On Thu, 17 Feb 2011 13:35:13 +0000, Arfa Daily wrote:

"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

Could refit with two EL34s. My Musicman 112 combo uses a pair of Mullards
at around 700v plate.



--
Live Fast, Die Young and Leave a Pretty Corpse