Fender Twin Reverb - Intermittent plate current on one power tube (bias probes)

[spunko]

Hello good friends!
I got my hands on this Twin reverb reissue. It came without power tubes, 2 burnt control grid resistors, and 2 burnt screen grid resistor on the same sockets (V7 and V9). I replaced only those (v8 and V10 test good), and using a bulb limiter I tested voltages without tubes, and everything seemed fine, so I installed the tubes and used my 2 bias probes to check.
I noticed V10 wasn't reading plate current, swaped probes and tubes with the same result. Hours later I tested the amp and I got the reading on V10, then I powered off/on, and then again no reading.

The title says intermittent, but on the 8 attempts swapping probes on the V10 socket, I manage to get the plate current reading only 2 times.

What could be causing this behavior?

PS: The first time I tried the probe on V10, it didn't give the readings, so I opened my cheap multimeter and the 500mA fuse was burnt. I don't know if it is related to the amp's problem. I changed the fuse and it works great, it read well on the other tubes and it hasn't blow again with all the tests I have done.
Also, I tried the other probe with its multimeter exactly the same on V10 and it didn't read current, nor blew the fuse.

[PRR]

> on the 8 attempts ...get the plate current reading only 2 times.

[spunko]

Well those attempts include swapping probes, swapping meters, changing tubes, and checking for loose wires or bad solder joints, but all seems fine.
I'm going to try new resistors tomorrow, although the others read/test fine. Maybe I'll try tightening the socket pins.

What could cause current stop flowing in that particular tube?

[PRR]

...Maybe I'll try tightening the socket pins.

What could cause current stop flowing in that particular tube?

I think you answered yourself.

[spunko]

Thanks! that fixed it

[Rontone]

PS: The first time I tried the probe on V10, it didn't give the readings, so I opened my cheap multimeter and the 500mA fuse was burnt. I don't know if it is related to the amp's problem. I changed the fuse and it works great, it read well on the other tubes and it hasn't blow again with all the tests I have done.

Did you solve this mystery? Was it the thing I keep doing, changing between voltage and current readings and forgetting to change the leads over on the meter

[PRR]

Do not measure current. Put an appropriate resistor in the chassis and measure voltage. MUCH-MUCH cheaper than meter fuses.

[Rontone]

Ah yes, I have seen that discussed for cathode resistor biasing, that has set it a bit more firmly in my mind now after a quick re read

[PRR]

.

[spunko]

PS: The first time I tried the probe on V10, it didn't give the readings, so I opened my cheap multimeter and the 500mA fuse was burnt. I don't know if it is related to the amp's problem. I changed the fuse and it works great, it read well on the other tubes and it hasn't blow again with all the tests I have done.

Did you solve this mystery? Was it the thing I keep doing, changing between voltage and current readings and forgetting to change the leads over on the meter

No, I didn't. It just happened one time, then no more fuse blown even with the problem still going on in the amp until it was fixed. 

[spunko]

Do not measure current. Put an appropriate resistor in the chassis and measure voltage. MUCH-MUCH cheaper than meter fuses.

What type of resistor you suggest? or any type will work?
I have a bunch carbon film and a few metal oxide film 1 0hm 1 watt

[Esquirefreak]

Any 1Ω resistor will do just fine on the cathodes of a fixed bias amp. Very little current flowing through there. Although the thicker leads on a 1w or 2w is often beneficial for when using probes.

/Max