Amp Static Crackling noises SOUND CLIP ATTACHED

[Ampzone]

Attached there is a sound clip of some crackling static noise with a tube amp, you can hear the static start once the amp get warmed up, so it may be a thermal or voltage issue. It is a Marshall plexi EL34 style amp. The coupling caps have all been tested and nothing appears loose. Screens look & test good. Tube swaps make no change nor do cable swaps. The clip has the same notes played over an over to get it to act up. Any help is super welcome.

[Enzo]

Ball up your fist and whack the top of the amp. Does that also make it crackle? That exposes loose connections.


How did you test the coupling caps? If you used a hand meter to measure the capacitance, that does not make a useful test. Noise like that does not come from a 0.02uf cap that measures out as 0.03uf. It might comne from a cap that leaks at voltage. SO you would have to do a leakage test at B+ voltages.

I'd be more inclined to think it was a loose socket or a broken solder connection. But it could also be as bad as an arcing output transformer

But do the basics, try a different speaker and cord.

[Ampzone]

Ball up your fist and whack the top of the amp. Does that also make it crackle? That exposes loose connections.


How did you test the coupling caps? If you used a hand meter to measure the capacitance, that does not make a useful test. Noise like that does not come from a 0.02uf cap that measures out as 0.03uf. It might comne from a cap that leaks at voltage. SO you would have to do a leakage test at B+ voltages.

I'd be more inclined to think it was a loose socket or a broken solder connection. But it could also be as bad as an arcing output transformer

But do the basics, try a different speaker and cord.

Thanks for the input. I replaced all the coupling caps one by and no change. Using the chopstick method and fist banging does not change the sound you heard. Harder playing makes it kick into the noise.

The output tranny measured good in my meter. No shorts that I can detect. Hmmm

[Ampzone]

The output transformer secondaries read appx 19ohm's from each plate lead to the center tap. The primaries read megaohms in resistance. I'm sure there are more involved test for arcing transformers, not sure what else to try for an output. The power supply filter caps give good readings with the cap tester. This is a bear of noise to track down.

[R.G.]

oscilloscope

[Jazz P Bass]

The output transformer secondaries read appx 19ohm's from each plate lead to the center tap. The primaries read megaohms in resistance.

I am not too sure what you are reading.
On a 'typical' output transformer, push pull, the Primary will read 100 some ohms from each plate lead to the center tap.
The Secondary is usually such a low value (1 ohm or less) that most meters cannot read the true value.
(if you must know the actual secondary resistance you can run a something K resistor in series, & then subtract that resistor).

[Joe L]

Move the speaker cab far enough away so there is little physical interaction with the amp. If it still does it just as much, its probably electrical as in an arcing tube, arcing transformer etc. Otherwise, it is a physical connection or a part that is failing microphonicly.

[Ampzone]

Move the speaker cab far enough away so there is little physical interaction with the amp. If it still does it just as much, its probably electrical as in an arcing tube, arcing transformer etc. Otherwise, it is a physical connection or a part that is failing microphonicly.

No scope I'm afraid. I will try another set of tubes. The total ohms for the plates is appx 50ohms. From center to plate it's about half 20-25ohms. The
Trash and buzzing comes when you dig in and push the amp. It sounds like something is loose but tapping every part of the circuit reveals nothing.

[Ampzone]

Ok, some updates. Since the beginning the noise takes about 5 min of hard playing to act up. A new test revealed (with the lights dimmed very low) one output tube was glowing blue way brighter than the others and lit (blueish flash) up much more when you struck a chord. So I replaced this tube with another and the noise stopped. I switched back, the noises came back after it warmed up (5min hard playing). I switched around about 5 times to confirm this was the culprit. The last time I did the switch with the new tube I saw a bright flash after playing for a few minutes, there was slight smell of carbon burn but the fuses did not blow. I switched the amp off and checked around this tube socket, the joints are tight, the screen resistor reads 1k and no sigh of burn, so I tightened the socket pins for a tighter grip as it was quite loose with the replacement tube. I played again and no issues, no sparks.

The only thing I notice is the bottom round disc inside of this tube (replacement tube) in one section a metal rod tip glows very bright blue when you play, the area about the size of a 12awg wire tip, the other 3 tubes do not glow blue in this particular spot. I would think the issue was solved after putting this replacement tube, but that one bright spark flash and this blue area glowing differently than the rest after tightening the socket pins has me concerned. It does however not make any noise as before now with the replacement tube.

Note the tubes are badly matched the replacement tube is around 20ma and the highest drawing tube is around 37ma, 32ma, 33ma with 450 on the plates. Could this be the source of much higher blue glow on the rod by the paltes.