Strange Hot Rod Deville breakup

[billings]

I've got a hot rod deville here that is acting strangely. The clean channel seems to break up a little early, and when you really crank it the distortion has a strong crossover sound to it that varies with amplitude that's not present at lower volumes.

I've tried swapping out power and preamp tubes to no avail, and the voltages on all the plates are within a few volts of what I'd expect.

The only real positive clue I have is something I can't make anything of. There's a small buzzing sound inside the chassis when the amp is on. It stays at about the same volume level whatever the rest of the amp is doing. I can't identify exactly where it's coming from, but it's definitely from some component on the interior of the amp.

I'd be much thankful if anyone can help out with this. For this evening, at least, I'm out of ideas.

[Enzo]

Look for an open resistor at the phase inverter tube. I'd bet one of the plate resistors is open. This is a class AB amp, so at low levels, one tube of the pair can handle the entire waveform, but as you increase the volume, you get to the point where the other tube is necessary to reproduce the entire waveform. If that tube is not in the loop, then you get distortion, yes crossover-ish distortion.

Of course ti could be something else...

As to the vibration, I wouldn't worry, most likely just the power transformer.

[billings]

Went back and double checked the plate resistors. They're working just fine.

Since that's not it, I guessed that it was maybe the input cap on the feedback input. The presence control works, though, so I don't think that can be it.

What about a failed coupling cap between the phase inverter and the power tubes?

[Don Symes]

What about a failed coupling cap between the phase inverter and the power tubes?

A failed solder joint on one of them would make sense (be most likely).
Next would be a cap that failed OPEN. Do you have a DVM with a cap tester function?
A PI coupling cap that fails short would take out (replate) the output tube it's feeding by driving the bias way high.

Hope this helps!

[sadgod]

I've had new tubes rattle and can hear it through the amp at certain frequencies. maybe it shakes em up at different volumes and frequencies.

[drewl]

How about the 470ohm screen resistors on the power tubes?
I've seen alot of these open up.
While there I usually reflow all the tube socket pc board connections (with tubes removed of course).

[chipaudette]

Hi,

You said "when you really crank it the distortion has a strong crossover sound to it that varies with amplitude". Can you give a few more words as to what crossover distortion sounds like?

I've got a Fender Deluxe Reverb Reissue (DRRI) that, I think, sounds a kinda funny when I drive it really hard. With humbuckers and with the volume past 5, I get some sweet overdrive but the sound is a little funny. I would describe it as a fizzy sound in amongst the overdriven guitar...a fizzy sound whose pitch decreases as the amplitude of the note decays. I've always wondered what it could be.

Could my problem also be crossover distortion? How can I tell? Is there an oscilloscope trace or something that I could measure?

Thanks,

Chip

[rphughes]

He drewl ( and everyone else who suggested this),

Thanks for the socket reflow tip. A week or so ago, my HR Deville smoked. I found that the printing on one of the 6L6's was blackened. So, after getting a new pair of 6L6's and replacing the fuse, and adjusting the bias, the amp was alive, BUT, now it had this wierd issue when I cranked it up. Fortunately I noticed that the heating filament on one of the 6L6's wasn't glowing. I gave it a little sidewards pressure and it came on. I swapped the tubes and the problem stayed with the same socket. Upon closer examination, I saw that one of the socket leads was definitely intermittent at the PCB. PROBLEM SOLVED!

Thanks a bunch!
Bob

[bnwitt]

This is a very common problem with the Hot Rod Devilles along with input jack solder joint cracks and footswitch jack solder joint cracks. I reflow both of the power tube sockets PCB pin joints and all jack joints every time I get one of them in for repair. Nothing worse than fixing an unrelated issue and having the amp brought back later with these problems.