Tube Amp 6L6GC Valve Peavey Mace VT Guitar Amplifier

[Wild_Bill]

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
..............

 

[Jeffrey Angus]

On 2/16/2011 10:19 PM, Wild_Bill wrote:

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.

Basically.... you'll be running 3X the plate load resistance.
So, you'll have to change the plate transformer. You could look
for one with 5K load, or.... if you have a 16 ohm output on the
current transformer, you might try loading THAT with the 8 Ohm
speaker.

WOn't be quite right, but it should work well enough.
At least for testing.

Might also want to go trhough and make sure the idling currents
on the two tube are correct and that the B+ hasn't risen past
their maxim ratings.

Jeff

 

[Phil Allison]

"Jeffrey Angus is full of Bull "

Basically.... you'll be running 3X the plate load resistance.
So, you'll have to change the plate transformer.

** Bollocks you will.

You could look
for one with 5K load, or.... if you have a 16 ohm output on the
current transformer, you might try loading THAT with the 8 Ohm
speaker.

** The worst possible thing to do.

Wot an idiot - mooooooooo.....


...... Phil

 

[Phil Allison]

"Wild_Bill"

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.

** No mod is needed.

But the speaker load impedance will need to be 3 ( or 4 ) times the
currently rated value.

If it is 4 ohms now, then the ideal is 12 ohms, 16 ohms is near enough.

So, get two 8 ohm guitar speakers of about 30 watts rating and connect them
in series.



...... Phil

 

[Fred]

"Wild_Bill" <wb_wildbill@XSPAMyahoo.com> wrote in
news:0m17p.239226$UC6.102660@en-nntp-08.dc1.easynews.com:

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.

 

[Phil Allison]

"Fred the Fuckwit "

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

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8s4emqFgqmU1@mid.individual.net...

"Fred the Fuckwit "


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

 

[Wild_Bill]

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" <arfa.daily@ntlworld.com> wrote in message
news:mu97p.56922$LO2.21988@newsfe20.ams2...

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

 

[Meat Plow]

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

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8s4emqFgqmU1@mid.individual.net...

"Fred the Fuckwit "


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

 

[Fred]

"Phil Allison" <phil_a@tpg.com.au> wrote in news:8s4emqFgqmU1
@mid.individual.net:

"Fred the Fuckwit "


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

Geez, Phil, asshole....thanks!

Assholes like you is the reason someone who comes here gets no help....

 

[Phil Allison]

"Meat Plow"

Could refit with two EL34s.

** Make any issues worse not better.

My Musicman 112 combo uses a pair of Mullards
at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.


..... Phil

 

[Phil Allison]

"Fred" <nobody@home.com>

"Fred the Fuckwit "

** Go away you PITA damn TROLL.



..... Phil

 

[Meat Plow]

On Fri, 18 Feb 2011 08:15:44 +1100, Phil Allison wrote:

"Meat Plow"


Could refit with two EL34s.

** Make any issues worse not better.

My Musicman 112 combo uses a pair of Mullards at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

It's very relevant, Fuckhead. Any moron knows you wouldn't
just drop a pair of EL34s in without some slight modification.
Hell i knew that when I was 15. Problem is you think everyone else is a
moron besides you. I've worked on just about every Peavey tube amp made
clear back to when you still shit in a diaper. Nevermind that you
probably still shit in a diaper.



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

 

[Phil Allison]

"Meat Plow the FUCKWIT "

Could refit with two EL34s.

** Make any issues worse not better.

My Musicman 112 combo uses a pair of Mullards at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

It's very relevant,

** Get stuffed.

Any moron knows you wouldn't
just drop a pair of EL34s in without some slight modification.

** Total red herring.



...... Phil

 

[Arfa Daily]

"Fred" <nobody@home.com> wrote in message
news:Xns9E8F934CEDD51nobodyhomecom@74.209.131.13...

"Phil Allison" <phil_a@tpg.com.au> wrote in news:8s4emqFgqmU1
@mid.individual.net:


"Fred the Fuckwit "


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





Geez, Phil, asshole....thanks!

Assholes like you is the reason someone who comes here gets no help....

Well, that's a bit unfair on the rest of us ...

Arfa

 

[Jeff Urban]

It's obvious you are looking for that sweet distortion at a lower
level. Whether the reason is to avoid a visit by the cops, or that the
speakers can't handle all the output doesn't matter.

One option is a power soak. These are just banks of high wattage
resistors in a box with switches to configure them for different
amounts of attenuation. This option removes the interaction between
the tubes/output transformer and speaker(s). For the purist, this
interaction is part of the sweet sound and is important.

Of course if you have to have it all that is not good enough. You have
options. Use a lower capacity speaker system to start with. The VC and
output Xfmr interaction is part of the cone excursion of the speaker.
To the amp is looks like saturation in a way, but it's not quite the
same. Much has been devoted to this and I am not prepared to go into
seventy years of history. But decoupling the VC from the amp will not
get you the same sound. Also, running the amp at a lower power will
never saturate the output transformer core, but eh, we do the best we
can.

Actually the best way one might think is to somehow reguilate the
plate voltage down. Doesn't matter voltage or current. But there is
another way. This will get you closest to what you want. I really
don't know how it will affect the output tubes, but I can guarantee
they won't overheat.

Lower the G2 voltage. Now if the amp is grid biased you will have to
adjust that, but if it is cathode biased it will adjust itself.
However, being familiar with cathode stripping in CRTs, I can't say it
won't happen to the output tubes. I don't THINK it's a big concern,
but don't take that to the bank.

The lower G2 (screen grid) voltage makes the tube behave more like a
triode. Thus it's output impedance is higher. This allows every
abberant change in current drawn by the load to interact. You could
literally have a switch on the back. If the outputs are cathode
biased, it should work just fine. Flip it one way for low power and
the other for high. If you're concerned about cathode stripping there
are two options. One is a simple megohm resistor in series with each
G1. To get fancy you could use a diode with the anode connected to the
control grid (G1). If you use the diode put about a ten meg across it,
no tube is perfect. But you do this separately for each of the six
tubes.

I think a one meg, or even a 2.2 meg would work just fine. In fact you
can go as high as you want until the interelectrode capacitance comes
into play. Always use a scope, because it could be oscillating. You
don't want that.

I don't think cathode stripping is the issue, but the operative word
here is think. Nobody wants to wreck $100+ worth of pentodes.

JURB

 

[Meat Plow]

On Fri, 18 Feb 2011 12:05:37 +1100, Phil Allison wrote:

"Meat Plow the FUCKWIT "

Could refit with two EL34s.

** Make any issues worse not better.

My Musicman 112 combo uses a pair of Mullards at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

It's very relevant,

** Get stuffed.

You first

Any moron knows you wouldn't
just drop a pair of EL34s in without some slight modification.


** Total red herring.

"Could refit with two EL34s"

Red Herring straw man burned at the stake.



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

 

[Phil Allison]

"Meat Plow"

Could refit with two EL34s.

** Make any issues worse not better.

My Musicman 112 combo uses a pair of Mullards
at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

No you post something that MAKES SENSE.

Surprise me.


..... Phil

 

[Meat Plow]

On Sat, 19 Feb 2011 08:47:37 +1100, Phil (Troll) Allison wrote:

"Meat Plow"

Could refit with two EL34s.

** Make any issues worse not better.

Go back to shagging your sheep you sodding pisswit.

You have no face to save here,,,troll.



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

 

[Phil Allison]

"Meat Plow is a Lying PIG "

Could refit with two EL34s.

** Make any issues worse, not better.

My Musicman 112 combo uses a pair of Mullards
at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

Now you post something that MAKES SENSE.

Bet you cannot.



..... Phil