Deluxe Reverb Buzzing

[dorrisant]

I have a '67 Blackface Deluxe Reverb (AB763). I own the amp, not a customer's. I have recapped it very recently, all electrolytics changed. New JJ power tubes and rectifier (GZ34) are installed and it is biased to spec. New power tube grid and screen resistors have been installed. All resistors remaining have been tested and pass for tolerance. Al signal caps have been checked to block DC as they should. All voltages are within reason. I have cycled several sets of known good preamp tubes through it. I cleaned and re-tesioned all sockets, jacks etc., changed the power cable to 3-wire, removed the death cap...c all of the things you normally do when refurbishing and getting it reliable.

This thing sounds fantastic except for a buzz. It seems to only occur when i play certain notes on the A string. When I play a C5 chord, I can hear this nasty buzz that doesn't necessarily trail the note, but seems to coincide time-wise with the root C (A string 3rd fret, 130.81Hz), although it seems to get nastier as I hold the note for a longer interval. Plucking that C by itself makes it even more apparent. It happens at the 9th and 10th frets of the A string as well, but not the frets between. It happens on other notes too but they don't seem as strong.

This happens with either channel, so it would seem to be global. It doesn't seem to be an issue if the channel volume is on 3 or below. Also, it happens with different guitars, in particular my USA Standard Strat and my Classic Les Paul, both from 1995.

I have aggressively chopsticked all components and wiring and my years of bench experience tell me that this is not pinpointing the issue. It actually has less reaction to the chopstick than most Fender amps of the same vintage. With a scope probe attached to the output, I see nothing out of the normal, whether it is getting signal from any of the guitars or my signal generator. I also bypassed the standby switch with no different result.

My thoughts: Could the JJ GZ34s be the culprit? I seem to recall this being present with the former set of tubes. It has been a while though. I traded for it and set it to the side for a good 6 months.

I can see that if I touch my meter probe (Fluke 1587) to the eyelet board near any high voltage terminals I can see up to 250-350 mV, where touching the chassis on opposite side shows less than 1 mV. I'm wondering if I may need to change the board. This wouldn't be that bad, at least I can make the board here rather quickly (CNC).

I think I have another OT here that I could sub in, but wanted to get some opinions before I go any further. Please let me know what you guys think. Thanks in advance!

[Phil_S]

I wonder if it's the speaker or the cab. Plugged into an external cab?

[sluckey]

Disconnect the internal speaker and connect an external cab to the main speaker jack. Don't put the cab close to the amp. Still buzz?

[dorrisant]

Thanks guys...

I forgot to mention that this was a problem in the combo cab. I have since had the same issue with two other known good test cabs.

So, it is probably not the speaker and it happens in or out of the cab.

[Andy Le Blanc]

Double check the entire amp pan for loose hardware as well. 67? Original OPT/PT? Give opt a thump test, eyeball the pt for rust.
The potting, paper formers and rusty laminations can do weird things with age.
I have had to replace the old wax pcb a few times for conductivity issues. Sometimes it improves with use, sometimes comes and goes with the weather, gets bad right out the cellar after storage.
I'd try to get signal into the power side, lift one end of the input cap on the phase invertor and clip in with the know freq. and if it is
not originating from the pre., its more likely the power side.

Double check any old carbon composition resistors , if any, in the signal path as well.

[dorrisant]

Thanks Andy...

Looks to be the original TX set. No dice on the OT thump test. There is a little rust on the PT lamination, but I have seen far worse be just fine.

Looks like a 130.81 Hz signal of 0.06v sine wave into the input of either channel, volume set at about 3 will trip the threshold that causes the noise. This signal will be increased to roughly 275 mV at the PI entrance when this happens. Not exactly scientific, but should be good enough. When I lift the 0.001 μF cap and inject the same signal, I can run it all the way up to around 600 mV before it really breaks up, and it is not the buzz that I'm hunting.

Divide and conquer, looks like it is before the PI.

[jjman]

Some time after I removed the anti-oscillation caps (aka tone-suckers) from the 6v6 grids of my ’71 DR, a similar issue arose. I changed those grid wires to shielded resolving the issue. You could install the caps as a test. ~1200pf

[dorrisant]

I have come to the conclusion that my ear is screwed up. Seriously. No one else hears the distortion that I am hearing and it is not showing up on a scope. About 9 months back I was too close to a gunshot... about 3 feet. I wasn't aware he was going to fire and didn't have my muffs over my ears. I guess it has not healed. Literally everyone who listens to this amp hears nothing in the way of distortion. Hopefully a bit more time will help. I did acquire a set of Peltor muffs that automatically shut down audio when you fire. They are great and I will never use anything else.

Thanks to all for helping me!

[pdf64]

Really sorry about your hearing damage.
Unfortunately the knowledge that hearing is a precious consumable is one of those many life lesson learnt a bit later than would have been truly useful.
One just has to protect what’s remaining and hope for the best.
The above, as ever, was typed whilst my ears were screaming ’eeeeee’ at about 2kHz, thanks to my similar lack of foresight in that regard, leading to tinnitus.

[thetragichero]

my ears eeeeee at almost a perfect C. all those years playing loud amps in seedy locations took their toll. auditioned for a band last weekend and i was the only one wearing earplugs even though most of em were decades older than me. it was actually nice because it blocked out the lead guitarist's terrible shrill tone

[TUBEDUDE]

My tinnitus, from a car accident, is at an F.
It has lots of slightly varying harmonics however depending on how much attention I give it. I was prescribed hearing aids for it. And they do suppress the tinnitus to the noise floor, but there's too much gain, and all the sounds, cars, neighbors talking, kids, dogs, TV, wife, etc., is too much to bear. Never used them again. I prefer the level ringing to all that discordant distraction.

[TUBEDUDE]

My tinnitus, from a car accident, is at an F.
It has lots of slightly varying harmonics however depending on how much attention I give it. I was prescribed hearing aids for it. And they do suppress the tinnitus to the noise floor, but there's too much gain, and all the sounds, cars, neighbors talking, kids, dogs, TV, wife, etc., is too much to bear. Never used them again. I prefer the level ringing to all that discordant distraction.

[dorrisant]

Thanks again guys. Ironically, I use ear plugs any time I even think I'll be around anything that might be questionable. Same with safety glasses too. I have an earplug station in the shop where the jam area is set up. I never joke about it, or pick at anyone who requests them.

This time was different. It was a 'new to guns shooter' and he was trying out a .38 revolver that seemed extremely loud for what it is. Caught me off guard. I will never be without my Peltors, especially with new shooters. The Peltors help because you leave them on all the time and never miss anything, conversation-wise.

Lesson learned, loud and not so clear.