Thanks for the concise reply, Arfa.. I believe your comments will apply to
the particular output stage of the Mace VT series.
I haven't received the amp yet, and wasn't able to find a readily available
schematic online (without extensive searching), but I did find a few others,
and the *Mace A series* uses the same tubes in a 2 x 3 configuration.. so I
was assuming that the power section is the same or very similar (same age,
same output power).

I had also seen the spec sheets for the tubes, and noticed that the voltages
shown in the *Mace A series* schematic would seem to exceed the maximum
specs for the same tubes.
This particular schematic only shows an adjustment pot for the -55V to #5
pins (from a -77V source).

The *Mace A series* schematic I'm looking at shows a difference in wiring
for one tube of each set, where the 100/5W resistor on pin #4 is wired only
to one of each set, and the 100/5W resistor is in series between the two
other sets for pins #4.
I have no idea what this would mean as far as using only 2 tubes for the
output section (but it looks like 2 of the sockets wouldn't be good choices
for only using 2 tubes).
The tubes aren't designated as V1, V2 etc, so I can't be more specific.

I no longer own any tube testers (other than a CR70 for CRTs), but I keep
seeing cautions about the need for matched sets. I've seen quad sets, but
not hex sets, although I'm certain that most sellers would match 6, but I
don't have a tester for an independent check of the existing used tubes.

--
Cheers,
WB
..............


"Arfa Daily" wrote in message
...

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