Bare Knuckle Pickups Forum

Forum Ringside => Tech => Topic started by: Antag on March 28, 2010, 04:50:29 PM

Title: Marshall valve assignments
Post by: Antag on March 28, 2010, 04:50:29 PM

Hi All,  quick question about the JCM900 SL-X (http://bareknucklepickups.co.uk/forum/index.php?topic=20645.0) I recently acquired.

Looking at the valves in this picture left to right:

(http://i252.photobucket.com/albums/hh23/george204/BKP/th_SLX_int.jpg) (http://i252.photobucket.com/albums/hh23/george204/BKP/SLX_int.jpg)

I'm guessing that V1 & 2 are gain valves, & V4 is the PI but what about v3?  Do Marshalls have valve-driven FX loops?  or another gain stage?

Title: Re: Marshall valve assignments
Post by: Twinfan on March 28, 2010, 05:04:32 PM

3 gain stages and a PI I reckon.....

Title: Re: Marshall valve assignments
Post by: Will on March 28, 2010, 05:16:02 PM

I had the 2100 std, I think the SL - X was Superlead Extended, with the diode clipping replaced by extra valve clipping

Title: Re: Marshall valve assignments
Post by: hunter on March 28, 2010, 06:08:04 PM

maybe the loop?

Does the amp still work if you pull V3?

Title: Re: Marshall valve assignments
Post by: FELINEGUITARS on March 28, 2010, 06:15:51 PM

Quote from: Will on March 28, 2010, 05:16:02 PM

I had the 2100 std, I think the SL - X was Superlead Extended, with the diode clipping replaced by extra valve clipping

That is correct
The JCM900 came in for some criticism that they had a diode clipping stage in the distortion chain - so Marshall came out with the SLX- I think the first two valves are 3 or 4 cascaded gain stages- v3 is the tonestack and V4 is the PI

Maybe someone could affirm or clarify this?

Title: Re: Marshall valve assignments
Post by: Antag on March 28, 2010, 09:06:29 PM

Can anyone make sense of this schematic (http://www.schematicheaven.com/marshallamps/jcm900_slx_100w_2100.pdf)?.  Damned if I'm any the wiser...  :?  :lol:

Title: Re: Marshall valve assignments
Post by: HTH AMPS on March 28, 2010, 11:23:24 PM

V1-3 are all gain stages (ish, but lets keep this simple).  The tonestack is fed in the usual Marshall way from a dc-coupled cathode follower from V3.  Then you have the opamp-driven effects loop, then V4 is the PI.  So you have four gain stages.