EL34's Glowing red hot!?

[Joe bags]

Hi guys

I have a problem with a Nolan tube amp that i'm trying to repair. When I turn on the standby switch one or
sometimes two of the valves glow red. I've tested the tubes with another amp and they work fine with that
one. So I don't think it is a valve issue. I've tested all of the components on the board resistors, caps and
diodes and they all seem to measure good. Anyone got any ideas i'm thinking that maybe it is a short on the output transformer. Have looked for schematics but can't seem to find much info on this amp.

Any suggestions or comments welcome.

Regards

Joebags

[big_teee]

I would check the bias circuit and Bias voltage.
Pin 5 needs -35 to -50 Volts.

Terry

[Joe bags]

I wrote down some measurements I took so I would remember them! There'a resistor between pin 3 and 5 this reads 460v on all power tubes. Pin 4 was reading -16.7v on all power tubes.

[Jazz P Bass]

EL34 pinout

I think you are confusing the pin numbers.
They are read from the bottom.
If indeed the grids are at -16.7Vdc, there is something amiss with the bias circuit.
Pull all of the EL34 tubes & test the pin again.

[Joe bags]

I think you are confusing the pin numbers.
They are read from the bottom.

Possibly. I'll check again tmw. Thanks for the reply.
Here's a photo.


DSCF1287 by Joebags1, on Flickr

[vibrovox7reverb]

Make sure the bias cap is oriented correctly too, or you will be apply a positive voltage on the grids. I did this recently on accident, and took a look in my amp, and while I hadn't reversed the cap I had reversed the leads.

[Gtr_tech]

Reversing the bias cap won't cause a positive voltage. It'll just ruin the cap.

The only way to get a positive bias voltage is to reverse the diode on the bias tap....

[Joe bags]

Hi guys

I replaced two caps that I think were part of the bias circuit. And it solved the problem. Pin 5 on the tubes are reading about -36volts which seems more appropriate. Amp is sounding great now.
Thanks for all your comments and suggestions.