Plexi crackling (moisture?), V2

hoss33

Member
Messages
610
Hi guys,

my self built Metro 50W Plexi started a crackling/frying bacon noise after about 5 years. The noise goes away when I pull V2.

It sits in a basement that has high humidity during the summer.

The noise is there even with the amp volumes all the way down and no guitar cable attached.

I have swapped out all tubes, opened the amp and checked for visual signs of problems (no corrosion) and cleaned the tube sockets.

The plate resistors are NOT of the carbon composite type (these tend to collect moisture).

Any ideas?

 
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WaltC

Member
Messages
2,126
doesn't go away when you pull V1? (just checking)

could be the tube, have you tried replacing it? Those circuits in the second position (V2) can be hard on tubes and not all 12AX7s do well in them. I'd not use a 12AX7LPS or a Tung Sol re-issue 12AX7 in that slot, I've had good luck with some of the Chinese 12AX7b's and Sovtec's 12AX7WA tubes in that position.
 

DaveK

Member
Messages
483
Have you cleaned the contacts of the input Cliff jacks ? When they get a little corroded, they don't shunt to ground and cause noise.
 

hoss33

Member
Messages
610
Thanks guys.

It goes away when I pull either V2 or V3, not V1. As I wrote earlier I have swapped all tubes, V2 with four different ones.

EDIT: I got a tipp concerning high voltage bleeding into the mix resistors through the board. Also these resistors are CC on my board. Going to check that out today.
 
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pdf64

Member
Messages
7,770
Does a shorting link between the mixing resistors junction turret and the adjacent ground (for V2 cathodes) stop the noise?
If not then the V2 plate resistor (100k mounted on the tube socket) seems a likely suspect.
Pete
 

hoss33

Member
Messages
610
Does a shorting link between the mixing resistors junction turret and the adjacent ground (for V2 cathodes) stop the noise?
If not then the V2 plate resistor (100k mounted on the tube socket) seems a likely suspect.
Pete
This guy was my first suspect. I will try the jumper. Thanks!
 

pdf64

Member
Messages
7,770
Sorry, forgot to ask, do the tone control settings affect the noise?
I assume so but if not then we may need to look elsewhere.
Pete
 

hoss33

Member
Messages
610
leaky silver mica cap in the mixer or tone stack?
I have a new mixer (two new metal resistors and new cap) as well as a new 100K plate resistor prepared and am heading to the band room :)

Update later. Thanks, guys.
 

hoss33

Member
Messages
610
Live from the amp room:

Treble actually does increase the noise.

Grounding the mixer doesn't change anything.

Putting a new plate resistor into V2.
 

hoss33

Member
Messages
610
Problem solved but I don't know why (or if it won't come back).

What did I do?

I connected various points in the EQ section to ground to see if it makes a difference.

After I touched the 3 lugs on the treble pot the noise suddenly stopped.

The amp is now absolutely quiet (and all pots work as they should).

:huh
 

hoss33

Member
Messages
610
maybe reflow the solders.
I have hit all connections with a plastic stick but that did nothing.

After touching the treble pot lugs with the ground jumper cable the noise went away. I can't say which lug it was.

Can one of the three caps connected to the treble pot be the culprit? Can a pot become capacitative?
 

pdf64

Member
Messages
7,770
Maybe a pot is bad, eg the crimp of a terminal lug at the track is slightly loose, or it's too tight and the track underneath has crumbled due to the compressive force. Or just a bad contact between wiper and track.
If so, most likely the treble pot but it could be one of the others, from the shock of the 'tap' being transmitted along stiff interconnecting wires.
Were the controls tweaked once in a while, or did they remain at one setting?
Maybe a squirt of quality cleaner / lube will keep the problem away.
Pete
 




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