Class AB amp Question - one tube loses input signal, other tube overheats??

[waspclothes]

Hey guys,
I came across a curious problem. I had an Orange OR80 that was burning out one of the two power tubes. All the voltages tested fine, bias voltage, etc.

Before the tube would burn out, the output was very asymmetric, so I probed the inputs of the tubes and only one of them was getting a proper signal.

This amp has a Master Volume after the PI - a dual-gang pot. I ended up finding a broken solder joint on one of these pots and after fixing that, was getting a proper symmetrical output of the amp and the amp was no longer burning a tube out.

So what I'm trying to figure out here is WHY?

- Why did the power tube that still had the input signal overheat?

[Leo_Gnardo]

This amp has a Master Volume after the PI - a dual-gang pot. I ended up finding a broken solder joint on one of these pots and after fixing that, was getting a proper symmetrical output of the amp and the amp was no longer burning a tube out.

AKA "PPIMV". A broken solder joint on one of the stacked pots would could a loss of bias voltage to one output tube. That's why it's a good idea to bridge wiper-to-no-signal tabs on those pots with a resistor, I use 2M2. In case the wiper loses contact there's still a route to the bias supply. Not a great one but something to hopefully keep the output tube from burning up.

So what I'm trying to figure out here is WHY?

- Why did the power tube that still had the input signal overheat?

That indeed is a mystery. I'd expect the one with the broken connection to overheat.

[waspclothes]

AKA "PPIMV". A broken solder joint on one of the stacked pots would could a loss of bias voltage to one output tube. That's why it's a good idea to bridge wiper-to-no-signal tabs on those pots with a resistor, I use 2M2. In case the wiper loses contact there's still a route to the bias supply. Not a great one but something to hopefully keep the output tube from burning up.

That indeed is a mystery. I'd expect the one with the broken connection to overheat.

Hey Leo, maybe I messed up which tube was which during the process of flipping the chassis over, etc. As far as I was measuring, the bias voltage was present at both tubes still.

Here's a schemo that seems to match the amp in question: http://www.orangefieldguide.com/OFG_SCHEM/ODschem_A.gif (That shows 4 output tubes, mine has 2. )

[d95err]

Here's a schemo that seems to match the amp in question: http://www.orangefieldguide.com/OFG_SCHEM/ODschem_A.gif (That shows 4 output tubes, mine has 2. )

The PPIMV pot is ground referenced and isolated from the powertube bias. The power tubes will have bias even if the PPIMV pot breaks. Something else caused the tube to burn.

[DRH1958]

I have the same problem with an old Bell PA amp. I've done some reading and think I might have figured it out. These amps have a cathodyne PI and when the power tubes are overdriven, they draw grid current. Since the cathodyne sends signal from both the plate and cathode, they react to this condition differently. The cathode acts as a cathode follower with much lower output impedance than the plate, which acts as a normal gain stage and has a higher output impedance. This causes different output signals. As long as the power tubes are not driven to draw grid current, the signals should remain balanced; they change with overdriven power tubes. Also see Merlin's page on this, especailly the second half: The Valve Wizard

EDIT: I also referenced great info from this book as well and they seem to agree with each other. Bias excursion is also mentioned which basically is caused by the different conditions of the cathode and plate. It causes one bias voltage to be lower than the other under overdriven conditions: https://www.ampbooks.com/mobile/books/power-amps/ With my Bell amp, I measured -55v bias on one power tube and the other tube moved all the way down to -70v with overdriven power tubes. So I'm guessing the one with -70v is in cutoff much of the cycle.

[Dave H]

- Why did the power tube that still had the input signal overheat?

Perhaps it didn't like having to do all the work with no contribution from the other tube?

[DRH1958]

Yep. That's the rest of the story, IMHO.