GAS Grandson - Bongiorno enhanced

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east electronics

east electronics

Member
Joined 2006
eastelectronics.gr
well ... since i have a similar amplifier to work with i may say that is a few parts of the procedure of the OP that i dont agree with and some other things that i would do in an other way ...

the use of such exotic caps at the specific areas could be more trouble than beneffit

for starts none of the so called exotic caps does do anyting importand in the operation of the amplifier since the one is simply a bias stabilizer and the other one is part of the zobel network that may as well be something of course better than a typical ceramic but nothing like that ...

on the other hand ....dead slow electrolytics are left as was in the input of the amplifier ( as far i can see from the fotos ) this only could cause a lot of issues to the sound of the amplifier than almost everything else ... single cap there but better quality may sound 10 times better

Also ...in the amplifier i am workinh with which is 220V version seems that both of these capacitors are original .... ceramics but 0.1mfd /50V in both cases especially in zobel the value of 0.1mfd /50V makes by far more sense than 0.5 (espeicially combined with 30Ω resistor ) in the bias i dont think that makes any change any way .....

this is what i ve seen so far i will proceed with testings and changes and post my results

One thing i would like to do very much since the amplifier is very handy to work with i will make changes and upgrades but to one chanel only make tests post scope readings before an after ...

Kind regards
sakis
 
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east electronics

east electronics

Member
Joined 2006
eastelectronics.gr
while working with the amplifier i noticed tha the all VU circuit beyon totally generic is actually working terrible ...not logarithmic at all and also needles moove too sharp that you get the feeling that are going to brake ..

will modify that with better rectification and skip the series resistor and make a shunt trimmer on it ...

been following the changes regarding the mica caps cobined with scope readings ive done originally with te ceramics ... and seems tha playing there with the values of the micas you cane make the amp a lot better ..

kind regards sakis
 
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dickl

Member
Joined 2011
i would be interested in the VU circuit. I have 2 Son of Ampzillas. One is in my office and is driven by an NAD preamp and powers a set of AR3a speakers. Sounds spectacular.....and to think the speakers are 35 years old and the amp is at least 25 or 30.

My other Son of..... was redone by James Bonjornio about 11 years ago. I believe he did all the tranistors and caps and connectors. It too is awesome sounding. It is hooked to a pair of Advent speakers and driven by a sort of homemade preamp. The Son of sounds terrific.

I would like more information on these products. I moved to Chatsworth CA in 1976 and some time in the next couple of years went to the GAS factory where an auction company was selling off all the old stuff. I did not have 2 cents to rub together at the time and I really did know what I was looking at......but later I bought up quite a few pieces of GAS gear in the pre-eBay days. I later got to know Jim Bongiorno and had him rebuild one of them.

I will never sell these, and sorry I sold off the 2 Ampzillas I had. I even had a Grandson for a while.

Keep the information coming.
 
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dickl

Member
Joined 2011
I was reading this post while listening to on of my Sons of in my office. I started hearing him on the left channel and the signal going of intermittently.....I started reading all the mods here in case I needed to fix this left channel.

Printed out a couple of diagrams and the mods......

finally it was making me crazy so I started checking connections......the RCA phono plug did not seem to be connecting well!!! I tweaked it a bit....deformed the shell for more pressure, and now is all great again.

I just want to restate how much I love this AMP.......really a wonderful sound.

My other SON was modified by James Bongiorno himsellf and it sounds even better.
 
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dickl

Member
Joined 2011
I should not have posted so soon!!! Now my other SON OF is acting up! Channel A is cutting out intermittently. I am going to take the cover off and check the fuses to Start. Will then go through the diagram to see if I find anything else.

What it does if go off when i have run it for 30 minutes or so. Let it cool down a bit and restart, now it bring in Channel A after being on for a couple of minutes.

Any ideas? This a James Bongiorno modified Son of.


Thanks, Dick Lague
 
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dickl

Member
Joined 2011
Sakis,

Thanks. I will check what you listed. I did take a look around and did not see anything obvious visually. I noticed the speaker output connectors have spade connectors inside. Maybe it is something as simple as a bad or dirty connection there.

I understand your point about the fuses. They are on or off unless a bad connection. They look intact, but are really buried down there.

Will let you know how I make out.

thanks for your help.
 
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dickl

Member
Joined 2011
I am still working on it. Have not found problem yet, but actually have been too busy to do more than just superficial checking. I have checked all the connections to the output terminals. I am going to look at the thermal protection, but the schematic I have is not to clear.....from the internet and an old copy I guess. anyonehave a better schematic of a Son of Ampzilla??

I got so frustrated with not having music in my studio that I pulled the big old Korean built 4 channel amp out of my old pickup truck and used an old computer power supply to drive the 12vdc......actually works pretty well.

It is a bit of a monster so running at no more that 25% is way loud and avoid the distortion at the upper end. I am driving this with Fubar 2000 music server software on my computer and a Behringer U-CONTROL UCA202 USB-Audio Interface. The latter is really cool $30 DAC that is basically and external USB sound card with dual RCA in and out and SPDF and headphone jack too. I also use it for my Virtins computer scope and signal generator.

the Preamp is an along1986090 with a pga 2311 chip and remote volume and LED.....cool little preamp!!

All of this sounds amazing with a set of homebrew 3 way speakers..... will post photos if anyone is interested.
 
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S

spocintosh

Member
Joined 2009
thespok.com
First off:
Never attempt to fiddle around with bias unless you changed the corresponding pots !!
These are rotten and will most inevitably break when you just think of touching it - which will have desastrous effect on your amp.

Here's what Mike had to say:

Mike Bettinger said:
Setting the bias for the channels:
Measure the voltage drop across the .2 ohm resistors connected to the emitters of the output transistors.
With no speakers hooked up and no signal the reading should be around 25mv.
You can raise this a bit as you warm the amp up. This sets the class-a crossover point of the output stage and a higher drop improves the sound but will make the amp run too hot when you start running it hard for extended periods of time.
Set it at the 25mv =setting and then monitor the temperature of the heatsinks as you use the amp normally. If they are not uncomfortably warm raise it to 30mv and monitor. the best setting is where it gets moderately warm to the touch in normal operation but cools back down to moderately warm when Idling with no music over time.
Click to expand...
 
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Tutor Turtle

Member
Joined 2016
Bias question

I am rebuilding this grandson for a friend. What is the bias setting and where do I measure it?

Thanks in advance, Steven


spocintosh said:
Hi 2 all !

Grandson rebuild finally finished.
I did most of the work about 3 months ago but after finishing it provided me with a dead left channel.
Due to loads of other work, interests (e.g. a Forté 1a) and holiday season I had to leave it until now as I had no time nor the nerve to investigate on that.

Turned out it was only a bad connection to the output daughterboards - phew.

So please have a look at the pix and the enclosed pdf to see what happened.

Grandson had blown up right channel many (MANY !) years ago and got repaired by a tech who obviously did his job but not really beyond the call of duty.
He replaced the blown MPSU57 with a BD530 and the 2SB556 with a 2SB681.
Ok, it worked and sounded fine but I now changed it back to an MPSU57 with a NOS part.
Output transistors were changed to the ON Semis James Bongiorno mentioned anyway.
I even managed to find a spare for the blown 2SB556 (just in case I wouldn't like the ON Semis and wanted to go back).

The i/p dual OP was different from what James had in mind. It wasn't a 1558 but a 1458. I changed it to an Analog Devices OP249 but have a BurrBrown 2604 at hand also which I'll give a listen also. Both did a very good job in replacing TL072's in my Harrison desk.

The matchboxes that you find on the pics are actually 50000pf ±.5% military grade silver micas.
All of the electrolytics were replaced with Nichicons and all ceramics are now silver micas as well.
Finally I replaced the bias pots.

The ceramic emitter resistors were already replaced couple'o years ago with Caddock MP930's which are thermally coupled on a common heatsink (huge improvement on detail and speed by the way).

All soldering was done with a special alloy silver solder that I order custom made to my personal preferences for some time now.

It is running for 36 hours now and plays great. Maybe a little too early to go into detail as none of the parts are anywhere near being burnt in but there definitely is a tendency towards extreme silvery high end.

Very detailed and sharp, almost laser-like. Just how I love it with my ring radiators. Extreme detail, even more than my SoA 2000.
Midrange sounds "new", meaning sounds like new parts that have to settle, so I wait with that description a little, it changes from one hour to the next at the moment.
Very lowest end hasn't improved much, it is still a little slow and "cosy/cuddly", meaning uncontrolled - might be just the lack of power.
Kick bass control, however, improved a lot - it is much less "boomy".

Imaging is impressive. It was before anyway but now it is like nothing I ever heard.
This amp delivers a sound field which is at best described as a "cloud" that surrounds the listener and expands waaaay beyond the listening room.
Yet it's not like other amps I heard with a wide image. If the're wide they normally lack precision of imagery on single instruments and voices due to phase instability.
Not so with Grandson. It projects sound events so precisely towards a point in your room that you shiver and have to look up check what's happening !

Al in all I'm totally happy. That wasn't a bad move at all and I'd like to encourage anyone owning a vintage baby to give it a try !

I'd also like to point out that I wouldn't have finished this without the kind help of Mike Bettinger of gasaudio.net who provided me with valuable information during the last stages of this debut project of mine.
My thanks go to this man who earns the greatest respect for his encouraging words, knowledge and commitment. Given the fact that I'm a non-customer this is even more impressive.

Only remaining thing now is a grounding problem that goes back to the above mentioned improper repair in the 80's and a broken right channel meter.

Here's some pix. Enjoy !

ImageShack(TM) slideshow
Click to expand...
 
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