Is there a 'Blackface' type ~15w clone?

[The Captain]

Tests done: 'BRR' stops when grounding out V2, pin 2 (both V1 pins 2 and 7 'BRR' with a 'pulsing' sound when grounding out I note here). I'll see if my brain can come up with any sense re. this this tonight.. probably not! cheers Mark,
Captain.

[MWJB]

So this tells you that the "Brrr" is being injected between V1, 6 and V2,2. If you pull V1 see if you still get "Brrr".

Pull V2, measure resistance to ground at V2, 2 - you should get 100K (your grid load resistor at V2,2). Then check continuity to top of coupling cap at V1, 6. If checks OK, suspect 0.22uf at V1, 6 (just replace it anyway, caps are cheap, make sure the cap legs are located in the relevant eyelets - not just resting on old component legs & soldered.)

If there's no change after that...I'll tune in again tomorrow...same time, same channel?

[The Captain]

So this tells you that the "Brrr" is being injected between V1, 6 and V2,2. If you pull V1 see if you still get "Brrr".

Pull V2, measure resistance to ground at V2, 2 - you should get 100K (your grid load resistor at V2,2). Then check continuity to top of coupling cap at V1, 6. If checks OK, suspect 0.22uf at V1, 6 (just replace it anyway, caps are cheap, make sure the cap legs are located in the relevant eyelets - not just resting on old component legs & soldered.)

If there's no change after that...I'll tune in again tomorrow...same time, same channel?

Ok I can understand prob lies between V1, 6 and V2, 2. Now I will try your suggestions above later tho im not sure exactly how Im testing; in meantime I just whipped out the .022 cap lying in this prob area replacing it with a .1 being only thing to hand: still bbrring.

So re the above tests- am I..

1. leaving V1 in or taking V1 out, or taking both out?
2. doing both the V2, 2 resistance check /and continuity from this coupling cap's top with amp on*, both switches flipped?
3. *but from where to where am I measuring this continuity? fwiw Ive measured continuity 10x across/ between each point on the preamp, 3x new bits added, indeed re-did the whole circuit again.. with amp off.. should this be on??

thoughrally confused of course.. Captain.

[MWJB]

For the resistance/continuity tests you don't need any valves in place, or the amp on.

Measure continuity from actual tube pin to component lead as it exits component body.

What 3 new bits have you added and why? Why have you "re-done the whole circuit", did you think the "whole circuit" is wrong? Because you are unsure, you are randomly throwing parts at the amp..STOP! You need to methodically find the specific cause of your problem and solve it. I don't care how many times you have measured continuity, if it was all connected correctly you wouldn't be having a problem.

[The Captain]

Ah apologies Mark. I meant I redid continuity over whole circuit, & the '3 new bits' added I meant just the new tone additions in Q. I was trying to explain Im thourough/ ALWAYS redo basic checks to rule out ANY page 1 dumbness you may be quizzical over, ie using the n/c pair of inputs, any bad solders, wrong R values etc etc.. too much info maybe.

So all as 5E3 with the added tone config: re diagnosed prob area- replaced the .022 cap, 100k grid R measures ok from tube pin, & general continuity around here all fine (all solders here checked with magnifying glass too) = still noisy.

[MWJB]

Leaving the wire from the 100K plate resistor to V1, 6 in place, and the 100K to ground from V2, 2 - remove the coupling cap from V1, 6 to V2, 2 from the board and physically mount it from one tube pin to the next, along the chassis floor. This will eliminate you picking up any noise from anywhere else with this component.

Replace V2 with any working 9 pin preamp tube (AY/AT/AX). Does the amp still "BRRR" with just one preamp tube in V2?

[The Captain]

Hi Mark. I just did the cap job direct from pin to pin- brrr still there.

And my 12AY7 solely as V2- brrr still there.

Bugger.

[MWJB]

grounding V2,2 and/or removing V2 stops the "Brrr"?

[The Captain]

grounding V2,2 and/or removing V2 stops the "Brrr"?

Yes exactly- gnding V2, 2 with tubes in- brrrr stops. Removing V2 only stops it too.

[MWJB]

If you still get the "Brr" with V1 removed, or v1 pin7 grounded, then it must be something around V1, 6 and V2. As you haven't changed anything around V2B and you have eliminated problems with the coupling cap from V1, 6 to V2, 2, I'd now look at your grid load resistor at V2,2.

Rather than run the long ground wire to your star ground at the RHS of the chassis, ground the 100K to a solder tag under one of V2's mounting nuts.

[The Captain]

If you still get the "Brr" with V1 removed, or v1 pin7 grounded, then it must be something around V1, 6 and V2. As you haven't changed anything around V2B and you have eliminated problems with the coupling cap from V1, 6 to V2, 2, I'd now look at your grid load resistor at V2,2.

Rather than run the long ground wire to your star ground at the RHS of the chassis, ground the 100K to a solder tag under one of V2's mounting nuts.

Just done this- still same (I swapped this resisitor from 470k to 100k fwiw too). Ive surely eliminated all avenues now? on a different track, I do notice valve 2 is not as 'iluminated' as the others, ie the friendly orange twinkles not as strong.. but as it works 'fine' as stock 5e3, and the voltages checked innitially here seemed in right ballpark i dont suppose this is important in context.

[MWJB]

No, the amount of glow from the heaters is rarely sign of a problem (unless they don't light up at all), even valves of the same brand and type don't always appear to glow the same.

OK, keep going through the preamp, you've eliminated V2, 2 & V1,6. Now look at V1, 7 - what resistance do you get, from middle tab to ground, on the volume pot turned when all the way down? Repeat continuity checks from V1, 1 to treble cap. Then look at the signal path from inpiut jacvks to V1,2.

I know you have done this before, but you MUST have missed something, otherwise the amp would work fine.

[The Captain]

OK, keep going through the preamp, you've eliminated V2, 2 & V1,6. Now look at V1, 7 - what resistance do you get, from middle tab to ground, on the volume pot turned when all the way down? Repeat continuity checks from V1, 1 to treble cap. Then look at the signal path from inpiut jacvks to V1,2.

I know you have done this before, but you MUST have missed something, otherwise the amp would work fine.

Hi Mark, just whittled the above checks off:

1. vol middle tab to gnd= 2.2 ohms
2. V1, 1 to treb cap (680pf)= 0.5 ohms

And continuity from top to toe on both inputs jacks (only one pair in now) as per hoffman pages. Dummy jack inserted:

1. tip to gnd on plug.. #1= 1m. #2= 135k.
2. tip to 68k junction on board.. #1= 34k. #2= 68k.
3. 68k junction to V1, 2= 0.5 ohms.

[MWJB]

Well that all seems in order. You're getting no signal from the guitar at all?

Measure the 6.8K middle resistor where it meets the bass pot tab, to ground (chassis).

[MWJB]

Have you tried installing a negative feedback loop as per the PR? Look at how the cathode at V2, 3 is wired. I suppose, in the absence of a better suggestion, that the additional gain/layout might be making the amp unstable and some NFB might be of benefit? Be aware that if you fit a NFB loop you may need to reverse the blu/brown wires at 6V6 pins 3.