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Discussion Starter · #1 ·
I had one of the amps I built a few years ago come back today, Suddenly no sound was the symptom.

It was easy to spot that one power tube wasn't lighting up, but when I checked heater power, it had power.
The problem turned out to be a 6V6 power tube with an open in the heater circuit. It was a Electro-Harmonix rebranded GT/Fender.

I did not know tube heaters failed.
I did not know that one dead 6v6 in a push/pull 6V6 system would be a show stopper. (AB763 DR clone)

Noted here as a public service. The learning never ends. (y)
 

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I did not know that one dead 6v6 in a push/pull 6V6 system would be a show stopper. (AB763 DR clone)
Loss of one power tube in push-pull will not kill all sound. The other power tube must not be working.
Try a single known good 6V6 in one socket at a time. I think you will find one socket is functional, the other is not. Perhaps an open screen resistor.

Loss of one power tube in push-pull will cause loss of power and early distortion, but you will still have sound (if everything else is functional).

Some amps (certain peavey models for example) will have heaters in series, so loss of one heater will cause other tubes to quit. But you said this is an AB763 circuit, so that should not be the case.
 

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Discussion Starter · #4 ·
Loss of one power tube in push-pull will not kill all sound. The other power tube must not be working.
Try a single known good 6V6 in one socket at a time. I think you will find one socket is functional, the other is not. Perhaps an open screen resistor.

Loss of one power tube in push-pull will cause loss of power and early distortion, but you will still have sound (if everything else is functional).

Some amps (certain peavey models for example) will have heaters in series, so loss of one heater will cause other tubes to quit. But you said this is an AB763 circuit, so that should not be the case.
good insight, thank you. I did change both power tubes with a matched pair of course, and everything suddenly came to life. I thought it should still push, even without a pull.
I did check voltages, and bias, but it sounds like I better check them again.
 

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Discussion Starter · #5 ·
Loss of one power tube in push-pull will not kill all sound. The other power tube must not be working.
Try a single known good 6V6 in one socket at a time. I think you will find one socket is functional, the other is not. Perhaps an open screen resistor.

Loss of one power tube in push-pull will cause loss of power and early distortion, but you will still have sound (if everything else is functional).

Some amps (certain peavey models for example) will have heaters in series, so loss of one heater will cause other tubes to quit. But you said this is an AB763 circuit, so that should not be the case.
I went back in again, didn't find anything wrong. I don't have a tube tester, so I can't say for sure, but both tubes must have died. It was biased a little hot when I got it back, 40 mV through a 1 ohm @450 volts. I knocked it back to 23mV.

I took the tube I thought was still good and plugged in into a V-champ. Surprize! virtually zero output and super distorted. So that tube was gone for a while already when the heater went open on the other tube and things got quiet. More learning (y) Learning is good (y)
 

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I went back in again, didn't find anything wrong. I don't have a tube tester, so I can't say for sure, but both tubes must have died. It was biased a little hot when I got it back, 40 mV through a 1 ohm @450 volts. I knocked it back to 23mV.

I took the tube I thought was still good and plugged in into a V-champ. Surprize! virtually zero output and super distorted. So that tube was gone for a while already when the heater went open on the other tube and things got quiet. More learning (y) Learning is good (y)
If you build amps, get a good old brand of Tube tester, rebuild and calibrate it! They are very affordable to buy if you have the skills to rebuild it. I rebuilt a B&K 707 and love it! Only cost me a few hundred and it's payed for itself more than once
 

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Discussion Starter · #7 ·
If you build amps, get a good old brand of Tube tester, rebuild and calibrate it! They are very affordable to buy if you have the skills to rebuild it. I rebuilt a B&K 707 and love it! Only cost me a few hundred and it's payed for itself more than once
I've been looking but I never seem to find one. What do you think of a B&K 606?
 

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I've been looking but I never seem to find one. What do you think of a B&K 606?
Interesting, do you have a line on one? I've never played with a 606 but from the pics I found online it looks like it must be very limited in tube types compared to the bigger 700,707 and 747 B&K's. I'd highly recommend any of these three as there is lots of literature and videos online on how to calibrate them.

I found mine on Ebay from a seller here in Canada. I converted mine to solid state rectification for better stability and also did a socket mod to specifically test 12AX7's. For some reason, when the stock 707 test's a 12AX7, it was only an approximate reading for that tube. I guess since these tester's were mostly for TV repair, they must have thought the 12AX7 wasn't an important tube to test accurately. It's an easy mod to do this.
 

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On a side note, the reason why I love this tester so much is that I've found more than one shady seller on Ebay stating claims of great vintage tubes when they were not :cool:
 
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