Odd popping noise

[casey73]

I just finsihed a round a mods to my old SF Princeton Reverb. Just in case someone is shaking their head wondering why I'd hack up an SF princeton, I got this at a pawn shop about 15 years ago, in not good working condition. The original cab had been cut down and the amp had become a Princeton "head".

I recently got a new reproduction cabinet for it, with a 12" speaker baffle. Over the years, I've added an adjustable bias (where the vibrato pedal plug is), recapped the electrolytics, replaced all of the coupling caps with orange drops and silver micas and replaced all the old carbon comps (many which had drifted) with carbon film resistors. I just installed a new matched set of JJ 6V6GTs (because I'd read they stand up to abuse). The 12AX7s are all RCA pulls from an old organ, tested good on a Hickock tester, and a Telefunken 12AT7 pull for the reverb driver. Today I installed at Mercury Magnetics Deluxe Reverb OPT (w/2,4, and 8 ohm taps) to replace the original Princeton OPT. I installed an old 4 ohm Celestion Vintage 30 for my speaker. I also did a mod on the PI where I moved the power supply to the next tap up on the filter cap, giving it a little more headroom. It looks stock from the front but is louder and more agressive than a standard Princeton.

The only problem is at high volume, when I had a cord, there is an annoying pop sound that coincides with the spike of the attack. The only thing I can think of is I might have a microphonic tube. Could this be a faulty tube? I've never run into this before. Anyone?

[Enzo]

Sounds more like a parasitic oscillation. Try moving the grid wires to the power tubes for starts.

[casey73]

Grid stoppers?

I'll try that out Enzo. Because I bumped the A+ up a little using a 5AR4 rectifier, I added 470 ohm screen grid resistors, but I didn't put in any grid stoppers. I think I'll add 1.5k to the grids also.

[casey73]

Some improvement, but.....

I seperated the leads as they come out of the fiberboard hole and added 1.5K grid resistors mounted on the tube sockets. The constant popping is now gone, but I still get an occassional splat on a loud low note, best described as "raspberries" sound. I think you are right about it being oscillation.

I'm thinking maybe I should move the leads from under the fiberboard and connect them to the output tube coupling caps above the board so I can seperate them further. I'm still using the original wiring. Since I've modified the output section, maybe the original lead dress is causing this issue?

[casey73]

Problem solved - odd popping

Today I moved the power supply for the phase inverter back to it's original position and solved my popping sound. The mod was supposed to allow the phase inverter to drive the output tubes a little harder. But it obviously made them a bit unstable? Not exactly sure, but the toublesome noise is now gone. Thanks to all for helping with this and my previous motorboating problem. The amp is finished....but I'll be making another change and that will be a different thread.

[Bruce / Mission Amps]

What is your plate voltage with a 5AR4... that amp is supposed to have a 5U4GB in it and the power tranny is wound to compensate for the larger voltage drop across the 5U4GB.
Normally a good sounding P Reverb is running about 420vdc on the 6V6 plates and idling the power tubes at around 20ma-24ma.
If yours is there with a 5AR4, much stiffer then a 5U4GB... you might have a power transformer problem.

[casey73]

Bruce - thanks for commenting on this one. The popping noise isn't an issue now, after I replaced all the preamp tubes with known good tubes. But, now I'm getting an intermitent power drop. If I put an over drive in front and start to crank it, the volume will suddenly drop a little and return again. Happens when I hit a loud note and hold it or a chord. I thought it might be a vibration thing, so I cleaned and retentioned all the sockets, and resoldered all the recent new connections I made to make sure I didn't have any loose/cold solder joints.

In another post I made about the hot transformer, you said that it's been your experience that Princeton PTs are prone to failure, sometimes because of people putting in 6L6s etc., which I did a long time ago. I also used a solid state rectifier in it for a while. I never played this amp for long periods of time back then, it was more of an experiment amp, but I could have strained the PT. It has never blown a fuse though.

Is there a chance this volume drop could be a symptom of a failing power tranny? I haven't monitored the PS voltage when it's happened so I don't know if that is what is causing it. In any event, I went ahead an ordered a beefy Allen amp drop in reaplacement for it. It's rated at 120ma, almost twice the original Fender. This will allow it to run cooler and give me the option of 6L6s if I want.