Peavey classic 20 Bias

[tdlunsfo]

Anyone happen to know the proper bias for the EL84's in this amp? I understand it is cathode biased but I am reading plate current at 45 and 41ma's. This seems excessive but also understand that a lot of companies love to set EL84's really hot.
peavey_classic_20.pdf

[Helmholtz]

What voltage do you measure at V3/V4 pins 3?

[tdlunsfo]

308v. I clipped one of the parralled 220ohm resistors and the bias is now closer to in range. Not sure why Peavey would had put two 220ohm resistors in parallel, didn't have any 110's? Regardless, the data I read shows a class A/B EL84 plate current at 70% around 27ma. Clipping one of the resistors got me to 28ma. Just curious why Peavey is running them so hot, am I missing something? Reason it has me concerned is these tubes are now really microphonic.. wondering if it was because they were being run full tilt for years..

[The Dude]

^^^^^^ That! One of the advantages of cathode biased amps is that the cathode resistor is already there for measuring. It's just ohm's law from there. Another good reference for biasing is here:

http://www.tedweber.com/webervst/tubes1/calcbias.htm

[The Dude]

308v. I clipped one of the parralled 220ohm resistors and the bias is now closer to in range. Not sure why Peavey would had put two 220ohm resistors in parallel, didn't have any 110's? Regardless, the data I read shows a class A/B EL84 plate current at 70% around 27ma. Clipping one of the resistors got me to 28ma. Just curious why Peavey is running them so hot, am I missing something? Reason it has me concerned is these tubes are now really microphonic.. wondering if it was because they were being run full tilt for years..

Sorry for any confusion, you jumped in while I was posting. 308V on the cathodes? Is that a typo?

[Chuck H]

The 70% thing is arbitrary. And ESPECIALLY so with cathode bias. And ESPECIALLY so at lower plate volts. And ESPECIALLY so with el84's.

One of my designs runs a pair of el84's at 365Vp (355ish plate to cathode) with a 160R shared cathode resistor for about 11.5W per tube, BUT...

That doesn't subtract screen current AND it doesn't account for how the bias voltage rises when the tube conducts.

70% for a pair of cathode biased el84's in push/pull is ice cold IMHO.

YMMV

[tdlunsfo]

Sorry that was plate voltage.... had 6v6 pin 3 on the brain. Cathode resistor drop was 10.6v prior to snipping the resistor. I usually use rob robinettes calculator for my numbers.

[Dave H]

308v. I clipped one of the parralled 220ohm resistors and the bias is now closer to in range. Not sure why Peavey would had put two 220ohm resistors in parallel, didn't have any 110's? Regardless, the data I read shows a class A/B EL84 plate current at 70% around 27ma. Clipping one of the resistors got me to 28ma. Just curious why Peavey is running them so hot, am I missing something?

The bias of cathode bias amps is usually set higher than 70% of Pd max. The EL84 data sheet for 300V plate and screen recommends a common 130R cathode resistor (I'd use 2 x 270R in parallel) to set the bias at 36mA which is a Pd of 10.8W or 90% of Pd max.

[tdlunsfo]

The bias of cathode bias amps is usually set higher than 70% of Pd max. The EL84 data sheet for 300V plate and screen recommends a common 130R cathode resistor (I'd use 2 x 270R in parallel) to set the bias at 36mA which is a Pd of 10.8W or 90% of Pd max.

All of these still seem more realistic than what I measured here. I was at nearly 14 watts and 116% with 45ma. I tried it both ways and the amp sounded the same to me.

[The Dude]

When you calculate dissipation, don't forget to subtract the cathode voltage from the plate voltage.

[tdlunsfo]

Right.. yes sorry. Minus the 10v it looks like I'm at 113% and 13.6 watts. I tried one of my vintage el84s and it was almost instant red plate. Maybe these new production tubes are more 7189's? Either way I think I will replace these tubes and play with the bias. Customer wants it "reliable" so I fill like I need to be more realistic with these settings.

[Helmholtz]

The bias of cathode bias amps is usually set higher than 70% of Pd max. The EL84 data sheet for 300V plate and screen recommends a common 130R cathode resistor (I'd use 2 x 270R in parallel) to set the bias at 36mA which is a Pd of 10.8W or 90% of Pd max.

Adding 4mA screen current per tube results in a total resistor current of 80mA and 10.4V across 130 Ohm. At max power the cathode voltage will be around 15V, meaning a resistor dissipation of 1.7W. It makes sense to use 2 resistors to better distribute the heat generated. The bypass E-cap doesn't like to get hot. Remember 10°C less doubles its lifetime.