1965 Pro Reverb: Driving me crazy!!!

[PrecisionB]

So a customer brought in a really nice, close to mint, 1965 fender pro reverb. He wanted an overhaul/3 prong power cord done due to some various pops and such.

I replaced all the electrolytic caps with Sprauge. I also replaced all the power resistors in the power supply with adequate modern counterparts, as well as the typical 470 ohm resistors on the power tubes.

Despite all of this, a loud POP occurs about one second after taking the amp off of standby. After that one pop, the amp seems to behave itself.

By simply pulling a 12ax7, I discovered that the pop disappeared. This was how I figured it was occurring in the preamp section of the amp. Seeing how it was not effected by the volume pot, I knew the problem was occurring somewhere post eq.

I zeroed it down to what I thought was a bad coupling cap (This is indicated in schematic circled in red) This made sense to me because it is isolating the high plate voltage. Pop are frequently caused by arcing, and the pop occurs when the plate voltage is applied (standby turned off)
I shorted the non-DC side of the cap to ground, and the pop was gone.

After replacing this cap, and EVERYTHING around it (100k plate resistors, various other caps) The pop is still there. Not sure what else to look for, and quite frankly, I'm getting a little tired of staring at it.
Any advice or insight would be greatly appreciated.
[IMG687]http://i1190.photobucket.com/albums/z46 ... 009d20.jpg[/img]

[passfan]

Have you replaced any of the tubes in the preamp section ?

[PrecisionB]

Yes. That was the first thing I tried. Same results.

[backpack]

You have a cap thats charging up. If it goes away after the intial pop thats what its been for me.

[JazzGuitarGimp]

Does the pop go away if you remove the 12AT7 reverb driver tube?

Did you replace the 25uF, 25V 'lytic at the cathode of the 7025 (the one that precedes the circled cap)?

[gingertube]

OK problem is definitely in that Vibrato Channel.

If you have not already done so replace the cathode bypass electrolytics, if you have already done so make sure you got the +ve lead on the cathode and the -ve lead to 0v.

Check the "bright cap" by turning off the bright switch to see if the pop goes away.

Next suspect would be that cap which couples the signal to the top of the treble pot.

Cheers,
Ian

[PrecisionB]

Cathode bypass cap have been replaced, as well as the resistor.
The pop remains even when the bright switch off, and the volume pot all the way down. So this pop has to be coming from somewhere post-volume pot... Right?

[PrecisionB]

Anybody have any more ideas?

[wicker]

Bad socket ?

[bluefireamps]

So does the pop go away if you remove the 12AT7? Grounding the non-DC side of the cap also grounds the grid of the 12AT7. If you just unsolder that end of the cap do you get the pop?
Dave

[Firestorm]

BF Fenders all pop when the Standby switch is flipped to the play position. This is because only the main filters are charged at power up and flipping the standby switch connects the rest of the power supply rail. The sudden rush of current makes the choke ring (one end of the choke goes to the standby switch; "flapping in the breeze" as Merlin would say.) The screen resistors damp the ringing in the output tubes (which would otherwise make a terrible racket). I think the only way to avoid the pop is not to use the standby (or use a different standby circuit altogether).

[Roe]

Anybody have any more ideas?

yes, soldier a 220k (or even 470k) 2w power resistor, preferably metal oxide, across the standby switch

[tsutt]

Roe, just reading your comment line of amps. Is you alessandro a clone or the real deal?

Todd

[Structo]

The standby switch also me be suspect as this type of switch doesn't really like to switch high voltage DC current.

What I have done on my Dumble amps is add a .01uF 1Kv ceramic disc across the standby switch.

The only problem with the resistor across the switch is that the amp will not be muted all the way when switched to standby.

[Roe]

Roe, just reading your comment line of amps. Is you alessandro a clone or the real deal?

Todd

a real rotty that I've sold.