Graaf GM20 mods - help!

[airtangent]

Hi Elberoth,

That's also something that is on my agenda for upgrading my Graaf GM20 clones.

However what I will use are Siemens MKV capacitors which were also used in my Vacuumstate RTP3 preamp and other
candidates are Miflex KPCU copper foil paper/propylene in oil types which are reasonbly priced and got very good reviews,
see here:

https://www.humblehomemadehifi.com/Cap.html

and here on youtube where they were tested in speaker crossovers comparing them with Rike caps and other brands:

But there are also other mods I'm considering like swapping resistors in the signal path ( preferably using Vishay naked Z-foils )
and I'm also planning to make the driver B+ regulated with a super reg ( CCS in combination with a shunt regulator ). This can
all be done relative easily in my mono block clones since they are completely hardwired, so no PCB's that could be damaged, see here:





Regards,
airtangent

 

[MikePP]

F.Y.I. on the GM20 amp, MV1 is a 82volt varistor (MOV)

Original is a Littelfuse P82Z2

 

[Audiomix]

Evening all - I'm afraid I'm a newbie...

I have a pair of Graafs GM20's (schematic via link) but the constant tweeking of bias and offset is driving me mad!

https://cid-a332bc3aefc89f6b.skydrive.live.com/self.aspx/Public/GM20 schematic.pdf

Is there a way of adding an auto bias circuit?

Also, can anyone suggest any tweeks / improvements? I've scoured the net but nothing forthcoming.

Ahh, can I reduce the gain / adapt the present circuit to suit a higher input voltage without using a pot and thus destroying the dynamics? I'm using a Weiss Medea DAC with +27dBu via XLR's....

Thanks guys 🙂

I built my own. It's rock stable. Important to use good tubes. Many 6C33 are not. Often thermal problems and run away. If you get two good pair they will last some 1000 hours.

 

[Audiomix]

Maybe you can use this:

http://www.tentlabs.com/Products/Tubeamp/page24/page24.html

Ronny

Not for OTL amps.