Filter cap limit?

[MikeT]

What is the limit on filter capatity to get rid of hum? Specifically, I have a 2-6V6 amp that has a good amount of 60z hum that I can get rid of with an 80mF cap to the b+ to the OT. A 22mF doesn't do the job too well. I know there is something going on that limits capaticitence. Any help would be appreciated. Thanks.

Mike

[R.G.]

Originally Posted by MikeT

What is the limit on filter capatity to get rid of hum? Specifically, I have a 2-6V6 amp that has a good amount of 60z hum that I can get rid of with an 80mF cap to the b+ to the OT. A 22mF doesn't do the job too well. I know there is something going on that limits capaticitence. Any help would be appreciated. Thanks.

Mike

The filter cap size is limited by the rectifiers. As you make the first filter cap bigger, the rectifiers must pass shorter but much bigger pulses of current. A tube rectifier has a strict limit on the peak current it can pass, and most tube rectifiers state in their datasheet the largest cap that is safe to drive with them.

The repetitive peak current on silicon rectifiers is in general huge, so there is no effective limit on the capacitance of the first capacitor if you use silicon rectifiers.

If you're getting 60Hz hum on your output, you have a different problem. A full wave rectifier makes the lowest frequency be 120Hz, two pulses of power per AC cycle. If one of the rectifier pathways is open, that gives you 60Hz hum. 120Hz ripple makes the existing capacitors twice as effective because it's only half as long between power pulses from the rectifiers. It's worth checking to see whether you're getting 60Hz ripple or 120Hz ripple - they're different problems.

[MikeT]

Thanks for the response. I'm using a 5Y3 in standard Fender configuration as found on a Music Master Bass as that is the chassis that I wired as a 6G2. I'm not sure if that is full or half wave, so I don't know what the problem is. But if I replaced the 5Y3 with (3) 1N4007s in series on each leg (red wires) coming from the PT could I then use the 80mF cap?

Mike

[Bruce / Mission Amps]

Originally Posted by MikeT

Thanks for the response. I'm using a 5Y3 in standard Fender configuration as found on a Music Master Bass as that is the chassis that I wired as a 6G2. I'm not sure if that is full or half wave, so I don't know what the problem is. But if I replaced the 5Y3 with (3) 1N4007s in series on each leg (red wires) coming from the PT could I then use the 80mF cap?

Mike

I think every Music Master Bass amp I've seen used a full wave solid state rectifier and power transformer.
Those amps had fairly low voltage B+ though and it would be way much worse with a 5Y3GT!!
Stick with two series wired 1N4007 solid state diodes in each leg with this amp if you are using the stock power tranny.
That way you can run 100uF to 200uF pretty easily of you wanted to.

[MikeT]

Bruce,

Its been such a long time since I got this amp I think I may have put the tube rectifier in and forgot. The PT has 5v taps, so either it already had them or I replaced it....the joys of age.... but the socket hole looks a little bigger than the socket so I suspect I put it in myself. Anyway, all I needed to know was that the uF limit is much greater with diodes so now I'm cool with it. Thanks.

Mike