JMP 50 Popping Sound

[Riffmonster]

Thanks!
I changed both silver mica caps and the 680nf cap but nothing changed. The wire going to pin 3 of V2 is definitely microphonic when lightly hitting it with a chopstick.

I wish I had an oscilloscope… any other ideas? Otherwise I might have to surrender and bring it to a tech. It’s a shame because the amp sounds absolutely great.

[bepone]

try to put in series some 10k close to pin 3 , V2 valve, if something will change? try to put 100pF 500v in parallel with all 100k resistors in V2 circuit, guessing there are oscillations ang guessing that 100pF will kill them

[bepone]

also gnd from pots reroute on cathode resistor gnd, on the other side like on pic

Screenshot_20220625-211826__01.jpg

[Riffmonster]

Thanks a lot for this tip. I put a 10k resitor at pin 3 and it reduced volume of the pop a bit! I have to check if I still have some 100pf caps here. If not, I will order them. Can I leave the 10k in when I add the two 100pf caps?

EDIT: I only have two 220pf silver mica, still worth trying?

[bepone]

it is not good to put silver mica on high voltage line, maybe it will fail.. but in case you want to risk ok . some ceramic will be perfect.. you can leave yes permanently grid stopper 10k, and also increase it to 33-47k and see what is the result! if you kill rhe provlem but also start to loosing highs then you can roll over to the smaller values

[Riffmonster]

I just shortly put the silver micas on the 100ks but that did not change anything, so I removed them again. Increased the resistor at pin 3 to 33k but not much difference either. The pop is still there but as I said at a lower volume now.

[wpaulvogel]

I use old Mullard I61 and I63 preamp tubes in my Marshall builds. From time to time I find a tube that crackles at power on. Sometimes it goes away and sometimes it persists. I also notice sometimes the new amp has some tonal behavior that seems off. I play the amp for a while before making any changes. Usually about 4 hours. Have you played it for any amount of time yet? Switching out the crackling tube corrects the issue and playing sometimes corrects it also. I call it break in. It’s really takes 50-100 hours to break in certain components like output transformers.

[Riffmonster]

Interesting! I have maybe played for an hour or so only, do maybe I let it rest a bit now and just play. Also, the pop tends to appear less frequently when the amp is warm after like 5-10 minutes.

[statorvane]

Are you sure those resistors in the image annotated by bepone aren't touching? Any chance the enamel on them or the cap insulation is chipped?

[Riffmonster]

I think they are touching, yes. I will change it and check if the insulation or enamel is damaged.

Btw, the trick with 10k on pin 3 of the V2 only worked because it reduced the overall volume of the amp. That’s why the pop seemed less loud too…

Thanks all for your help!!

[bepone]

hmm. lets try one more thing. maybe the pot is having a bad contact.
lets put 2M2 or 1M even better to bypass pot, and see if will be some improvement..

otherwise maybe 100k in anode V2 is faulty? with the stick one -two hits on it and see if will create this pop

Screenshot_20220625-211826__02__01.jpg

[bepone]

also will be good in future investigations put voltmeter in all high voltage nodes and see if there is some drop of voltage (increase of current). then will be possible to isolate circuit where can be found some discharge from high voltage line to zero. maybe faulty socket, tube, electrolytic cap flashing to gnd or heater line

[Littlewyan]

What valve are you using for V2? As most russian valves can't handle the high heater to cathode voltage. I've had weird issues with this in the past. Once I had a problem where an amp would motorboat when I turned it off and it was the valve in V2.

[Colossal]

What valve are you using for V2? As most russian valves can't handle the high heater to cathode voltage. I've had weird issues with this in the past. Once I had a problem where an amp would motorboat when I turned it off and it was the valve in V2.

Good point. The older 12AX7 datasheets list h-k (heater to cathode voltage) limit as 200V. New tubes are 100V. I always elevate heaters to a DC reference for any amp with a cathode follower (and most any amp, because it's easy to do and only takes a few parts).

[bepone]

this was my first test here but with the lover voltage (reducing cathide follower resistor value), Riffmonster reports that popping is worst. tube replacement can give more info