biasing a Jet City 20 EL84 amp

[Randall]

What would be the best/easiest way to bias this output section? Transformer shunt?

JCA2112RC POWER.pdf

[The Dude]

I would use this method instead of the shunt method. (copied from Aiken Amps so I wouldn't have to write it out)

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

It's sort of the same as measuring the voltage drop across a cathode resistor, except you're using the resistance of the OT instead.

[Enzo]

I suppose I am incorrect in the usage, but what you just described, I usually call the shunt method. I know officially the shunt method is the parallel the winding side with an ammeter, but inevitably we leave the thing in current mode than try to measure voltage later and blow the expensive little fuse.

Also please observe the power tube circuits are drawn INCREDIBLY incorrectly. B+ to heater? Unwired screens? Screen and plate symbols swapped inside the tubes. OT wired to screens, but have correct pin number there at least.

[The Dude]

Nice! I had only glanced at the schematic long enough to see if there were cathode resistors present. That schematic is a mess!

[Jon Snell]

If the output stage has good cathode resistors, no setup is required. It is not fixed bias.
A 3 year old could draw a more accurate schematic I would say.

[Dave H]

A 3 year old could draw a more accurate schematic I would say.

Looks like they didn't have a pentode EL84 symbol so they used a dual triode 12AX7 symbol for the EL84s.
Also D2 and D4 will short out the transformer.

[Chuck H]

I think the message here is that the schematic can't be trusted (I have to wonder why it was even linked?)

That said... If it's at least accurate in that the amp is cathode biased I would agree with John. Just measure the resistor and then measure the voltage drop across it. This doesn't separate the screen current from the plate current but that's no great shakes.

Provided the amp IS cathode biased, the best way to bias the amp is to use the appropriate cathode resistor value. As yet unknown pending measurements.

[Chuck H]

Maybe THIS is more accurate? (in which case it is NOT cathode biased) Randall, please report.

[Randall]

Thanks guys. I used the Aiken amp method. Clunky, but effective. I can confirm this is not a cathode biased amp, cathode is at ground. Considering Soldano designed this amp, you'd think he would have at least stuck a 1 ohm resistor in there to make it easier to deal with.

Another flaw in this design is an arcing point from the left output tube plate to a ground trace that gets too close ala JCM2000. This poor amp had two shorting output tubes, a charred arc point, a dead 12AX7, and another one with one side barely breathing. Biasing was the easy part.