Removing 2 of 4 Power Tubes

[NOGE]

I think there are two reasons amp manufacturers don't recommend this.

1, because you need to "mismatch" your amp and cab impedance and it's confusing for most people.

And 2, you DEFINITELY need to re-bias the amp because the plate voltage will rise if 2 tubes are removed and your tube dissipation will therefore increase. Also, though probably not a major issue, the preamp voltages will be higher than stock even after a re-bias.

 

[RickyLee]

I think there are two reasons amp manufacturers don't recommend this.

1, because you need to "mismatch" your amp and cab impedance and it's confusing for most people.

And 2, you DEFINITELY need to re-bias the amp because the plate voltage will rise if 2 tubes are removed and your tube dissipation will therefore increase. Also, though probably not a major issue, the preamp voltages will be higher than stock even after a re-bias.

Man, this is a tough room for sure.



And now I will sacrifice myself and let the flaming begin on me.

What NOGE posted is good advice.

The primary reason you will not hear any (well, that much of) change in volume when pulling two of the four power valves is because the entire amp power supply is now running at a slightly higher voltage. This is why you should check the bias when doing this. The plate voltage is now higher and the bias supply voltage is also slightly higher BUT not high enough to compensate for the higher plate voltage and put the bias/idle setting back to the same setting it was at while the four power valves were in the amp. Also, the heater filament circuit is now slightly higher voltage as well. The preamp B+ will be slightly higher changing the bias of the 12AX7 preamp valves. In some amp designs, the higher preamp B+ voltage will have an influence on the tone and feel of the amp as well.

BUT . . . if you can lower the MAINS voltage or your wall voltage slightly, THEN you will be safer doing this and hear a slightly audible change for a lower volume. Yes, I know the lower volume thing is not the reason or concern here. I used to do this same exact thing in a few 100W amps so I only had to run two power valves. BUT, I was using a VariAC to compensate and keep the amps power supply voltages (mainly the heater voltage) in spec. The VariAC thing became a pain in the ass when playing out and that did not last long. And then like the others pointed out, you have to remember to compensate the impedance setting to half the load.

Last year I got two quads of new Svetlana EL34's shipped for $93. I got them from Musicians Friend. The prices have went up a bit. But they have Ruby EL34's right now for quite cheap. You can get a quad for $58. And they are decent sounding EL34's. I have them in my 2555 Jubilee Clone.

Ruby EL34 Matched Power Tubes | Musician's Friend

So, my advice would be to keep all the power valves in unless you are savvy to checking bias ect AND, your wall voltage is below the average to where your heater voltage is not running high along with the rest of the amps power supply/B+ voltages. Basically, this stuff should all be checked out by taking voltage measurements inside the amp and such and most people do not have experience doing this type of thing. This is why most people just tell the people that ask about pulling two of four power valves not to do it . . . because obviously if someone is asking if it can be done, they do not have experience doing it already and they more than likely do not have a generalized understanding of how a valve amplifier works.

Remember, this is all just my opinion. It can be done but you need to consider is it really worth the hassle when you do the math on the $ you are saving over the long time frame of the life of the valves?

 

[anitoli]

I've said this before and i'll say it again, the half power switch and the output tube fail circuit on the 6100/900 series amps does the EXACT same thing as pulling tubes especially the 6100's half power mode. When engaged in half power it lifts the cathode connection and kills the tube. I have measured the voltage rise by using the switch and pulling the tubes and the difference is nil.

Now my question that never seems to get answered is why doesn't Marshall tell you to change the impedance when using the half power mode?

 

[RickyLee]

I've said this before and i'll say it again, the half power switch and the output tube fail circuit on the 6100/900 series amps does the EXACT same thing as pulling tubes especially the 6100's half power mode. When engaged in half power it lifts the cathode connection and kills the tube. I have measured the voltage rise by using the switch and pulling the tubes and the difference is nil.

Now my question that never seems to get answered is why doesn't Marshall tell you to change the impedance when using the half power mode?

Yeah, I know you have brought that up quite a few times. And I am with you on the thingymybob of Marshall DOES NOT mention to compensate the impedance on the OT secondary.

I have not ran my 6100 in that mode except for just a very few minutes to hear the difference, and I did not check voltages when I did. But the 6100 has 11 total valves, and I think that is why you did not see too much change in the power supply voltage under load when removing two valves. Also, were the power valves 6L6 or EL34? Two 6L6 missing is a much less difference on the heater supply circuit than pulling two EL34's that draw a combined 3A or slightly more just for the heaters. What I am getting at there is, you will see a more abrupt change in voltages in a 2203 when pulling two power valves for example. Heck, my DSL100 & TSL100 had much higher running voltages when I yanked two EL34's out of them.

And as for not adjusting/compensating the impedance on the 6100, I would think that would make it even harder to hear an audible drop in volume vs halving the impedance. I have tried switching the impedance switch back and forth in this scenario, and I heard an audible change in volume level even though it was very subtle.

Which brings me to another 6100 question. Is the power transformer in these amps rated high enough for the heater demands of four EL34 and seven 12AX7's? I was looking at replacement PT for these amps and the heater specs were actually too low for running four EL34's. I will not give the name of the PT brand as they are close to home here.

 

[marshallmellowed]

Santiago (Marshall Design Engineer) has stated that it is perfectly safe to pull 2 tubes from a 100 watt amp and re-bias the amp using 50 watt values (referencing a JVM). He also stated the same procedure as others have posted, regarding the speaker connections (16 ohm cab to 8 ohm tap...). I've been running both my JVM and JMD with 2 tubes removed with no issues (as have many JVM owners on the JVM forum). Bottom line, if you don't feel comfortable doing it, then don't. On the flip side, if you don't have proof of damage resulting from running on 2 tubes, then it's just an opinion, not supported by facts.

 

[Ken]

Santiago (Marshall Design Engineer) has stated that it is perfectly safe to pull 2 tubes from a 100 watt amp and re-bias the amp using 50 watt values (referencing a JVM). He also stated the same procedure as others have posted, regarding the speaker connections (16 ohm cab to 8 ohm tap...). I've been running both my JVM and JMD with 2 tubes removed with no issues (as have many JVM owners on the JVM forum). Bottom line, if you don't feel comfortable doing it, then don't. On the flip side, if you don't have proof of damage resulting from running on 2 tubes, then it's just an opinion, not supported by facts.

I would trust Santiago's knowledge at face value, but point out that the JVM is not the same design as other Marshall amps, just like the 6100 has unique aspects as anitoli has pointed out. I consider it dangerous to say, "Well it's okay on a JVM, I'll just pull 2 tubes on my 800" which is why I err on the side of caution and advise people not to do this. Despite the fact lots of people pull tubes and have had no problems for years, the stresses on the amp could be shortening its long term survival rate. I frequently make the analogy between cigarettes and mismatching impedances: one won't kill you but there is a cumulative effect. My sense is that applies to pulling 2 tubes on at least some Marshalls as well.



Ken

 

[EdHopper]

Well, I've been using my Marshall Super Lead 100 watt with 2 of the 4 tube pulled since 2014 with no problems. Amp sounds great and there is no problem with volume or tone. And to Sinner 13, I say, nice try Troll.
You don't need 4 power tubes - unless your playing with no sound reinforcement at a outdoor, very populated, rock festival with 3 drummers, and 3 other guitar players and a horn section that IS amplified.

 

[johan.b]

I've said this before and i'll say it again, the half power switch and the output tube fail circuit on the 6100/900 series amps does the EXACT same thing as pulling tubes especially the 6100's half power mode. When engaged in half power it lifts the cathode connection and kills the tube. I have measured the voltage rise by using the switch and pulling the tubes and the difference is nil.

Now my question that never seems to get answered is why doesn't Marshall tell you to change the impedance when using the half power mode?

I'm not really answering your question either, but. .The impedance mismatch is a lot harder on the remaining tubes than on the transformer( i think that is addressed in the 6100 manual? ). pulling 50+ watts through a 100+watt transformer isn't that bad...but the reflected impedance to the remaining tubes, the math doesn't add up...static B+ voltage, max watt dissaparion then suddenly the impedance gets cut in half. .that would be a lot more current (a tube delivers what it can, a transistor would die instantly ). ..when/if the tube dies through an internal short, it takes out the transformer.
Just make sure the reflected impedance match the tubes and it's perfectly safe running 50 watt through a 100 watt transformer. When things are matched a 100w unit is just a bigger lump of iron than a 50w unit.
People think of the transformer as a magical device, but basic math still applies
..but as Madampscientist points out, there is little reason to go there. Choose a less efficient speaker instead or go with a 1x12 instead of 4x12..move less air
j

 

[EdHopper]

For those who have put forth the theory that "The Tube Amp Book" by Aspen Pittman is telling us it's OK to pull two of the four tubes is so they can sell more tubes, I say this: you have flawed logic. Pulling out 2 tubes saves tube life since the tubes you remove are not being used. The 2 tubes you are using in the amps will be the ones getting the wear but they will not have to "work harder" and when these tubes do wear out, you will have 2 tubes ready to replace them. Therefore, Grooves Tubes would be selling LESS tubes.

 

[EdHopper]

One more thing - I agree that the bias should be checked and adjusted when removing 2 of the 4 power tubes - but I do this anyway. I measure the plate voltage and plug that into a formula to get a target number. I then use a bias probe and my meter to adjust the bias.

 

[chiliphil1]

I've said this before and i'll say it again, the half power switch and the output tube fail circuit on the 6100/900 series amps does the EXACT same thing as pulling tubes especially the 6100's half power mode. When engaged in half power it lifts the cathode connection and kills the tube. I have measured the voltage rise by using the switch and pulling the tubes and the difference is nil.

Now my question that never seems to get answered is why doesn't Marshall tell you to change the impedance when using the half power mode?

Does it do that or does it switch them to triode rather than pentode? I know on some amps (new DSL) it's labeled as "pentode/triode" all tubes keep working and the impedance remains the same but it changes how it uses the tubes..

 

[charveldan]

Id say anyone "pulling tubes" has the wrong amp for the job.

 

[RickyLee]

Regarding the 6100 (and many Mesa amp models use/have his feature as well) and its half power switch, it takes out two of the four power tubes by lifting the cathode connection to Ground on those two two tubes. But, those two tubes are still in there with their heater filaments on. That is completely different than the pentode/triode switch, which the 6100 also has that feature as well. I do not like the idea of two of my tubes sitting there with their heaters burning away and not getting used. I would rather just run the tubes.

And this half power feature does not do any adjustments/compensating on the impedance mismatch.

 

[EdHopper]

Yes, the switch from Pentode to Triode is a different deal all together.
What we are talking about here is taking a Marshall Super Lead 100 watt what (4 power tubes) and converting it to a 50 watt. Please be aware that it does matter which tubes you remove. One should not just remove ANY 2 tubes. I have removed the outer two tubes in my Marshall Super Lead 100 Watt and believe me, you won't even notice a difference in volume. I've been using this method for about 2 years now - extending the life of my quartet of EL34s. One must also remember to change the impedance selector to 1/2 of whatever your load is. All this being said, please try to remember the words of the wiseman: "It's not about the hardware" - it's about the music!

 

[RickyLee]

Yes, the switch from Pentode to Triode is a different deal all together.
What we are talking about here is taking a Marshall Super Lead 100 watt what (4 power tubes) and converting it to a 50 watt. Please be aware that it does matter which tubes you remove. One should not just remove ANY 2 tubes. I have removed the outer two tubes in my Marshall Super Lead 100 Watt and believe me, you won't even notice a difference in volume. I've been using this method for about 2 years now - extending the life of my quartet of EL34s. One must also remember to change the impedance selector to 1/2 of whatever your load is. All this being said, please try to remember the words of the wiseman: "It's not about the hardware" - it's about the music!

I run my Twin Reverb with only two of four 6L6's. Run it into my 8 ohm cab where originally it would need a 4 ohm load.

But I have to mention part of the reason you and many others say they do not hear much volume change.

When you completely remove two of the four EL34's, your B+ will rise a bit due to less load demand, less heater current demand ect. I seen my DSL100 rise from about 480VDC to well over 490VDC going from 4 EL34 to 2 EL34. My heaters went from around 6.7V to about 7.1V. BUT, my bias idle was now much higher/hotter by quite a bit. The B+ rises, but the bias supply voltage also rises . . . but not enough to keep up with holding the bias at 60% idle to match the now higher B+.

Then what I did was ran this test with the amp running only two power tubes of the four with the VariAC adjusting my amps B+ down to the value it was at with four running EL34's. I now could hear some volume drop while turning the VariAC up and down between the two different B+ values.

So, when you pull two tubes, you should re-bias unless you are going to adjust the wall voltage/mains to keep the amp running at the same voltage. If you do this, you will hear a slight volume decrease. I have tested this. But to anyone reading this, you should check the bias as it will be now running hotter if you are at the same wall voltage as before.

ADD/EDIT: I wanted to also mention that everyone should be aware that some amp models are running certain components close to their max voltage ratings, such as 450V & 500V filter cap cans. If you have an amp like the DSL100 and TSL100 that runs quite high when you have a wall voltage like my area, of 125VAC and up, you might see well over 480V on your power tube plates. If you then remove two power tubes, that plate voltage will rise and be closer to that max capacitor rating (This applies to many other makes/models of amps like the 2203, Jube 2555, 2210, old Fenders).

To go even further, for people that work on, test their amps and turn them on with NO tubes/valves installed, this risk of higher than normal B+ is exaggerated even more. Just something to keep in mind . . . .

 

[RickyLee]

I forgot to mention something else that gets overlooked. When pulling two power tubes, the HT fuse should then be reduced by half and the MAINS fuse reduced by a bit less than half as well.

 

[anitoli]

Does it do that or does it switch them to triode rather than pentode? I know on some amps (new DSL) it's labeled as "pentode/triode" all tubes keep working and the impedance remains the same but it changes how it uses the tubes..

The Pentode/Triode switch changes the screen grids supply voltage from being parallel to the anode ( triode ) to after the first voltage dropping resistor that gets the voltage feed from the primary side of the OT ( pentode). The switch does nothing to affect the impedance of the OT. This is what Marshall says about it, go to section 8:

http://home.myfairpoint.net/vze1u14s/sitebuildercontent/sitebuilderfiles/30thanniv.pdf

 

[ampmadscientist]

I've said this before and i'll say it again, the half power switch and the output tube fail circuit on the 6100/900 series amps does the EXACT same thing as pulling tubes especially the 6100's half power mode. When engaged in half power it lifts the cathode connection and kills the tube. I have measured the voltage rise by using the switch and pulling the tubes and the difference is nil.

Now my question that never seems to get answered is why doesn't Marshall tell you to change the impedance when using the half power mode?

No, the half power switch does NOT do the same thing as pulling tubes. Get your facts in order here:

The half power switch on 6100 / 900 changes ALL 4 TUBES to triode. This yields an output of approximately 40 watts.

* Triode being less efficient, produces a noticeable drop in loudness.
** ALL 4 TUBES continue to operate, and the impedance match is maintained in 40 watt operation.
*** This is considerably more intelligent than pulling 2 tubes out. (which yields the resulting mismatch problems)

"...why doesn't Marshall tell you to change the impedance when using the half power mode?"

Because: there is no change in the impedance, ALL 4 TUBES are still operating!

Got That? I hope your question has been (definitively) answered.

THE
amp that actually shuts off 2 tubes for 1/2 power, is MESA BOOGIE.
AND I'm not even going to try to excuse that (rather idiotic) idea.

 

[anitoli]

S

No, the half power switch does NOT do the same thing as pulling tubes. Get your facts in order here:

The half power switch on 6100 / 900 changes ALL 4 TUBES to triode. This yields an output of approximately 40 watts.

* Triode being less efficient, produces a noticeable drop in loudness.
** ALL 4 TUBES continue to operate, and the impedance match is maintained in 40 watt operation.
*** This is considerably more intelligent than pulling 2 tubes out. (which yields the resulting mismatch problems)

"...why doesn't Marshall tell you to change the impedance when using the half power mode?"

Because: there is no change in the impedance, ALL 4 TUBES are still operating!

Got That? I hope your question has been (definitively) answered.

THE
amp that actually shuts off 2 tubes for 1/2 power, is MESA BOOGIE.
AND I'm not even going to try to excuse that (rather idiotic) idea.

You are not always right dude. There are two switches on the 6100, the HALF POWER and the PENTODE/TRIODE. Have a look and see why you just made an ass of your self! Your Statement has been definitively rebuked.

 

[chiliphil1]

THE
amp that actually shuts off 2 tubes for 1/2 power, is MESA BOOGIE.
AND I'm not even going to try to excuse that (rather idiotic) idea.

Nothing idiotic there. Mesa does it the way it should be done. Not to mention that they are not sensitive to impedance mismatches.