Switching global NFB on/off (pentode LTP)

[ChrisM91]

I am building Pete Millet's DCPP "Engineers Amp" and would like to add a switch to turn the global NFB on and off.

See attached for an annoted schematic snippet

Is this the correct way to switch the global feedback on an amp with a pentode long tail phase inverter?
In the NFB off position the V7 grid is grounded via 1K+ 3.3K || 47K, which is ~4K. I think this should be fine.

Thoughts?

Full schematic for reference

 

[jan.didden]

Well yes but you do realise that that upsets the amp? For starters, the gain will shoot up, distortion explodes and the freq response will be a mess.
Is that what you want?

Jan

 

[Fuling]

The Engineer's Amp has a healthy dose of "Schade" feedback, it should be fine without GNFB.

 

[Osvaldo de Banfield]

With a SPST switch shorting 3.3K R70 is sufficient. Grid can be grounded directly.

 

[ChrisM91]

Thanks all for the confirmation

I guess now I am questioning myself if it will be worth it to drill a hole in the chassis for this GNFB switch. Wondering if I'll find I prefer it in one setting (most likely NFB engaged) and just leave the switch there, rendering it rather useless.

Maybe it is worth just experimenting with, making a decision then hardwiring the amp that way.... 🤔 🤔 🤔

 

[Lingwendil]

In previous builds where I implemented a switch I have never preferred the sound of pentode outputs with NFB switched off, so I personally wouldn't drill a hole for it. If anything maybe wire in a switch offboard and see how you like it first before committing.

 

[stephe]

Maybe have a switch with different amounts of GNF or switchable amounts of the "Schade" feedback if it is employed? I have found adjusting the amount of Schade feedback used changes the tone of the amp.

 

[lcsaszar]

See my adventures in the "with or without GNF?" field:

https://www.diyaudio.com/community/threads/push-pull-807-amplifier-without-global-nfb.384100/

Spoiler: I found "with GNF" is better (and UL is better than triode or pentode).

 

[rkoonce]

The Engineer's Amp has a healthy dose of "Schade" feedback, it should be fine without GNFB.

I've built several of them and they are fine with no feedback.