Unmarked mystery cap can

[Resonator Guy]

On my bench is a 1950 RCE G-5 amp. It has a 6SL7, 6V6, 5Y3 line-up and works very well after replacing the rotten two prong cord. It has two cap cans: a 2x20mf @ 450V on the left (the little one) and a huge unmarked single one on the right rear. There is not a marking on it anywhere. It, the clamp and the solder all look original and untouched. Can anyone advise what size this cap could or should be? Since a Champ with a similar tube layout has 3X8mf filter caps, I am very unsure what to replace this one with if replacement is required. Checking the wiring, it seems to hook up with the rectifier and the OT.
I would appreciate any thoughts.

[Tom Phillips]

Thoughts based on the information you provided.
The "mystery cap" is most likely the input reservoir cap. I'd say typical value would be 10uf to 20uF at a voltage rating sufficient to withstand the max reached in your amp. 450V should do it. Old data sheets list a maximum allowable input filter capacitance value of 32uF for the 5Y3. I think that 20uF would be fine for your amp.
Cheers,
Tom

[custom_amp_30]

Can you use a capacitance tester to verify the uF? I know you would have to drain the charge, but a tester would at least give you the ranges and you can measure applied voltage to get within the voltage range required.

[Resonator Guy]

I don't have a capacitance tester so can't verify it.
Tom, any idea why this appears so big? I am just curious as it is a monster.

[Tom Phillips]

...Tom, any idea why this appears so big? I am just curious as it is a monster.

One possibility is that it is not an aluminum electrolytic. I have some 1950's era caps that are single section cans about 1 1/8" diamiter by 3.5" tall. They are 4uF @ 600V and the description in the manual (An old Hewlet Packard power supply) bills them as "fixed paper" types.

Edit: I found one of those old "paper" 4uF / 600V can caps. It was much larger than I remembered. 1.5" diameter and 4.5" tall. It is date coded 1955.

[Resonator Guy]

Thanks Tom, I will do some reading on those.

[Rhodesplyr]

Hammond used oil-block capacitors from 1935 through the 1950s rather than electrolytics, but I've never seen these in a guitar amp. They usually still measure like new with better DF/lower ESR than electrolytics.

You're sure it's not a potted choke of some sort, right? (Purely a wild speculation)

[Resonator Guy]

No, it's definitely a cap, and it is a Canadian made amp with a Hammond transformer.

[Rhodesplyr]

Must be a different Hammond, the ancestor of the current Hammond Mfg. transformer company, perhaps? I meant the organ company.

Perhaps Santa will bring you a capacitance meter :-)

[R.G.]

Hammond used oil-block capacitors from 1935 through the 1950s rather than electrolytics, but I've never seen these in a guitar amp. They usually still measure like new with better DF/lower ESR than electrolytics.

That is the realization that was behind my including motor run caps in the "Immortal Amplifier" concept. Electrolytics have an inherent wear-out mechanism; non-electrolytics do not.

As a suggestion, modern polypro/oil motor run caps are much smaller for the capacitance-voltage product than older paper/oil caps.

[tedmich]

Here is a bunch of immortal 20uF motor run caps
Lot of 50 Motor Start Capacitors CBB60 SH 20uF 240VAC | eBay
for $0.68 each

They're probably good to 600-700VDC and take 1 3/8" cap clamps and you can paint em black with Fusion paint

(they say motor start but they are CBB60 PP motor run)

[Tom Phillips]

Here is a bunch of immortal 20uF motor run caps
Lot of 50 Motor Start Capacitors CBB60 SH 20uF 240VAC | eBay
for $0.68 each...

Best to read that again. There were 50 available when the auction started but they are priced at $18.00 each plus $15.99 shipping.
Tom

[tedmich]

Best to read that again. There were 50 available when the auction started but they are priced at $18.00 each plus $15.99 shipping.
Tom

Don't think so Tom, here's his listing for singles
Motor Start Capacitors CBB60 SH 20uF 240VAC | eBay
$2.00 plus $2.99 shipping

other is lot of 50; they have boatloads

[Tom Phillips]

Don't think so Tom, here's his listing for singles
Motor Start Capacitors CBB60 SH 20uF 240VAC | eBay
$2.00 plus $2.99 shipping

other is lot of 50; they have boatloads

I understand now but before seeing the other listing the seller's wording for the "lot of 50" sure is confusing.

Edit: How do you figure that the 240 VAC rated cap is good for 600-700 VDC?
I get more like 336 VDC.

Tom

[Enzo]

Just a guess: as motor caps they are not rated for rectifiers and filter caps, they are for AC use and so would be rated for peak to peak voltage if the AC mains.

Maybe?