A couple of Princeton Reverb questions...

[bentonar18]

A friend of mine has a silverface princeton reverb that needs a small repair, then he plans to sell it. My 2 questions:
1) There is a crackling noise when I turn up the reverb. I have swapped all the tubes and the noise is still there. Any suggestions as to what to swap next? I'm guessing a cap or two: this thing is all original, including the tubes. Only makes that noise when the reverb knob is past 2 or 3, making it virtually unusable.
2) Any idea on the value or worth of the amp? I'm considering buying it myself and want to make a reasonable payment. It looks like, well, a 35 year amp. I mentioned the above noise; beyond that it looks like reasonable care has been taken. I checked the serial number and it looks like it was made in the early to mid 70's. As I remember, the caps are the black ones, not orange. What, $300-400 as it sits today?

Thanks. And no, I'm not selling it, so don't ask!
John

[Darkbluemurder]

I would swap the coupling cap which connects to the Reverb pot. It is probably leaky. If that doesn't get it I would swap the plate resistors.

[bentonar18]

Cool, thanks. Uh-oh, looks like the schematic is showing a .003 value capacitor, and I don't have one of those. Could I substitute some other value there?

John

[sliberty]

I have heard that crackling can also be caused by the tube sockets. You might try cleaning and retensioning them.

[rfgordon]

Crackling can also be caused by resistors that have slowly baked their way to oblivion over the years, and is common among old carbon comps. With the chassis on and speakered up, tap and gently nudge each resistor with a non-conductive probe, like a chopstick. I once worked on an amp that had this noise, and, lo and behold, a plate resistor turned to dust when probed!
Ciao,

[krash]

The reverb tube must be a very robust tube or it'll fry really quickly. It's hooked up as a power tube. You have to put a 12AT7 or AU or whatever the chart says. a 12AX7 will die quickly. So don't count tubes out just yet.

That said that .003 cap is a 3,000pF. You can get a 2700pF it's close enough, or a 2200pF + a 1000pF in parallel. I think my amp had a 1000pF in it from the factory.

FWIW I think $300-400 is a real bargain. $500-600 probably closer to market.

[bentonar18]

Sorry to take so long to get back to this. Had to put everything aside for awhile, but now I'm back.

I'm about to order some parts and was preparing to order a .003 cap. I looked at the chassis on the bench, but I don't recognize the brand of capacitor or it's type. It's not an orange drop or mallory cap; it's black...? What should I buy to replace the .003: OD or mallory? Or does it even matter? Thanks again.

John

[krash]

yeah it could matter.

I don't know who the vendor is either. I have the old ones I pulled out of my amp. any modern cap should work ok. I'd put a polyester/mylar cap in there to preserve the "Fenderness".

[UR12]

The reverb circuit has two tubes. The 12at7 is the driver and it doesn't have a plate resistor it has the reverb tranny primary as a load. The 12ax7 recovery stage, (the tube after the reverb) has a 100k plate resistor and I would change it first before looking at caps.

Just my 2 cents

[David Root]

The yellow 630V Mallory 150s are good caps for an old Fender and don't cost an arm and a leg either. However I don't think you can get them in 0.0033, which is the modern equivalent to 0.003. I would do an 0.0022 in parallel with a 0.001.

In NOS caps, and I'm guessing here, look for a 600V Cornell Dubilier Black Cat, that should work fine if there is space to fit it. It will be bigger than the Mallory though.