GAS Grandson - Bongiorno enhanced

[spocintosh]

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 ?

Thanks,
spocintosh

[llwhtt]

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.

Craig

[wahab]

Originally Posted by spocintosh

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...

[spocintosh]

Originally Posted by llwhtt

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

Originally Posted by llwhtt

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 ?

Best,
spocintosh

[lineup]

Fully symmetrical amplifiers are a pleasure to see.
And with transistors of good complementary we have today
it is not difficult to design symmetrical amplifiers.
Even there are now powerful output devices with very good NPN-PNP match.

[spocintosh]

Originally Posted by wahab

Btw, this amp is a symetrical differential topology,
and those ones are very rare [...]

Originally Posted by lineup

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.

Thanks,
spocintosh

[llwhtt]

All of the pf values are comp. caps. As I said from the schematics there are NO .1uf bypasses so just forget about them. Looks like all you need is some 10pf, 100pf and 220pf silvered micas.

Craig

[spocintosh]

OOPS...that's gonna be expensive then...means 22 micas...argh !
I only ordered 4 now: The .05uF C114/214 and C115's on the daughter boards.

[wahab]

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...

[spocintosh]

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 !