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sizzlingbadger wrote: ↑Sat Jun 11, 2022 10:55 am Full disclosure... I have Covid induced brain fog this morning, but the more I look at R3 the more I reckon it would work as NFB without any any real ill effects despite no DC de-coupling. Maybe a bit more hum, and the pre-amp anode voltages may creep up a bit. I'd still remove it to start with and then maybe see what effect it has, someone must have liked it in the past

Is this what you are thinking?

Anode second triode is out of phase with anode of pentode. When input signal forces pentode anode voltage up, R3 is trying to lift triode anode voltage despite that triode itself trying to lower the anode voltage as per same input signal.

If so, then that can be described as a NFB loop.

yep - pretty much

V1b is not really enclosed in the NFB loop. But R3 (plus the OT primary winding) is effectively in parallel with R4 to the B+, lowering the V1b anode load resistance to around 110k, which will decrease its gain. The feedback component of R3 is the output of the EL84 feeding back to it's input, through C5. This effectively the pentode version of this: http://www.valvewizard.co.uk/localfeedback.html

I'm talking about global feedback, from the power amp to the pre-amp, not the local feedback in the power amp stage itself. Either way its a not a conventional way to design feedback

sizzlingbadger wrote: ↑Sat Jun 11, 2022 12:32 pm I'm talking about global feedback, from the power amp to the pre-amp, not the local feedback in the power amp stage itself. Either way its a not a conventional way to design feedback

So from RA and SB posts, we can assume that R3 has quite an impact on the amp's character at higher volumes. Noting that the original speaker is only 8" the amp without R3 may have sounded too 'rough'. I shall test this with a GH12 as well to see how it sounds with and without R3 in the circuit.

I wonder who dreamed up and soldered R3 in...

Fun times

Speaking with the owner who has had it for over 50 years, he remembers a repair way in the distant past but can't recall what the problem was.

Could we be looking at a fix by an electrical repairman (yes, there used to be such a creature) who understands electrical circuits but has no concept of an amp as a musical instrument?


I will also add with some confidence that the owner is a player not a collector. Any changes that make the amp sound better will be what he'd choose.

You can put negative feedback in anywhere really, it's plausible that someone might have even just selected a high value resistor and decided to linearaise the power stage by ac coupling the plate to the grid using the grid input cap that was already there, and be damed that it was dc coupled to the plate of the previous stage already.. I dont know what that interaction would do exactly, but as mentioned it wouldnt hurt anything and maybe it sounds good!

That tone cap - is that the mustard that says 0.22? 220n not 22n if so? That's a lot of filter...

Optical wrote: ↑Sat Jun 11, 2022 11:17 pm That tone cap - is that the mustard that says 0.22? 220n not 22n if so? That's a lot of filter...

Both the tone pot and tone cap are not original imo. Why use a gang pot? It is wired in parallel... the only explanation I can think of is that, whoever put it in, didn't have the right value single pot.

It is 220nF... Might try a 47nF if I have one lying around.

You guys rule.

Jay wrote: ↑Sun Jun 12, 2022 9:30 am

Optical wrote: ↑Sat Jun 11, 2022 11:17 pm That tone cap - is that the mustard that says 0.22? 220n not 22n if so? That's a lot of filter...

Both the tone pot and tone cap are not original imo. Why use a gang pot? It is wired in parallel... the only explanation I can think of is that, whoever put it in, didn't have the right value single pot.

It is 220nF... Might try a 47nF if I have one lying around.

Just to be clear, if what you see makes no sense, delete it.

GrantB wrote: ↑Sun Jun 12, 2022 9:41 amYou guys rule.

Best thing about the forum - community knowledge and help

Jay wrote: ↑Sun Jun 12, 2022 9:30 am Both the tone pot and tone cap are not original imo. Why use a gang pot? It is wired in parallel... the only explanation I can think of is that, whoever put it in, didn't have the right value single pot.

It is 220nF... Might try a 47nF if I have one lying around.

I’ve seen double gang pots in other IA-series Jansens- but usually for tremolo. One gang wasn’t connected to anything and the pot was def stock, so I think Jansen did use random part selections back then, based on whatever they could get their hands on.

I mentioned the pot in an earlier post.... parallel will double the wattage it can cope with, maybe that is a reason. 220nF may be fine as its in a much lower impedance circuit, being as its in the power amp.

RectifiedAmps wrote: ↑Sat Jun 11, 2022 12:25 pm V1b is not really enclosed in the NFB loop. But R3 (plus the OT primary winding) is effectively in parallel with R4 to the B+, lowering the V1b anode load resistance to around 110k, which will decrease its gain. The feedback component of R3 is the output of the EL84 feeding back to it's input, through C5. This effectively the pentode version of this: http://www.valvewizard.co.uk/localfeedback.html

I measured all resistors without power applied. Most have a higher reading.

As you said, R3 and R4 are 'parallel' and I read 134Kohm over either.

Fascinating stuff. I'm going to shuffle off to the security my lawn tractor gearbox rebuild.