Can A Resistor Measure Good But Be Bad?

June 11th, 2010, 12:51 AM
I have an amp that has two 100k resistors for the voltage drop to the reverb recovery tube. They measure 100k but the voltage drop is only 4V.--from 406V to 402V.

I will get some new resistors, but I have never run into such a thing before. Have you?

June 11th, 2010, 01:11 AM
Sounds like the tube isn't drawing very much current. What does your cathode voltage read?

June 11th, 2010, 01:40 AM
Scott--I will check tomorrow. I put the amp back in its cab for tonight. I am stumped and, obviously, this problem is above my level of knowledge.

I built this amp as a clone of an Allen Old Flame. David Allen sent me the board and I wired it into a SF Bassman chassis.

Why would a tube not draw current? Short on the cathode preventing completion of the circuit? Bad tube?

Weird. I just retubed the whole amp and played three gigs with it. It sounded great. Then, tonight, nada. tubes v2 and v3 are not even glowing.

Bear with me. I may not know that much, but unless I ask questions, I'm sure not going to increase my understanding.

June 11th, 2010, 01:53 PM
Ahh... well if the tubes aren't glowing, the heater filaments aren't getting their daily dose of 6.3v. Without the tube heaters going they won't draw any current.

First things first, try cleaning the contacts on the tubes and tube sockets. There might just be a little gunk on there preventing a good contact. If that doesn't fix it check the heater wires for life.


June 11th, 2010, 04:36 PM
Well...well...well. Turns out that the problem was a bad tube. One of the new 12ax7's I put in just gave up the ghost. Now the voltages read properly and I have an amp again.

I didn't think it was a heater issue because V1 and V2 had power. What I failed to understand is that the tube takes voltage from the supply and the supply voltage was high precisely because the tube was not using any voltage...

There. I've learned something new. Thanks everyone for your thoughts and insights.