Son of Ampzilla - Keep or Return?

dr*audio said:

The hiss problem is not likely caused by bad caps, it's bad transistors in the differential pair in the front end of the amp. This is common. You have to buy 20 or 30 replacement transistors and match them for HFE so you are installing matched pairs. If you short the inputs of the amp and you still hear hum and hiss from your listening position, the hiss is the transistors and the hum is power supply caps. If you only hear hiss and the hum only happens when you connect the preamp, it's either the cable between the preamp and power amp or you have a ground loop. Put a 3 prong to 2 prong adapter on the power cord of the preamp to lift the ground. Only one connection to power line ground is necessary.

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All the noise, both hiss and hum increases significantly when shorting the inputs (compared to connecting the inputs to the preamp). When connected to the preamp, the hiss isn't too bad, the hum (from the speakers) dominates and sounds like it could be fixed by recapping the power supply.

dr*audio said:

The hum coming from the chassis is from the power transformer. Sometimes the screws that hold the transformer together get loose, sometimes it's the internal laminations that are loose (shellac dries up) and nothing can be done.

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This is my biggest concern. From the photos I've seen, that transformer looks big and expensive. Any idea what a replacement would cost (if they are even available)?

Going through the thread, it looks like all the bases have been covered. Power outputs hopefully/usually don't need replacing but I did recently see a bunch of prized original and unused Sanken outputs specifically for this amp on ebay for cheap. The guy has a lot. The transformer hum will always be present albeit only with your ear agasinst the top cover when all is well. The transformer hum will be louder with the other issues, especially bias/offset which is tied to the front end as mentioned. If it plays and makes music, I would not push it but pull the plug and rebuild it. The key to a competent rebuild is commensurate with your due diligence in researching your circle of peers preferably in finding not just a competent tech, but someone who's heart is in it and is also an audiophile. I recommend someone here. Mike Bettinger, a member of the dedicated GAS Ampzilla forum here heads up GasAudio, a company dedicated to the preservation of this gear. Search the archives and find him. He did a full rebuild on my GAS500, a discription of which is in Barter Town North. He also rebuilt my Grandson of Ampzilla. His workmanship is second to none. He was a principal tech at Great American Sound when it was still around. I would refer my questions to him. Great guy, very personable.

Btw, that big ole transformer lights the way to a 250wpch 2ohm load on that amp.

Yikes! I just noticed Mike at GasAudio has an ad for Son rebuild at $600.00 on Ebay. Nevertheless, imo, a new Son is easily worth a grand sq wise.

Superampman said:

Yikes! I just noticed Mike at GasAudio has an ad for Son rebuild at $600.00 on Ebay. Nevertheless, imo, a new Son is easily worth a grand sq wise.

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Sounds like a pretty good deal to me. Lots of work and parts in rebuilding that.

True. The GAS500 all told cost me $2400. It is a Son, only bigger. I had a good conversation with Mike on that issue. The baby is the Grandson. All three have a lot in common.

MCM_Fan said:

All the noise, both hiss and hum increases significantly when shorting the inputs (compared to connecting the inputs to the preamp). When connected to the preamp, the hiss isn't too bad, the hum (from the speakers) dominates and sounds like it could be fixed by recapping the power supply.



This is my biggest concern. From the photos I've seen, that transformer looks big and expensive. Any idea what a replacement would cost (if they are even available)?

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Well over $120 for a new replacement transformer. You can get something that will work but it will not be original. I say return it.

dr*audio said:

Well over $120 for a new replacement transformer. You can get something that will work but it will not be original. I say return it.

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I did, earlier today. The suspect transformer, combined with the $250 initial cost, was the deal breaker for me. Completely hassle free return and refund. I explained the problems I'd observed with the amp, especially my concerns about the transformer noise, and was immediately given a full refund. Didn't even bother to plug it in and verify what I told him.

I'm a little bummed. This really is my dream amp. To me, it looks the way an amp SHOULD look, and I've heard so many great things about the sound quality. Unfortunately, even though this one was in great cosmetic condition, it was looking like a real money sink with all the work and replacement parts that would have been required to get it performing up to it's potential. As it was, it really wasn't any better playing music than the NAD 3150 it was slated to replace and the noise from the transformer was entirely objectionable in a bedroom system. I would have had to invest several hundred more dollars just to get it to be as quiet as the NAD. Although I'm sure, with a proper restoration, it would have handily out performed the NAD, I don't have the time or >$1k budget to tackle a major restoration at this time.

So, back it went. Hopefully, it will end up with someone who has the time and budget to get to fully restored. Although they are rare around these parts, I'll continue to keep my eyes open for another. Thanks to all for the help

It's baaaaaaaaack....

A week after I returned it, I noticed the seller had listed it on Craig's List at $125.00. So, my lust for one of these classics got the better of me and I went back a few days later, to their new location and picked it up.

I know it needs work, and quite possibly a very expensive transformer replacement, but I figured at $125.00 for a functional amp, I could afford to put some money, and a whole lot of my time, into restoring it to better than new condition.

I know this is going to take a LOT of my time. In fact, it already has. But, even though it's only September 1st, this will be my big winter project. I'll proceed in a slow, methodical fashion, do as much as I can, and if I get stuck, I'll take it to one of the skilled local professionals (I'd really like to avoid shipping it back and forth across the country) and pay them for their talents.

In any case, you can follow my progress here:

G.A.S. Son of Ampzilla Restoration Thread