BF Pro Reverb Shutting Itself Off

[TubeGuy]

I replaced the switch with a brand new 15 amp Carling. If anything, the Pop is louder than before.

Its definitely not coming from the power amp. It mostly goes away if the phase inverter is removed. If all the preamp tubes are removed, its dead silent. If all but V4 and the phase inverter are removed, its more of a loud thud than a pop, but still unacceptably loud.

[TubeGuy]

Ok so just for giggles, I re-replaced the new filter caps and all filter cap resistors I installed in case something was bad. No change.

This is driving me nuts. Everything I've been able to research as a possible cause has been fixed/replaced, and the problem remains.

[Steve Conner]

What problem? The amp works, doesn't it? When you consider what the standby switch does, it's a miracle that it doesn't make a loud thud or bang in every amp.

Have you checked the dropper resistors and bypass capacitors to the preamp stages? If these are in good order and wired correctly, the plate voltages on the preamp should come up slowly, over a period of a second or two after flipping the standby switch. If you had a bypass cap open or a dropper resistor shorted, the plate voltage to that stage would come up instantly, which might explain the pop. If you replaced the dropper resistors with the wrong value, or ones with too low a voltage rating that are arcing internally, there's another possibility.

If you were feeling really OCD, you could borrow a digital storage scope and compare the turn-on transients in every stage to your other Pro, until you find the difference. ;-)

[TubeGuy]

The pop is loud enough to make me fear for the vintage speakers.

All bypass caps are new and correct, plate resistors are new and correct, all filter caps and resistors thereof are new and correct.

Not sure what you mean by "dropper resistor"; are you talking about the pre-amp tube biasing resistors?

[Steve Conner]

In the AA165 schematic on FFG:

http://www.ampwares.com/ffg/schem/pr...a165_schem.gif

the dropper resistors are the ones between nodes B, C, and D: the 1K 1 watt and 4.7K 1 watt ones. Changing these resistors for higher values and/or making the 20uF caps bigger might be one way to reduce the popping problem.

However, did you check that your PI tube has both sides working and reasonably matched? Also, do the volume controls affect the pop?

[TubeGuy]

I see; yes those were all replaced with 2% w watt fireproof; no effect. Those are what I was referring to as filter section resistors. Both 220k, the 1k, and the 4.7k were replaced, no difference at all.

I havent measured the PI or tried a balanced PI, although I did try several different new tubes there, no difference.

Also, I used 80uf Spragues for the big caps for 40uf total (actually measured a touch more) and tried two different brands of 25uf/500v caps for the pre-amp and screen grid filters; again, no effect.

Ultimately, the source of the pop is a big slam of AC on the Power Tube plates at start up; I know its coming from the pre-amp somewhere, but where, I dont know. Most things I can think of that would cause that, other than what I've tried, would result in contant hum etc; i.e. bad coupling caps, bad tone caps. The amp is fairly quiet after about 30 seconds, but that first pop is VERY loud.

[TubeGuy]

I put in a .047uf/600v both across the standby and from the standby to ground. That eliminated the big pop when the standby goes to standby mode, but not when it goes to play mode.

I've never had a choke go bad, but theoretically, if they do, what symptoms are you likely to see?

[Amp Kat]

I doubt it. Try another switch which could be part of your other problem also. Pops are caused by bad connections and a semi cold solder joint anywhere on the HV side could cause it. A loose ground connection from the HV source to ground on that ground side will cause it also.

[52 Bill]

You said that the amp did this before all of the work was done on it, so I would guess that the problem lies elsewhere.

Have you tried to isolate the source of the pop? Does removing any tubes have an effect on the pop? If you remove all of the tubes does it still pop?

I've had chokes go bad by shorting to the case (ground), maybe yours is arcing internally and will eventually fail. Try swapping the choke with another one. Another source could be the output transformer, same idea, maybe an arcing primary winding.

Whatever the cause, trying to isolate the problem should be your first step.