Fender Bassman 100

[CJ]

just a quick look at this silverface amp, Model CFA7100

dude had a 20 amp fuse instead of a 3 amp slo blo because he lost the bias, and the tubes were melting down, pwr transformer drippin black ooze, you know musicians, the show must go on,  ;D

OPT and choke ok,

there are 3 models of the Bassman 100, two are transistor,

 

[CJ]

two channels, one for bass, one for guitar, deep sw on ch1, brite sw on 2,

stripped down chassis makes life easy,

bias circuit had a shorted 70uf. 80 volt lytic, so we put in a 10 uf and 15 uf at 100 volts , non ploarized, you can have a few milli-volts ripple on the bias supply because it gets dropped by grid resistors and also cancels in the push-pull circuit, original blackface circuit just has one cap at 25uf,

tore out the good cap because it was the same type and was probably ready to go,

"Blackfaced" the bias and inverter circuits, and fixed a noisy vol pot and it sounds great but probably could use some new pwr tubes after the bake out,

 

[CJ]

gotta love the ham radio shield trick,

ripped out that ceramic disk as it is a tone killer,

schemo-does not have tone kliller cap,

does have cool AC voltages, inject 16 mv and follow it down the line, notice how much juice the tone stack steals, signals drops from .85 to .12,

http://music-electronics-forum.com/attachments/27515d1392425733-bassman_100_schem.pdf

 

[PRR]

> the ham radio shield trick

Is that from factory?

 

[CJ]

yes, factory wired,

 

[joaquins]

Nice amp, a friend found one in a trash, took it to his home, plugged it in and it was working like a charm... who knows.
Where was the tone killer cap exactly?

JS

 

[CJ]

err, well, looks like i pulled out a positive feedback cap, the tone killer caps, circled in green in the pic below, were not found on the chassis, maybe they discontinued using them,

this looks like a bit of a bass feeed back thing, 220K/.01, circled in red,

 

[CJ]

here is Gerald Weber's wonderful guide to converting a silverface Fender,

hope you can read it alright,