Help - Power Tube 1 registers 0 MV on bias

[Scaryfast]

I have an RM100. It seems to play and sound great, but I wanted to try switching from 6L6GC power tubes to JJ EL34II tubes. I followed the instructions in the RM100 manual and checked the current 6L6 tube bias voltage. Tube 1 read 0 MV. I turned the screw both directions as far as it would go and it made virtually no difference. The tube glows but is not nearly as warm as the others.

I switched out the tubes to the brand new EL34II tubes thinking that maybe it was an issue with the tube, but it isn't. The other three tubes bias and adjust fine, but tube number 1 still does not register anything on the tester and is much cooler than the other tubes (but is glowing). I cleaned the contacts in the tube socket (twice) just to see if that was it, but that didn't make a difference either.

Any thoughts? Is this a known or relatively common issue? Again, the amp sounds awesome (I think) and is MORE than loud enough.

Thank you in advance.

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[CrazyNutz]

Did you check the fuse for that tube? Most likely that is your problem.

I have an RM100. It seems to play and sound great, but I wanted to try switching from 6L6GC power tubes to JJ EL34II tubes. I followed the instructions in the RM100 manual and checked the current 6L6 tube bias voltage. Tube 1 read 0 MV. I turned the screw both directions as far as it would go and it made virtually no difference. The tube glows but is not nearly as warm as the others.

I switched out the tubes to the brand new EL34II tubes thinking that maybe it was an issue with the tube, but it isn't. The other three tubes bias and adjust fine, but tube number 1 still does not register anything on the tester and is much cooler than the other tubes (but is glowing). I cleaned the contacts in the tube socket (twice) just to see if that was it, but that didn't make a difference either.

Any thoughts? Is this a known or relatively common issue? Again, the amp sounds awesome (I think) and is MORE than loud enough.

Thank you in advance.

[/img]

 

[Scaryfast]

The tube still glowed, so originally, I didn't think it was the fuse, but I did check it anyway. Swapped fuse from Tube 2 (which was biasing fine) to tube 1 and no change. Still no reading on tube 1. Thanks.

 

[Capt_Tippy]

There are fuses inside the amp too, but I think if a fuse were blown the amp would not have sound coming from the speakers. Maybe the wire/connector inside the test port is damaged?

 

[CrazyNutz]

The tube will still light up even if that fuse is out, because that is a separate circuit called the heater/filiment circuit.

That fuse is the high voltage (B+) fuse for the tube. You will get no bias reading if it is blown.

Since you say the fuse is good, next thing I would check is the Screen Resistor for that tube. You have to remove the chassis. It will be a big white rectangular 1k 5w resistor near the tube socket.

The tube still glowed, so originally, I didn't think it was the fuse, but I did check it anyway. Swapped fuse from Tube 2 (which was biasing fine) to tube 1 and no change. Still no reading on tube 1. Thanks.