AC10 SRT Clone - power tube bias

[mwaller]

Hello -
As you probably know, I'm trying to sort out multiple issues with my AC10 SRT clone. I recently decided to check the power tube bias, and I believe they are running very hot.
This amp has a cathode biased pair of EL-84 tubes with a shared 130-ohm cathode resistor. I'm currently seeing 360V on the plates, and about 12V across the cathode resistor. Should I look to increase the cathode resistor? If so, what are some reasonable values to try?
Thanks!

[pompeiisneaks]

Vox tended to bias them pretty hot, but remember cathode biased can be biased much higher, say near 90% vs grid biased. at around 360VDC, that's 30mA per tube.

you said your measurements were 'about 12V' that's not super exact. To be sure you need to know two things:

1. the exact measurement of the cathode resistor, is it 132 or 129 or?
2. the exact measurement of the voltage drop across the resistor.

If you have those well within spec, then yours is very hot.

12/130 = .092 A or 92mA which is like 131% of max diss.

To adjust you'll want to increase the resistance. It's a bit of a trial and error process. Try doubling it for now, since you're so high, say 250 ohms or so, but buy a few around the range and you can step up/down until you get it dialed to where you won't be cooking the tubes so fast

~Phil

[mwaller]

Vox tended to bias them pretty hot, but remember cathode biased can be biased much higher, say near 90% vs grid biased. at around 360VDC, that's 30mA per tube.

you said your measurements were 'about 12V' that's not super exact. To be sure you need to know two things:

1. the exact measurement of the cathode resistor, is it 132 or 129 or?
2. the exact measurement of the voltage drop across the resistor.

If you have those well within spec, then yours is very hot.

12/130 = .092 A or 92mA which is like 131% of max diss.

To adjust you'll want to increase the resistance. It's a bit of a trial and error process. Try doubling it for now, since you're so high, say 250 ohms or so, but buy a few around the range and you can step up/down until you get it dialed to where you won't be cooking the tubes so fast

~Phil

Thanks! I ordered a bunch of alternate values to try. So, 90% of max dissipation is a good target?

[pompeiisneaks]

Yes, with cathode bias, you can run the tubes much higher dissipation with no ill effects, it's with grid bias that you want to limit it to 70% unless you want extra gain at the cost of tube life

~Phil