"Sizzle" sound in my SE homebrew

[casey73]

The amp - homebrew SE, essentially champ platform with hammond 125ESE output to accomodate an EL34 or 6L6. Preamp is a 6SL7. But, I deliberately kept plate voltage low using a hammond 261G6. With solid state rectifier I get 188VDC on the plate. The trannys are way big I know, and it operates at about half capacity.

I works well, almost too well. I wanted cranked overdrive tone at low power. It is difficult to get clean tone. The bigger problem is, I have this ever present sizzle, that I can best describe as "frying bacon" that rides over each note and tails off as the note fades out. All parts are new, layout is clean. I've built quite a few amps, so I know most of the rules about lead dress, shielding, etc. Tons of filtering. I've swapped out all tubes and speakers to illiminate them as the problem.

Because the plate voltage is so low I'm wondering if I need to consider reducing the preamp gain since the output tube has such a low ceiling. Before I start playing with the preamp values, I'd like to know if this is the first place to look. I've dealt with feedback and other weird oscillations before, but this is the first time I've had a "sizzle" thing going on. Any suggestions?

[stokes]

First isolate where the problem is coming from.Pull the preamp tube and if it stops it is there.You can isolate each side of the preamp tube by using a jumper wire across the plate resistor of each triode.This symptom is caused quite often by a bad plate resistor,particularly a carbon comp resistor.

[Enzo]

And try a different power tube as well, if the preamp tube is not it. Just because parts or tubes are new, doesn't mean they can't be bad.

[Bruce / Mission Amps]

The amp - homebrew SE, essentially champ platform with hammond 125ESE output to accomodate an EL34 or 6L6. Preamp is a 6SL7. But, I deliberately kept plate voltage low using a hammond 261G6. With solid state rectifier I get 188VDC on the plate. The trannys are way big I know, and it operates at about half capacity. ...
Because the plate voltage is so low I'm wondering if I need to consider reducing the preamp gain since the output tube has such a low ceiling. Before I start playing with the preamp values, I'd like to know if this is the first place to look. I've dealt with feedback and other weird oscillations before, but this is the first time I've had a "sizzle" thing going on. Any suggestions?

Is it the power tube's plate at 188vdc or the preamp tubes?
If it is the power tube at 188vdc, I'd suspect that the power tube is grossly overbiased and running class B 'ish

[casey73]

Bruce - yes, the 188VDC was the output plate voltage. I've changed out all the tubes several times, so I know that isn't the problem. I went ahead and changed the first stage set up. I changed the 100K plate resistor to a 150K and changed the cathode resistor from 1.5K to a 2.7K resistor. This improved the situation but didn't eliminate it. I then changed the cathode resistor on the output tube (6L6) from 250 ohm to 120 ohm. Now the output tube plate is at 175V, with 7.5VDC across the 120 ohm cathode resistor, which tells me it is idling around 11 watts if my math is correct. The sizzle is not as bad, but I haven't eliminated it. I'm wondering if it's just a bad idea to run a power tube at such a low plate voltage.