Biasing a 5E3

[snewman2010]

I've read that when using the cathode method of biasing an amp, a 1 ohm resistor is inserted between the cathode and the ground. However, a 5E3 already has a 250 ohm resistor and a 25 uF cap running in parallel to the ground. Where does the 1 ohm resistor get inserted? Before the 250 ohm/25 uf circuit? After? Do you bypass the 250 ohm/25 uf circuit? Thanks...

[MWJB]

NO, do not bypass the 250ohm/25uf cap with 1ohm, you will destroy your tubes.

I would use bias probes.

You could put the 1ohm resistor between the tubes pin 8 and the 250ohm/25uf assembly & read accross the 1ohm resistor. A good W rating (>2W) would be an idea to cope with the physical stress of you prodding it with the meter leads.

If your plate (pin 3 to ground) voltage is <380vdc you probably don't need to rebias.

Most folks would simply measure the dc voltage at pin 8 of the power tubes & divide that voltage by the value of the cathode resistor (250ohm), for instance you might get 20v/250ohms = 80mA. Divide that by 2 tubes & multiply by plate voltage less cathode voltage. E.G. 40mA*(370v-20v), therefore .040*350 = 14.8W per tube, deduct about a W for screen current and you get 13.8W. This method isn't that accurate but you rarely have to be spot on with cathode bias.

[cminor9]

You are thinking of the 1-ohm bias method, which is good for fixed bias amps. Cathode biased amps such as a 5E3 are a different animal entirely.

In general, you can tell the difference between a cathode biased amp and a fixed biased amp by looking at the schematic. On the power tube(s), is the cathode sent directly to ground or is a resistor and a capacitor attached to the cathode before ground? If the latter, then it's cathode biased.

Changing the bias on a cathode biased amp involves changing the value of this resistor. As MWJB mentioned, this will kill your power tubes. So long as you have a 270-330 ohm resistor there, your amp is biased properly for 6V6s. If it's not, then you've got some voltage problems elsewhere (like at the plate).

[Five_E35]

After you figure the current for both tubes as posted above, you can compare the plate voltage of each. If they're within ~ .5v they're pretty well matched.