Taking tubes out of a Mark IV

[alphadog808]

I was reading that if I set the amp to "Class A" and "Triode", the amp only uses the 2 outer Power tubes. That being said:

1) Can I remove the 2 inner power tubes and still run OK?

2) If I can remove the power tubes, what would happen if either the "Class A" and/or "Triode" switch were changed to "Simu-Class" and/or "Pentode"?

 

[zeppman]

I called mesa and asked them the same thing. They said it wouldn't be the best idea, I don't remember exactly why though.. They did say though its the equivalent of just having those two inner tubes on standby. What I do though is swap the inner ones with the outer ones every so often, to try and get even wear. I was looking at a schematic of the mark iv once, and while I'm not an expert by any means, the "class A" really didn't look like a class A circuit. It just looked like they ran those tubes a lot hotter than the inner tubes (hence my swapping). but please, someone correct me if i'm wrong.

 

[Blueracer]

The class A/simulclass switch effectively takes the inner two tubes out of the circuit by lifting the connection of these tube's cathods from their ground connection. When the switch is in the class A position the tubes will not conduct current. This is effectively the same as removing the tube from the circuit. The only effect that would occur by physically removing the tubes is a decrease in the heater current seen by the transformer. I would question why Mesa sees removing these two inner tubes as a problem. Further if the tubes are not present and the switch is in the simulclass position it is effectively the same as being in the class A mode. There should be no problem in doing this.

The triode/pentode switch has nothing to do with the inner two tubes. What it does do is switches the mode of the outer two tubes between pentode and triode mode. This switch will function whether the inner two tubes are present or not. It has nothing to do with these two inner tubes.

That Mesa equates the class A/simulclass switch to the standby mode is a bit misleading. The standby switch removes the B+ voltage from the plates of all the tubes in the amplifier. Hence none of the tubes conduct current when in standby. This certainly is different than the class A/simulclass switch and is relying on a different mechanism than this switch uses. I will admit that in both cases the tubes don't conduct but the similarity ends there.

So much for the theory. I hate to advise you to remove your inner two tubes in as much as Mesa says that it might be a problem. However I believe enough in what I wrote above that I "risked" my Mark IIC+ by taking the two inner tubes out and playing it while switching between class A and simulclass. The amplifier worked just fine. Note that the Mark IIC+ does not have a triode/pentode switch so I could not try these combinations out but again this switch has nothing to do with the inner two tubes.

Sorry to be so long winded but I wanted to provide enough information so that you could make an informed choice. Hope this is in some way helpful.

 

[phyrexia]

Have you ever played a 60/100 II or III, rabies?