2xEL84 amp with long-tailed pair PI - questions

[stormbringer]

Hi All,

I just fired up a new project amp with a 2xEL84 fixed bias output section, 2204 preamp, and long-tail pair PI with feedback. I tried to implement the 'preferred' presence design from the Blencowe preamp book. My first impressions of the amp are positive, but the Presence control does nothing. I chose the feedback resistors somewhat randomly, so that is not a huge surprise.

The PI values that I have used are shown in the schematic below (the sketch is based on Soldano Atomic 16 schematic).
My layout is shown in the second illustration.

I have two questions for starters:
1. are the values for feedback resistors (Rf and Rs) reasonable? (notes: 1. feedback is attached to the 8R output jack; 2. I increased Rf to 100k).

2. Is there something wrong with the layout?

[loudthud]

Are you aware that the post PI Master volume will hinder operation of the Presence control when that Master is turned down? Could the output stage be oscillating?

[eschertron]

I'll have to go look at some other designs, but my suspicion is that 25K for the presence pot is too low.

[stormbringer]

Are you aware that the post PI Master volume will hinder operation of the Presence control when that Master is turned down? Could the output stage be oscillating?

I was unaware that the bootstrapped MV that Kevin O'Connor has sketched out in one of the TUT books (and sells as the LPMV) would hinder operation of the Presence control. Actually, I thought I read that it would not hinder performance. However KOC has also informed me that it will. So I will test with the MV at max.

[stormbringer]

I'll have to go look at some other designs, but my suspicion is that 25K for the presence pot is too low.

I'm interested in knowing what you find. Thanks!

[eschertron]

I'm interested in knowing what you find. Thanks!

Mister Excel Sheet says you're probably OK with the 25K. If you've got a 50K or 100K you could try it to see if there's a difference. It is audio taper? I think you'll want that. The lowest affected freq is about 400Hz with your values. Raising Rs a little, to about 5.6k, might help offset the extra parallel resistance. JM2C

[stormbringer]

Mister Excel Sheet says you're probably OK with the 25K. If you've got a 50K or 100K you could try it to see if there's a difference. It is audio taper? I think you'll want that. The lowest affected freq is about 400Hz with your values. Raising Rs a little, to about 5.6k, might help offset the extra parallel resistance. JM2C

Thanks for going the extra mile!