Power Supply Caps question.


The story:

I picked up a Fender Bandmaster blackface back in, eh, 1984 or so. Used it to back up a Marshall 50, and for the occasional C & W gig. After a while, it started blowing fuses. Quickly got progressively worse, until the fuses would blow as fast as a slo-blo blows. Put the amp up, gonna get it fixed someday.

Someday arrived this month. Got it out, noticed 2 of the PS caps are rolling around in the bottom of the cabinet. Not good. The 3rd is hanging by one lead, and it is obviously cold-soldered by someone who doesn't know what is going on, much like me.

I obtain a copy of A. Pittman's book, and find the schematic. The PS caps are supposed to be 20 Microfarad, 525 volts DC. I know I should recap the thing, but impatience and stupidity gets the better of me. I resolder them, and CAREFULLY fire up the amp, all controls on 1. OK in standby. Let it warm up 10 minutes, plug a guitar cord in, use insulated pliers to flip on the standby switch.

After 20 seconds of careful observation, I discover a little smoke out of the PS caps (saw that one coming, didn't ya?) Power down, unplug, fan smoke away (wasn't really that much).

Then I go on the cap hunt.

So far the closest I have come is 450 volt 20 microfarad. The schematic seems to indicate 440 will be the highest voltage in the chassis, but +- 20%.

OK to try, or order the right (or close) caps online?

John Phillips

You need to go with the 525V cap voltage rating (or at least 500, anyway) since line voltages are significantly higher than when the amp was built - I'd expect the internal voltages to be at or above what the schematic indicates.

Remember to change the bias supply cap too (on the little board behind the pilot light) - a 100V cap is a good idea. You probably don't need to do the small electrolytics on the main tag board unless you want to, but it won't hurt either. All electrolytic caps degrade with time, especially if not being used.

And please fit a proper 3-wire power cord. You most likely didn't need the insulated pliers for the standby switch, but you don't want to find out that you did while you're playing guitar through it.

Carl Zwengel

What you need to snag is a set of Sprague Atom caps. They're pretty common in the 20uF@500vdc ratings as well as about anything else you're likely to encounter in tube gear. I'm trying to remember exactly how that particular amp is configured but another option you've got is to use a stacked pair of 100uF@350vdc caps with bleeder resistors across them ala Super reverb. I'm pretty sure the power supply board has eyelets to support that arrangement but don't hold me to it! Also, as suggested, it wouldn't be a bad idea to replace the bias supply cap as well. Upgrade that sucker to 100uF@100vdc Sprague as well.



FINALLY received the 500v 20 micro from Triode (he ran out, nice guy) and recapped along with new filters (80micro 450volt) and new bias cap (100v,80micro).

1. Gosh, it's quiet! PS and filter caps work!

2. I'll have to admit this is a modded amp. I (personally- can't believe I didn't do myself or the amp in) installed a master volume circuit. 17 years ago. Got the schematic out of Guitar Player mag. It works, but don't ask me for it. I lost the mag in a divorce. :D I also had to borrow a cab for it, and it is a 1-15 jbl E-series (8 ohm) handmade cabinet, so I don't have a really accurate picture of what's going on, tone and volume wise.

3. I have a bunch of noisy jacks and pots to work through, along with a complete retube ($$$$$).

So far, the tone is really nice. It needs tweaking. It's not very loud.

It's nice to have a project.;)

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