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JCM800 build with GZ34 Rectifier filtering question?

Discussion in 'Building the Classics' started by Kelia, Jun 27, 2021.

  1. Kelia

    Kelia Well-Known Member

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    Hi guy's , I had a butchered JCM800 chassis with an empty JMP head and was also given a 2203 circuit board from the era. I've been buying parts here and there and now it's getting pretty close.
    I want it to be GZ34 rectified so my question is , can I go 3X 32-32 for filter caps for more sag or can I go with a mix of 32's and 16-16 for preamp ?

    Reason why I like 32's for main is that I tried this in my 1980 JMP 2204 and it had a nice natural sag that was great feel wise !

    Thanks for your inputs !

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  2. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Use 32uF plates, 32uF screens. I’d use 32 all the way through. It’s going to work good already with that power transformer.
     
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  3. South Park

    South Park Well-Known Member

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    With that GZ34 you will not need all that filtering from the caps. So I would look at the specs of other amps that use that tube and see what combo of caps and choke .
     
  4. Kelia

    Kelia Well-Known Member

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    Thank you William ! .. I bet it will sound good , would like to have a resonance knob
    on the back and a loop , maybe a Granger ?
    I have a 3 way switch for SS-Standby-Recto but I know I'll always be on the recifier mode
    so don't think I'll install it , and when I say i install ,I mean that I'll have someone do the work
    for me as I'm only the dreamer and the player ..lol wish I was a tech , so interesting !
     
  5. Kelia

    Kelia Well-Known Member

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    Thanks ! Someone told me that with that board , I would need another cap inside the amp ?
     
    Last edited: Jun 29, 2021
  6. mickeydg5

    mickeydg5 Well-Known Member

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    You want to stay 60uF or under with a capacitor input 5AR4/GZ34 rectifier design.

    In the Tweed Bassman Fender used two 20uF in parallel for 40uF total.
     
    Last edited: Jun 28, 2021
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  7. 2L man

    2L man Active Member

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    GZ34 datasheet calls 2x 75ohm current limiter resistors to both anodes for 700V CT when max 60uF RC filtering is used (no Choke on immediate Cathode circuit) but you can install higher value resistors! There come some voltage drop but also they form RC filter with first capacitor although there come tube diodes between R and C. They also are part of Sag-effect and sag is only reason I use tube rectifier.

    Current transformers do not necessarily sag as much as PTs which were manufactured when tubes were the only option and perhaps current tubes are not as durable as old were? Now NOS are expensive so I don't want to wear them too fast.

    Current limiter resistor "flattens" sine wave peaks and current flows longer and it is how they limit the peak current tube delivers.

    It is good practise to also install SS diodes to rectifier tube anodes.
     
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  8. Kelia

    Kelia Well-Known Member

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    Want to ask , there was a switch installed on the front from a previous owner,
    leave it like this or install another switch for something , and if yes, what would you guys do ?
     
  9. Pete Farrington

    Pete Farrington Well-Known Member

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    I can’t see an additional switch on the photos?
    There’s a small hole between the treble and mid controls. I suppose it could be used to switch to a different tonestack option, eg slope resistor, or to change the treble peaking cap, eg to a different value or completely disconnect it.
     
    Last edited: Jun 29, 2021
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  10. Kelia

    Kelia Well-Known Member

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    Thanks , exactly the small hole, great ideas.
     
  11. Stephen H

    Stephen H Member

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    Hey Kelia, sorry I'm late to the party, but how'd this project turn out?

    Interested to hear, as I built myself something similar but with a 5u4GB to bring the V's down even lower. I used 32uF for reservoir and 22uF for the rest. Like you I realised I would never ever use the ss diode side of a rectifier switch, so since I was building it for myself that was one less complication to worry about.
    Anything in that extra switch hole yet? I don't have any spare holes, but I do have spare switches, so toying with a 3-way Tone stack, although I'm happy with the feel and sound as is, I'm just a chronic tweaker and built with the vague idea of using it as a test platform for my own personal evaluation of individual and/or combined mods.

    Cheers
     

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