No plate current, but why?

[wizard333]

I have a Peavey Alpha Bass on the bench, 6x 6L6 power.

Two tubes show no current. Both have plate voltage. One has a blown screen resistor, which is the first thing I look for in that case.

BUT

The other does not. Plate voltage.....check. Screen grid voltage......check. Heater voltage......check (although its low on all the tubes, 5.6VAC). Bias voltage.....check. Grounded cathode.......check.

Yes I switched in different tubes, no difference. Checked my tester, everything works.

Can't find a thing wrong with any reading on that socket, except its not showing any current flow.

What stupid thing am I missing here?

[MWJB]

Power down, drain caps, check socket/pin tension at cathode pin?

[wizard333]

Pin tension has been checked, thats not it.

[Atomic]

replace both plate and/or screen resistors on the affected tubes and see what happens. just for giggles. also check your anode connections thoroughly. maybe even replace the anode wire if there is one. if there is a break in the wire somewhere your tube wont have a proper ground. without a proper reference to ground the tube wont draw current.

[Enzo]

OK, take the no-current tube and stick it in one of the working sockets. Does it draw current there? If not, then it is a bad tube. If it works over there, then your socket has an issue.

How did you check voltage on the socket, at the solder tabs? Empty the tube socket, and from up top, probe the female socket pins for voltage. It is possible the solder tab has broken free of the rest of the pin, so voltage is at the solder conection but not getting to the tube itself.

[wizard333]

As I noted above, the tube was checked. No matter what tube goes in there, no current is drawn.

I checked voltage at the socket itself, not the solder tabs. The tubes sockets in this amp face up; the solder tabs are under the board and the board would need to be removed to check them.

Directly at the socket: Plate voltage 487v Screens: 480v Grid bias: -57v Heaters: 5.6vAC Pin 8 shows a direct ground connection with an ohm meter.

If the screen resistor was tits up, you wouldnt see a constant steady 480v there. If the anode was an issue, you wouldnt see a steady 487v there.

Shows the same with my amphead probe plugged in to the socket; so if its getting to that, its getting to the tube pins as well.

So all the lights are on, but for some reason, no one is home. Seems impossible.

[wizard333]

Oh and the schematic shows nothing that would cause the other tube not to work due to the screen resistor being blown in the other.

[Steve Conner]

Maybe the screen resistor on the mystery tube just went very high resistance, rather than blowing out completely. As in, it lets through enough current to operate your DVM with its 11M input impedance, but not enough to power the tube.

Or maybe you replaced the wrong screen resistor, got the sockets mixed up etc.

If that's not it, that board is coming out for sure...

[MWJB]

OK, how does the amp sound? Run it with 3 tubes, two on each end of the primary are you getting a proper push-pull signal?

A friend of mine has an amp in which we can never measure the current on one tube. The amp works perfectly & we ensure that it has matched tubes...just can't get a reading on one socket. Could this be your situation? A probe issue perhaps?

[wizard333]

It sounds ok because the two tubes that are not working are on oposite sides of the push/pull, so its operating as a 4 tube push/pull at the moment.

But

That doesn't change that 2 tubes dont work. One is obvious, the screen resistor needs replacing. Shows 0v at the screen, 480v other side of the resistor. No brainer there

The other...........

Pulled the board, I don't see any obvious bad solder joints, burned traces, etc. Everything tests connected with an ohm meter.

They used 100ohm screen grid resistors which I think I'll replace with std Fender values on all 6 anyway just because that much voltage on the screens makes me nervous to begin with.

I just don't see any reason that tube has 0 current, except that it does.

[MWJB]

I meant run it with 4 tubes (or even 2 with an appropriate speaker load) INCLUDING the tube that shows no current...to decide whether it is a real issue, or simply a measurement issue, stranger things happen.

[Gtr_tech]

Check gnd at the cathodes of the 2 that won't pull current yet? Use yer meter, not yer eyes. If the output tubes are pcb mounted, there could be a broken solder pad that you can't see by looking but will show an open gnd at the socket.

[Enzo]

I like Steve's idea, measure the resistance of that screen resistor. If it were 100k instead of 100 ohms, for example, then you;d still get the B+ at the tube socket, but no screen current would make it through all that resistance.

I just had a fellow bring in his Fener something a few nights ago. He was baffled over a loss of power. Among other things, one of his 470 ohm screen resistors measured out at 700. VOltage was there though.

[micri5446]

ditto what enzo and steve said....i have experianced high screen value with correct voltage but, no tube output...changed screen resistors and...welllah all good. i use good practice to replace all screen resistors if one is faulty cause a $2.00 resistor is worth it in regards to the time trying to trouble shoot a mystery...lol good luck

[wizard333]

Measured the screen resistor in question, its fine - 99.7 ohms out of a 100 ohm spec. Also noted above that I inspected the solder joints, traces, etc, all test fine but just for giggles I re-flowed them.