negative feedback switch on ceriatone ef86 36w

[chrisblues]

hello happy builders
I just finished a 36w ef86 inspired by ceriatone model
I'm looking for more headroom but the nfb switch does not change the sound at all.
any ideas?
thanks
Chrisblues

[zaphod_phil]

Depends how the NFB has been implemented. I've seen some schemes used with 18W amps that wouldn't really have much effect at all. Unfortunately Ceriatone doesn't publish schematics, so it's hard to know what's been done. However, just reducing the 470k resistors at the EL84 grids to 220k will give you more clean range out of the amp. And don't forget to turn up the master volume with the preamp gain down, then just increase the preamp gain to the level you want.

[Chubsman]

Pretty sure the missing clean headroom is a result of the EF86 preamp more than the neg feedback configuration. You could always convert back to 12ax7 if it's a real prob for you.

[chrisblues]

thank you for your answers
I still change the el84 grid leak resistors to 220k.
I'm surprised by the value of the nf network.
I use 820 and 100+22µF.
the amp works very fine but I want just more linearity when I switch this.
and for me it doesn't alter the sound at all.
I build it as a combo amp with 2 x 12" speakers.
here is one pic of it
Chrisblues

[Plexi]

Looks like Nik has changed the feedback circuit.. the ones i looked at will not make any noticeable difference..
set your up like this.. and you will have a much cleaner sound or headroom when the feedback is engauged..

http://mhuss.com/18watt/schematics/36wTMB.gif