Amp Repair - GAS Son of Ampzilla - diyAudio
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Old 27th September 2004, 10:15 PM   #1
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Default Amp Repair - GAS Son of Ampzilla

I have a few GAS Son of Ampzilla amps. I love the sound of these amps. One of them recently developed severe distortion in one channel.

I am NOT an electronics tech but I can solder/desolder parts. Anyway, every time I have had one of these amps repaired it was ALWAYS the output transistors that needed replacing.

Since there are 4 power transistors per channel (2 NPN and 2 PNP) I figured I could buy all for parts for around $20. I can easily desolder the old and solder in the new parts.

If this doesn't fix the distortion then I am only out $20 in parts. Like I stated, it has always been these parts that was the source of the amp failures in the past.

On the other hand, I realize that the distortion problem could be caused by the drivers or bias circuits. Does anyone have an educated guess as to whether my DIY repair would be successful or even worth trying?

Thanks.
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Old 27th September 2004, 10:42 PM   #2
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Hi Horizons

Do you have an ammeter to measure current? When you replace the outputs and verify that the speaker output has no DC on it, then connect the collector of each transistor through an ammeter to the appropriate rail voltage. Measure the idle current through each device and make sure they are the same. This being a BJT amp, the idle shoudn't be very large, less than 100mA probably.
I am not familiar with this particular circuit, but judging from the output power it sounds like it is servo biased and probably has a small trim pot in the board someware around the driver stage that controls the amount of idle current in the outputs and can be adjusted. If you turn the idle to low then you will get crossover distortion and if it is too high, then the outputs produce lots of unneeded heat and will draw more ripple noise from the power supply. Measure the current and adjust the pot.
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Old 27th September 2004, 10:48 PM   #3
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I am of the opinion that you don't blindly throw parts at an amp in hopes of fixing it. No, your replacement of the outputs will not solve the problem. The bias IC isn't bad either or the amp would have blown up. It could very well be a driver or another problem.

It is time to go ahead and replace all the transistors and go thru this amp very carefully.

Check the SAE forum, someone should have a schematic for the amp. I have one somewhere. If I can find it I will post it.
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Old 28th September 2004, 02:28 PM   #4
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Default GAS Components

Hello Everyone,
I was looking thru the solid state forum and came across this thread. I own a Son of Ampzilla,Thoebe preamp and Goliath phono preamp. The Son of Ampzilla still plays but has some hum
in it now,my Thoebe preamp has started to do the same. I looked at the SAE forum some years back and saw that the designer of these products was offering restorations for them. But the cost of of his services seemed to be a little steep for me at that time. I too, have enjoyed these products very much and would like to see them working again properly. The schematic for the Son of Ampzilla,Theobe and Goliath would be very helpful. Worst case being,having to pack them up and send them to Jim B.
Sincerely,
Cambe
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Old 28th September 2004, 03:11 PM   #5
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Hi,
You can find the schematic for the lovely Son of Ampzilla and the Thoebe at my website www.audio-circuit.dk under Schematics
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Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
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Old 28th September 2004, 03:49 PM   #6
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Default Gas Components

Thank you ACD,
On the SAE forum Jim B. talks about how difficult it is for the average tech to work on these products.
Sincerely,
Cambe:
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Old 28th September 2004, 03:54 PM   #7
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Default Son of Ampzilla Repair

I tried to get one of my son of ampzilla's fixed only to be told it could not be fixed. I made the mistake of letting them keep that
one for parts.
Cambe
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Old 28th September 2004, 04:36 PM   #8
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Default Couldn't be fixed............?

It was designed in the 70s. We didn't have great parts back then. May not be able to get the exact part, but I can not see any reason why it can't be fixed. Might need a minor tweak if parts are substituted.

The Grandson may have been to best of the 3 amps in the line.

Jocko
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Old 28th September 2004, 05:13 PM   #9
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Hello Jocko,
You may be right. Esp. since yours is working properly. James B.
said many technician's don't have the skill's to perform the necessary repairs.
Cambe
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Old 28th September 2004, 09:21 PM   #10
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Send me an email. I'm sure I can help. I have fixed over 100 Ampzilla's and Sons over the years. They aren't difficult like James says.
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