BF Pro Reverb Shutting Itself Off

[TubeGuy]

I have a BF Fender Pro Reverb that has been behaving oddly. It started out by shutting itself off, then it either 1) Wouldn't come back on at all, 2) Would come back on but shut off again when the standby was flipped.

I can think of a LOT of things that can cause an amp to shut off, but very few that will do it without blowing the fuse, and the fuse does not blow (its the correct value). There is no loose wiring in the AC to PT section, no shorts, etc.

Installation of a different rectifier tube alleviates this, but the tube in it was a brand new JJ and lasted about a week, my guess is the bad tube is a symptom not the cause. Other symptoms: There is a pop when the standby is turned to play mode, and the tubes flare blue briefly when coming off standby.

Whats been done: I just refurbished it with new filter caps (power, pre-amp, bias filter), new cathode bypass caps, new plate resistors, 3 prong chord, all new tubes. Biased at a conservative 50% plate dissipation at idle. All JJ tubes. Vibrato ticking fixed per Fender's service bulletin.

I'd suspect the first stage filtering, but they are new Sprague 80uf caps, which is well within the first stage capacitance that a GZ34 can handle (caps in series, 40uf). The pre-amp caps are 25uf each. I tested all the filter resistors, all are fine, measured the caps on a meter, all are very close to spec. Choke reads 97ohms.

The screen grid resistors are suspect and I'm going to replace them, but I cant see where they'd be the culprit here.

Any ideas?

Thanks guys

[52 Bill]

I hate when that happens!

When you say "shutting itself off", what do you mean? Is the whole amp going dark (no power) or is just the audio cutting out?

What are the supply voltages (AC and DC) doing when this happens?

[TubeGuy]

It goes dark. No lamp, no heaters, no nothing, its as if the power switch was turned off, but it isn't. Its also not a power down sort of phenomena, where you leave the standby in the play position and turn the power off and it dies off slowly; its pretty much instantaneous when it happens. Acts a lot like a blown fuse, but the fuse does not blow, which is what has me scratching my head. I've seen bad rectifier tubes blow fuses, bad PTs, shorts any number of places, but I've never seen an amp just go dark without blowing the fuse, much less come back to life afterwards as if nothing happened.

It seems to be the rectifier going bad thats causing that, as another rectifier tube gets it to come back to life, although whats causing the tube to go bad in the first place is a mystery. I suspect its the same thing that causes the "pop" and the momentary blue flare when the standby goes off, but what is causing that, I don't know. The caps and resistors all check ok and the caps are new; I'm not sure how to check for a bad choke but at least the choke is not 'open', its reading 97ohms.

Of course over capacitance in the first stage can cause a rectifier tube to give up, but its a GZ34 seeing just over 40uf at the first stage, thats well within what it can tolerate. Thats why I was suspecting a bad choke not isolating the first stage from the screen grid filter, which would push that up to around 66uf, but thats only a guess.

I'm going to replace the screen grid resistors in any case as they are old stressed carbon comps; but if they were bad, I could see the tube not conducting and no sound or bad sound, but not really blowing a rectifier tube.

[52 Bill]

Well, if the whole amp goes dark, it's not the rectifier tube, or the screen grid resistors causing the problem.

The problem must lie in the AC primary wiring for the symptom that you describe to happen. Switching on or off the standby is probably just causing whatever is loose to make and break contact. Try this if your meter has an audible continuity test setting. Attach the meter leads to the ac plug's hot and neutral prongs, and turn on the power switch. Your meter should start beeping, as the total resistance of the primary wiring should be well under 50 ohms. In fact it will probably be under 10 ohms. Now while it's beeping and driving you crazy, start flexing the AC plug where it attaches to the wire. Also flex the two prongs of the AC plug itself. If the beep does not drop out, move on to where the AC cord passes thru the chassis strain relief. Keep going along the path of the primary wiring, pushing and pulling on every wire and connection you see, and hopefully something will cause the continuity to break and there will be your problem. Try cleaning the AC switch, as sometimes they can become carbonized or oxidized. Check the fuse holder and cap, sometimes they don't quite fit together correctly or the internal spring gets soft.

Best case, is a wire has come loose, worst case is a problem inside the power transformer.

Hope this helps.

[Bruce / Mission Amps]

I had an old Fender in recently that did this and it turned out to be a bad fuse holder.
When the holder got a little warm, from being a bad connection I presume, it would just open up and let go of the actual fuse, creating an on-off switch.

[Enzo]

Yep, you are losing the mains voltage. SOme connection or part in the primary circuit is intermittent - bad cord, bad fuse holder, bad fuse, bad switch, bad PT primary, bad solder anywhere along there.

[TubeGuy]

Primaries was my first thought as well, but I couldn't find anything there at first. I did the test for continuity and nothing seemed to throw it off. I checked the fuse holder, and the spring had disappeared, so I replaced it, but I'm not really sure this was the culprit.

If the PT primary is bad, how can I test for it? Is it possible for it to go bad intermittently? I've had PTs go bad before and blow fuses like no tomorrow, that I know how to test for; but I've not seen one be intermittent and not blow fuses, so I'm not sure how to check for something like that.

Thanks for the replies guys!

[52 Bill]

The wires going inside the transformer end cap are often crimped connected to the actual coil wires of the transformer. These can come loose or the lead wire can break internally, where it enters the end cap.

You can try pushing and pulling on the leads or tapping the transformer end cap to see if it will cause the connection to break. If the transformer isn't filled with the black tar-like stuff, you can remove the end cap itself and check the connections.

[TubeGuy]

Assuming that the standby POP is a separate issue, any ideas? Here's what I've done there: Replaced the screen grids with 5 watt 470ohm wirewounds; replaced the grid stoppers with NOS 1.5k carbon comps, tried putting a 600v
.047uf cap on either side of the standby (gator clipped in to see if it would work before soldering). Tried a different set of output tubes.

No joy, it still pops loudly when the standby goes off and has a little aftershock a few seconds later as well.

Filter caps are new. Anything else I should try?

[Twist]

If it does it often you could pull the chassis out and connect your AC voltmeter to the two primary wires of the P.T. While watching the meter turn the amp on and wiggle the AC cord and tap the fuse holder and power switch. If the amp dies and you still see 120 volts then it's probably your P.T.

[TubeGuy]

Did that, havent been able to get it to die since I replaced the rectifier, although I haven't pushed the amp either.

Right now I'm trying to nail down that pop on standby issue; once thats fixed I'll turn it up and put it through its paces and see if it stays on or not. Just wiggling things etc., at low volume, it seems just fine.

[TubeGuy]

Anyone have ideas on the standby pop? THat one has me stumped, did everything I could think of and nothing seems to help it.

[Steve Conner]

Well, did it pop before you replaced all those components? Some amp circuits probably always popped. It's actually quite a difficult design gotcha to make the standby switch work silently.

I've seen many amps where the tubes flash blue when B+ is applied. I don't think it means there's a problem.

[TubeGuy]

It did that before.

All Fenders have some little pop, but I have quite a few vintage Fenders and this is well outside the 3 std. deviation range for volume; its LOUD. I have another Pro that is less than a year older among other things, same circuit (AA165), no pop; so it shouldn't be a design issue.

[Enzo]

Switch it on and off with a clip lead, see if that makes a difference. It could be something as simple as pitted contacts on the switch itself.