Filter Cap question - Silverface Princeton Non Reverb

[jmp100watt]

Got a question for you guys. I have a couple of 70's Silverface Non Reverb Princetons. On my favorite one (1975), I decided to replace the original Mallory 20/20/20/20 UF capacitor due to age. Keep in mind there is nothing wrong with the amp, tubes, bias all good and the usual electrolytics (bias cap/etc) were
replaced as well. I tried the new CE brand filter cap and also the F&T brand Authenticap and both new capacitors work great, low noise but after both
cap changes the amp sonically sounds dull and less sustain than the original cap. I ended up going back to the Mallory and everything is great again. I did
this with my second amp as well (1979 NR Princeton) with the same results. I know most people are going to say "if its not broke dont fix it" and I agree,
but the old Mallory cans are getting up there in age in these amps and Id love to keep the sound of the old Mallory but with new caps for reliability. Is there
a way I can measure the old caps with a ESR meter or something to see what it is I like about them (specs/measurements...etc) so I can try and recreate
a cap to match the older ones? Hope this makes sense. Thanks

[Bill Moore]

I have one of these that seem to work well!
https://www.tubesandmore.com/product...f-150-9-ranges
I'm not sure that you will be able to duplicate the old cap values, as they are probably in between standard!

[phantomman]

I'm not sure that you will be able to duplicate the old cap values, as they are probably in between standard!

+1

Component drift in vintage amps is a known and quantifiable phenomenon -- have you checked the values of your power-rail resistors?

[jmp100watt]

+1

Component drift in vintage amps is a known and quantifiable phenomenon -- have you checked the values of your power-rail resistors?

yeah had both amps gone thru by competent techs. Seems the filter cap makes all the difference

[bluesroom]

Don’t you just hate this! I always feel like the mojo is lost, especially with orange drops. I go through the same thing when I replace an inefficient speaker / although that’s easily reversible.

[Offshore Angler]

If the filter caps are making that much of a difference you may want to try a power conditioner upstream of the amp.

Does this cap change have the same effect on all stages or only at home?

[phantomman]

If the filter caps are making that much of a difference you may want to try a power conditioner upstream of the amp.

I was thinking about that last night but I'd posit that a variac would be more appropriate. Set the output to 117 VAC (what Leo designed his amps to operate on) and see if that restores the "mojo" you perceive to have been lost.

[Offshore Angler]

I was thinking about that last night but I'd posit that a variac would be more appropriate. Set the output to 117 VAC (what Leo designed his amps to operate on) and see if that restores the "mojo" you perceive to have been lost.

I need to chew on that one. I don't know if a Variac has a filter section. Besides the voltage differences, it's always a matter of how clean (sin wave) the power is. The filter caps are eliminating ripple in the current. That ripple may be the mojo he's missing, i.e., the amp was repaired and sounding correct but it wasn't what he wanted to hear.

Anyhoo, I just know from doing large shows whenever we're running off generators you definitely need to condition or your power or amps may sound weird and even worse, your expensive electronics may be damaged.

Chuck

[phantomman]

A variac is a variable voltage regulator -- it plugs into a wall socket then the amp is plugged into it. The amp would still be relying on its rectifier and filter array to provide the ripple-free DC that the amp depends on.

As for the caveat re using generators at remote venues, agree 115%. Power conditioners are a must-have at all times.

[Bill Moore]

Did you re-bias after the cap change?
After setting to 70% always play for a while, and adjust bias, (up or down) , to get the sound that you like!

[phantomman]

Adding an adjustable pot to the bias supply might be a worthy mod -- Princetons are often cantankerous when it comes to setting the bias due to the type of tremolo circuit used. Ir can be performed without any permanent alterations to the amp, totally reversible if desired.