Sound City 30C Mark 6 - occasional soft motor boating

[mikeydee77]

Hi, I have one of these Chinese re-issue combos not sure of age but assuming 5-10 years old. It is a 6L6 pp based amp with two pre amp valves and reverb. I can't locate a schematic for it and not really sure who actually made it. It's pretty cheap and cheerful.

Original fault was a bit of microphonic noise from the power valves (pair of 6L6GC). Turned out that the glass envelope was detached from its base. So the power valves have been replaced and generally amp sounds ok apart from an occasional mild motor boating thang.

As you switch on the HT the amp will "sometimes" start phutt, phutt, phutting... This is not full on motor boating but something a little softer which fades away after say 10-30 seconds. Once or twice it sounded a little like a tremolo effect being added.

Its difficult to get it to repeat and I have not been able to record it. Seems to happen more frequently when the amp is cold or not been fired up for a day or two. I tried freezing the caps and droppers to see if I could get it to repeat but no success.

I presume that it is something to do with the filter caps and decoupling caps but just wondered if there was anything else to consider especially to make it more repeatable?

PS If anyone has a schematic then that would be useful.

[pdf64]

Which control settings affect it?
If nothing else, at least try all at full CW and all at full CCW.
It's most likely a B+ cap; to to try and identify which one it is, try tacking a low value eg 10uF cap of suitable voltage rating across each B+ cap in turn and check for the fault.

[mikeydee77]

Which control settings affect it?
If nothing else, at least try all at full CW and all at full CCW.
It's most likely a B+ cap; to to try and identify which one it is, try tacking a low value eg 10uF cap of suitable voltage rating across each B+ cap in turn and check for the fault.

Hi pdf64, thanks for replying,

Unfortunately the fault is short lived and difficult to repeat but I will try your suggestions if possible. I had hoped that freezer spray on the caps might have brought it out of its shell.

In the circuit there is one large 220uF reservoir cap and then three 22uF decoupling caps for the pre amp. I was able to check them out on an ESR meter and of course they measure fine, which I realise is obviously no guarantee of working at circuit voltage, but I was hoping might identify an iffy one.


Mike