Marshall Valvestate 80V Pop Sound

[misterc57]

After fixing the clean channel and getting some play time on this amp, I notice an occasional popping sound. So far I have only noticed it on the gain channel. Happens once every 3 to 5 minutes.

Similar to but not as loud as the pop the amp makes when being shut off.

If I tap on the on the amp top or sides it does not happen.

Pots had been cleaned. Solder connections on bottom of board looked good.

I know this is a tough one to diagnose since I cannot recreate it. Any ideas?

Thank you. MC

[Jon Snell]

Try a new valve first.

[J M Fahey]

IF it always happens after 4 or 5 minutes, scope the output to be certain you catch it and also see shape, height, etc.
And wait 4 or 5 minutes

Then repeat going backwards, towards input jack.

Slow but safe.

Eventually youŽll catch the stage causing it.

OR

it is not an amp problem at all, maybe it catches some ioutside intereference , remember gain channel is way more sensitive.

Does it do it plugged somewhere else in the house?

[misterc57]

Still happening. Pulled the chassis, replaced the valve, played for 20 minutes on the bench.

Seemed good and put the chassis back in the cab. Played again for a couple minutes then again a pop, few seconds later another pop. Another few minutes of playing no pops.

When chassis is out, playing on bench, there is no reverb tank connected. I am also connected to a different speaker cab. Not to say it could not have popped when on the bench,.

JM, I will try another location in the house. So far I have only heard the pop in OD2 mode. I have limited scope experience. I could try putting the output on the scope. Not sure it would happen when I am not playing through it, and if playing through it I would have trouble catching it on the scope. Maybe if I scope it at idle? Also not sure how I would see or save the scope image when it happens, maybe there is a scope feature that does that?

Aside from this muffled pop, the amp seems to be working well and sounds great!

[Jazz P Bass]

You need to find where it is happening (or not)
Try going in to the FX Return jack.
That should bypass a whole lot of the front end.

[Richardvet]

Hi, sounds like voltage spikes in the mains supply. Do you have another amp you can turn on at the same time? If both pop then it’s the household mains supply. If your second amp doesn’t pop then more likely to be a problem with the first amp. Marshall valvestates need a good quality 12AX7 valve. Cheap and cheerful 12AX7s don’t cut the mustard. I use a Hughes and Ketner one, which solved many background noises in my valvestate. Be aware that the two channels have different background noise levels as a norm in these amps. Another possibility is a broken/partial ground in the guitar cable, or a weak solder joint in the input jack onto the board ... both of which will be random but more likely as you move the cable.

[misterc57]

Lots of good advice, thank you all. More trials..

Marshall effect "SEND" signal to another amp's (power amp in). Effect knob to max dry. Pop in "other" amp.

Other amp "pre-amp out" signal to Marshall "RETURN". Effect knob to max wet. Pop in Marshall amp.

Played second amp alone on same power strip. No pops.

Played Marshall alone. Second amp turned ON, input cable plugged in to keep second amp active. Both on same power strip. Marshall popped. Second amp did not pop.

Played Marshall on CLEAN channel. Pop.

I have not tried a different house location yet but this location is where I test all my amps. Only this Marshall is having a pop noise.

In summary, Marshall is popping regardless of SEND or RETURN effect in use. Marshall is popping on both CLEAN and GAIN channel.

Wiggle input cable/jack on Marshall does not cause issues.

Maybe I should reflow all the heavy 5W resistors on the board. Tough none act loose.

[Helmholtz]

Could be a bad power supply filter cap. Those things are chemical reactors and if electrically leaky, internal temperature and gas pressure rise. Eventually the seal breaks and they blow out.

[misterc57]

Could be a bad power supply filter cap. Those things are chemical reactors and if electrically leaky, internal temperature and gas pressure rise. Eventually the seal breaks and they blow out.

Probably the original caps in there.

So the 2200 uF/50V C61 & C62? What about the 1000 uF/16V C40 & C41? Any others?

Schematic attached. Thank you


Marshall valvestate 8080_8100.pdf

[Helmholtz]

So the 2200 uF/50V C61 & C62? What about the 1000 uF/16V C40 & C41?

Yes, these 4 are the prime suspects as they impact preamp and power amp.

[Enzo]

The fact other amps do not pop doesn't tell us much. Not all amps are sensitive to outside noise. I had an amp on my bench once that worked OK, but about once every 90-100 seconds it made a funny hum for about 10 seconds. I went nuts trying to find out why. The amp seemed OK, but there was nothing in our building I could find that corresponded. NO furnace or blower coming on, no neon signs, nothing. Then I stepped on my front porch for a breath of air, but left the amp running, and I could hear it. I happened to notice that the noise was occurring only when the traffic light at the corner was yellow for one direction. A noisy triac in a traffic signal a half block distant was being picked up by this amp. Nothing else had ever reacted before.

[misterc57]

Yesterday morning I reflowed the 4 power supply filter caps. 30 minutes of playing both yesterday and today and no pops! Hopefully that was it.

Thank you to all for the help! MC