Choosing Ck for cathode biased 6L6's

[wagdog]

Greetings all,

I have a twin project amp in which I have (currently) 4x6L6's using cathode bias. It sounds very sweet, nice and round, with a soft edge to it. Single notes compress nicely and just sort of swell as they sustain. I actually had a switch at one time to run fixed bias or cathode bias and found I never used fixed bias.

I'm using a Ck of 50uf as that's what I had handy at the moment.

I tried hooking up a switch to take Ck in and out of the circuit, and quite honestly, could not hear a sonic difference with it in or out.

I'd actually like to hook up a spdt on-off-on switch with a couple different Ck's for a switchable high-end or none or low-end boost. What would be proper way to come up with a value(s) for Ck in such a circuit?

Thanks!

[MWJB]

Listening tests.

Larger the bypass cap then the tighter the amp's response, going bigger than 100uf may not make much difference. I'd perhaps hook up the switch with 100uf, 25uf & no cap? Use 100v rated caps.

[trobbins]

Merlin gives a good background description on the effect of bypassing cathode bias stage:
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

It's worth putting a bleed resistor to each cap on the switch, so that they are kept at the DC bias bias point - then you won't get any sudden current surges as you change switch positions.

Ciao, Tim

[imaradiostar]

Gut feeling says 50uF might not be enough for a 6l6 to be flat across the guitar range but I hadn't done the math- as said above Merlin shows how to calculate it.

Was there a noticeable high-frequency volume change when switching out the cathode caps?

jamie

[wagdog]

Listening tests.

Larger the bypass cap then the tighter the amp's response, going bigger than 100uf may not make much difference. I'd perhaps hook up the switch with 100uf, 25uf & no cap? Use 100v rated caps.

That sounds like a good plan. I had a switch w/no cap and the 50uf, and really couldn't hear much difference.

Merlin gives a good background description on the effect of bypassing cathode bias stage:
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

Got the book. I think the bypass cap has a more pronounced affect on a preamp stage then a power-amp maybe not, but that's just what I'm seeing in my amp anyway.

Was there a noticeable high-frequency volume change when switching out the cathode caps?
jamie

Not really any perceivable audible change to be honest.

I guess I'm going to hook up a spdt on-off-on switch and dig into my parts bin to find a bunch of different values to try. Thanks for the tip on the bleeder resistors.

[Chuck H]

What is your plate voltage, cathode resistor value and voltage on top of the cathode resistor?

[wagdog]

What is your plate voltage, cathode resistor value and voltage on top of the cathode resistor?

Plate -> Cathode is 425v
Rk is 250R (20w)
Vk is 43v

This is for four 6L6EH's.

I'm seeing 43ma across 1ohm resistors between the cathodes and the top of Rk.

[pdf64]

The pre amp cathode equations don't work as expected on push-pull power amps with shared cathode resistor, because the inverse signals from the phase splitter appear at the cathodes and mostly cancel out, so the cathode resistor doesn't do much. Except that as the signal level increases and the tubes cut off for some of the wave, then the cancellation stops and the cathode bypass cap comes into play.
So as advised, listening tests at high signal levels are the key thing, there's no simple frequency breakpoint.
Have you scoped the output?
Pete.

[wagdog]

The pre amp cathode equations don't work as expected on push-pull power amps with shared cathode resistor, because the inverse signals from the phase splitter appear at the cathodes and mostly cancel out, so the cathode resistor doesn't do much. Except that as the signal level increases and the tubes cut off for some of the wave, then the cancellation stops and the cathode bypass cap comes into play.
So as advised, listening tests at high signal levels are the key thing, there's no simple frequency breakpoint.
Have you scoped the output?
Pete.

Thanks for that, very informative. That may be why I'm not hearing much effect from the different Ck values.

I'm afraid running this amp to the point that the power amp would clip might be REALLY loud! Actually, I'm certain it would be really loud.

I do have a scope from my ham radio work in the past, but have not scoped the output - I should do that, be interesting to see.

[ThermionicScott]

Plate -> Cathode is 425v
Rk is 250R (20w)
Vk is 43v

This is for four 6L6EH's.

I'm seeing 43ma across 1ohm resistors between the cathodes and the top of Rk.

Kind of OT, but that's a little cool for cathode bias. I'm figuring 16.4 watts per tube at idle, and that's on the cool side for fixed bias.

With a handful of 8-ohm/20-watt power resistors, you should be able to scope that output (and see the Ck's effect at different frequencies) without making much noise.

[Chuck H]

I'm afraid running this amp to the point that the power amp would clip might be REALLY loud!

Hmmm... If your not turning it up the amp may not even be operating AB1 yet (though it is biased there). Small drive signal may not even have the power tubes in cutoff at the level your ear testing. If the tubes are operating in what amount to class A operation there will be little or no difference in operation between no bypass cap, big bypass cap or small bypass cap.

[wagdog]

Hmmm... If your not turning it up the amp may not even be operating AB1 yet (though it is biased there). Small drive signal may not even have the power tubes in cutoff at the level your ear testing. If the tubes are operating in what amount to class A operation there will be little or no difference in operation between no bypass cap, big bypass cap or small bypass cap.

OK... that also might explain what I'm hearing.

Kind of OT, but that's a little cool for cathode bias. I'm figuring 16.4 watts per tube at idle, and that's on the cool side for fixed bias.

Plate -> Cathode is 425v (not plate to ground, which is ~v468)

With 43ma across 1ohm resistors between the cathodes and the top of Rk I calculate 425*.043, ~18.3 watts - still a little cool, but I've found that twin style amps sound better on the cool side.

With a handful of 8-ohm/20-watt power resistors, you should be able to scope that output (and see the Ck's effect at different frequencies) without making much noise.

Yeah, that would be cool. I dug into my junk box and found a 15ohm 100w power resistor that I could use. Slight mismatch, but it would work.

[Gingertube]

Gingertubes experience and opinions (feel free to debate the later).

Remember that when switching out the cathode bypass cap the amp output stage is geting some signal level self balancing being applied via "common mode" feedback.
I actually like that sound.

In my HiFi Tube Amp designs which are all fixed biased I also add a common cathode resistor for each pair of a value of around 15% of the value what each individual cathode bias resistor would be (Example EL84 push pull would normally have a 270 Ohm cathode bias resistor on each tube - I used a 39 Ohm common resistor + fixed bias). This was intended to reduce 3rd and high odd harmonic distortions (via common mode local feedback) BUT what showed up on a spectrum analyser was not just a reduction in odd order harmonic distortions but an almost complete elimination of Intermodulation distorton, in particular the "sidebands" due to power supply ripple were reduced significantly.
So there is more happening here than you might expect. I have NOT tried this on a guitar amp - YET. (You could say that this method applies 30% cathode bias + 70% fixed bias).

I built a guitar amp for a friend which has 4 off 6V6 output tubes in parallel push pull with switchable fixed bias / cathode bias for each pair (that is a single "common" cathode bias resitor for each pair). With bypass caps on the shared cathode bias I could not really tell any sonic difference between fixed and cathode bias. With the bypass caps deleted there WAS a sonic difference and I then preferred cathode bias.

Big cathode bias bypass caps can also cause blocking distortion problems when overdriven - another good reason to leave them out entirely.
HiFi boffins (like where I came from) will tell you that unless you are running Class A Output Stage you MUST use cathode bypass caps (from a minimum distortion perspective) - my experience with Guitar Amps says you can leave them out and that extra output stage self balancing offers worthwhile improvement.

Having said that (the counter arguement) - I did a full restoration on a VOX AC30 (the old copper front panel model). On that, I rebuilt the output stage with 2 x 470R 2W in Parallel (235 Ohms 4 Watt) with 47uF bypass on each of the 4 Output Tubes. In that case I was (of-course) trying to retain the VOX "sound".
That also was stunning so you can't make hard and fast rules to cover every situation.

Cheers,
Ian

[Chuck H]

Gingertubes experience and opinions (feel free to debate the later).

There's no debating opinion... Especially WT guitar amps

Remember that when switching out the cathode bypass cap the amp output stage is geting some signal level self balancing being applied via "common mode" feedback.
I actually like that sound.

See

In my HiFi Tube Amp designs which are all fixed biased I also add a common cathode resistor for each pair of a value of around 15% of the value what each individual cathode bias resistor would be (Example EL84 push pull would normally have a 270 Ohm cathode bias resistor on each tube - I used a 39 Ohm common resistor + fixed bias). This was intended to reduce 3rd and high odd harmonic distortions (via common mode local feedback) BUT what showed up on a spectrum analyser was not just a reduction in odd order harmonic distortions but an almost complete elimination of Intermodulation distorton, in particular the "sidebands" due to power supply ripple were reduced significantly.
So there is more happening here than you might expect. I have NOT tried this on a guitar amp - YET. (You could say that this method applies 30% cathode bias + 70% fixed bias).

Interesting. Nice R and D. Could be a very effective method for making these inexpensive and generally underperfoming tubes behave. But keep in mind that the intermodulation distortions of EL84's is part of their sound. Part of why they sound so "jangly". As guitar amps go odd distortions can become part of the instrument. We're not dealing with sound reproduction here but rather sound developement.

Big cathode bias bypass caps can also cause blocking distortion problems when overdriven - another good reason to leave them out entirely.

How? Unless operating in class A big caps generally sabilize the bias condition. All things being equal a smaller cap would allow for more bias shift and promote crossover distortion. So I suppose it's a trade off. Just playing devils advocate really, but the point is that designing the circuit to behave under certain drive conditions is the key to good overdrive. And that's a very subjective matter. Also, there's no steadfast rules or classes that teach how to get "good" distortion. It's pretty much all about the elimination of distortion as study material.

HiFi boffins (like where I came from) will tell you that unless you are running Class A Output Stage you MUST use cathode bypass caps (from a minimum distortion perspective) - my experience with Guitar Amps says you can leave them out and that extra output stage self balancing offers worthwhile improvement.

Many here would agree. TONS of posts here about omitting the bypass cap from AB1 cathode bias guitar amps to achieve something specific.

I did a full restoration on a VOX AC30 (the old copper front panel model). On that, I rebuilt the output stage with 2 x 470R 2W in Parallel (235 Ohms 4 Watt) with 47uF bypass on each of the 4 Output Tubes. In that case I was (of-course) trying to retain the VOX "sound".

OOoohh... Not really a strong argument to "retain the VOX sound". The alteration of the power tube arrangement and cathode R values changes the impedance of the circuits significantly. And therefor the knee frequency of the 47uf value that, while stock in the vox amps is presenting a very different knee frequency and effect in your above described circuit.

[wagdog]

Sort off the original subject... but w/a b+ of 470'ish, what would folks recommend setting the bias at?

I tried it this morning w/a 500R and 300R in parallel (187.5R), got 433v plate->cathode, 41v cathode to ground and 54ma through the cathode 1R resistors, approx 23w per tube idling, or 77% of max dissipation. Sounds really nice, almost squishy but not quite, very responsive to pick attack.