Possible to add NFB to this schematic?

[888guitars]

I tried to figure this out but I'm at a loss. I've never had a amp where the PI tube was fed from separate channels. I'm wondering if anyone has an idea?

[nosaj]

I tried to figure this out but I'm at a loss. I've never had a amp where the PI tube was fed from separate channels. I'm wondering if anyone has an idea?

Don't understand what you mean fed by separate channels. To me it looks like the PI is fed right at C7.
Here is a basic variable NFB you could add in.

nosaj

[Steve A.]

Don't understand what you mean fed by separate channels. To me it looks like the PI is fed right at C7.

The TMB channel is fed into the PI at C11.

I've seen some amps (Vox or Matchless) with the reverb return fed into the "other" PI input.

I have no idea how to add NFB to this amp but perhaps it doesn't need it...

Steve Ahola

[Richard]

It looks like you could feed the NFB to pin 2 of V2a. That's where you would send it in any other LTP PI.

[frus]

No you can't, and Steve is right - it doesn't need it, not having NFB is its "thing"

In fact, you "could" feed the output to V3A cathode, and have the NFB on TMB channel only (I think...?)
Normal channel doesn't really need it.
In fact, I would rather put in the tone control from standard 18W tremolo channel, it shaves a bit more bass off and has a bit more punch

[J M Fahey]

No you can't, and trying to some to V3a cathode would affect *all* of the amp anyway.

Plus any NFB will kill the amp sparkle, bite, punch, gain .... do you *really* want that?

[potatofarmer]

Why wouldn't it be possible to inject NFB directly to one of the grids, like the Fender Bassman AB165?
schem: http://www.davidsonamp.com/sf/images/bassmanab165.gif

The two channels being mixed at the LTP are out of phase so it seems to me the situation is similar to how LTPs are typically used; the outputs from the PI will be of the same phase regardless of which channel is used.

Plus with both grids in use you can just switch which grid you're injecting NFB into instead of having to swap the OT primaries. Which I guess is an inconsequential benefit, if it even works.


edited to add: Just had a thought - will this idea not work because of the lower open-loop gain?

[888guitars]

I wanted to put it on a switch to see if it would tighten up the bass. And see if I liked it. I have trimmed coupling and cathode bypass caps but it's still a bit flu by on the low E chugging along. It may be the MM OT tranny? What if I bumped the main can cap from 32/32 to 50/50?

[potatofarmer]

Try removing the cathode bypass capacitor on the power tubes first; you might be pleasantly surprised.

[888guitars]

I did bring it down to 20uf. I will take it out and see. Is there any problem that may occur without the cap?

[Dave H]

I wanted to put it on a switch to see if it would tighten up the bass. And see if I liked it.

The Normal channel bass should be OK but the TMB channel has far too much bass. I'd try reducing C5 (it could be on a switch) until the bass tightens up. You may have to reduce it a lot. VOX had 500p there on the TB channel of the AC30.

[888guitars]

I have a .0047uf there. (C5) maybe go down to plexi bright .0022uf

[888guitars]

I tried it on the cathode of V3. No change at all. From 1k-100k no change?