1962 Bassman electrolytic question

traynorbassmate · #1 05-05-2010, 02:42 PM

I'm repairing a dead brownface '62 bassman head...bass-ically a power section rebuild. New diode section, and one filter cap was shorted so the whole lot are being swapped.

I haven't double checked but I'm pretty sure the sprague Atom 20mfd 600V caps won't fit (they're 4" long). Originals are 525VDC...what is the consensus about using 500V caps instead?

Anyone run into this problem?

thanks

BassmanPaul · #2 05-05-2010, 02:52 PM

IIRC the B+ on those amps was in the range of 450V. They were designed when AC voltage levels were 115-117V so with todays 120 odd volts the B+ will run a little higher. If my memory is correct then 500V caps would be fine. Changing out the electrolytics is a good idea but don't forget the bias supply and cathode decoupling caps as well.

Paul

MAMMOTHvolume · #3 05-05-2010, 03:41 PM

Quote:

Originally Posted by BassmanPaul

IIRC the B+ on those amps was in the range of 450V. They were designed when AC voltage levels were 115-117V so with todays 120 odd volts the B+ will run a little higher. If my memory is correct then 500V caps would be fine. Changing out the electrolytics is a good idea but don't forget the bias supply and cathode decoupling caps as well.

Paul

+1 on all this. Also keep in mind that modern caps are generously underrated voltage handling wise so don't worry.

johnk_10 · #4 05-05-2010, 03:59 PM

sprague atom 500V caps are fine in a '62 bassman. i've recapped 100's of fender amp with them.

traynorbassmate · #5 05-05-2010, 04:10 PM

Cheers guys! Very helpful.

Re: bias supply and cathode decoupling caps....which are these on the schematic? there are two 25+25mfd (25V?) caps inside the tag board (one at the far right, one in the middle), one has been replaced already, i'm going to replace the second as well. Are these what you're referring to?

http://www.schematicheaven.com/fende...6g6b_schem.pdf

I'm also swapping the 470 ohm screen resistors - they both read over 1K now (they look a little burnt), is it normal for their tollerance to increase with this type of wear?

BassmanPaul · #6 05-06-2010, 10:50 AM

There are four 25µF 25V caps on the cathodes plus a 25µF 50V cap in the bias circuit. I personally don't like the 470Ω screen resistors although they do work. I prefer 1KΩ 5W wire wounds in that position.

Paul