"Wild_Bill" wrote in
:
I bought a Peavey Mace VT guitar amp (bare, no enclosure) which has a
power output capability of about 150 watts, by way of a power section
utilizing 6 each, 6L6GC tubes, arranged in 3 paralleled pairs (2 sets
of 3), feeding the center-tapped output transformer.
I'd like to just use 2 tubes (~ 50W) for the power section, but don't
know if the power circuit would need to be modified for only using 2
of the tubes.
The amp is capable of just using the non-tube/solid state pre-amp
section with the power section off (standby mode).. which will meet my
needs while practicing.
I'm not sure what I'll end up mounting the amp guts in.. I suppose
either an amp head box, but maybe in a combination amp-speaker(s) box.
I don't expect that finding a couple of high power capable 12"
speakers will be inexpensive, so using the amp with only 2 tubes will
be more than adequate for now.
I can't remember the last time I actually worked on any tube
equipment, and haven't retained any tube related experience/info, but
I have looked into checking the bias for power tubes.. which I'm
likely capable of doing.
--
Cheers,
WB
.............
Hmm....I don't think this is a good idea. These amps are built really
cheap with common cathode bias, which is probably all going through one
big HOT power resistor with a big cap across it. If you start unplugging
tubes, the idle current drops and so does the bias voltage, so the tubes
that are left start conducting harder to hold up the bias voltage.
Unplugging 4 tubes will surely make for some RED HOT PLATES on the two
tubes that are left, just eating the tubes in short order unless
something inside melts, like the screens, and causes a fuse blowing short
to the HV power supply, probably taking out the solid state rectifiers
before the fuse blows.
Nope....not a good idea at all.
Turn out the lights with the amp running with no audio input. Turn down
the gain to zero to make sure. Now, look at the 6L6s and see if you see
an internal blue glow inside the plates....a little nuclear physics in
action. The holes in the plates will also allow high speed electrons to
zoom past the plates through the holes and make the glass glow inside,
too. The tubes get quite hot....real finger burners....in these cheap
amps. The tubes run this way for decades, by the way. Every tube amp
Leslie speaker on the planet uses cheap common cathode bias on the 6550
big power tubes, too. A 10 watt white resistor through the metal chassis
hole is their bias resistor on Leslies. This amp probably has one
similar that's really hot if you carefully feel around the chassis after
it's been on 10 minutes.
This is why I don't think it's a good idea. The purists will tell you
the impedance matching in the plate circuit will be all screwed up on 2
tubes, but it'll run fine like that....except for the horrible plate
current from the bias problem I describe here.
A pair of 6L6s with 300VDC on the plates is only about 15-17 watts before
the distortion starts to make your guitar sound like a fuzzbox is inline,
just before the plates arc from the output transformer inductive kick at
cutoff arcs back to the beam forming plates. 6 tubes will be about 45 to
50 watts of honest power (not furniture store "music power" nonsense)
Here's the schematic:
http://www.lh-electric.4t.com/projects/nfg-amp.htm
R4 is the cheap cathode bias resistor, 240 ohms to get 19.5VDC, smoothed
by C5 to prevent inverse feedback eating up the output power. You COULD
change out whatever cathode resistor is hooked to the pin 8 of the 6
tubes and copy R4, 240 ohms, 5 watts is plenty and use 2 tubes, but
you'll have to swap it back when you plug the tubes back in to get the
bias right. With the transformer winding mismatch for 2 tubes, it would
probably put out 10 watts when the buzzing started....enough for a living
room without the neighbors calling the cops, unless Les Paul is playing.
Just run the 50W 6-tube amp at a lower volume level and forget all this
nonsense. It's not going to run up your electric bill unless you use it
for a room heater all night. (By the way, as this amp draws plate
current all the time, playing at low volumes does NOT "save tubes" at
all.)
If you find a deal on KT-88 pairs or the big 6550 beasts, they are also
direct replacements with bigger plate ratings if the price is cheap.
http://tubesandmore.com/
has 6L6 for about $15 each. I've never heard any difference in the $250
set with the fancy boxes over the cheapest tubes. I like RUSSIAN tubes
because the Soviets made them to operate in a tank, the kind with the
gun, and will take a horrible shock if your handle falls off the amp
carrying it to a gig. I hate Ruby and other Chinese tubes because too
many of them came back to bite me in the ass on service recalls many
years ago. I've never replaced a Sovtek I installed, which is great for
customer relations but kinda hard on profit margins....even on Leslies
run wide open throttle for years in an AME Church, here in South
Carolina. The Leslie drive belts wear out before the tubes!
Well, thanks for the memories....Let's build a serious guitar amp out of
a pair of 4-1000A broadcast tetrodes and some modern kilowatt speakers
the neighbors won't soon forget:
http://en.wikipedia.org/wiki/4-1000A
Imagine how the audience at a big gig would react to your custom amp
driving a half dozen huge speakers using two tubes glowing bright red
behind a neat window in the front of the 6' tall 18" rack like the
transmitter tube in the website picture. They'd remember you and those
tubes lighting up the stage for the rest of their lives. 3800 watts is
pretty....well....deafening! The filaments for 2 tubes is 7.5V at
42 amps! Most impressive! Airflow sockets keep the wind noise cooling
them down. Nobody could hear it, anyway, with all that screaming...(c;]
Transistors my ass......hee hee.