Fender Hor Rod Deville 212 buzzing with certain notes

[RMD]

I have an old 60W Deville tube amp, made in 1999 I believe. When I bought the amp I replaced both 6L6GC output tubes. The issue with the amp, which otherwise works and sounds great, is that certain notes played on low E and A string resulting in cabinet buzzing or farting-type sounds. Seems to happen around the 10 - 12 frets. The buzzing may or may not be coming out of the speakers, I cannot say for sure. Thinking it's some type of microphonics phenomenon, I shotgun replaced all (3) 12AX7 tubes, but the issue remains. Anybody ever run across a similar issue?
Thanks a lot.

[Jazz P Bass]

Use your DVM & check for excess Vac ripple on R4, the non plate side. (Marked X on the schematic)

If the ripple is high, then C36 is bad & needs to be replaced.

[g1]

As far as microphonic phenomenon, in my experience lower frequency sounds (rattle, rumble) are more likely from power tubes, higher frequency sounds (squeal) more likely from preamp tubes.

[soundguruman]

I have an old 60W Deville tube amp, made in 1999 I believe. When I bought the amp I replaced both 6L6GC output tubes. The issue with the amp, which otherwise works and sounds great, is that certain notes played on low E and A string resulting in cabinet buzzing or farting-type sounds. Seems to happen around the 10 - 12 frets. The buzzing may or may not be coming out of the speakers, I cannot say for sure. Thinking it's some type of microphonics phenomenon, I shotgun replaced all (3) 12AX7 tubes, but the issue remains. Anybody ever run across a similar issue?
Thanks a lot.

Replace 100K plate resistors on preamp tubes. Known to cause farting noises...
Also, bias for output tubes my be too cold. Causes crossover distortion, sounds like fart on low notes.
Also, remove tubes and re-solder tube sockets. Known issue with UN-soldered or cold connections on tube socket pins.
Check the ribbon wire connectors, between the main board and the tube socket board. May have a broken wire...

[Chuck H]

Aside from all the suggestions made so far...

Let's find out what the problem is first. Doing ANYTHING other than testing before identifying the actual cause is asking for more trouble. Start by plugging the amp into a different cabinet. If the problem still happens this isolates it to the amp. If it goes away this isolates it to the speaker/cabinet. With a third possibility being that it's a bad connection (dirty tube socket or ribbon pin contact, a cold solder joint or damaged board trace or a microphonic tube). In this case it is the amp, but only happens when it's subjected to vibration. To see if this is the case, turn town the reverb, ball up your fist hammer at the amp to see if you can induce a noise.

[RMD]

A little update after getting through the holidays. I did a quick ripple check on R4 with a Fluke DMM, per the suggestion from Jazz P. This particular meter has a pseudo-analog movement (bar graph) at the bottom of the display, the idea being is that it updates faster than the digital display and thus easier to catch something that happens fast. With amp volume at 4, the instant of pick striking the strings (E and A), I could see the ripple spike up to about 0.7VAC, then immediately settle back down to near zero mV. I think it idles around 0.002 VAC.
Would this be considered normal, or is C36 not doing its job?
Also, I did connect this Deville to a 4 x 12 cabinet and did NOT hear any odd sounds whatsoever, which tells us something. Perhaps I should go about re-soldering all the tube pins.
Thanks for all the suggestions.

[Jazz P Bass]

C36 appears to be fine at 0.002Vac ripple.

What you where observing is the signal actually riding on the cap.

It's when the Vac does Not return to zero that you have a problem.

I would be interested to know if Any tone controls affect the problem.

[g1]

Also, I did connect this Deville to a 4 x 12 cabinet and did NOT hear any odd sounds whatsoever, which tells us something. Perhaps I should go about re-soldering all the tube pins.

Good idea and this is a common issue with these amps, especially the power tube sockets (more heat). But even before that, while the problem is happening grab the power tubes with a rag or oven mitt in case it is just a rattling 6L6.

[Chuck H]

Have you tried playing whack a mole on it yet? There's no guarantee that this will detect a cold joint or dirty contact but if it does induce a noise you can be almost sure that's what you have. Do it with the amp plugged in to the other cabinet just to eliminate one more variable.

[RMD]

Good news- I re-soldered all tube socket pins for the 2 output tubes and all is well with my amp. I cannot get the same buzzing sound now by playing, or by banging on the chassis.
Thanks to all who offered suggestions.
Rob

[Jazz P Bass]

I believe that is Enzoism # 49.

'Resolder the output tube sockets.'