Bias problem RM100

[RADE01]

I was checking the bias on my RM100 with 6l6's and got my usual readings of about 32mV for the inner two tubes. But the outer tubes wouldn't get a reading at all. Maybe 1-2mV at the most, and it doesn't change at all by adjusting the bias. I've checked them many times before and always had readings in the appropriate range. I've changed the power tubes and they still give these readings. The amp seems to sound fine, and there are no tube failure lights on. The tubes all light up as usual. I'm worried there may be something wrong and don't know if I should crank it up. Has anyone had this problem? Any help would be appreciated.

 

[RADE01]

No one's had this problem? I read somewhere on another forum that it could possibly be a bad connection or resistor in the test point. Anyone heard of this?

 

[eyeball987]

Sorry I have not had this problem. I do have one tube on the outside that does not hold a bias well but I blame that on the tube probably going bad.

 

[JKD]

Did you check the tube fuses in those two sockets?

Pull the fuses and check them with a meter, don't rely on visual inspection.

The fact that it's a pair of tubes gone makes me think it might not be so simple as that as each is fused individually.

 

[RADE01]

Fuses look OK but I'll check them with a meter tonight. Actually first it was just the tube closest to the phase inverter that wasn't reading, then while I was checking and rechecking, the other of that pair went too. I only know this from checking with the meter, it all looks fine. But there has been a strange soft crackling sound coming from this amp for a few months that I only notice when not playing. It sounds like a dodgy lead, but it's from the amp.

 

[JKD]

Fuses look OK but I'll check them with a meter tonight. Actually first it was just the tube closest to the phase inverter that wasn't reading, then while I was checking and rechecking, the other of that pair went too. I only know this from checking with the meter, it all looks fine. But there has been a strange soft crackling sound coming from this amp for a few months that I only notice when not playing. It sounds like a dodgy lead, but it's from the amp.

I suspect one pair of tubes isn't giving any output....since I think the heater can be on (glowing) without the HT working (what the fuse is protecting)

Checking the fuses and replacing the tubes is typically where you go first...since tube failure can cause fuses to blow without opening up the circuit causing the tube fail light to go on (again...not 100% sure)

Could be the tubes...or something more serious like arc'ing inside due to dirt or component failure.

 

[RADE01]

I was actually just about to replace those tubes when I noticed this. I changed them and it made no difference. Still gotta test those fuses. Thanks for the help.