"Phil Allison" wrote in message
...

"Fred the ****wit "


Hmm....I don't think this is a good idea. These amps are built really
cheap with common cathode bias,


** No they are not.

Strictly grid bias.

You trolling imbecile.




.... Phil




I've gotta say that in this case, I agree with Phil. The schematic that I
have for the 6 x 6L6 Peavey output stage, has the cathodes decked, and a
fixed negative 60v bias supply, injected into the grid circuits via a pair
of 220k resistors. This being the case, the bias on the remaining tubes will
not be affected at all by the removal of the other four. Most tube guitar
amps that I come across have the cathodes decked and either a fixed or
adjustable independent bias supply.

Whilst there are some theoretical issues with output tranny impedances and
plate loads, in practice, I would be surprised if it gave any problem
running with just two tubes. Over the years, I have seen many four tube
output stages being run with just two tubes fitted, without a problem.

My biggest concern would be the plate voltage. It appears to be fed directly
from the bridge on the end of the power tx high voltage winding, and is
shown as a nominal 500v. The screen voltage is not far behind it at 490v.
500v is already up at the original design maximum for a 6L6, and 490v on the
screen actually exceeds the quoted maximum of 450v.

It may be that the 500v shown on the schematic is nominal unloaded maximum,
and when it is running with 6 tubes in place, the actual value is lower than
that, in which case, running with just two tubes fitted will not be a
problem. If, however, the 500v is the loaded voltage, then it could rise to
well above the maximum rated plate voltage for the tubes with only two
fitted, which is not good news. If that did turn out to be the case, you
could put a resistor in line either on one leg of the AC input to the
bridge, or immediately after the bridge, to drop the maximum available
voltage. Be aware, however, that it would need to be a beefy resistor to
drop say 50v at a current demand of a couple of hundred mA. Something around
220 ohms at 10 watts maybe. You could always fit a switch across it to put
the supply back up to the full amount, if you refitted all six tubes.

Arfa