Marshall JCM 900 4501 mods - advice please

carl999pop

Hi

Picked up a nice and cheap 4501 to as a reasonable portable amp for gigs when I can't bring a half stack. I used it for a couple of gigs over the weekend and found the following;
- The amp sounds very bright and even harsh, not much low end and the mids sounds ...hmmm..."honky"
- Ch 1 sounds great when dimed or almost dimed
- Ch 2 sounds very "fizzy" and didn't work that well with pedals in front
The amp seems to loose "ooomph" now and then and I could clearly here comeing and going....odd

I have planned to do the following:
- Service + re-valve. Should I get L6L's or 5881's? or can it be modded for El34's? What's the benefit of this?
- Maybe change the speaker (G12 75) to something a bit "warmer: and "looser" sounding

I understand there are other mod's out there and I'm keen to learn what could help me make the amp sounds less harsh, warmer and Ch2 less fizzy. also looking for good tech in London who can sort the amp out.

Thanks!

ICBM

Yes, have it modded for EL34s - they sound much better and lose some of that thin fizziness, and generally just sound more 'Marshally'. You will need to change the screen resistors and a resistor in the bias circuit, but it's pretty straightforward.

The V30 and Classic Lead 80 are the best speakers I've heard in them, but you might want to consider the new Creamback M65 or H75, since they're in the right power range.

The volume drop is most likely caused by dirty contacts in the FX loop jacks, although occasionally the High/Low power switch can cause trouble as well - and arc, in bad cases.

The two channels are actually essentially pedal circuits - it's a hybrid amp really, although don't let that put you off straight away! Channel 1 is something like a Tube Screamer/Boss SD-1, channel 2 is something like a Rat/Boss DS-1. So they can be a bit fussy with which pedals sound best into them, in the same way as pedals can be when stacking in different orders.

carl999pop

Hey
thanks for advice and input! think I'll go down the EL34 route.
I love V30's but wil check out the Creambacks.
What do you think of Svetlana Winged C's in the poweramp and JJ ECC's in preamp?

ICBM

I like Winged Cs a lot, but not JJ preamp valves really. Although in an amp without enough midrange and with too much treble they may be a good choice.

carl999pop

that was my thinking as I find them a bit "darker" than other valves

Cheers - very useful!