Distortion issue – question about bias

[überfuzz]

I was asked to check an amplifier with a distortion issue.

The tubes, a quad, measures within 5% voltage: 455V Amps: 40mA. Giving me less than 20 watts per tube, wish would be 66% of max, more accurate - 60%. Which to me seems to be well within normal bias. However, the distortion did wear off as I raised the bias…

Is it possible that this is crossover distortion (translated from Swedish, don’t know the English term) even if the bias is over 60%? How high is it sensible to raise the bias..? I usually bias as low as auditive possible.

[Jazz P Bass]

What amp is it?
What is the bias voltage at pin 5?
What is the actual amplifier output (maximum clean) into a dummy laod?
Do you have a scope?
You should be able to see the "crossover" distortion on a scope.

[MWJB]

40mA per tube @ 455vdc in fixed bias, 6L6/5881/EL34 makes crossover distortion unlikely. You shouldn't be running into that at clean W RMS ratings until you get MUCH lower, under 30mA?

I'd check the other dc voltages (specially screens, PI plates) to see if anything else was upsetting the balance of the PP output.

[pdf64]

I've noticed that EL34 (winged C at least) tend to crossover distortion at full power output, unless biased rather warm. Pete.

[überfuzz]

I didn't have any tools, so I couldn't gut the thing. I just probed it. Is the negative grid control likely to cause this kind of distortion?

By the way, I'm not sure but by listening to the distortion it seemed to be more evident on the mid stings, G and B, around fret 3 to 6. It's quite possible that this is just my perception though.

[überfuzz]

pbf64 - I missed your post. Ill try to raise the bias further to see. However, I'm a bit reluctant to raise the bias to high. My experience is that the tone break up starts to be somewhat loose.

[überfuzz]

I forgot to mention, the distortion happens when I hit two or more strings, hard.

[Jazz P Bass]

I forgot to mention, the distortion happens when I hit two or more strings, hard.

O/k.
At what volume are you duplicating the distortion?

[R.G.]

I forgot to mention, the distortion happens when I hit two or more strings, hard.

This describes intermodulation distortion pretty well. Could be power supply ripple, not crossover distortion.

[Ronsonic]

As others are pointing out, it could be bad filtering, either the B+ or bias supply. It isn't uncommon for a push pull amp to cancel the hum from a noisy power supply but still have it affect the sound.

Get AC measurements with no signal at the B+ and bias supplies to check.

[überfuzz]

Thanks all!

I won't get around to tinker with this amp until this week end, possible even later. I'll report back when I checked for ripples or/and AC leakage.

We're not just talking power section now are we, has it widened to all stages..?

[pdf64]

Power section ripple, eg reservoir cap, screen node or bias supply.
Power supply ripple is a common mode signal in a differential (push-pull) amp, so tends to get cancelled out at idle. But the cause of the ripple, eg bad cap, makes bigger problems at high power levels.
Poor grounding schemes / connections can cause similar double stop weirdness also. Pete.

[überfuzz]

Today I ended up jamming in the rehearsal studio where this amp is situated. It turns out that the issue I described occurs when a new Hagström guitar is plugged in. It has humbuckers, and I guess very strong signal output. So I figured, poor vintage amplifier having to deal with a state of the art guitar, no wonder it screams, from the pits of hell.

We tried a bunch of different guitars but we couldn't recreate to distortion, with any guitar but the Hagström. I'm not sure of what to do now, convince the owner that everything is hunky dory or look for gnarlys in the amplifier..?

[überfuzz]

Suggestions? Anyone had the same issue with a new guitar and vintage amplifier. Possible fix..? Ehh... Bump...

[kg]

well you could condition the signal somewhere else in the amp... ie, a wideband pad/attenuator, or a frequency dependent rolloff. might be as simple as changing a couple of cap values, and/or voltage dividers in the preamp.