Graaf OTL 20 amp

[William Brinsmead]

Hi Folks, Here in Reno Nevada USA, I just aquired a Graaf 20 with one buzzing chanel nice otl amp but sadly this link does not work any more.
I am experianced and I am sure i can fix it but it would be so much betterwith a schematic.

dead link: https://cid-a332bc3aefc89f6b.skydriv...0schematic.pdf

any help would be greatly appriciated,

Thank You, William Brinsmead

 

[Nightpuma]

First, have you adjusted the bias and offset?
If you have, do you have the later model with the brass bars connecting to the heaters?
PM me your e-mail adress, and I’ll send you the schematic.

 

[William Brinsmead]

Thank you so much, yes it is the modle with the copper filiment supply bars
Someone did a poor repair befor I got it, so a scematic will help imensly. Checked
bias and balance is ok. My Email is ....deleted....
Much appriciated , Bill

 

[Koonw]

That is the only schematic floating on the net. Heard it's used to be problematic, fine once is fixed.

 

[Nightpuma]

Thank you so much, yes it is the modle with the copper filiment supply bars
Someone did a poor repair befor I got it, so a scematic will help imensly. Checked
bias and balance is ok. My Email is ....deleted....
Much appriciated , Bill

Sent you the schematic.

Because of the heat, the big PSU capacitors can cause hum and need to be replaced once in while. Also some of the electrolytics inside. If I got a humming GM-20 with unknown story, I would have replaced all the electrolytics. They may be close to 20 yrs.

 

[as_audio]

Edit for the search: GRAAF GM 20

The name of the company is GRAAF, this is an abbrevation.
Puma, does your schematic contain the parts values ?

PS thread title was corrected now ..

 

[Nightpuma]

Yes the schematic is complete with parts values. It is not the offical schematic from GRAAF, so you should probably not build a clone after it, but for repair it will be very handy.

 

[as_audio]

So it should probably be published here. Mine is from the factory and I am not sure..
Additional information and link to another forum here:
Graaf GM20 6C33

 

[Nightpuma]

Here is also a nice thread with some modifications. If you have any trouble, get in touch with @Audiomix in the thread, he has repaired an rebuild a lot of them
Graaf GM20 mods - help!

 

[as_audio]

Do you agree to publish your schematic? Mine is the official one (see post 4) and I added the parts values myself. It is on paper only.

In the other thread you mention you can see contributions of people who do not know the amplifier.

This is is dangerous, for amp and speakers, but I will not comment more..

You write "I am the guy with the gm-20 page". Where can we find it ?

 

[Nightpuma]

Ragarding the thread I linked to, I agree with you, but some of the information is useful.

graafgm-20otl - theothor

Schematic is attached. If it is against the policy of diyaudio, please let me know moderator and I will remove it. GRAAF with Mr. Mariani is out of business, and schematics of other designs are readily available.

 

[as_audio]

Thank you for the schematic. I have no time to cross check the parts with what I have.

I think that the one of post 4 is from the factory. So I see no copyright issue with yours.
Is it made by some german fellow?

Looking in my company documents the drawing style is a little bit different among models.
I believe this one (#4) is accurate however. I have no idea of the original characteristics of
NTC MV1.

This said I can help with repair of other GRAAF amps also, but the old schematic paper
copies I own are bad and useless to scan or duplicate after more than 20 years.

Perhaps we should start a GRAAF schematic collection here at DIYA.

( GRAAF : Gruppo Ricerche Audio Alta Fedelta ) HiFi Research Group ..

 

[colmancer]

Dear Nightpuma!

I have the similar situation as William Brinsmead.
It would be a great pleasure me if you sent me the schematic you have. Some PCB layout should be handful too if reachable.
my e-mail is: deleted....
Thanks and best regards: Roland Bagi

 

[savu]

Hi there Nightpuma,

Can I also have the Graaf schematic? I want to see how they do driver stage to output stage biasing... My email is ......deleted...
Thanks in advance,
Savu

 

[as_audio]

It is in post 4 and in post 11 ..

 

[TonyTecson]

posting email addresses in threads are not encouraged, you can get spammed....please send your email thru pms....i will delete your emails as they appear here....from now on do it by pm's...

 

[colmancer]

Dear Nightpuma!

Thank You very much for schem!

Thanks: Roland

 

[Nightpuma]

You're welcome🙂

Remember the schematic in post #11 is not original from GRAAF, so care should be taken

 

[larsjaric]

You're welcome🙂

Remember the schematic in post #11 is not original from GRAAF, so care should be taken

yes, i think the offset trimpot should be 200ohms

 

[as_audio]

Confirmed, offset trim is 200 ohms.