Jet City bias and preamp valves

HaychHaych Frets: 5595

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?


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Comments

  • Yes, Jet City amps are fixed bias. Fixed bias = need to adjust every time you change the power valves. Cathode bias is the one where you don't need to.

    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.
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  • ecc83ecc83 Frets: 1589

    " 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. 

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  • HaychHaych Frets: 5595
    ecc83 said:

    Yes, paradoxically, "fixed bias" is generally adjustable. "Cathode bias" is not (unless you change the cathode resistor).

    Ah, interesting. Everyday is a school day. I always thought “fixed bias” meant just that - that it had a set value and cannot be adjusted. 

    There is no 'H' in Aych, you know that don't you? ~ Wife

    Turns out there is an H in Haych! ~ Sporky

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  • ICBMICBM Frets: 71956
    ecc83 said:

    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.

    Yes, they're over-spec by comparison with what most other amp companies use.

    https://res.cloudinary.com/powerreviews/image/upload/f_auto,w_1024,h_356/prod/rgf9m5i2sem9ujmphzyd.jpg

    Haych said:

    I always thought “fixed bias” meant just that - that it had a set value and cannot be adjusted. 
    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

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • HaychHaych Frets: 5595
    ICBM said:
    ecc83 said:

    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.

    Yes, they're over-spec by comparison with what most other amp companies use.
    Forgive me for being inept but does that mean it’s safe or unsafe to pop an ECC81 in?
    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

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  • It will work but life of valve will be affected I believe as an ecc83, 82 and 82 do not work off exact same voltage. A lower gain ecc83 would be more appropriate (7025?)
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  • HaychHaych Frets: 5595
    Right, ok - think I’m with you. Thanks. 
    This is educational if nothing else. :D

    There is no 'H' in Aych, you know that don't you? ~ Wife

    Turns out there is an H in Haych! ~ Sporky

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  • ecc83ecc83 Frets: 1589
    It will work but life of valve will be affected I believe as an ecc83, 82 and 82 do not work off exact same voltage. A lower gain ecc83 would be more appropriate (7025?)


    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.

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  • ICBMICBM Frets: 71956
    Haych said:

    Forgive me for being inept but does that mean it’s safe or unsafe to pop an ECC81 in?
    Safe.

    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

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • tekbowtekbow Frets: 1686
    The 7025 isn't a lower gain, it's a ruggedised 12ax7/ECC83. To be fair most ECC83's from the 70's on would have taken on the characteristics of 7025's, so most current valves are 7025's.

    The lower gain 12ax7 variant is a 5751, with a mu factor of 70% to the 12ax7's 100%.

    And thankfully, the NOS one are not quite as expensive as full gain ECC83's.
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