Relationship between output valve resistance, OT and speaker load

SunDevil

Alas, adding a 4K7 across the presence pot did't help - still the same issue

It actually sounded very much like it did running 16 Ohms into an 8 Ohm load - buzzier, more compressed OD but the this oscillation still present

I'll need to get some appropriate caps to try the valve grid mod, but @JPF amps is kindly lending me an amp this weekend without NFB, so hopefully that will go some way towards exercising the NFB theory

Thanks for the suggestions / help none the less

ICBM

SunDevil said:

I'll need to get some appropriate caps to try the valve grid mod, but @JPF amps is kindly lending me an amp this weekend without NFB, so hopefully that will go some way towards exercising the NFB theory

You could just try disconnecting the NFB entirely. It might cause other oddness, but if it stops that exact oscillation you will at least know what the cause is.

DJH83004

Totally agree with @ICBM temporarily disconnect the NFB loop altogether and note the difference. To be honest this is a mod I have done on many Fender amps over the years, using the redundant 'ground' switch as NFB on / off. As you would expect makes the amps more lively and crisp sounding.

Arkless_Electronics

What frequency is this oscillation at?

SunDevil

Don't know, but we've already established that it's not ghost noting if that's possibly where you were going?

Given that the amp is 100% when un-attenuated and it sounds great with EL34s even when it is, I'm going to look at fix that amp agnositic

I think for my next step, I'm going to plumb it into a resistive load and feed the line in on the Power Station with that

BP

Arkless_Electronics

I've no idea what you are talking about there!

ecc83

Arkless_Electronics said:

I've no idea what you are talking about there!

And I am unanimous in that!

Dave.

SunDevil

:-)

..Translation - try a resistive load rather than a reactive one, but take a line level out and re-amp it, so the speaker isn't loaded down