Running a JCM800/2203 on one tube question

I'm not an expert, but I would bet $100 that you can't run the amp with only one tube.

 

You can't do that. Either 4 tubes or 2 tubes.

AL

 

You will unbalance the OT and destroy it if you run just one tube. One thing you could do is ground the grid of one tube. That would effectively make the amp run in SE class A mode. It can sound pretty sweet. I put a switch on one of my amps to allow me to do that at will (you just don't want to flip the switch with the amp on--it's the mother of all pops!).

 

Although it's probably bad for the amp, VHolic did quite a few one tube Marshall clips.

 

Well, this is typical...

A quick search at the Plexi Palace and here's what came up.

Posted by Cerrem

"OK...so you want to run the 100 watter on one tube but you want to do it safely....
No problemo...
First to prevent the OPT from blowing and also maintain descent bottom end...you will need to run the amp with 2 tubes installed...BUT with the sound going to only one tube...The other tube will just sit idle to balance the DC quescient current so to keep the OPT happy as well as maintaining inductance for good low frequency response....
So basically you are running the amp on one tube for sound...
You need to maintain DC bias on both output valves but need to kill the AC signal going to one valve..To do this, simply unsolder one PI coupling cap from the PI plate, such as the 100K side of PI and then solder it to ground...make sure to properly bias tubes..
You could also leave all 4 valves in the amp and only send signal to one valve....this way maintianing some power supply draw..
One thing I forgot to mention was the biasing situation....Technically if you run a single valve in this 100W amp situation you should really bias it to run in Class A....since leaving it where it is it will be going into cut-off when really cranked up....But biasing into Class A at those plate and screen voltages is not a reality with these valves used...So deal with the asymetrical cut-off and see if you like it...
Regards
Chris"

And, from Kevin O'Connor

"by Kevin O'Connor of London Power.
Q: In a magazine Q-A, a player wanted to pull tubes to reduce power, but the "expert" said this would cause a meltdown of the remaining tubes. Of course, it was suggested that the expert's attenuator product was the preferred way to go. Is any of this true?
A: This is a person who should know better!
Removing tubes from a multi-tube fixed-bias output stage is never a problem. You can remove any number of tubes, and yes, that means you can take one tube out of a two-tube amp; one, two, or three out of a four tube stage, et cetera. This sounds heretical to techs stuck in the mire of convention, but it is something that has been known since tubes were invented.
The even-number tube extractions reduce power symmetrically. Neither the tubes nor the transformer will be damaged. Power will be reduced and so will frequency bandwidth - you will lose some bass and some treble. This is the point that switching the impedance selector to a less-than-load setting is supposed to correct, but it is completely subjective whether you should. The only 'should' of the matter, is do I like it this way, or do I like it that way?
In the uneven tube extractions, asymmetric power reduction occurs. Conventional thought says "the one tube on one side of the circuit will be trying to match the output of the two tubes on the other circuit half". This is wrong. The single tube can only produce so much power, and that's all it does. It doesn't melt down. The transformer does not blow up.
So, what's missing from conventional thought? The realization that tubes are "self-limiting power governors", which was stated in The Ultimate Tone (TUT), and explored in more detail in TUT2 and TUT3. TUT4 explores all of this in great detail. Our "expert" should get a copy.
In the end, you can pull tubes to reduce power, unless the amp is cathode biased - then you have to split the bias resistor. In any case, you do not have to worry about the impedance selector either."

 

Your JCM800 is fixed bias. According to KOC's logic, you would only need to split the bias resistor on a cathode biased amp if you have more than 2 output tubes.

The first excerpt describes what I recommended. Grounding the output of one side of the PI (via its coupling cap) or the grid of one tube is the same thing, just a different way of saying it. That will make the amp operate in SE mode, but ususally needs a rebias to get the best tone. In this case, the OT is running with relatively equal current on both sides.

I would be very afraid to run an OT with vastly different amounts of current on each side. I really think you run the risk of blowing it under those conditions--especially considering the standards to which mass produced OTs are made.

To each his own, I guess...

 

...You won't hurt anything, but you will only be getting what portion of the signal the phase inverter is feeding it. Works in some amps better than others....even if it doesn't sound that great, It's an avenue worth "exploring".
Most don't like it, but make up your own mind about it.. so the next time this question is asked, you'll actually have a "tested" response.....happy hunting!

Jeff Seal

edited: To s2amps, I think you may have not intended to say "ground the grid" in your earlier post, I don't believe this wiill work very well, or am i wrong here?

 

I could be wrong, but I think the danger to the OT comes from having an unloaded half primary winding. Combined with the much higher static magnetic field from the DC going through only one tube (normally the two halves cancel each other out), I'm guessing that this could cause far higher than normal voltages to be induced in it, which might arc internally.

(This won't occur on the secondary side when the amp is running at 4 ohms, even though the 16-ohm portion is also an 'unloaded half', because there's no DC on that side, and the induced votages will be much lower anyway.)

So you do probably want to ground the input of the other tube, which then remains in the circuit as a low-impedance load on the other side of the transformer and damps it, and will draw the right amount of DC - mostly, although it will still get a bit unbalanced at high signal levels in the 'working' tube since it's a Class AB amp.

I'm really not that interested though. I wouldn't run an amp with only one power tube because I just have a bad feeling about the consequences. Did EVH? I don't know. He may have used other extremely risky - and in at least one case downright dangerous, and not just to the amp - methods to get more saturation as well, and didn't seem to be aware of (or care about) the risk. Great player, terrible tech. If you want to fry your amp trying to get the same tone, go ahead.




FWIW, I have friend who ran a Fender Princeton Reverb II with only one power tube for years, for blues harp. No mods to the amp, just pulled the tube. Sounded great for harp, crap for guitar, and didn't blow the transformer. So who knows...

I'd also be interested to know how the Mesa Lone Star Special works - it can switch from four- or two-tube push-pull to one-tube single-ended on the fly. According to Mesa's website this is patented (pending), so it may not be as simple as it appears.

 

The London Power amps often have a single ended/push-pull pot (that is, continuously variable between SE and PP). I haven't seen a schematic but I'd guess it progressivly grounds the grid from one of the power tubes...

 

That is exactly how it works. The pot replaces the grid to ground resistor on one side of the driver and progressively moves that grid toward ground. It is a really cool effect and something I built into one of my personal amps. I really like it.

 

why?

 

Because it's a different sound. Why not?