Ampeg Mercury M12A

[stokes]

Anybody ever been in one of these? I have one on the bench and am a bit perplexed at what I see. The schem shows 3 x 20uf in the power supply in a can cap. The can has the 3 20uf/450v and a 25uf/25v for the cathode cap. In addition to the can there is an additional single 20uf/450 on the board that is wired parallel to the input cap.Its definately stock, the cap is one of those old numbers with the brown cardboard casing, glued to the board with that typical brown adhesive they used. In addition the wire leading from the extra cap is wrapped with some other wires and glued to the cahssis with the same brown adhesive so I'm sure its stock.The schem in the amp and the one online from Joe Piazza are the same and neither shows that paealleled cap.Any thoughts?

[The Dude]

A schematic or schematic link would help. Aside from that, maybe they decided they needed more filtering at some point in the production run. No big deal.

[mozz]

Ampeg, Gibson, Premier and others have many variations of schematics under the same model number.

[The Dude]

Found this (directly from ampeg's site):

https://ampeg.com/support/files/Sche...Schematics.pdf

It shows an additional paralleled cap like you describe.

[stokes]

Yeah, Dude, thats it. Same schem from the back panel in the amp except they didnt pencil in the addition on mine. I thought it was odd that it appeared to be factory installed and original and wasnt on the schem. Its not like its a minor addition. I've seen Fender labels with penciled in changes but never anything as major as doubling the input cap.

[g1]

I'll have admit I was a bit confused by the 'input cap' terminology. Now I see it is referring to the first cap off the rectifier. I was thinking of the filter cap for the node that supplied the triode closest to the input.

[stokes]

I'll have admit I was a bit confused by the 'input cap' terminology. Now I see it is referring to the first cap off the rectifier. I was thinking of the filter cap for the node that supplied the triode closest to the input.

On the tube data sheets, there is a recommended "input capacitance", I've always called it the "input cap"

[g1]

On the tube data sheets, there is a recommended "input capacitance", I've always called it the "input cap"

Yes, and they also refer to 'choke input' or 'capacitor input'. Just something I haven't come across much, and seems to be most common where there is tube rectification.
There's always something to learn here.

[Helmholtz]

Is there a problem with the amp?
If so it's hardly due to an added 20µ reservoir cap (provided the cap is not leaky).

[stokes]

Is there a problem with the amp?
If so it's hardly due to an added 20µ reservoir cap (provided the cap is not leaky).

So far the only problem with the amp is that it hasnt been used in at least 30 yrs and dont expect any problem from the added filtering, just found it odd that it wasnt on the schems I was seeing.The Dude found it penciled in as I would expect such a main component addition. Not concerned if its leaky or not, its coming out.

[Helmholtz]

Not concerned if its leaky or not, its coming out.

But don't be surprized if ripple hum increases - especially with unbalanced power tubes.

[The Dude]

......The Dude found it penciled in as I would expect such a main component addition. Not concerned if its leaky or not, its coming out.

I'm not sure why you'd take it out except to replace it. The fact that Ampeg themselves thought it was a necessary addition should be enough info to indicate it should stay. How many years has the amp worked with that extra cap inside?

[catalin gramada]

There is matter of rectifier used 10uF for 5y3ga and 20u for 5y3gb, gt as tipical. Found somewhere 40uF allowed wit minimum 50ohm plate. Wanna use with 40uF better put some fuses in PT secondary and check the winding resistance.Maybe possible with high impedance PT secondary...who knows ? Install some fuses anyhow.

[catalin gramada]

Late: Any build I saw around 5y3 works with 20uf as reservoir. If Ampeg used 40uF they did in certain conditions. The PT winding resistance will be the first suspect.

[stokes]

I'm not sure why you'd take it out except to replace it. The fact that Ampeg themselves thought it was a necessary addition should be enough info to indicate it should stay. How many years has the amp worked with that extra cap inside?

Didnt mean I wasnt going to replace it, I'm replacing all the electros.Was just curious as to why it wasnt at least pencilled in on the schem, as your link showed.