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Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: pontiacpete on August 29, 2026, 11:07:02 AM

Title: Sunn coliseum 300 pre amp trouble
Post by: pontiacpete on August 29, 2026, 11:07:02 AM
I am tracing through the pre amp and finding that the signal stops at IC5 SSM2012a. I could  be wrong about this but it looks like this chip is bad. Since these are long unavailable what are people doing for a substitute?

The schematic is :
https://www.prowessamplifiers.com/schematics/images/Coliseum_300_11_x_17.pdf
Title: Re: Sunn coliseum 300 pre amp trouble
Post by: J M Fahey on August 30, 2026, 04:00:32 AM
It might be wrong of course, but remember SSM2012/13 are current output ICs, so scope who reads voltages only can show "no signal" there.
Try output of Next op amp, pin 7 of IC6b.

From what I read modern substitutes are certain THAT ICs but are not pin compatible and require redesigning.
MAYBE the Synthesizer guys make a replacement board based on modern stuff. Remember Peavey made a replacement board for their obsolete analog switching chip.
Title: Re: Sunn coliseum 300 pre amp trouble
Post by: pontiacpete on August 30, 2026, 09:08:10 AM
Unfortunately there is no signal on IC6b. Pin 5 and 6 just a flat line and pin 7 has a small wiggly line.
Is that it for this amp unless I can fit another chip in place of IC5?
Title: Re: Sunn coliseum 300 pre amp trouble
Post by: J M Fahey on August 30, 2026, 10:04:05 AM
Look what the cat brought:

https://britishaudio.com/products/ssm2013-precision-monolithics-pmi-vca-nos?srsltid=AfmBOoqTUnd39el9qiksIqwS56QvfSlzfY9EzT4xgYLJcs6vbn7CBclC
Title: Re: Sunn coliseum 300 pre amp trouble
Post by: J M Fahey on August 30, 2026, 10:12:34 AM
Absolute worst case, IF your amp is muted AND you need it NOW for a tour, a rehearsal studio, etc. you can fully bypass the compressor section by lifting the right leg of C1, the left leg of C7 and joining them with a piece of wire.
Amp will work fine, except compressor.
Right or left as seen on schematic,  they can point anywhere on PCB
Title: Re: Sunn coliseum 300 pre amp trouble
Post by: pontiacpete on August 30, 2026, 10:38:14 AM
Quote from: J M Fahey on Today at 10:04:05 AMLook what the cat brought:

https://britishaudio.com/products/ssm2013-precision-monolithics-pmi-vca-nos?srsltid=AfmBOoqTUnd39el9qiksIqwS56QvfSlzfY9EzT4xgYLJcs6vbn7CBclC

How did you find that??
Thank you very much!



I think I will do the temporary fix for now, also to see if it's working
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