Graaf GM20 schematic

If they glow, this is the place to be
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pre65
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#1 Graaf GM20 schematic

Post by pre65 »

Hi-i asked this question on the "other" forum some time ago without success but has anyone got a copy of the Graaf GM20 amp circuit WITH component values ?

Has anyone ever had a chance to compare them with "other" valve amps ?

It seems "Dr" John Caswell is a fan of them.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
John Caswell
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#2 Graaf GM20

Post by John Caswell »

Hi Philip,
Really unable to answer this on the other forum, however, cct diagram is commercially sensitive so am not prepared to release it at all, but the following bits may help.
Output impedance is quite high between 6-8R so needs high sensitivity LS. Sounds brilliant on my workshop rubbish LSs but absolutely awful on my IPL S5TLs. On a pair of Klipsch at a customers he has a GM20 on each, mono blocked, and the results are to die for but he also has a big high room as well, ex rag trade workshop.

Generally are reliable providing you
1) Check bias and DC offset monthly
2) Replace bias and offset controls with 20 turn devices
3) Run bias at 20mV not 30mV as printed on PCB
4) 6C33C-Bs really should be matched and the 6922 I have found best is the Philips JAN one. Try to get some good NOS EF184s (Mullard or Mazda)
5) Lots of ventilation required and I mean lots.
6) Do not run with the valve cover on it restricts ventilation a lot
If I owned one I would bring the 4 pots plus test points out to rear panel as on the GM200

John Caswell
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pre65
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#3

Post by pre65 »

Hi-i can understand why Graaf would not want a rival manufacturer to produce copies.

Just wanted to see what it might cost for a poor person like me to make a working copy.

Never mind,there are so many ways to go with this audio nirvana quest. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
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