Peavey Classic 50 - 212

[Jay D]

Hello:
I have a Peavey Classic 50 -212 that I need to install new output tubes (four EL84's).
I checked the bias voltage, plate voltage, and cathode currents and average them. I measure -15.6 VDC at the junction of R16 (18K ohm) and R18 (33k ohm), and at the input voltage to R16 = -24 VDC. If I use the voltage divider rule to check the bias voltage, I then calculate -15.53V.
The cathode currents average to: 30.25 mA, and the plate voltages average to: 392VDC. Now, If I calculate correctly, the plate dissipation for the EL84 is 12 watts.
I have a plate voltage of 392V; therefore: 12/392 = 30.61 mA per tube I normally bias my amps between 65% and 70% of this max dissipation number, so the measured data from above puts me at 100% at Idle. 70% would equal: 0.7 x 30.61 = 21.43mA per tube.

Does anyone know the factory bias voltage and measure the cathode current settings?

[Jazz P Bass]

That -15Vdc bias sounds about right.
I know Peavey runs these amps hot.
I have seen the Classic 50 pull 100 watts from the mains at idle.

[Amp Kat]

Factory is -14 volts so -15 is pretty dang close. 65% would be 19 ma's and I don't think that amp has a bias pot but you should be fine there. Those amps are set for plug and play.

[Chuck H]

el84's are funny to bias. They are ALWAYS set up hotter than you think they should be. This could be the screen current throwing the measurement since the "max" that everyone uses for biasing is the plate dissapation and not the total tube screen and all. Vox and other cathode bias amps often have el84's idling at 14 watts!!! Biasing on a scope (which I only do as an exercise to see what's going on, final bias is set by ear) even with over 350Vp I usually end up over 11 watts total tube dissapation by the time it "looks" right.

I've never measured screen current for an el84 at idle but it might be interesting.

As noted, the amp should be fine with new tubes as long the bias reads similar to where it is now and there are no red plates.

[dcline]

I just replaced a Power Transformer, bias cap and tubes in a Classic 50. It's fired up. My bias probes say it's running at about 43 to 45 mA. That seems really high to me. Plate voltage is about 445 VDC with no tubes in.
No red plates but I haven't played through it yet because I don't want to fry the new tubes.
Any opinions?
Thanks in advance

[mac dillard]

I have a Classic 50 that I recently sent back to the factory for a check up after I had done some work on it. I had measured 399.9 volt for B+ going to the OT and measured a bias voltage of -15.5 volt. The amp came back with a report that said everything met factory specs. As the old cowboys use to say, this amp has been rode hard and put up wet. It has taken a lot of abuse and keeps on ticking..Maybe Enzo will chime in. He would know for sure.

[g1]

What is the DC voltage on the power tube grids?
And the bias cap is installed with + end to ground, yes?

[Enzo]

The B+ with the tubes out is meaningless. The tube current draw will pull the B+ down to more reasonable levels.


43ma per tube? That does seem high at 400v B+. What is your B+ with tubes? What is the bias voltage at the tubes?

[The Dude]

My bias probes say it's running at about 43 to 45 mA.

For clarification, is this measurement per tube?

[dcline]

Yes it is 43 mA per tube... and yes the bias cap is installed with + to ground.
When running at idle,
G1 is -15.45
G2 is 400 VDC
G3 is 0
Plate is 402

[g1]

Did you happen to check the bias before you did the work? The tubes are running hot, but it's not so uncommon to see EL84's run this way. You need tubes that bias up colder, or you need to increase the negative voltage at the grids to bring the idle current down.

[Pedro Vecino]

Intensity in each EL84 can be very variable but mostly tending upward dissipating much heat in that amp. It is best to use an adjustable bias circuit and adjust them to 24/25 mA with 400V (I always adjust measuring them through the anode). I usually remove R18 (33K) resistor and connect in parallel with the electrolytic C10 a line of 27K resistor (fixed) and an adjustable of 25K in series. With this system I can put intensities in this range.

It is possible that in your case:
- Their EL84 are high degree (have higher emission)
- Your transformer delivers a lightly higher voltage

[dcline]

Thanks again everyone for all the info. The amp came to me with the power tube board burned to a crisp, the bias cap completely fried (visually) and with a blown power transformer. I found a power transformer on ebay for 40.00.
I ended up doing a similar mod that Pedro suggested but did not have any 25k pots laying around. I did have a nice 50k so I ran that in series with a 10k resistor and it now dials right in. Nice sounding amp too!
Thanks again! This forum rules!