A question about caps for old solid state Ampeg

[glebert]

I acquired a project 1969ish Ampeg ST-25 solid state head which had been worked on by a tech with questionable judgement. The power caps installed are big Sprague screw terminal caps, of the right capacitance called out by the schematic (2500uF) but 75V rated and the voltage rail on this is 80V, schematic calls for 90V rated caps. I can't figure out why someone would put $60 worth of underrated caps into this, unless that is just something he had laying around(?) Who knows.

My question is do I need to replace with screw terminal caps? I've read that they have lower ESR and can handle more ripple curent than something like a snap in or axial lead cap, but most amps don't use them.

Here is the schematic if anyone cares. The parts list does not call out anything other than the capacitance and voltage rating. https://ampeg.com/support/files/Sche...Schematics.pdf

[Helmholtz]

Don't rely on what you've heard, compare the datasheets for ESR, ripple current, temperature and lifetime ratings. I don't think there is such general rule.

[Jon Snell]

Screw fixed large electrolytic capacitors have a lower ESR in general than other types. The ESR is reduced if the screw tightness does not comply to the stated torque in the data sheet. (Normally hand tight plus 20 degrees).
I would fit Kemet
ALS40A222DB100

2,200uF 100volts low ESR high ripple current.
as I would require reliability.

[Enzo]

Remember, it is just a guitar amp, not lab equipment. Those old "computer grade" caps were often selected because other formats had less selection in the large values. But these days caps are cheap and available in all manner of ratings. 90v or 100v caps would be a good idea. But what matters more than that I think is temp rating. Use 105 degree caps instead of the common 85 degree ones for longer life.

[g1]

I can't figure out why someone would put $60 worth of underrated caps into this, unless that is just something he had laying around(?) Who knows.

I'm guessing an owner wanted the exact same looking caps of original brand, and the 75V are a current Sprague offering, the 90V version no longer available?
I almost seem to recall reading about it on a forum.

The 75V is working voltage, maybe 80V is within the tolerance, not sure but they seem to be rated up to 95V surge.
Maybe they will be ok in there, I guess you are concerned from a reliability standpoint?

[glebert]

I'm guessing an owner wanted the exact same looking caps of original brand, and the 75V are a current Sprague offering, the 90V version no longer available?
I almost seem to recall reading about it on a forum.

The 75V is working voltage, maybe 80V is within the tolerance, not sure but they seem to be rated up to 95V surge.
Maybe they will be ok in there, I guess you are concerned from a reliability standpoint?

Mostly I don't know if they were installed and working OK at some point or if he put them in while trying to revive this from the dead. Same guy that was working on this amp before he passed away unexpectedly: https://music-electronics-forum.com/...mp-i-have-seen

I guess it would be a concern of reliability, really don't want to get this thing working and then the caps fail and take out the PT or something.

[g1]

There's nothing that says they must be bad if they are working above their rating, so I would say if they don't appear to be venting or measure short, carry on, fix the amp, and worry about it later.
A while back I realized that with modern (higher) line voltages, it's not unheard of to see amps working with caps running above their voltage ratings. A manufacturer builds an amp with no leeway on the B+ cap rating, at 120V line. Now I plug it in with our 125V line and there is 70V on the 63V caps (example).
How often do we measure the B+, then double check the cap rating to make sure it's not being exceeded?

[Jon Snell]

Remember, it is just a guitar amp, not lab equipment. Those old "computer grade" caps were often selected because other formats had less selection in the large values. But these days caps are cheap and available in all manner of ratings. 90v or 100v caps would be a good idea. But what matters more than that I think is temp rating. Use 105 degree caps instead of the common 85 degree ones for longer life.

As my example shows; 105℃ in post 3.