Cap values for old Mallory replacements- silverface era Fender

[Stratoblaster]

Hi everyone…. I’m working on an old, ugly 70’s Fender PA 100 tube amp I bought over the weekend as a “basket case”. Despite being barn fresh and gross looking, the transformer values are looking good after firing up on a variac and bulb limiter. I plan to replace the old filter caps and coupling caps as the first order of business and then see where I’m at with it. If I should modify that plan at all, appreciate all suggestions.

So…could anyone set my mind at ease and confirm whether those old 70’s era “blue drop” type Mallory caps are listed in uF or mF? The schematic I found doesn’t specify either, unless I missed it. I’d rather not be off an order of magnitude before ordering them!

My experience with amp repair and old analog electronics is fairly minimal, but I’ve now “restored” 2 Peavey Classic VT hybrid amps with the help of the amazing knowledge base while lurking on this site. They sound amazingly good after a total lytic and coupling recaps and I eventually modded the bias with a pot so those tubes aren’t running so cold all the time.

Anyway thanks again for your guidance and helping my understanding of what I’m looking at. Old fenders are new certainly new territory for me. I’m hoping this will be a great amp to get my feet wet, since it shares some design and parts with some iconic amps, and seems to have great tinker/mod potential.

[mozz]

Schematic should be online. uf would be microfarads. mf is not popular (milli) way of describing those caps.

[Helmholtz]

Those "blue drop" film cap are polyester which don't age and hardly ever fail. I would leave them alone.
Values are in µF, so the cap in the pic is a 0.022µF.

µF means microfarad and mF would mean millifarad. 1mF = 1000µF.

[Stratoblaster]

Thank you. The Mallories in the doghouse under the chassis at least have mF printed on them, so at least no ambiguity. The schematics I have found blanket specify “at least 600vDC”, but don’t specify units. .022uF seemed like a tiny value to me, but guess not. Glad I checked in…thanks!

[g1]

mF (milli-farad) is a fairly modern value. Such high capacitances as 1 Farad were only theoretical until the 90's, so there was no need for milli-farads, although 20,000uF was getting cumbersome to write out. Now the mF designation is seen much more often, especially shopping for components at wholesalers.
But some very old equipment (1930s and 40s) will sometimes use MF or even MMF on the schematics, MF for microfarad and MMF for picofarad. So it is important to clarify whenever you run into the mF or MF designations.

[g1]

Thank you. The Mallories in the doghouse under the chassis at least have mF printed on them

Not uF ? What value number is on them?

[Helmholtz]

Thank you. The Mallories in the doghouse under the chassis at least have mF printed on them, so at least no ambiguity.

There are no millifarad caps in a tube amp!

Older caps were sometimes wrongly labeled as mF, while actually meaning microfarad (µF).
The ecaps in the doghouse are probably 22µF.
Schematic values are typically µF (look for notes).

MF would actually mean megafarad according to modern standards.

[Stratoblaster]

Oh yeah good catch on my typo there… the large, tan cardboard Mallory filter caps are all labeled 100 MFD and 20 MFD. that’s exactly how it’s printed. So 100uF and 20uF to order up.

As for the little drop caps, the schematics don’t specify a thing other than numeric value, and all caps 600VDC. I guess I’ll hold off on those anyway, do the filter caps and go from there, since they are reputed to be long lasting.

Thanks everyone.

[Helmholtz]

Please post the schematic.

[g1]

You may find 22uF easier to find than 20uF.

[Stratoblaster]

Thanks, I’ll keep that in mind, and appreciate all the help on this from everyone. It’s not letting me post the schematic I’ve got at the moment, I’ll try again later when I’m back home.

[g1]

schematic attached

[Helmholtz]

You didn't mention if there's something wrong with the amp.

As a wise guy used to say: "Don't fix something that isn't broken".

[Stratoblaster]

Yeah so It’s a resurrection of a literal barn find. I tried uploading some pics to give a sense of condition. To my eye it’s unmolested inside, but I’m really a newbie to this stuff. I brought it up slow on the variac and bulb limiter as described in multiple places. The voltages looked expected from the main transformer. I saw redplating and current in excess of 100ma on socket 1&2 for the power tubes. Figured it was the coupling caps, but I did a deeper dive just now and had no bias voltage on pin 5 of socket 1&2. Tracing it back lead to the 4 lug pot for the bias balance. 1 leg was putting out -50v to sockets 3&4 and nothing at all on the leg to sockets 1&2. I opened up the pot and pieces of the carbon track fell out. Totally shot. I soldered a 10k pot in series to the existing 15k on the old balance pot and set it for 5k and wired as a voltage divider. I clicked in some tubes I had, fired it up and I see them pulling around 18ma each. No more redplating, so very happy about that. I strummed a guitar through channel 1 with the channel volume around 5 and the main volume around 3. It sounded nice and clean. I kinda expected that to be a lot louder, but, lots more to check out on this thing. For now I guess I’m going to table things until I build a cabinet for it and replace the old power caps. Also, the ground wire from the power cord attaches to a solder pad that has broken away from the chassis. The weight of the giant transformer stressed this area I believe. I’m running a jumper for the moment, but still need to replace the cord and safety ground the chassis properly before going any further. So I’m happy….things are looking up. 3 more channels to check, 10 more pots, and see if the reverb is still good, etc. I appreciate all the help here, and glad I didn’t just replace those coupling caps blindly. Thank you.

[soundmasterg]

Those blue coupling caps are great quality and sound very nice. I personally wouldn't replace any of those unless it was bad.