12AX7 Heater to Cathode Resistance?

[imjonwain]

Background: I am working on a fender super sonic with a heater related issue. I got this amp cheap with the only info being it "didn't work" and it looks like it took a tumble hard. The cabinet has a nasty chip and power transformer was held on by 3 of the 4 bolts and the mounting legs for it were bent. No clue how it happened.

The amp SEEMS to work fine but periodically the 6.3V is dropping out while all other voltages stay high. Weirdly, the first maybe 2 times I saw this happen it didn't blow the 10A heater fuse but did eventually.

I can't find any obvious shorts anywhere so I measured the heater to cathode resistance of all the tubes in it. Most measure open, a few 12AX7s measure 20-40k, and one 12ax7 is measuring 12k on one triode and 20k on the other. Could it possibly have an intermittent or temp related shorting issue? thoughts?

[Stevem]

That amp took a good hit!

First off because of that if the amp is relying on any circuit board traces to deliver heater voltage to the tubes I would cut those at the tube socket pins and hard wire that heater supply and see what that gets you.

[imjonwain]

All tubes are actually chassis mounted sockets connected with wire. This thing is decently built for a mass produced amp imho. All the switching is relay based too.

[R.G.]

I can't find any obvious shorts anywhere so I measured the heater to cathode resistance of all the tubes in it. Most measure open, a few 12AX7s measure 20-40k, and one 12ax7 is measuring 12k on one triode and 20k on the other. Could it possibly have an intermittent or temp related shorting issue? thoughts?

Yes, it could have an intermittent/thermal issue. If I were messing with this, I would replace the 12AX7s that don't show open to the heaters with others that measure open. The cathode and heaters are supposed to be open circuit, with at least a 300V withstanding voltage. A resistance between them could well indicate that the impact moved the internal elements into almost-contact, and heating does the rest.

I would be very interested if replacing the "resistive" 12AX7s doesn't fix it.

[nuke]

Yes indeed, it could have a heater related short. Hence the advice on older amps to disconnect the filament supply center tap from ground and install a pair of 100-ohm resistors to form a "virtual center" for the heater supply.

I tend to suspect though, a bigger short somewhere if it is blowing the 10A fuse.

[stephen_w_keller]

If the tube is out of circuit, then the heater to cathode resistance should be close to infinite. In circuit, it depends on the context.

[imjonwain]

Seems that replacing the 4 tubes I was talking about has solved the issue. Two of them were VERY noisy and microphonic. I plan to replace all the tubes once I get some more. The amp is loaded with Fender branded GTs that I think are all the original tubes. The tubes all have 2006 as the sovtek date code and the amp was made feb of 2007.

Never seen this happen before but I guess I haven't tried sliding an amp down a staircase before either.