Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ritchie200 on October 08, 2010, 10:51:14 pm

Title: Feedback question...
Post by: Ritchie200 on October 08, 2010, 10:51:14 pm

Typical Marshall - why feedback from the 8ohm leg for EL34's and 4ohm leg for 6550's?

Thanks,
Jim

Title: Re: Feedback question...
Post by: Geezer on October 09, 2010, 06:18:07 am

Don't know the technicalities, but I suspect it has to do with the tube differences in what is optimal secondary imp for each tube type.

EL34's want to see ~3.4k 2ndary imp, so if you have a 4/8/16 ohm OT, take the NFB from the 8 ohm tap.

6550's want an ~6.6k 2ndary imp, so the 4/8/16 now becomes ~8/16/32 ohm, so move the NFB tap to (what was) the 4 ohm tap (now the 8 ohm tap) for similar feedback response.

$.02, grain of salt, etc.

Geezer