Hammond power supply cap subs

[Randall]

Got a Hammond M3 amp in for buzziness. It has it's original filter cap cans and a couple of other electrolytics, so I'll start there. Is there a reason not to up C62 50/50/450v to 60/60/450v? I know I can get Marshall style 50/50/450 cans, but I don't want to drill the chassis and install a mounting strap.

What about upping C63 and C66 from 20uF to 40uF, would there be any advantage in doing this, like maybe less hum?

[Leo_Gnardo]

Got a Hammond M3 amp in for buzziness. It has it's original filter cap cans and a couple of other electrolytics, so I'll start there. Is there a reason not to up C62 50/50/450v to 60/60/450v? I know I can get Marshall style 50/50/450 cans, but I don't want to drill the chassis and install a mounting strap.

What about upping C63 and C66 from 20uF to 40uF, would there be any advantage in doing this, like maybe less hum?

Your plans all look good to me. Ordinarily I'd avoid going over 50 uF on a 5U4, but there's a nice resistor in the way before the current hits the cap, plus you're only looking at 320V, so I wouldn't worry. That, and tolerance on original filter caps may have had that cap at 60 anyway. Also no worries on upping to 40 from 20. Less hum is always a good thing. I'm assuming you'll be using CE caps, as they're the only brand I know of that packages caps this way. If the voltages involved were higher I'd say go for Marshall style 50+50/500 but that isn't the case. I've always been not terribly impressed by CE's caps of this type, however you're using them in a circuit that will not apply near max voltage stress on them so I expect they'll work OK.

[Randall]

Thanks Leo. Yes, CED has 40/40/40 and 60/30/30 I can parallel up to get 60/60/450.

Now for anther question: it just occured to me there is no AC fuse on this amp. Really? The original plan was to put a 3 prong AC cord on it, and now I should install a fuse. However, over on some Hammond boards there are guys that say this is not a good idea. How can this not be a good idea? Something about floating grounds and possibly burning out the PT. This schem looks like a 'normal' tube amp to me, not a widowmaker.

Convince me I shouldn't install a power fuse and 3 prong plug. And then tell me what size fuse. 5U4, two 6V6s and 8 pre tubes.

[Leo_Gnardo]

Convince me I shouldn't install a power fuse and 3 prong plug. And then tell me what size fuse. 5U4, two 6V6s and 8 pre tubes.

I don't see why not, both fuse and grounding. If you were to convert the amp to guitar use, who would think twice about it? Of course there may be some real Hammond expert out there who can fill us in on real reasons to avoid fusing & grounding, not just some old wives' tale stuff.

I'd think 2A slo blo would be appropriate. Of course you could monitor the current draw on that amp as is & let that decide your fuse value.

FWIW many years ago - 1982 - I converted a Leslie 122 amp to a 147 with an extra preamp stage so it could accept normal line level signals. Fuse in that, grounded too, you betcha!

[Randall]

" Of course there may be some real Hammond expert out there who can fill us in on real reasons to avoid fusing & grounding, not just some old wives' tale stuff. "

Anyone else care to comment on any reason why not to fuse and ground this amp before I dig into it? I'm with Leo as for thinking I see no reason not to.

[Helmholtz]

I am not familiar with the amp's construction and grounding scheme but from the schem cutout it is clear that the cathodes of C62/C62A as well as the HT CT must not be grounded as they are at -22VDC wrt ground.

[Randall]

Interesting, I missed that. C62 and C62A are a cardboard covered can with it's cathodes twist locked onto a fiber mount isolated from the chassis. The other can is metal with it's cathodes twist locked into the chassis.

[Randall]

Well, I have installed the caps and grounded AC cord and fuse. It seems happy enough, but I won't really know until it goes back in the organ.

It is idling at 0.93 A. Does a 2A fuse still seem reasonable, or maybe a 2.5A?