Amp Crackle

[Littlewyan]

Got an odd issue to run by you all. My amp it has a weird crackle as soon as I drive it into distortion. Literally on the edge of breakup it will crackle when I hit certain chords. Back off and it’s perfectly clean.

The amp has two channels, a TW songwriter preamp and a Fender normal channel preamp mated to a 6V6 cathode biased output stage with a LTPI. It happens on both channels so I know it’s in the power amp. If I drive the amp harder it doesn’t get worse per say, I think the distortion masks it.

I’ve tried:
- Changing valves
- Changing output valve sockets
- Different Cab
- Reflowing solder joints

I’ll get a recording of it when I can. I’d say the crackle isn’t directly related to the amp’s distortion, it sounds unrelated to the notes I’m playing, almost like a bad connection. But I can drive the amp flat out no problem. Any ideas?

[R.G.]

Maybe speaker vibration/sound level affecting a contact or loose joint, or socket contact?

[romberg]

Got an odd issue to run by you all. My amp it has a weird crackle as soon as I drive it into distortion. Literally on the edge of breakup it will crackle when I hit certain chords. Back off and it’s perfectly clean.

The amp has two channels, a TW songwriter preamp and a Fender normal channel preamp mated to a 6V6 cathode biased output stage with a LTPI. It happens on both channels so I know it’s in the power amp. If I drive the amp harder it doesn’t get worse per say, I think the distortion masks it.

Your description matches the symptoms I had with an express preamp into a 6sn7 power amp pretty much exactly. In my case, I was able to observe the noise as a very short lived oscillation on the PI plates. With a scope, I could see on the PI plates, a small higher frequency burst on the waveform just exactly at the point the power tubes started to clip. The wiggle lasted maybe 1/16th of a wavelength and maybe had an amplitude of about 1/10 or 1/20th the main signal. It was pretty small. But you could hear it pretty well when notes decayed. Heavy overdrive tended to mask it. I'm guessing it was caused somehow by grid current on the power tubes.

I *fixed* it by increasing the fizz cap across the PI plates to around 100pf or so (don't remember exactly). So, the same might work for you. Try adding or increasing the value of the fizz cap across the PI plates and see if that works for you.

Mike

EDIT: Found the thread: https://ampgarage.com/forum/viewtopic.php?f=6&t=27369

[Littlewyan]

So I've narrowed the issue down a bit more. The amp has a Vox cut control in it, which if you think about it can act as the capacitor between the PI plates. I turned this all the way up, full cut and while the guitar tone became very muffled, the crackle was unaffected and became very clear. So the issue must be occuring post PI, given it's not affected by the Cut control and isn't affected by volume (except for the fact it only occurs when turned up enough to distort the power amp).

I'd also say it's not affected by vibration as I have an attenuator I use and even on the lower setting (below talking level) it still occurs.

[Littlewyan]

Here’s a recording with cut on full so you can hear it clearly. It sounds like static on top of the notes.

[Stevem]

Please post up 3 or 4 good photos of the pi and output section of the amp.

[Littlewyan]

Here you go.
Given it seems to be occuring post PI there really isn’t much left. I’ve already swapped out the 6V6s and the valve sockets. So that really leaves the power supply, OT, grounding and any wiring inbetween.

[Littlewyan]

Also used my scope to check for oscillation and I can't find anything. And I tested again with the cut control, it is affected by it but only very slightly.

[Stevem]

I am curious about the condition of the OT.

If you pull out one 6v6 at a time does the issue still take place.

Note that with only one tube in the amp will distort very early.

[bepone]

where is speaker gnd connected?

[Littlewyan]

I’ll have to wait until next week to test the OT, will need to replace the bias resistor to test otherwise I’ll be running a single 6v6 a bit hot! I have checked the speaker output with my scope though and it looks perfectly normal, distorts pretty evenly on top and bottom of the waveform. Also checked voltages and current on the 6v6s under heavy drive conditions, both sides seem to be normal.

Speaker ground is connected to the PI ground. Connects to the 4.7K resistor on the board.

[bepone]

is it possible to make a oscilloscope recording during live playing those riffs where you can hear the crackling?
like rec, and then zoom in from scope memory, to see if ther is a sign in waveforms for oscillation bursts?

if you did oscilogam on resistor load, this means nothing, only reactive load (speaker) is valid for that troubleshooting..on resistor load all amps are fine

how i hear can be crossover distortion, bad bias, or unsimmetric clipping in output stage.. if you didnt say anything i would assume it is one preamp tube /stage in unsimmetrical cliping (only one half and only little bit in clipping) and all other in clean

[alnight]

I have encountered a similar issue that turned out to be a coupling cap. Quite frustrating. The cure ended up being just a slightly higher value.

[Stevem]

This just dawned on me here !
That Vox type cut control with that caps basic placement across the PI plates is at high volumes getting hit with the full audio swing from both plates.

Caps have a ac voltage limit also, not just dc, and this ac limit is about 1/2 the caps dc rating.

In a nut shell here you may be really overdriving that cap your using in that cut circuit!

[samtech60]

When I have crackling problems I usually substitute even the preamp tubes as well as power tubes . The output transformer I usually would condemn for last .

Check all solder joints and connections throughout the signal chain even tube sockets clean them with deox spray clean up all the pins on the tubes first and re seat in socket. If you still have not repaired it coupling caps are rare but possible so for the cost of a few dollars you can just simply replace one at a time . I hope this helps