Mark IV Modifications

Is there any available modifications for the Mark IVA or IVB ?

yanking out ceramic C11, c12, c31,c32 , tone stack caps swaping by polystyrene on the IVa leads to a great higths tone improvment !

What I am really inyterested in is a lot more gain and saturation in RHY2 without using a boost or overdrive. The clean and lead channels work perfect for me the way the way they are.

stephen sawall said:

What I am really inyterested in is a lot more gain and saturation in RHY2 without using a boost or overdrive. The clean and lead channels work perfect for me the way the way they are.

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there is a voltage divider between v1b / pin 6 and v2a / pin2, r1 (R101 3.3M) on r2 ( R362 680k) for the Rhy2 gain channel !

increasing R362 to 1.5M will increase gain by around 40% , i would test this , becarefull to drop in 1/2 watts resistor !

http://www.freeinfosociety.com/electronics/schemview.php?id=287

as other mods, to get a vintagier tone, i changed the choke with this one

http://www.classictone.net/40-18058.html

the regular PT and ot by schumacher are great , the real choke is a lil under powered 95ma 95ohms for a 3H even if it seems enough for regular plays.

with new choke, it does limit ghosting tone when the amp is pushed louder , especialy if you use class A el34 pair !

this choke doesn't change the tone, but the feel, this classic tone choke can bare low and high volume plays without losing to much "feel" !

as to get a lil more vintage feel , i changed .047 uf tone stack cap on both channels tone stacks , with .040 6ps 600v polyester caps !
i find polypropylen very cold feeling in the mids, too modern !

and i yanked out all pf ceramics caps from tone stacks with styroflex, it brings softer hights, remove ice picking !

and of course after 20 years filter caps with mesa parts !

now this Mark IV sing like a diva

Thanks BlackBoxy :idea:

I'd be interested in a bias mod, maybe??... and I wonder why no one seems to do this with Mesa amps?? It's done with Peavey's all the time, especially the 5150, but I've never heard of a bias modded Mark IV, or even a bias modded Rectifier??

rgx612a said:

I'd be interested in a bias mod, maybe??... and I wonder why no one seems to do this with Mesa amps?? It's done with Peavey's all the time, especially the 5150, but I've never heard of a bias modded Mark IV, or even a bias modded Rectifier??

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Mesa Mark amps tubes aren't so coldy loaded, and by the fact you can change safely plate voltage with tweed/full + simul/class A + triode/pentode makes a bias pot useless, to my own view

I have seen a adjustable bias mod on many Rectifier.

stephen sawall said:

I have seen a adjustable bias mod on many Rectifier.

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most people who test bias pot on, said it wasn't worth the change