, power tube screen resistor, preamp pentode screen resistor, cathode follower, or something else?

[bepone]

@bepone You seem very knowledgable – what would be your favourite way of generating compression in an amp? Vari-mu tube, power sag resisor, power tube screen resistor, preamp pentode screen resistor, cathode follower, or something else?

vari-mu sounds strange (EF83), from all other options the best works tiny output transformer with thin wire, + the same for power transformer..read like 5E3 power trafo but running 6L6's instead 6V6.

small output, like on bandmaster and is rated for 40W!

[Tobyk]

Thanks! Not many tiny OT’s in the market nowadays though, it seems. As for under spec PT – could a sag resistor create the same effect?

[Tobyk]

@bepone You seem very knowledgable – what would be your favourite way of generating compression in an amp? Vari-mu tube, power sag resisor, power tube screen resistor, preamp pentode screen resistor, cathode follower, or something else?

vari-mu sounds strange (EF83), from all other options the best works tiny output transformer with thin wire, + the same for power transformer..read like 5E3 power trafo but running 6L6's instead 6V6.

small output, like on bandmaster and is rated for 40W!

@bepone Did you try cathode follower with low Rk for compression, and if so, did you like it?

[bepone]

Thanks! Not many tiny OT’s in the market nowadays though, it seems. As for under spec PT – could a sag resistor create the same effect?

sorry didnt see.. yes with every anode on put tubes put 100R 5W in series woth OPT and see what will be effect

@bepone Did you try cathode follower with low Rk for compression, and if so, did you like it?

i didnt because is biasing hot , and i like more colder bias (SOldano SLO 39k stage for OD+sustaiiiinnnn)