Funny bias problem in PP EL34. Please help

[OzMikeH]

I've been asking for help on the correct forum but I'm not getting much assistance there.

http://www.audioasylum.com/cgi/vt.mpl?f=diyhifi&m=8226

Basically my bias keeps wandering and I've checked just about everything. I'm stumped.

Moderators: If it isnt ok for me to post a link to another forum delete this post, I'll extract the relevant text and repost it here.

[Joe Curcio]

There are two basic types of output tube bias configurations - fixed bias and self bias. Since you don't indicate what configuration you have I'm going to guess fixed bias.

With fixed bias there are only two factors that determine the bias in your output tubes: input grid voltage and screen grid voltage (the plate voltage has little effect when near normal operating range). The cathode voltage is also contributory but with cathode resistors this should be determined by the other two voltages. So your job is to find which of those is varying.

I would suggest that you go to my webpage and download my paper on output tube bias theory & troubleshooting here:

http://www.curcioaudio.com/Output%20...Final%20R0.pdf

[HollowState]

Originally posted by OzMikeH
(from the other forum)
Bias set after 20 mins warm-up then increases from 0.43V to 0.56V after an hour or so.
I also have a very faint whistle from that channel, about 13kHz audible from 2 feet away from the speaker. The sound is there even with the amp in mute mode.

Just about the only thing left is the coupling capacitor as you mentioned. You could monitor the G1 voltage on that tube to see if the negative bias voltage drifts lower. Or you might want to replace the coupling cap too, even if it's a new amp. Don't worry about an exact match yet. Just use a known good 600 volt cap of a similar size to see it things stabilize.

The high pitched whistle concerns me since it might be a parasitic oscillation of some type. You'll need an oscilloscope to check for this at the output. If there, work backwards trying to find where it stops.

I would also look closely, in a darkened room, at the inside of the EL34B in question. Look to see if the screen grid is getting hot. (glowing dull red) It should not.

One last far-out thought. If this amp is ultra-linear configured, check to see that the plate and screen leads were not inadvertenly switched when it was built. I once found this on an amp that worked, but strangely.

Victor

[OzMikeH]

Excellent article Joe. Thanks for that. It is a fixed bias system and I've eliminated pretty much all of the bias supply.

Victor, it is Ultralinear. no glowing grids but there is a faint light blue glow inside.

Sound like I should be lifting the valve up slightly in the socket and measuring voltages while it is running. It's a difficult amp to handle at close to 30kg.

[ray_moth]

The problem you describe sounds like the bias instability that can occur if the grid 'leak' resistor for the misbehaving tuibe has too high a value. It may be apparently the same as the grid resistors of the other tubes that are behaving properly, but it's worth checking the actual value, because resistors can be faulty.

[Hi-Q]

Are there any grid stoppers in the design?
Apart from that, the slow drift over time leads me to believe one of the EL34's may be playing up, it is possible for one to effect the other if the fixed bias is being manipulated by it. Every other possibility seems to have been covered.

[Joe Curcio]

The whistle is suspicious. Pull the driver tubes (they shouldn't affect the bias anyway) for the rest of your troubleshooting (once you stabilize the bias you can return them to their sockets.

Be sure the bias supply is stable - monitor it at the far side of the bias voltage injection resistors (can range in value from about 100K to 470K) while you are monitoring the bias current thru the output tubes.

Once you confirm that the bias voltage is stable (look closely since your bias current shift is relative tame) then (assuming you have confirmed that the tubes are OK) the coupling caps from the driver board become questionable.

[Miles Prower]

Originally posted by OzMikeH
Excellent article Joe. Thanks for that. It is a fixed bias system and I've eliminated pretty much all of the bias supply.

Victor, it is Ultralinear. no glowing grids but there is a faint light blue glow inside.

With a fixed bias topology, and a working bias supply, a failure of the finals to settle down is usually due to gassy VTs. I had that problem with a couple of cheap 6BQ6GTBs, and those tested gassy even though they did not glow. That light blue glow pretty much cinches it. Your finals are gassy, replace them.

[chrish]

With all of the valve amps I have made, all of the valves have a blue glow on the inside to some extent or other. Do a search on this, you will see it is not a problem.

Cheers,

Chris

[Miles Prower]

^^^^

There's a big difference between the sky-blue cobalt glow, and what was described here: a light blue glow that fills the inside. That is a big problem, especially when coupled with being unable to stabilize the bias. Those are gassy tubes and need to be replaced.