No plate current, but why?

[Enzo]

OK, this isn't rocket surgery, so we are missing something.

First, replace that bad resistor, don't leave it until later.

Second, fight the temptation to change resistor values until AFTER we have the amp working. You may feel uncomfortable with your screen voltage, but thousands and thousands of Peavey tube amps are designed and built the same way, and as far as I know, none of them are 6L6 eaters. SO there is no reason to add potential confusing elements to the troubleshooting process. You can always go back and alter them once the amp works.

Pull ALL the power tubes. You know which tube sockets are working, so pick one. If I read you right, all six power tubes work in "good" sockets, right? Install ONE power tube in a good socket. Verify it makes sound, verify it conducts current. Don;t push it, we are not trying to get any power or tone, we are merely establishing a basis. In the absence of the other tubes, your voltages will be a bit higher.

Now, put a good tube in the "bad" socket. All by itself, no other power tubes. What happens? SOund or silence? Current or none? The lonesome tube will concentraste the troubleshooting right on the tube.


And never assume your measuring process is not involved just because it works on the other sockets. Consider there may be something about the socket adaptor that is not agreeing with that socket. You said you checked voltages at the top of the socket, did you actually also take the readings at the adaptor socket?

You have gone past the "bad resistor" step of repair, something else is amiss. So we have to check everything. No this and that so therefore so and so. We have to measure the so and so, not just infer it from other stuff. SO for example, we can't say to ourselves the socket has all the voltages, and the probe adaptor works in the other sockets, therefore all the voltages are present on top of the adaptor. We have to actually check. No therefores.

I have a little sign that goes up during training sessions, "Never think up reasons not to check something."

And when all seems OK but the problem persists, take the measurements a different way. Using a bias probe in the socket? Take the current reading in shunt of the output transformer then.


A story I think I have told before, but hey... One time I had a Crown 300 amp through the shop. Dead channel. Older model amp, it had 5 fuse holders across the access panel. Two power rail fuses per channel and a mains fuse. Dead channel, loss of V+ on the channel. To make the story short, all 5 fuses OK, and removed and measured OK. V+ on the supply side but not the circuit side of the fuse. Fuse good. Different fuse and the circuit works. Try the "bad" fuse in any of the other fuse holders and it works fine. For some reason that particular fuse would not work in that particular fuseholder. In other words, all I had to do was swap fuses between holders, and the thing was repaired. But I had to get past the "I know the fuse is OK, so that's not it" stage.

[jrfrond]

Never assume ANYTHING, especially with resistors. We are talking a dollar's worth of parts. Change them, period. I've banged my head against the wall far too many times in the old days with static testing on components that then fail under load.

I know you said that you had heater voltage (5.6VAC is fine; no concerns there), but are the heaters actually glowing? No heater would cause the problem you describe. Also, are you getting a voltage drop across the screen grid resistor? If you are, then you are drawing screen current and the tube is actually working.

[wizard333]

Enzo I've checked both the socket directly, and the adaptor plugged into the socket directly, voltages on both are as quoted above. Tighted up and cleaned them to, etc.

Since my bias/plate voltage probe has 2 sockets, I also tried both and measured both, as well as the tube socket directly. Same result, the socket appears to be ok and the adapator (both sides) appear to interface with the socket just fine; voltages measured on the adaptor match the voltages measured at the socket.

Voltages are the same on working sockets.

I've also measured the resistor, as noted above.

Heater elements are glowing. There is a voltage drop across the resistor.

I've switched the tubes around as well, any tube works in good sockets, not in that one though.

I haven't tried putting just one tube in the bad socket and leaving the 5 others out though, what exactly am I looking for there?

I guess I can try shotgunning it and just replacing resistors to see if there is any affect, since there doesn't seem to be a logical explanation why a tube with heater working, screen and plate voltage, bias and a grounded cathode wont conduct.

[MWJB]

You have only so far determined that you can't measure the bias at that socket, you have not ascertained whether or not the tube still functions normally, this is why Enzo & I are suggesting running with either just the socket in question, or a socket either side of the secondary & assessing whether you get good PP operation...Enzo's idea of running 1 tube is more cut & dried...if you get any sound at all, it works.

Don't start shotgunning parts until you have done this.

[Enzo]

We have six tube sockets and some of them work, that means the amp is working all around the problem. WHy have all that in the way? You have a dead socket - or at least something that seems like one. SO let us look only at that without anything else going on that could possible mask the evidence we seek.

We have not been able to come up with a "logical explanation" for this. Fine. But our job is to fix the amp, not solve intellectual puzzles. Oh I love the riddles as much as the next guy, sometimes I feel guilty here, because I get to solve other peoples' riddles, or at least try. But ultimately what we want is a working amp. And once we find the proble, we can go back and find out what was preventing us from seeing the logical situation.

The whole essence of troubleshooting is to isolate the problem. By having only the bad socket in use, that is a good first step in isolating, in my view.