#1

Hi, so I've pulled two tubes from my jvm210 and saw that there was a fifth, smaller power tube. It has 4 EL34 and one ecc83 (which is the same as the preamp tubes) in the power section, as well as four ecc83s in the preamp. What is the effect of the fifth tube on the signal? Can anything happen to the amp with these three power tubes when there are normally five? Thanks.

#2

That's the phase inverter. It's in the power section but it's not a power tube. Your amp won't work without it.

Hopefully you did a little research on what you need to do to safely pull power tubes. In case you didn't - pull the inner two (or the outer two), and use the next lowest impedance output (e.g., if your cab is 8 ohms, plug the speaker cable into the 4 ohm output).

Hopefully you did a little research on what you need to do to safely pull power tubes. In case you didn't - pull the inner two (or the outer two), and use the next lowest impedance output (e.g., if your cab is 8 ohms, plug the speaker cable into the 4 ohm output).

#3

Right, I pulled the outer two and went from the 8ohm jack in the amp the the 16 in the cab. Thanks!

#4

I thought the OT has to be tapped? Or is that what you meant- if 8 ohms with 4 tubes, 4 ohms with 2 tubes. And the cab still has to match the output right?That's the phase inverter. It's in the power section but it's not a power tube. Your amp won't work without it.

Hopefully you did a little research on what you need to do to safely pull power tubes. In case you didn't - pull the inner two (or the outer two), and use the next lowest impedance output (e.g., if your cab is 8 ohms, plug the speaker cable into the 4 ohm output).

*Last edited by Will Lane at Dec 4, 2016,*

#5

Some Marshalls allow the outer power tubes to be taken out but I don't remember exactly how everything else worked (impedance, etc.).

From what I remember it was business as usual just half the power...

From what I remember it was business as usual just half the power...

#6

I thought the OT has to be tapped? Or is that what you meant- if 8 ohms with 4 tubes, 4 ohms with 2 tubes. And the cab still has to match the output right?

I don't know what "the OT has to be tapped" means and I suspect a tad that you don't either

The impedance outputs on the JVM, like all impedance outputs, are OT taps. When you pull two power tubes you double the plate resistance of the power section. Obviously you need a different OT ratio at that point.

The labels on the outputs don't magically change to reflect that change in impedance, so the cab has to be plugged into a jack that doesn't "match." OP has his 16 ohm cab, the OT ratio stays at N:16 but the new N is twice the old. Plain as day you can see that since you can't change the tubes' output impedance (N or N*2) and you can't change the speaker's impedance (16), the only thing to do is to double the OT ratio by plugging into the labeled "8 ohm" tap which is now actually a 16 ohm tap.

For example, using made-up numbers for simplicity: plate resistance of 4 tubes is 2400 ohms, cab is 16 ohms. The 150:1 OT tap is labeled 16 ohms because under normal operation that's what it is. 2400/150=16, you can see everything working as intended. Now pull half the power tubes. Your new untransformed output impedance is 4800 ohms. If you use that same tap, you get 4800/150 = 32 ohm output impedance. Obviously that's not a match for your 16 ohm speaker so you need to use the 300:1 tap. I won't ruin the surprise but I bet you can figure out which one that is.

Will Lane

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