plexi cap tweaks...

[HTH]

been messing with my '76 Superbass this afternoon to get it tuned in a bit more to the way I 'want' it to sound.

the circuit is pretty much the old plexi one from the '67 era. I've bypassed the cathode of V2a with a 220uF cap - this gave quite a bit more overall gain.

I also lowered the screen filtering to 16uF and the PI filtering to 32uF - this removed the overbearing bass and stiff feel. it's now brighter and more mid/treble focused (I still need to run the bass around '2' though). It's much closer to the vibe my old '67 plexi had - I'm much happier with the amp now.

The feedback is directly connected to the output jack - I like this hookup alot more, the amp isn't as loud and it feels softer in the treble.

Just thought this info might be usefull to those wanting to mod their Marshalls to plexi specs.

[dai h.]

hey Ian if you have some really good sounding originals, maybe you could A/B and try to mod one so it sounds like the good one then document your efforts if not to post at least for yourself so you can mimic the well performing ones with other amps. Also, the big cap on V2a cathode--I think this is useful but you don't get too much bass?

[HTH]

Originally Posted by dai h.

hey Ian if you have some really good sounding originals, maybe you could A/B and try to mod one so it sounds like the good one then document your efforts if not to post at least for yourself so you can mimic the well performing ones with other amps. Also, the big cap on V2a cathode--I think this is useful but you don't get too much bass?

Dai, I've modded quite a few different era Marshalls to get them sounding right and it really does depend of the particular amp.

with a JCM800-era 1959 I had, the 250uF cap bypassing V2a was WAY too much and it sounded farty (47uF was perfect for that amp). it was also sensitive to the amount of feedback otherwise it sounded stiff - 56k on the 8ohm tap was spot on.

with this '76 Superbass I've got now it had way too much bass and sounded really stiff (lowering the filtering really did sort that out and has balanced the feel and tonal response). since it's already close to '67 plexi specs I decided to try the 250uF cap on V2a and it really is spot on, still got shared cathodes on V1 too. I've left the 100k feedback resistor in as I want plenty of breakup but have moved the feedback wire to the 16ohm tap - seemed to make the amp a little quieter and softer (still gonna play with this aspect)

this Superbass also has 82k bias splitter resistors so that's suppoded to make the amp breakup a bit easier too. must say I've changed these back to 220k on another amp I had and didn't really notice much/any difference.

bright cap on the volume pot is 100pF but it could do with a bit more as it gets a bit dull below 6/7 - maybe 250-500pF would be right.

[dai h.]

lowering the filtering should have helped reduce lows and the 82ks must help from having too much bass as well. I'm using 82ks in an MV converted to a later 1987 cct. with split cathodes, etc.

[HTH]

Dai,

I'd say you're right on both counts - I've never actually heard an amp's tone change so much from just lowering the filtering.

I've just took delivery of a new overdrive box too (MI Audio Crunchbox) and it's absolutely fantastic with the Superbass.