#1
Hey guys, I've got a bit of a problem with my JCM 900 MKiii first off, after about 20 mins. of usage, my amp "crackles" when I play single notes and when I'm playing powerchords. could this be the amp vibrating the gain sens. knob? (this is a bit scratchy, also the reason I think this, is because that knob is quite loose and turns "faster" than the others probably due to wear 'n' tare).
Or is it the power valves showing signs of age? or is it them being moved sliglty by the vibrations?
they feel quite loose but don't fall out if i remove the retainer. (i get confused between power and pre amp valves, im talking about the EL34's.)

secondly, I think I may need a valve swap, it looks like the amp still has its stock valves (they say marshall on 'em). Any recommendations on brands?

And finally, does anybody based in the U.K know of any amp techs (near cardiff, south wales) and how much it'd cost to get this is amp serviced (replace the scratchy pots, give it a re-bias and re-valve (if it needs it, I aim to replace the valves with matched sets) and give it a look over for any other obvious problems?

thanks for reading, i tried to explain the problems the best i could
gear
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#2
Cracking is 'usually' indicative of a bad preamp tube. At a minimum, you can try swapping some of them around (smaller 12AX7s) and take notes of what happens. Second, you can buy a new preamp tube online and do the same thing.

It could also be a ground, pot or solder issue.

As far as EL34 power tubes go, I'd look to either some Mullards or JJs. Then Sino, Winged =C= and Tung Sol. I have a similar amp with Mullards for power and JJs for preamp. Darkish tone. Contact hotroxuk.com and see what fits best for you tone wise and cost wise. I doubt the crackling is coming from the power tubes btw.


here....this might help a bit too...

http://s545.photobucket.com/albums/hh384/buckethead_311/Amp%20stuff/?action=view&current=Microphonicpreamptube.flv

I also have a biasing Blog in my profile if you want some basic run down stuff.
#3
Cool, thanks. I should also mention that there seems to be a bit of a volume issue.
I can play the amp with the master vol. around 5, the pre amp vol full and the gain at about 15-17 (5-7) and comfortably play in the bedroom.
But when i set it up for a clean sound (master 5-7, pre amp vol 2-3 and gain at 0), theres a considerable vol. drop. Is this a valve issue also or just becasue i've lowerd the gain and pre-amp vol? if so which valve/s seem the likely candidates for this "diagnosis"?
gear
Fender highway 1 tele
v
Korg pitchblck
Zoom Uf-01
EHX Big Muff
EHX Russian Muff
Made By Mike fender blender
Mi Audio Crunch box
Malekko Ekko 616
Ibanez CS9
BYOC Flange
v
Fender 75

welsh?

FOR SALE: PEDALS