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random crackle ? (McMartin MA-20)

the fuhz

Member
2008-02-06 6:49 pm
Hello all,
This is my first post on your forum. Im new to the tube amp scene.
I found your site while researching an amp that I have: McMartin MA-20 (20 watt tube PA amp). I am having some issues with the amp, and Im hoping that someone can help...

Anyway, the amp essentially works as it should. All tone and volume knobs are fully functional. However, there is a consistant 'crackle' that is occuring. It is very similar to the sound that a dirty pot/tone knob would make when being turned. I have cleaned everthing, replaced all of the tubes, and the 'crackle' still occurs. I dont know what else to try, any ideas?

Another problem is that there used to be a bad buzzing noise coming from the amp. I learned that when I turned down the volumn knob, the buzzing would die down as well. I assumed that one of the two pre-amp tubes were bad, so I replaced them. However, I was only able to solve the problem when I removed one of the two preamp tubes from the circuit entirely. So now, Im just running the amp with one 12ax7a tube and there is no more buzzing. This cant be normal/good for the amp. Any idea what this means??

BTW I noticed that a member by the name of 'Porkchop61' has one of these same amps and supposedly did some guitar mods to his. Would love to see what he did.

If anyone has any ideas or suggestions to these problems, please let me know. thanks, tf
 

Shoog

Member
2002-08-15 10:16 pm
Eire
The crackling can be caused by poor valve to base contact. You may find that using a pair of pliers to close the socket holes up a bit may solve the problem. The crackling in this case would be caused by arcing between the valve and its base.
Give it a try.

Shoog
 
Hi Mr fuhz :)
Here is what I did to my MA-20.
McMarting MA20 modified for guitar
The schematic is not complete, as I changed mostly the preamp section. I did add bias pots and 1 ohm bias resistors to make it easier to bias the amp. I think I also added a decoupling network to the B+ for the preamp.
I also got rid of the 6AV6 tube and replaced it with another 12AX7. The 6AV6 tubes are very microphonic.
The preamp section is a copy of the "High-Octane" from AX84.com
NICE preamp overdrive from this setup!
This is a pretty good sounding amp with these mods. Mine also came with vintage GE 6L6GC tubes as a bonus:D
Good luck.
Glenn