Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: imgumby001 on February 18, 2023, 08:37:22 am
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Hi everybody!!
If someone has the time to look over what I’ve done with this little Jca20h I’d really appreciate it. I modded it according to Roosville’s thread over on Valvesphere. I’ll include his schematic and write up along with the stock schematic and pics of what I’ve actually done. The issue is this- the amp sounds terrible. Definitely NOT high saturated gain. I’m most in question of how I’ve placed the components in question, I’m very much doubting my interpretation of his schematic. As there is no actual layout I’ve done the best I can, but the outcome is the amp is lifeless, has little on the gain side, is bassy/woofy, and has a lot of noise. I tried to find pics of other mods done to this circuit board, but nothing is exactly like what I’ve attempted, or I haven’t been able to find it. On a side note, if anyone has experience with this head, or suggestions of what might be done better/differently to achieve a more SLO-ish sound with this circuit I’m very, very open to suggestions. Thanks to you all!!!
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Never built a high gain amp but if it was my amp and it was lifeless and boomy and did have no wiring errors i'd swap R43 for a smaller value, maybe 820R and also omit the bypass cap on that cathode or make it much smaller.
And don't be disapointed, it works to a degree and sound tweaking is where all the fun is happening.
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Thanks boss. I’m assuming I’ve misunderstood the schematic. There’s an error there somewhere, but I can’t identify it. Been trying
To understand more and more everyday.
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Thanks boss. I’m assuming I’ve misunderstood the schematic. There’s an error there somewhere, but I can’t identify it. Been trying
To understand more and more everyday.
V4 and V5 cathodes are connected together but don't connect to ground. Instead, the cathodes are connected to the plates!
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Thanks boss. I’m assuming I’ve misunderstood the schematic. There’s an error there somewhere, but I can’t identify it. Been trying
To understand more and more everyday.
V4 and V5 cathodes are connected together but don't connect to ground. Instead, the cathodes are connected to the plates!
Ok, thank you. I really must be not understanding what I’m looking at. I didn’t even realize I had modified that part of the circuit.
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I was just pointing out an obvious drawing error on the schematic. Not saying YOU made an error with your mods.
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I was just pointing out an obvious drawing error on the schematic. Not saying YOU made an error with your mods.
Ok, wheew!!! I was really starting to doubt myself there for a min!!! Ha!!!
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going off your jca20h schematic this is what i've done on the jca50 combo that is basically a slo (two channels instead of your one, but the overdrive channel didn't do it for me):
c26 47nf (can leave as 22nf to keep it tighter)
r53 470k with 470pf in parallel
r25 1k (throw a bypass cap over it for some more gain)
c24 47nf (200nf here is likely an error. 22nf can work here too)
r32 680k
c23 220pf
r44 220k
r13 remove (that 220 has gotta be a typo for 220k)
r14 short/jumper
if you need more low end c5, c11 can be changed to 47nf
sound ends up being like a hot rod plexi, nice with an overdrive/boost pedal in the front
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going off your jca20h schematic this is what i've done on the jca50 combo that is basically a slo (two channels instead of your one, but the overdrive channel didn't do it for me):
c26 47nf (can leave as 22nf to keep it tighter)
r53 470k with 470pf in parallel
r25 1k (throw a bypass cap over it for some more gain)
c24 47nf (200nf here is likely an error. 22nf can work here too)
r32 680k
c23 220pf
r44 220k
r13 remove (that 220 has gotta be a typo for 220k)
r14 short/jumper
if you need more low end c5, c11 can be changed to 47nf
sound ends up being like a hot rod plexi, nice with an overdrive/boost pedal in the front
Dude!!! You’re the man bubba!!! Basically I started pulling things FROM the other schematic, and the replacing values with the ones you’ve listed above(at least the ones I had on hand). She is 100% more ueseable now! The tone is still a bit sterile(for lack of a better term), and she’s very noisy. The gain on its own is better though. Not where I was hoping, but definitely more SATURATED and a bit more compressed. I was really wanting it to NOT need a boost, but, with the mxr classic od out front, maxed out, and the tone shaped up on the amp I am getting really, REALLY good tones out of it! Many thanks to you thetragichero. I’m also grateful for everyone’s else’s input.