Rebel 20 Troubleshooting question

[RichR]

If I have a bad 6v6 or el84, should I be able to make the pop,crackle,hiss of the bad tube go away by putting the tube selector all the to one side, or the other?

I would have thought that to be the case, but experience seems to be a little different.

I started getting the telltale pops, crackles, hiss of a bad tube. Happened off-and-on at first. But more and more frequently.

The first thing I tried was to try the tube selector all the way on EL84, then all the way on 6V6. Still got bad tube noise. Started to think, 'well, maybe a preamp tube'.

I had some spare 12ax7 tubes around, and tried each preamp/PI position, but problem persisted, so moved on to power tubes.

I pulled the 6v6s, then put the selector all the way on EL84. Sounded great and no noises. Just to be throrough, pulled the EL84s and put the 6v6s in; put selector all the way on 6V6-- yep, snap, crackle, and pop, with a side order of hiss.

Had a lightly used set of 6v6s on hand, installed rebiased, and good to go.

I guess my question is: Isn't putting the selector all the way to the EL84 side totally cutting off the 6v6s? I was surprised to get different results when I actually pulled the tubes.

Or maybe a matter of the tube having a bad short, and causing problems just by 'being there'?

Amp is working great now, so just a curiosity.

 

[dan desy]

That's what I would think, as well. But I'd love to hear Bruce chime in on this.

 

[bruce egnater]

That woul NOT be the case. The tube mix control does not actually shut off the tubes in some magical way. It is essentially a pan control (with a few twists) the routes the signal to the inputs of the respective pairs. All of the power tubes are always on and connected so, if one of the power tubes is making noise, the tube mix knob will have no effect. You definitely had a bad power tube.

 

[RichR]

That woul NOT be the case. The tube mix control does not actually shut off the tubes in some magical way. It is essentially a pan control (with a few twists) the routes the signal to the inputs of the respective pairs. All of the power tubes are always on and connected so, if one of the power tubes is making noise, the tube mix knob will have no effect. You definitely had a bad power tube.

OK, that makes sense. If the mixing is on the input to the tube sets, and both pairs of tubes remain on and always fully engaged and contributing to output. I can wrap my mind around that.

Any harm/downside to pulling one pair of tubes, and flipping the selector 100% to the remaining tube pair. Say, to finish a gig after a surprise tube failure.

Thanks for the response, and the great amp.

 

[Wil]

I'm having what may be a similar issue. No snap, crackle and pop here, but I've noticed and extra (and fairly unpleasant) harmonic lately. It seems to be worse on the EL84 side, but never really goes away. Could this be a power tube issue?