Valve preamp circuit question

[ikay]

The schematics below show part of the Orange AD200B preamp circuit compared with the Terror Bass 500 Mk1 preamp. The TB preamp is supposedly based on the AD200B but there are some noticeable differences. One that jumps out at me is that the TB signal is fed back to the input of the valve via capacitor C5 (47p) which doesn't happen with the AD200B. Can someone explain what this 'feedback' loop does and why it's there? Thanks for any input from all you valve amp experts out there!

[Woodinblack]

Isn't this part of the HF rejection circut?

http://www.valvewizard.co.uk/gridstopper.html

 

[ikay]

Thanks for the link. I guess it could be, although the added cap mentioned in the HF rejection circuit goes direct to ground...

[TwoTimesBass]

42 minutes ago, ikay said:

Thanks for the link. I guess it could be, although the added cap mentioned in the HF rejection circuit goes direct to ground...

Yes, it's close but it would go to ground not to the cathode.

It's puzzling, best guess is it's somehow shaping the harmonic response of the valve, it certainly doesn't need to be there for the amp to function...

[ikay]

15 hours ago, TwoTimesBass said:

It's puzzling, best guess is it's somehow shaping the harmonic response of the valve ...

Thanks, maybe that's it...

[basstone]

On 02/10/2020 at 17:27, ikay said:

Thanks for the link. I guess it could be, although the added cap mentioned in the HF rejection circuit goes direct to ground...

If its purpose is to offset the Miller effect, it would be better going directly to ground but C3 is effectively a short circuit at all but very low frequencies so probably does not make any difference. It would be interesting to know if it makes any audible difference disconnecting it. It could also assist with HF stability if the circuit layout is not particularly good.