Fender Hot Rod Deville - Octave distortion effect on More Drive

[stereomonostereo]

Hey guys, I'm working on a Hot Rod Deville that's having a weird issue. When playing on the More Drive setting, it produces an octave distortion effect, like an effects pedal. My cathode voltages on V2 are good, both at 1.7, and all of the switching is working properly. I jumpered across R23 and R24 and only seem to get the issue when both are engaged. Doing one at a time doesn't produce the problem. I've had a few in the past that were oscillating due to a bad C36, but that cap is working fine. I'm pretty stumped. Has anyone encountered this before?

[g1]

The only way to know if C36 is okay is by replacement. It may appear to be working fine yet still cause such issues.

[stereomonostereo]

I jumpered in a replacement and still had the same issue with no change at all

[g1]

With the original disconnected?

[dwmorrin]

I'd be curious as to what it looks like on the scope.
Do you have a scope, and can you trace a sine wave through?
Just to clarify: Are you getting octave up tones?

[stereomonostereo]

The original was disconnected. I'm not great with a scope, but I'm gonna try running a wave through. And yes, it seems to be an octave up

[stereomonostereo]

Also, if I unhook one leg of the cathode caps, C8 and C9, I lose the octave effect. I was under the impression that when More Drive is engaged, it bypasses both of those caps from the circuit

[52 Bill]

Also, if I unhook one leg of the cathode caps, C8 and C9, I lose the octave effect. I was under the impression that when More Drive is engaged, it bypasses both of those caps from the circuit

Just the opposite, when more drive is turned on the two bypass caps are added to the circuit increasing the gain at V2A and V2B.

I will assume that you have tried different tubes?

[dwmorrin]

You can get an octave up effect anytime you have parallel and 180 deg out of phase signals combining. (If perfectly matched, you get cancellation, but anything less than perfectly matched can produce odd results.)
I see the potential for that due to the split after C2, then recombining at K2A. Of course, they aren't supposed to be combining there, so I'm still stumped. But the octave up thing is a really big clue...

I posted awhile ago about a SG that was producing an octave up effect straight out of the guitar. When I opened it up, someone had wired the four conductor humbucker for "parallel opposing" operation. Probably a mistake on the part of the installer, but a cool trick!

[stereomonostereo]

Yep, I've swapped out the preamp and power tubes

[stereomonostereo]

I've never heard of that with a guitar pickup. That's interesting!

Whoever worked on this amp prior was a hack. A bunch of leads on the PCB board were destroyed due to poor soldering and then jumpered with wires. From what you're saying, I'm wondering if maybe a trace is crossing somewhere.

[dwmorrin]

My coworker who services these all the time suggests checking the big power supply caps by jumping in a known good one across each one (one at a time!).
He's had several with weird drive problems that ended up being bad filter caps.

[stereomonostereo]

I jumpered across all of the power supply caps with no change

[stereomonostereo]

However, jumpering across C44 eliminates it. I removed C44 and it tests out at 68nf like it should. The schematic shows it connecting to a diode, CR16, which this amp doesn't have. In it's place on the layout is a 10M resistor labeled R106.

[dwmorrin]

Try this schematic for Hot Rod III.
This matches for the 10M R106.