HELP!! strange issue

[krash]

hey folks-

I just built up one of my "72" amps for a customer and it's got some kind of weird failure that I can't sort out. Driving me nuts!!

The symptom is like this. With the amp cranked (80% vol or more), it gets an intermittent kind of "flappy" or "chattery" kind of sound. It's sort of like motorboating but more staccato and high frequency in nature. It only does it with humbucker guitars. Both channels do it the same way (it has a "normal" channel as above and an overdrive channel with a whole lot more gain). At first I thought it was the voice coil of the speaker running out of excursion but I tried it with a different cabinet with 2x 75W speakers and it still does it the same way. Otherwise the amp sounds fantastic.

My second thought was that maybe it was blocking distortion, but it does the same thing with either 1.5K or 10K grid res. on the EL84s and there's nothing abnormal about the preamp stages. I have checked all of the voltages and connections and they are right on the money. FWIW the amp sounds absolutely great other than this little niggle. I have also swapped tubes and it doesn't seem to be related to a bad tube unless somehow it is ALL of them.

If you unplug the guitar cable and touch the tip with your finger it gets either the normal loud 60Hz hum you are used to, or this 60 Hz hum along with this flappy/crackly sound. It is sort of alternating. It has this kind of feeling to the problem like a capacitor is either charging or not fully charged or something when it does it.

The amp has a normal channel and an OD channel and they both do exactly the same thing so I think I can isolate the problem to the PI or later.

My current guess is maybe I have a ps cap on B+2 or B+3, maybe the screen filter cap? They all measure ok. Nothing is arcing, no red plating, everything works 100% other than this issue.

Any hints? Anything I could test for? Maybe I'll make a recording of it and post it. I am sure one of you guys has run into this before.

And BTW, yes I've built several of these amps with only very minor variations and never run into this problem before. I'm not yet convinced it's a design problem but I'm open to all suggestions

[bueller]

I have had a very similar situation in a cathode biased 6V6 amp just this week. A harsh brrrraapp noise but only at higher volumes when the driving signal was larger. A first I thought it was a broken speaker because that's what it reminded me of, but swapping that proved it wasn't. Then I changed power tubes, nope. I went back through traced every single solder joint, but it was still doing it. Finally while rebuilding the front end for other reasons, I found two things.
One was a cold solder joint of the filter cap bleeder resistor, it was attached but loose. And the other was more odd. I just happen to be poking at everything looking and checking the joints, when I flicked the 10w cathode bias resistor, it made a vibrational noise. Replaced the cathode resistor and soldered the bleeder back on, the bad amp noise was gone.

Hope that helps

[krash]

that sounds like my problem. I'll be sure and check that out

I also thought maybe my screen resistors are too small and too few in quantity. Tonight I plan to put in some 2K screen resistors directly on the pins, see if that helps.

[LOUDthud]

When you tested the amp, was there enough isolation from the speaker to eliminate vibration as an aggravating factor or do you think vibration is inducing the sound?

[krash]

yeah amp's on a table, cab is on the floor 10 ft away.

[Phil_S]

Sounds like there is intermittent contact somewhere. Time for the chopstick test and see if you can make it happen at lower volume setting.

[krash]

I changed the screen resistors to 2K, one per tube. That fixed it.

[teleamp]

Did you try your normal value resistors at the tube socket, that could have been it too.

[krash]

no but I'm beginning to think that 100 or 150 ohms for screen R's on modern production EL84s is just too small.

anyway 2K fixed it definitively and the amp absolutely rocks so there's no hesitation in my mind about making a permanent design change.