Too high supply voltage in Deluxe Reverb

[bob p]

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Using a variac and a limiting resistor I've "restored" electrolytic caps that were 25 years in storage and I still use 'em in my own stuff
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Rob, I hope that you don't mind, but I'm going to start a new thread on reforming caps and use your post as the anchor.

[BDM]

How would I know if I have a transitional SF, or if the transformer was for a BF? The amp does NOT have the power boost pull switch. The transformer codes indicate 1972.

I see there was another thread here last year where a SFDR had the same overvoltage on the preamp plates. Guess I'm not unique.

I see a 1972 SFDR sold for $1,182 on eBay two days ago. Seems like a lot.

[bob p]

Yikes! That's way too much money IMO for a SFDR. 5 years ago $1000 was a high price for a BFDR. But then Tweed Deluxes were $1000 then instead of $2000 now. This vintage amp stuff is getting insane.

How would you know if you had a SF amp with a BF PT? If your voltages under load are normal, forget about it -- its not a problem. If your voltages are abnormally high, then you could do a simple test.

To check your PT, I would look at the back of the amp and read the voltage specification for the power mains. Then I would use a variac to feed the amp AC input voltage at that value. Then I would take measurements of the B+ rail in the amp and compare them to the published specs. If all of the votlages seem right, then you have the PT that was spec'd for the silkscreened input voltage on the chassis and there is no "transitional" mismatch between the PT and the silkscreening on the chassis.

OTOH if your amp says 120 VAC on the back and you have B+ voltages are way too high with a 120VAC input voltage, this suggets that you might have an amp that was built with an older PT spec'd for the lower mains voltage. I think that this is less likely, as an amp like that coming from the factory would be a rare bird. More likely is the possibility that someone might have replaced the PT in the amp. If that does turn out to be the case, a external bucking transformer might solve the overvoltage problem.

[Rob Mercure]

Bob,

To return to the theme of "historic/classic/vintage" wide tolerances, I've run into several Blackface and Tweed Fenders whose PTs still produced quite high voltages (at least, as compared to schematics and company literature) even when fed 110 VAC, or lower (105-108VAC) using a variac. So the "problem" (which in the specific seems more theoretical than practical) might be one that cannot be "solved without replacing the PT on a vintage amp. Over the years the only times I've ever noted problems with too high PT voltages in older Fenders was when someone had already replaced the PT, for whatever reason, and it was cooking components "down stream." While you would think that amps running the B+ 20% over "spec" would "eat" tubes and fry filters that hasn't been my experience when the amps came from the factory that way. I dunno, perhaps I'm giving past "Murphy" engineering philosophy too much credit but I also suspect that the PT variablility was built into overall design and that there was as much experiential "what you can get away with" wisdom incorporated into the calculations as there was careful review of component "maximums."

Rob

[stokes]

Having the caps on the "hot" side of the standby saves the rectifier from some stress,instead of the rectifier having to supply all the current to charge all the caps at one time,the first cap gets charged and when the standby is flipped to "on" the rest of the caps draw from those first caps,makes it easier on the rectifier.Like Enzo said,the caps can stand a certain amount of "overvoltage",and remember when you are in standby mode you have no current draw on the B+ so that 498 volts isnt going to be as detrimental as it would be under load,but if it eases your mind to use 500v or better caps and you can fit them then why not do it.