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.
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.
"Wild_Bill" wrote in news:0m17p.239226$ snipped-for-privacy@en-nntp-08.dc.easynews.com:
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:
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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.
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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:
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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;]
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.
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.
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.
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.
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