Blown power tube on Randall 667

[JConstantineLV]

Biasing is for the power tubes, not preamp tubes.

 

[CrazyNutz]

Did you test the fuse for continuity with your meter? They can look fine but still be blown. Did you swap them around to see if the problem follows the fuse?

Other than that there are a few components around the power tube that can blow when a power tube blows. The screen resistor is a common one, and in those test points there should be a 1ohm shunt resistor that can blow.

 

[CrazyNutz]

thanks for the info. the tubes are fused in pairs so I figured if one worked then the fuse was okay, but I'll check it anyway. I've got spares in any case.

I bet it's the shunt resistor. A problem with the screen resistor would show up other ways wouldn't it, not just the bias test point? I replaced the blown tube and it's partner, and the amp is back to 100%, except for the bias test point.

Well the shunt resistor would be the only path to ground the tube cathode has, so if that is blown that tube will not be conducting current and would not be contributing to the amplification. A blown screen resistor would cause the tube to not work as well.

Basically what I'm saying is you hear sound, and all seems well, but that tube may not be working at all. You should at least pick up a bias probe, and see if that tube is doing anything at all. You can get the ones that are inexpensive and plug into your multi-meter.

Either that or take it to a repair shop, you could cause damage to your amp in the long run if there really is a issue.

 

[CrazyNutz]

Just a followup:

We had a break, so I took the amp to a tech, and he replaced the shunt resistor for the bias test point. Everything else checked out fine. Amp is back home, biases fine, and sounds great.

Thanks for all of your help.

No problem, I figured it was the shunt resistor. It's actually nice to have a resistor blow, and not your output transformer.

 

[Maisie]

Hi and thank you for the info above!This
 is essential as for me and thank you thank you again!