GAS Grandson - Bongiorno enhanced

Bias 1mv !!!! :-(

Hello everybody.
It 's my first post in this forum and I apologize for my English, I'm Italian and I use a translator to help me ..... the results are sometimes ridiculous.
I have also completed the modification of a Grandson following this tread .... then all the replaced capacitors and those of small value with silver mica except for 0.05 uf which have been replaced with 0.044 uf polypropylene. I used new Caddok 0.20 Ohms as emitter resistors and replaced the final transistors with ON MJ21193 -21194.
I have problems adjusting the Bias .... with the original pair (2SD426 - 2SB556) was not already adjustable over 10 mv .... but with the new pair the maximum value I can get is about 1 mv !!! :-(

the 1k r128 trimmer has been replaced with a new one of good quality.

I think it is necessary to change the value of R119 and R120, but this goes beyond my knowledge and so I would like to ask for help and suggestions to the friends of the forum ....
Hi Craig, thank you for answering me. Tomorrow I will measure the tension as you suggested. Yes the integrated mpq6001 should be working, in doubt I bought a new one and replaced it. Nothing has changed. Tomorrow (now in Italy it is night) I will let you know the value of tension between the bases of Q107 and Q108.
Well, I measured the capacitor C115 - C215 (located between the bases of the Q107 and Q108 - Q207 and Q208) and the values obtained are 2.08 v and 2.28 DC.
Qli MPQ6001 are 4 transistors in a single container. Before mounting them, however, I did a rough check with a tester and seemed to work properly.
The capacitor C115 (0.05 mf ceramic) that actually on the circuit was a 0.10 mf, was replaced with a polypropylene torque 0.022 mf in parallel ..... so it should be a 0.044 mf but I do not think it makes a big difference ... ...
Son of Ampzilla settings?

Hi All,
I am in the process of fixing and upgrading a SOA. Does anyone know how the Protection circuit is setup? That is the pots R129 and R130. Also What value and where to measure the offset voltage, R131?
Thanks in advance


  • hfe_gas_son_of_ampzilla_schematic-CLEAR.pdf
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I've asked many people, techs from GAS and other experts on GAS equipment, nobody knows the VI adjustment. I usually leave all of those components out when rebuilding a SOA as the general consensus is that the SOA doesn't need it. DC offset is measured at the speaker terminals, adjust for 0VDC.

That schematic is a modded version, are you rebuilding it that way? I have two more of own SOA to rebuild and I'm going to try the mod on the next one.

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Hi Craig
Thanks for your reply.
With respect to the DC offset; Is this done with an input signal? I would guess a 1KHz at line level.
I intend to rebuild in its original state. Although I will be replacing things like the Electrolytics with Elna Silmics. Do you have an unmodified schematic that I can get from you? You might notice mine is quite difficult to read.
All the best
I'll try to get my original schematic scanned, it's 11" x 17". I still haven't figured out why the modified schematic is the only one online. DC offset is ALWAYS set with no input and usually no load. Put some shorting plugs in the input jacks if you have any. Also if you are replacing the input jacks they MUST be grounded to the chassis.