Author Topic: Amp biasing ? jcm900 sounds cold / brittle (Read 5004 times)

Will · « **on:** August 12, 2007, 05:28:48 PM »

So is this what I need then?
its fine at low levels, but then when I crank it to above 2 or 3 it loses all the warmth
suggestions where to have it done aswell please and how much it will cost?
I live near Aylesbury / MK - would marshall be expensive

indysmith · « **Reply #1 on:** August 12, 2007, 05:47:43 PM »

Marshall revalved, rebiased, and changed the pots on my marshall jcm800 for a very reasonable £83.
Go for it.
Also - if they're old speakers, you might want to try some others. made the world of difference to me

Will · « **Reply #2 on:** August 12, 2007, 05:55:45 PM »

How many tubes was that for?
Speakers should be new, the cab isn't too old

Woogie · « **Reply #3 on:** August 12, 2007, 08:07:14 PM »

Indysmith, howcome you changed the pots? Did it affect the tone much?

Cheers

HTH AMPS · « **Reply #4 on:** August 12, 2007, 09:32:38 PM »

If you want more balls from your 900 get some 6550s in there.

:twisted:

indysmith · « **Reply #5 on:** August 12, 2007, 09:51:47 PM »

pots were changed because they were old and scratchy. the tubes that were changed were 2 el34s and 3 12ax7s

Will · « **Reply #6 on:** August 14, 2007, 11:37:25 AM »

marshall won't fit 6550s
anywhere else that would?
how much more expensive are 6550s going to be?

the_bleeding · « **Reply #7 on:** August 14, 2007, 09:33:44 PM »

jcm900 typically comes stock with 5881's or 6l6's. It was designed around the el34, and then after marshall released it, they stopped making el34's, so marshall quickly switched them over to 5881's. I have the same issue with my 30th anniversary. Get some el34's in there.

HTH AMPS · « **Reply #8 on:** August 14, 2007, 10:03:28 PM »

Quote from: Will

marshall won't fit 6550s
anywhere else that would?
how much more expensive are 6550s going to be?

Any decent tech that knows his arse from his elbow will be able to do this. I remember when Marshall switched to 5881s - the supply of EL34s was a bit poor and the quality was terrible. The Sovtek 5881s were (are) very sturdy valves, so naturally lots of amp manufacturers started using them. Problem with those 5881s is that they're very average sounding - almost any 6L6 will sound better.

If you hunt around, you should get 6550s on ebay for around £60 for a matched quad. check these out, I've used this seller on ebay and he's a cool guy (seems to know his valves)... http://cgi.ebay.com/Matched-Quad-of-TUNG-SOL-6550-Tungsol-KT88-Power-Tubes_W0QQitemZ120077209734QQihZ002QQcategoryZ121164QQcmdZViewItem

The Tung-Sol 6550 is cool because it's got the midrange of an EL34 and some of the same breakup character - not so harsh as other 6550s can be to some ears.

:twisted: