Capacitance questions

[Randall]

In this following power supply circuit, I am not sure why the filter caps are stacked as they are. I am replacing all the filter caps, and have questions about what ratings I can use. The schematic shows 220uF/350v reservoir caps, but what the originals in the amp are 220uF/450v. Am I correct that the total capacitance for the two is 110uF, and that each sees an equal voltage amounting to 1/2 of the B+? What would be the better choice for replacements, 220uF/385v or 250uF/500v? JJ offers both at CED. Same question for the 220/350v at node B, lower voltage rating or higher capacitance? The dual 50uF replacement is rated 450v.

And what about the 32/16uF on nodes D and E, would there be any trouble with making it a dual 32/450v?

https://hiwatt.org/Schematics/DR504OL.gif

[Chuck H]

Schematic?

And... With "totem pole" capacitor arrangements the caps do share voltage (provided there are "balancing resistors"). This should be sufficient information for capacitor selection. A pair of 350V caps in series with balancing resistors is good to 700V. Unless the amp in question has more than 700V you'll be fine with the schematic spec. Why there were 450V caps in the amp is anyone's guess. What they had on hand? What they got a good deal on? Doesn't matter.

The only other thing that might be a consideration is derate. If you have a capacitor located in the amp such that it's going to get hot, like right over the power tubes or a power amp cathode bias resistor, you'd want to take derate due to heat into account. At 700V for the 350V units in totem arrangement I think you're alright

[Leo_Gnardo]

It's always a good thing to have leveling resistors, they will steer the voltages across the series caps so that each sees half the high voltage, even if one starts leaking a little current. Cheap insurance. That being said, I don't see any worries about using 350V caps as long as the max voltage across the pair doesn't exceed 700V. Although ... in those MusicMan amps that sport a 700V B+ I always use 450V caps as the manufacturer did. Y'know, just in case the amp user plugs in somewhere the line voltage is a little high. Or more than a little - it happens.

Also I see no worry about using the 32/32 uF as you mention.

[Randall]

Schematic?

Whoops! Fixed above.

The schematic shows 506V B+, which is just about what I am getting. And I proposed 385V replacements, so even better?

edit: I just realized the 385v cans are pc snap in type, and a little smaller, so scrap that idea. But here is a new question, is there any reason I couldn't use 250uF can in place of the 220uFs? Not much more than 10% over.

[Chuck H]

250/2=125uf
220/2=110uf

I think it'll be alright

[g1]

Cans often have the shell connected to the negative terminal. This can make fireworks used in a totem pole arrangement.
Are they negative shell, or more like the Marshall LCR type where the shell floats?

[Justin Thomas]

Although ... in those MusicMan amps that sport a 700V B+ I always use 450V caps as the manufacturer did. Y'know, just in case the amp user plugs in somewhere the line voltage is a little high. Or more than a little - it happens.

Or in a more liky for us scenario, we have to pull the tubes with the amp powered up for certain tests...

Anyway, that's the circumstance <I> aim for when picking cap voltages.

Justin

[Randall]

"Cans often have the shell connected to the negative terminal. This can make fireworks used in a totem pole arrangement.
Are they negative shell, or more like the Marshall LCR type where the shell floats?"

OOOH, hadn't ever thought of that! These are JJ and CED mounting clamp type. I don't believe ground is tired to the can, plus I think they are both plastic covered like LCR. They mention Marshall and LCR in their descriptions.

[Leo_Gnardo]

"Cans often have the shell connected to the negative terminal. This can make fireworks used in a totem pole arrangement.
Are they negative shell, or more like the Marshall LCR type where the shell floats?"

OOOH, hadn't ever thought of that! These are JJ and CED mounting clamp type. I don't believe ground is tired to the can, plus I think they are both plastic covered like LCR. They mention Marshall and LCR in their descriptions.

Don't we have a sticky about that? * see below...

I would avoid assuming the can shell is floating in any can cap. In spite of anything the seller/manufacturer may infer. What I do, is put a couple wraps of quality (3M) electrical tape around the part of the can that's gripped by the clamp. A similar fix could be done if you have any way big heatshrink on hand, 2 inch ID for instance.

Avoid the fireworks, enjoy the sound. And having the bill paid.

In some repairs I've seen done many years ago, the tech wrapped the entire - bare - can of a twistlok type cap with electrical tape. OK until some years later when time & heat undo the wrapping, then you have a shock hazard if that can's hot compared to chassis ground.

* Whups, that sticky is at our friendly neighbor, The Amp Garage. Here's a link:

https://ampgarage.com/forum/viewtopic.php?f=6&t=31852

Bee safe not sorry. Shocks & sparks, who needs 'em. 4th of July is over. 'Til next year.

[Randall]

With regards to can cap shells being tied to ground, HOLY SHITE!. I just checked this Hiwatt, and indeed the LCRs are exposed at the bottoms, and show they are tied to neg. On two of them there is also shunt to chassis at this point, so no problem, but on one of the 220uF it is not. Only the plastic case wrap insulating from the clamp, and with the exposed bottom a hair from the chassis, only held away by the thin layer of wrap around the edge. That's CRAZY. The double 50uF has a plastic disk on the bottom held in place by the edge of the plastic wrap, so that looks OK.

I am shocked that this is considered safe enough, what happens when the wrap insulator gets cut by a clamp or otherwise? Why don't the exposed bottoms arc to ground on the totem cans that are not sitting at ground? Why don't these cans have covered bottoms? This is just nuts.

Thank you for opening my eyes to this, I never knew.

[Chuck H]

I am shocked... that this is considered safe enough, what happens when the wrap insulator gets cut by a clamp or otherwise?

No pun intended?

I've even seen this even with axial caps. The ground is the case and there's only the thin, brittle and sheer shrink wrap around it. Most of the time this is not a problem because the "-" end of the cap is firmly soldered to 0V, BUT... Of course that's not the case when they're used as signal caps or (holy shit) in totem pole arrangements. Indeed, some care should be taken beyond the paltry and relatively fragile shrink wrapped label on the cap. As Leo indicated, I've always used a couple of wraps of electrical tape and payed attention when mounting and spacing things.

[ric]

Electrical tape's been know to come unwrapped. They make a self fusing rubber splicing tape to go over it.

Edit: Uh, no pun on self fusing either. It sticks to itself and becomes un-unwrappable.

[Randall]

"Electrical tape's been know to come unwrapped."

It's not going to unwrap if it is only under the clamp band, which is what I think Chuck and Leo are talking about.

[ric]

"Electrical tape's been know to come unwrapped."

It's not going to unwrap if it is only under the clamp band, which is what I think Chuck and Leo are talking about.

Understood, just thought I'd mention it.

[Leo_Gnardo]

"Electrical tape's been know to come unwrapped."

It's not going to unwrap if it is only under the clamp band, which is what I think Chuck and Leo are talking about.

That's why I mentioned, use the quality stuff, 3M, not shoddy dime store junk.

After some tuff nut repairs over the last week, I think I'm becoming unwrapped.