# Running With 2 Power Tubes Instead of 4



## Tone Chaser (Mar 2, 2014)

I came across a 120 watt 6L6 tube amp that is running on only 2 installed power tubes, instead of 4 power tubes as designed.

Are there any drawbacks, or is there actually any benefit, (like lowering the output to a more user friendly volume?; or perhaps easier to overdrive the tubes?)

Is this safe to do so, (2 tubes vs. 4)?

Inquiring minds want to know.


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## guitarman2 (Aug 25, 2006)

Don't run with power tubes. You could drop them.


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## Cups (Jan 5, 2010)

It is safe. It's an old trick to reduce power and it'll also reduce bandwidth. Probably wouldn't hear much of a difference.


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## Stratotone (Jan 29, 2014)

Better read this first
http://www.premierguitar.com/articles/20713-ask-amp-man-removing-output-tubes-to-reduce-power

Let us know how it works out. I thought about doing this when I had a 6505+ head but never got around to it.


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## JCM50 (Oct 5, 2011)

The pentode/triode switch works well. I had one on a custom build and my Marshall Jubilee has one too and its easy enough to do.
In terms of removing two tubes its always best to compensate for primary impedance by setting the impedance selector (if you have one) to half its "correct" setting with all power tubes present. You won't be saving much on volume though. The difference between 50w and 100w is only 3db.


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## Cups (Jan 5, 2010)

Pentode/triode is a very different animal. Changes tone and sometimes adds buzz. 
You also don't need to change the impedance. I'm on my phone right now but I'd share a link to A London Power article. When I get on my computer I'll link it up. 
I believe he answer those questions in his FAQ.


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## Budda (May 29, 2007)

I'd just run at 4 - it's going to sound its best.


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## Chitmo (Sep 2, 2013)

I'd just get an attenuator if your amp is too loud.


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## Budda (May 29, 2007)

Chitmo said:


> I'd just get an attenuator if your amp is too loud.


I'd just get earplugs and a jamspace haha.


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## Disbeat (Jul 30, 2011)

Also keep in mind if you are running 2 instead of the full 4 it changes the ohmage you need to run at.
Something else ya wanna look at as well.

Edit. After further inspection I see JCM50 already touched on that.


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## greco (Jul 15, 2007)

Disbeat said:


> Also keep in mind if you are running 2 instead of the full 4 it changes the ohmage you need to run at.
> Something else ya wanna look at as well.


Really...are you sure? Why?

Cheers

Dave


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## Disbeat (Jul 30, 2011)

I honestly can't answer that, and it's not something I've ever done with my amps but anytime the subject has been brought up that's what people say.
I'm sure someone here can shed some light on it though. 



greco said:


> Really...are you sure? Why?
> 
> Cheers
> 
> Dave


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## jb welder (Sep 14, 2010)

The output transformer matches the impedance of the power tubes to the impedance of the speaker.
If you change the impedance on the tube side (by removing 2 power tubes) you should also change the impedance on the speaker side to restore balance.
When you remove 2 out of 4 power tubes, you are doubling the impedance on the tube side of the OT. So to balance it out, you need to double the impedance on the speaker side of the OT. This is accomplished by setting the impedance switch to half the value of the speaker load. As far as the OT is concerned, it is now running into double it's impedance, and the primary and secondary sides are now balanced.
(in case you are wondering why removing 2 of 4 power tubes doubles the primary impedance, the power tubes act like parallel resistances)


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## greco (Jul 15, 2007)

jb welder said:


> The output transformer matches the impedance of the power tubes to the impedance of the speaker.
> If you change the impedance on the tube side (by removing 2 power tubes) you should also change the impedance on the speaker side to restore balance.


Thanks for taking the time to explain this in such a detailed manner. 
Good information to know.

Cheers

Dave


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## Cups (Jan 5, 2010)

Here's what Kevin O'Connor has to say on the subject:
*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.*


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## jb welder (Sep 14, 2010)

Kevin is right in as much as you believe you should never have to worry about the impedance selector at all.
If you want to keep the same frequency response & bandwidth, and max. power output, then you want to stick with matched impedance.
If you do not have an over-spec'd OT, then you may not want to risk running high power levels at mis-matched impedances.
I like Kevin and he was always very nice to deal with, but sometimes he likes to make pretty broad statements.
If you design your own amps and over-spec, like Kevin suggests in his books, you won't have any problem with mis-match.
Or if you have an amp that can handle it. There are Ampeg's and Boogies and even others that even suggest doing it and choosing whichever impedance setting sounds best to your ears. (and I plead guilty of this)
But there are also amps that stress their OT's and get played at "11". This is where I say, better safe then sorry. And if you want to stay on the safe side, the way I described is how you do it.


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## Tone Chaser (Mar 2, 2014)

Thanks for all the insight.

My internet search came up with too many contradicting opinions, or views that lead me to question other concerns, that my inquiring mind can come up with.

I don't really need another big, heavy, not much loved amp; but, other than 2 missing tubes, it is all there at a somewhat intriguing price. It is supposed to be a very clean amp. It has a solid state preamp section, and the only tubes are the power tubes. I have read that this was common '90's thinking.

I have been doing my due diligence for several months, and the next step is to take one of my well known guitars to test drive it.


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