EL34 PP triode mode hums

[slideman82]

I decided to install a switch for triode/pentode operation in a EL34 based guitar amp, with 500VDC on plates. It sounds normal in pentode mode but hums a lot in triode, and quiescent current in one tube rises too much. It's not a matched pair, and I think they're quite old... could be this the cause of humming?

 

[SemperFi]

Triode mode gives the tube lower rp, which means b+ ripple now rides on the OT more than it does the tube. So you get more hum out to the speaker.
I'm not sure if triode mode exaggerates an unbalance and thus creatse more hum, but very unbalanced tubes will have less PSRR than well balanced dittos.
You should always connect the screens to the plates with a power resistor of say 100-500ohms to protect the screens. That should also help balance out the iq since the tube drawing the most current will have a little less voltage on it's screen...much the same as using cathode resistors in parallel tubes.

 

[DF96]

Yes, triode mode will give more hum, and lack of balance will give more hum. How much does the g2 voltage rise when in triode mode - you may be pushing the valve over the limit. A 1K screen resistor should be fine.

 

[slideman82]

Yes, triode mode will give more hum, and lack of balance will give more hum. How much does the g2 voltage rise when in triode mode - you may be pushing the valve over the limit. A 1K screen resistor should be fine.

Thanks a lot! But I'm already using a 1k 5W resistor from anode to g2... I think they're just too unbalanced... I tried once in an old Fender Bassman 135 and sounded great as triode mode instead of tetrode... even more linear!

 

[directdriver]

500vdc on the screen is too high. The problem with triode or ultralinear mode is that when operating at high voltage like 500v for a tube like EL34, the screen gets about the same voltage at 500v and that's too high. In pentode mode, one can set the voltage at a more conservative level, say, 400v and the tube will last longer. I only recommend triode mode when the plate voltage is below 430v for EL34. Anything higher than 450v is pushing your luck.

 

[slideman82]

500vdc on the screen is too high. The problem with triode or ultralinear mode is that when operating at high voltage like 500v for a tube like EL34, the screen gets about the same voltage at 500v and that's too high. In pentode mode, one can set the voltage at a more conservative level, say, 400v and the tube will last longer. I only recommend triode mode when the plate voltage is below 430v for EL34. Anything higher than 450v is pushing your luck.

Thanks for the tip! This is really helpful! This amp is about 80W rms, for guitar use, and I made my friend a huge cabinet and has a lot of SPL, that's why we wanted to reduce output... it would still be a lot of power but a bit less kick.