GAS Grandson - Bongiorno enhanced

Hi 2 all !

Sorry for reposting - I tried this before however it seems to get a bit lost at the end of this other thread.

I am currently refreshing my trusty old Grandson with a little help of "The Bongiorno" himself but I still could use a little help from some of you more technically knowledgeable enthusiasts.
I am therefore happy to share James' suggestions that I am sure will be of interest for some of you.

I really think these units deserve to be kept alive because for me a long experience clearly shows that - if ever - in terms of musicality only the most ridiculously expensive high end gear even comes close to what these machines delivered more than 30 years ago.

Back story is that I got the Thalia/Grandson combo in 1985 and I am obviously totally addicted to the sheer musicality of it.
I tried loads of other amps, even insanely expensive high end types (Pioneer, H/K Citation, Forté, Hafler, Bose, Chord, Yamaha, Crown, Magis, Bryston etc.) and while finding some preamps that I could live with (and finally went for a Technics SU 9070) power amp-wise I always came back to the Grandson.

So I got used to the fact that James Bongiorno seems to be able to do something in his designs that has become vital for my life as a musician and producer/engineer: Musicality.
But I needed a bigger amp.
Living in Europe however makes it pretty hard to find a decent GAS or SUMO as there are veeeery few 220V units on the market anyways and to find one in mint condition is nearly impossible.
The best I could find was a Crown K2, it is awesome - but still it doesn't really have this kind of "Groove" that Grandson adds to the musical performance.
Yeh and then one day I found out that Guru James introduced Ampzilla 2000.
Decision was clear within milliseconds - but to get one over here is not really the easiest of tasks.
Now the Son Of Ampzilla 2000 arrived and it does exactly what I expected. This is clearly the best amp I ever heard. 'nuff said.

Now for the Grandson that is powered off for the second time now in 30 years.
After all these years of non-stop delivery it needs some refreshing. Bass precision has gotten quite poor and also there is a strong hum component which I can only get rid of by running the left input cable in a special loop across the transformer to cancel it out...

Of course I encountered the same problems with the schematic and board mismatching as others before.
The boards in most of the units seem to be 721, whereas the schematic on the web is for 721 B which I now think maybe never made it into production...

As I am in contact with James Bongiorno himself about the Ampzilla 2000 anyways I decided to just ask him about rebuilding Grandson and in return of (hopefully) getting some expertise here I'd like to share with you what he suggested:

"Dear Spok,
I myself, do not do upgrades anymore as I just don't have the time.
If you want to tackle this then merely replace the ceramic compensation caps with silvered micas.
Then replace ALL the bypass caps with .1ufd/100V green drop polys (Panasonic).
If the output devices are Sankens, then leave them alone. If they are something else, then replace them with ON Semis MJ21193 and MJ21194.
The rest of the semis should be OK.
Replace the 1558 type servo dual opamp with a FET input type like a LF353, TLO72 etc. This should give you some small improvements."

Then I found Toshiba output transistors in my unit and did a web search for "green drop polys" which I couldn't find anywhere. They seem to be grey now...
And instead of the 1558 there is a 1458 in my unit.

This is what he says about it:
"The output devices should definitely be changed to the ON (formerly Motorola) devices. The grey Panasonics are OK. The 1458 is a bipolar and definitely should be changed to a FET input type opamp."

So I ordered a bunch of spare parts, except for the Micas which don't seem to be available over here in Europe so easily.

I have to say that I don't really have a history in servicing vintage amps, so any help on how to adjust bias and DC offset after replacing the OP and output transistors is greatly appreciated.
The schematic does not give any hints, test points or voltages.

Also I am not 100% sure if I identified the bypass and compensation caps right.
I believe that (on the schematic) C114 and 214 (.05uF) and the ones on the output daugther boards 722 and 723 are the compensation caps that he wants to be replaced with Micas but honestly this goes beyond my knowledge of reading schematics.

Another question is, does he really mean that ALL the ceramic caps which on my PCB have values of 10pF, 100pF and 220pF are to be replaced by .1uF polys ? Does this make sense for you guys ?

Instead of the TL072 I thought of going for an Analog Devices OP249GPZ which I believe to remember sonically did a great job in my Harrison console.
Not 100% sure though.

Any confessions ?



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I have an Ampzilla IIa and a SON, there are several .1 uf ceramic bypass capacitors scattered everywhere on both amps. Looking at the Grandson schematic I see NO .1uf bypasses anywhere. Just do as JB said replace the pf ceramics with the same value of silvered mica and the .1uf, if any, with the film type. Change all of the electrolytics while you are at it, if you haven't already.

Another question is, does he really mean that ALL the ceramic caps which on my PCB have values of 10pF, 100pF and 220pF are to be replaced by .1uF polys ? Does this make sense for you guys ?

No, you must use the same values..
Particularly, the 10pF must be about 500V rating
since they are used for the amp compensation,
with high voltage swings.
Also, use high quality for these ones..

Btw, this amp is a symetrical differential topology,
and those ones are very rare compared to the
ubiquitous, not to say rampant, single differentials,
since the latters allow for low THD with less effort
and a little less costs...
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Change all of the electrolytics while you are at it, if you haven't already.
Thanks for your reply !
Just this moment I'm at it :)

Just do as JB said replace [...] the .1uf, if any, with the film type.
In Grandson there is not one .1uF, just 10pF, 100pF and 220pF. That's the reason why I asked. Shall I just plaster it with the .1uF's instead ?

Btw, this amp is a symetrical differential topology,
and those ones are very rare [...]
Fully symmetrical amplifiers are a pleasure to see.[...]

Well Bongiorno built fully symmetrical amps a lot, even held patents for that, but Grandson isn't one of those, as we can clearly see from the schematics.
Some SUMO's and Ampzilla2000 sure are, however.

Can anyone point out by the schematic maybe which are the "compensation caps" and which ones are "bypass" ? I'm still a complete newbie to this.

C103/104 C106/107 (10pF), C105 (100pF) and C109/110 (220pF)
are the compensation caps of the first channel.....

You have surely noticed that they are marked 2XX on the
other channel.

Anyway, check the voltage rating of the caps that are used on your amp.
If they are rated a few hundreds volts, just keep them as they are, no use
to change them...

Edit :
i just read that the original ones are ceramic..bad ones, indeed..

For bass losses, you can recap the power supply caps which are
now somewhat old..
If there s some room on the amp, keep them as they are, and
add another pair of 10000 uF in paralel...
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Ha !
Thanks, man !
I already changed all electrolytic caps, including the 10000uF ones.
Have been told that changing it to greater values might induce other problems so I decided to use the same value though.

Edit about the Micas: Just found out that these small values are relatively cheap. I was looking up .05uF the other day and they were around 40 to 50$ each. So I decided to try some of these NOS russian military .5% types.

Also I replaced the ugly ceramic resistors R114 and 115 by Caddock MP930's a while ago.
Highly recommended - secret weapon indeed.

Will come back when I got all the parts in.
Thanks so far !
Mission accomplished.

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 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


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I have a Thalia and a schematic for it but what I don't know if it's a I or a II. Does your's have Thalia II on it? My does not have a I or a II. Electronically mine has an IC for the power supply regulator. The other Thalia uses discrete components for the regulator, but which one is which. I can send you my schematic if you what. It has all of the basic circuits on it, phono, line, tone controls and regulator. What I don't have is how it all interconnects, don't know if there is a schematic for that.