changing filter caps in solid state amps????

[loosecounty]

Hey everyone I have a question. I have a couple old solid state bass amps , an acoustic 136 and a carvin sc3000 . They both are totaly stock. They both have rather large electrolytic caps in them. Now if they were tube amps I would change these old caps, But I dont know what they do in a solid state amp or if I should change them. What difference would I hear if I did change them. The reason I ask is the acoustic starts distorting at half volume and the carvin has a very noticable hum. They re both from the 70's. I know my tube amps very well but this solid state stuff is a bit foreign to me. Thanks for any help!

[km6xz]

What frequency is the hum in the Carvin? If it is a primarily 120hz and occurs even with the gain control down, it is likely that filter caps are not doing their job. Filters in solid state amps run a long time, due to being of more modern design that have lower ESR to start with, which translates to less heat and more effective filtering.
The biggest killer of electrolytic caps is heat and tube amps generate a lot of it. The large "Computer Grade" caps in old solid state power amps last a long time from my experience. Even in tube circuits of reasonable design caps should last 30++ years. There are a few specific designs of caps that failed prematurely primarily due to poor seal designs or unstable electrolyte.
Luckily, caps are easy to test, with known loads and open circuit ripple, you can calculate the storage capacity, and ESR. Capacitor testers usually are not very revealing of the condition of capacitors since they do not put a stress close to expected operational conditions for the devices under test. In-circuit is the best way to test most devices...caps, resistors, transistors, tubes, etc. Measure its influence on the circuit.

[loosecounty]

The hum in the carvin isn,t really a "hum" but more like a noise presence. You turn the unit on and there is a backgroud noise even with the volume down. Its not exactly like the hum you would hear in a tube amp. You could never record with it. Both of these units are 30+ years old and I may change the caps regardless, but wanted to get an idea of what to espect with new caps.

[km6xz]

The filter caps are not the cause of the noise. Don't change anything until you get the noise taken care of. Changing good parts can introduce more ambiguity in repairs and make your job harder. Does the noise change with tone settings?

[J M Fahey]

Agree and add:
Your Acoustic may be fine, simply old amplifiers were designed to reach maximum at around 6 or 7 on the volume pot; a modern bass with good pickups might make it clip somewhat earlier.
Simply said, that clipping was not nice.
They could (and did) be used in a band situation, where the general noise level masked that buzzy noise, but played on their own ... ugh !!
That's why most later bass amps incorporated some kind of compressor or limiter.
Don't think it's related to your filter and output caps.

[Ronsonic]

Hey everyone I have a question. I have a couple old solid state bass amps , an acoustic 136 and a carvin sc3000 . They both are totaly stock. They both have rather large electrolytic caps in them. Now if they were tube amps I would change these old caps, But I dont know what they do in a solid state amp or if I should change them. What difference would I hear if I did change them. The reason I ask is the acoustic starts distorting at half volume and the carvin has a very noticable hum. They re both from the 70's. I know my tube amps very well but this solid state stuff is a bit foreign to me. Thanks for any help!

In general, for a SS amp, if both supply rails are equal in voltage and you aren't seeing big AC on them the caps are good.

[Mark Hammer]

I recently bought anold Heath TA-16 solid-state amp. It hummed. I replaced all the electrolytics that were 100uf or bigger. Now it doesn't hum.

Since the era when the Acoustic was made, electrolytic caps have gotten smaller for any given value/rating, and cheaper. So if there is a 3900uf cap in thepower supply, you could easily stick a 6800uf cap of a higher voltage rating in that same slot for a couple of dollars at most.