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finally went ahead with some of the more common mods to my Ultra 60 212,
namely:
swapping out the C76 coupling cap after V1b from the (overly shrill) 1nf -> 2.2nf
changing the parallel C75 470pf/470k before V2a to 2.2nf/470k
upping the V1b C35 0.47uf cathode bypass cap to 1uf PP film
(on the JSX these caps are C3, C8 and C21 respectively)
for giggles I included a Ultra/JSX switch which puts a 75k resistor across the Ultra's 220k R7 which gives it the 56k value found at JSX R6
as is my practice I bagged all the pulled parts labeled it and gorilla taped it to the inside of the head for total reversibility.
I didn't add the ice pick 10nF cathode bypass cap the JSX has at V3b since a +10kHz boost seems a real bad idea...with many JSX mods removing this.
end result:
huge low end bloom with beautiful sustain and transition into controllable violin like feedback; the shrill "bee in a can" 5150 flavor is gone from the Ultra channel. Through my little ported cab this amp now makes a thick breathing rumble but with high end harmonics jumping out.
The "JSX switch" does what is expected; dumping more signal to ground it sounds like one ball is lost; it instantly thins up (maybe why the JSX adds the 2.2nf/2.2M switched fat cap just upstream).
We are talking the Ultra channel tone here...clean tone? WTFC as I never use it (or crunch).
Didn't get pre mod clips so post mod seems pointless But subjectively I am VERY happy.
now to add the 750-1k screen resistors and the JSX bias circuit so I can try kt77s!
what a pain having to drill the rivets on the power tube board!!!!!!!...I 'll post when I have a good fastening strategy for it.
namely:
swapping out the C76 coupling cap after V1b from the (overly shrill) 1nf -> 2.2nf
changing the parallel C75 470pf/470k before V2a to 2.2nf/470k
upping the V1b C35 0.47uf cathode bypass cap to 1uf PP film
(on the JSX these caps are C3, C8 and C21 respectively)
for giggles I included a Ultra/JSX switch which puts a 75k resistor across the Ultra's 220k R7 which gives it the 56k value found at JSX R6
as is my practice I bagged all the pulled parts labeled it and gorilla taped it to the inside of the head for total reversibility.
I didn't add the ice pick 10nF cathode bypass cap the JSX has at V3b since a +10kHz boost seems a real bad idea...with many JSX mods removing this.
end result:
huge low end bloom with beautiful sustain and transition into controllable violin like feedback; the shrill "bee in a can" 5150 flavor is gone from the Ultra channel. Through my little ported cab this amp now makes a thick breathing rumble but with high end harmonics jumping out.
The "JSX switch" does what is expected; dumping more signal to ground it sounds like one ball is lost; it instantly thins up (maybe why the JSX adds the 2.2nf/2.2M switched fat cap just upstream).
We are talking the Ultra channel tone here...clean tone? WTFC as I never use it (or crunch).
Didn't get pre mod clips so post mod seems pointless But subjectively I am VERY happy.
now to add the 750-1k screen resistors and the JSX bias circuit so I can try kt77s!
what a pain having to drill the rivets on the power tube board!!!!!!!...I 'll post when I have a good fastening strategy for it.
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