Recapping Bassman 50 Questions

[DesmondWafers]

A question to all of you vintage Fender amp gurus: how do I get rid of this nasty brown stuff around all of the eyelets?

Also, the cap set I bought was missing a 16uF 475 cap. For some reason, the person who capped it last only replaced one, so I have an extra Sprague 20uF 500V cap lying around. Would this be fine to use in it's place? If not, I'll simply leave the last Mallory in place until I can get a replacement. Thanks!

[JSett]

A question to all of you vintage Fender amp gurus: how do I get rid of this nasty brown stuff around all of the eyelets?

That's flux residue. Isopropyl alcohol and a Q-tip is a good way to remove it.

Where in the circuit is the 16uF you're replacing?

[DesmondWafers]

Thanks for the tip! It's the smallest filter cap. I have already replaced all of the other 5.

[JSett]

I'd just wait the few days it'll take to get the proper size one (it'll likely be 16uF 500v, but the voltage doesn't really matter as long as it's same or higher).

I just like to keep things as close to original spec where possible.

[DesmondWafers]

Funnily enough, the schematic I found says 8uF and the amp has the 16uF from the factory, but you're right; better to be patient and leave well enough alone for a few days. Thanks for the tips! This was my first recap and the hum is already much improved.

[Dr Tony Balls]

Would this be fine to use in it's place?

Yes.

Funnily enough, the schematic I found says 8uF and the amp has the 16uF from the factory

You need to be looking at a Bassman 50 schematic: https://schematicheaven.net/fenderamps/ ... sman50.pdf

[øøøøøøø]

20µF @ 500v is perfectly fine to use as a substitute for a 16µF.

Electrolytic caps have a fairly wide tolerance, and old electrolytic caps tended to be even wider.

A typical modern part will have a tolerance of ±20%, a premium part may have a tighter tolerance.

Old electrolytic caps often had even looser tolerances, and often these tolerances were skewed toward the positive (e.g. "+70%, -20%")

20µF is essentially about 20% away from 16µF and errs on the high side. This is therefore more than adequate for use in a Fender tube ampifier (I'd certainly use it in mine).

Power supply filters are not a precision-critical application. The value just has to be high enough to move the corner frequency of the lowpass filter low enough, and not so large as to substantially change the sound of the amplifier or cause arcing within a tube rectifier (you generally need to go double or more before those things even begin to be a concern).

In the modern era, a series of standardized values have become accepted industry-wide. In the mid-20th century this wasn't yet the case, so there are lots of older values that aren't standard anymore--for example, a 25µF cap will always get replaced with 22µF. Due to the wide tolerances involved, this is effectively the same part.

TL;DR-- nominal values on electrolytic caps are far less precise than many assume, especially in old tube amps from the 20th century. Just go for something that's within 20 or 30% and you'll be fine in almost every case.

[OffYourFace]

Normally I'd say just convert it to AA864 specs but IME, The Bassman 50 sounds great as is. That is, as a clean pedal platform amp.

[DesmondWafers]

Thanks to everyone for the help! All the filter caps are now fresh and it's sounding great. It may not be the most sought-after Bassman, but this thing has massive-sounding cleans.