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There are people here far more qualified to answer this as I have never worked on a tube amp, not once in my life, but I will take a crack at it while we wait for the professionals.

It would seem (to me anyhow) that those pentodes are being run in a tetrode class-c configuration (both screen and suppressor grids grounded with a negative voltage at the control grid). When the RF waveform at the filament drops below the control grid voltage, the tube conducts. If your audio suddenly got garbled and you think you may have bumped that bias pot, I would assume it got turned in a way that raised the voltage on the grid making it conduct for more of the RF waveform. I think you need to adjust that pot so that the grid voltage goes more negative so that the tube conducts for less of the cycle. With the voltage on the control grid too close to ground, more of the negative half of the RF waveform can turn on the tube and that probably has an effect similar to saturating a transistor (flat topping and distortion). My recommendation (if a better one doesn't appear) would be to make that control voltage as negative as you can (with respect to ground) and then increase it until it sounds good with decent output power, backing it back down if distortion occurs. This seems like a job for a scope honestly.

<blockquote data-quote="brandon7861" data-source="post: 856627" data-attributes="member: 53724">There are people here far more qualified to answer this as I have never worked on a tube amp, not once in my life, but I will take a crack at it while we wait for the professionals.It would seem (to me anyhow) that those pentodes are being run in a tetrode class-c configuration (both screen and suppressor grids grounded with a negative voltage at the control grid).&nbsp; When the RF waveform at the filament drops below the control grid voltage, the tube conducts.&nbsp; If your audio suddenly got garbled and you think you may have bumped that bias pot, I would assume it got turned in a way that raised the voltage on the grid making it conduct for more of the RF waveform.&nbsp; I think you need to adjust that pot so that the grid voltage goes more negative so that the tube conducts for less of the cycle.&nbsp; With the voltage on the control grid too close to ground, more of the negative half of the RF waveform can turn on the tube and that probably has an effect similar to saturating a transistor (flat topping and distortion).&nbsp; My recommendation (if a better one doesn&#039;t appear) would be to make that control voltage as negative as you can (with respect to ground) and then increase it until it sounds good with decent output power, backing it back down if distortion occurs.&nbsp; This seems like a job for a scope honestly.</blockquote>

[QUOTE="brandon7861, post: 856627, member: 53724"] There are people here [B]far [/B]more qualified to answer this as I have never worked on a tube amp, not once in my life, but I will take a crack at it while we wait for the professionals. It would seem (to me anyhow) that those pentodes are being run in a tetrode class-c configuration (both screen and suppressor grids grounded with a negative voltage at the control grid). When the RF waveform at the filament drops below the control grid voltage, the tube conducts. If your audio suddenly got garbled and you think you may have bumped that bias pot, I would assume it got turned in a way that raised the voltage on the grid making it conduct for more of the RF waveform. I think you need to adjust that pot so that the grid voltage goes more negative so that the tube conducts for less of the cycle. With the voltage on the control grid too close to ground, more of the negative half of the RF waveform can turn on the tube and that probably has an effect similar to saturating a transistor (flat topping and distortion). My recommendation (if a better one doesn't appear) would be to make that control voltage as negative as you can (with respect to ground) and then increase it until it sounds good with decent output power, backing it back down if distortion occurs. This seems like a job for a scope honestly. [/QUOTE]

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