Second gain stage distorting heavily on output, Help!

[Ripthorn]

After more than a year of moving several times and several thousand miles, I have settled back down to where I can do amp stuff again. I pulled out my first build that I finished some time ago, and the output was very low and very harsh/distorted/nasty. I traced the signal path with an audio probe and found that the first issue is that after the output cap of the second gain stage, the signal is clipping very harshly and nasty. The cathode resistor and bypass cap are fine, as are the plate resistor and output cap. The input is not terribly strong, and when it was working completely, it never had this problem. Because I was checking the signal at solder joints, I think they are okay.

After some poking around, I noticed something very odd. This build uses submini tubes and the supply voltage to the preamp section is 105V. The first gain stage has a plate resistor of 220k and 10k cathode resistor and I am reading a plate voltage of about 58V. The second gain stage uses a plate resistor of 150k and cathode resistor of 4k7, but I am only reading a plate voltage of about 37V. This seems like it could be an issue, since the lower voltage allows for less clean headroom. However, the distortion produced is not smooth at all like the typical distortion from this amp is.

For those interested, here is the schematic. Tubes are 6111. Any help would be greatly appreciated.
[img612]http://i652.photobucket.com/albums/uu24 ... eclean.png[/img]

[Structo]

Well have you tried other known good tubes to eliminate the tubes as the problem?

[tubeswell]

After some poking around, I noticed something very odd. This build uses submini tubes and the supply voltage to the preamp section is 105V. The first gain stage has a plate resistor of 220k and 10k cathode resistor and I am reading a plate voltage of about 58V. The second gain stage uses a plate resistor of 150k and cathode resistor of 4k7, but I am only reading a plate voltage of about 37V. This seems like it could be an issue, since the lower voltage allows for less clean headroom. However, the distortion produced is not smooth at all like the typical distortion from this amp is.

All other things being equal, when you lower the value of the plate resistor or the cathode resistor (and in your case both have been lowered), you allow more tube current. Increasing the tube current decreases the 'internal' plate resistance of the triode. This will lower the potential (voltage) between the plate and the cathode. Lowering the plate resistor value will also increase the current at the following coupling capacitor, which increases the propensity to produce blocking distortion at the grid of the following stage (not that I'm suggesting that this is your problem)

[jjman]

What's the cathode voltages on each? Are the plate resistors appropriate at that PS voltage for a 6111 ?

[Structo]

Are you paralleling the triodes or is that only one tube shown on the drawing?

[Ripthorn]

The strange thing is that this used to work perfectly. It wasn't until about 6 months after I finished that this problem started. And sometimes, after playing for several minutes, it goes away. I know the caps aren't bad (at least they) and the resistors aren't bad. I tried a new tube when the problem first cropped up and that didn't fix it. I probed at solder joints, so I don't think those are bad, but I am open to any ideas.