Weird crackling from amp.

[indysmith]

There's a strange sort of crackling sound that comes from my amp when I pluck more than one string. It's quite a quiet effect, below the actual amplified note, but it's really getting on my nerves as it is very noticeable (to me at least) strumming low-gain rhythm.
It sort of crackles when you first strike the note, and then after about a half second fizzles out completely very quickly. It's kind of like a really nasty clipping. It happens with more than one guitar, with more than one cable. The only thing I haven't ruled out is that it's the speakers, but I don't have a spare cab here, and I'd make a clip but I don't have any decent recording equipment since i'm staying at my gf's house.

Obviously my first thought was that it's a bad valve, but I don't know enough about valve amps to diagnose it. What do you think? Time for another service? (Last was about a year ago).

Thanks

[Twinfan]

Sounds like a faulty valve to me.  Try switching their positions to see if that helps, especially the preamp valves.  If it goes away/gets worse you know what the problem was/is....

[indysmith]

Sweet! Thanks Twinfan; I changed the positions of all of the preamp valves, and the crackling has gone. However, I have a slightly 'harder' sound, which i'm not really into.
What's a good preamp valve that I can replace the three in my JCM800 with? I'd like something very warm and low gain.

[Twinfan]

If you're after warm and low gain, I've heard a 12AT7 (in place of the standard 12AX7) can work wonders, especially in V1.

I'd try a 12AT7 of some sort in V1 and JJs in the rest of the preamp.  Should be less than £40 for that lot, and you can experiment by swapping them round a bit.  Of the newer production valves, I've found JJs to be nicest.

I don't bother with old valves as I can't be bothered spending a fortune messing about with different types.  I know what I'm getting with JJs.

[indysmith]

I was thinking 12AU7s were for lower gain. Does anyone know what's the difference between these and 12AT7s?
Also, how do I know which spot is V1?
(sorry I really don't know about the technicalities of valve amps - that's in the next year of my course!)

[HTH AMPS]

12AX7s (ECC83s) have a mu/gain factor of 100
12AT7s (ECC81s) have a mu/gain factor of 70
12AU7s (ECC82s) have a mu/gain factor of 30

I personally find 12AT7s in V1 (the first preamp stage) to be a bit lifeless.  I'd go with a 12AX7 that is know to be not quite so hot.  For example, most Chinese 12AX7s are very gainy whereas something like the JJ or Mullard reissue is less gainy (with better tone imo)

fwiw, I'd go with some reissue Mullard ECC83s, they're nice preamp valves.

[indysmith]

Well in that case maybe I'd be best off getting the deal that HotRox have going atm for a 50W JCM800 - 3xJJ 12AX7 and 2xJJ E34L all for £42. Seems reasonable :)
Thanks all!

[indysmith]

$%&#

I put the amp on today, and I've got all sorts of weird hiss and pops and clicks. Sounds like the valves are definitely dead!
I could take it to the Marshall Factory for a service again, but the valves they put in last time didn't sound as good as the ones that were in it before and they didn't last very long; besides, I'd really like the JJ's. I could buy the JJ's and then get Marshall to install them I suppose...

Does anyone know how to bias a JCM 800 (in simple terms)?

[Roobubba]

If you live anywhere in the south east of England (like between Kent and Oxfordshire), then you could take your amp to Steve of Steve's Amps.  He took a look over my Peavey 5150-II after I got it and installed my new valves for me. I didn't fancy biasing it myself, and frankly, he was great. Not sure if it was something he was going to carry on doing - he's really a full-time teacher, just does amp repairs/etc in his spare time!
About a year ago or so, his charge was 25 quid an hour, and it took less than that. I wouldn't like to suggest what his charge might be now, but for a professional opinion, that was good price, IMO.

Hope this helps,

Roo

[Will]

I agree with you about Marshall valves, they do often get people bring their own I believe.
and sorry, don't know of any amp techs around the area :(