Bassman 70 bypass capacitor

[Axtman]

I am recapping a late 70's Fender Bassman 70 (UL) amplifier. The "Bass Instrument" channel has a 750uf / 3v bypass capacitor. What would be a good subsititute value for this capacitor?

[Chuck H]

The ubiquitous 25u/25v value used by Fender in all applications is essentially fully bypassing with only a tiny affect even at DC frequencies for the amp. Marshall chose as large as 330uf just to be sure (?). I'd say you're good for whatever that cap is supposed to do in that circuit with anything over 100uf and go ahead and get a 25V rated part just because it's a common voltage, parts are smaller now and in this application it makes little difference.

[Jon Snell]

47u, 100u, 220u 330u, 470u, 6.3v, 10v, 16v
any of the above will be perfect.

[Helmholtz]

While a 22µ bypass cap should be large enough regarding frequency response, a much larger cap value can help to lower heater hum.

The modern equivalent to a 750µ cap would be 680µ.

[Axtman]

I checked the other Fender Bassman models and this is what I found out: Most Bassman models use a 25uf/25v bypass capacitor on the Bass channel except:

Bassman AA731: 1uf/16v
Bassman 50: 1uf/50v
Bassman 70: 70uf/3v
Bassman 135: 5uf/50v

[Chuck H]

The whole point with the huge uf is that it bypasses down to the power supply frequencies. Any difference between 100uf and the spec'd 750uf is ignorable. Anything over 100uf and rated over 5V should be perfectly suitable in this application.

[Helmholtz]

For good bass response 22µ would be completely sufficient (I'm sure Leo knew).

But for heater hum suppression the 60Hz impedance of the cap should be as low as possible in order to short any hum voltage to ground.
60Hz impedance of 22µ is 120 Ohm while 60Hz impedance of 750µ is 3.5 Ohm.

Heater hum is more likely to be audible with a bass amp/channel.
Heater hum suppression is most effective at the input tube, so that's where you sometimes find large cathode cap values.

Sensitivity to heater hum varies with amp and individual tubes.