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I have a JCA50H from about 2009. I recently clipped the bright caps on both channels and while I had the amp chassis out of the I noticed it has a bias pot in the middle of the board - now I thought Jet City amps were all fixed bias?
As I've replaced the output tubes since buying the amp would I get the best out of the it by having it re-biased?
Also, while I replaced the output tubes the original (I assume) Chinese preamp tubes all seem to be ok - no noise, microphonics or anything else to suggest one or more is duff. However, if I could I'd like to tame the overdrive channel a bit if possible. I assume this is possible by swapping one or more of the preamp tubes out for a lower gain version - perhaps ECC81? Can anyone confirm this is the case and which tubes I'd need to swap?
There is no 'H' in Aych, you know that don't you? ~ Wife
Turns out there is an H in Haych! ~ Sporky
Bit of trading feedback here.
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I've tried an ECC81 in V1 on my JCA50H before just to try it, and yes...it does tame it a bit. That's generally the opposite of what I want to do, though, so I put the original valve back in.
" I noticed it has a bias pot in the middle of the board - now I thought Jet City amps were all fixed bias?"
Yes, paradoxically, "fixed bias" is generally adjustable. "Cathode bias" is not (unless you change the cathode resistor).
I don't know the amp but will give my general warning NOT to change pre amp valve types unless you know the circuit components are up to it.
ICBM or one of our other resident techs will know.
Dave.
There is no 'H' in Aych, you know that don't you? ~ Wife
Turns out there is an H in Haych! ~ Sporky
Bit of trading feedback here.
https://res.cloudinary.com/powerreviews/image/upload/f_auto,w_1024,h_356/prod/rgf9m5i2sem9ujmphzyd.jpg
It means the bias is fixed by a separate circuit and not allowed to vary with the signal conditions in the valves.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Apologies for the dumb question
There is no 'H' in Aych, you know that don't you? ~ Wife
Turns out there is an H in Haych! ~ Sporky
Bit of trading feedback here.
This is educational if nothing else.
There is no 'H' in Aych, you know that don't you? ~ Wife
Turns out there is an H in Haych! ~ Sporky
Bit of trading feedback here.
There made be variants I am unaware of but AFAIK the 7025 (hg) is identical to the ECC83/12AX7 except that is is said to have better hum and noise figures. I certainly used the TAD version as a direct replacement and had no problems. And! I used to specc check the gains!
I think ICBM is saying that an 81 or 82 will be safe. Going to an ECC81 only drops mu from 100 to 60 but when you take all the other "wrongness" of the circuitry into account you may not notice much loss of gain. The ECC82 with a mu of 20 (iirc) SHOULD be noticeable but that will pull the most current and be even "wronger"!
Dave.
The limiting factor is the power dissipation rating, and sometimes the voltage rating, of the plate resistors. The ones in the Jet Citys are at least twice as big as they really need to be.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein