Choosing resistors and capacitors for mods???

Discussion in 'The Workbench' started by uncle sixer, Nov 24, 2020.

  1. uncle sixer

    uncle sixer New Member

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    I have some mods I want to try on my JCM 800 50W combo (4104/2204 in 2x12 combo form).

    So far, it has been switched to EL34s from 6550s by a tech and rebiased, he also added a snubber cap to V1 (I read that stability could be a problem with any mods that add gain). I removed the bright cap, which seemed to be a step in the right direction for the tones I want…

    I want to get rid of brittle ice-pick tones when the gain is around 4-6, running through low input.

    I would like some more gain/sustain when running through high input.

    My ideal spectrum of sounds would be like Allman Brothers to Led Zep on the low input and AC/DC to Guns n Roses on the high input (I hope that analogy makes sense). I am probably asking too much, but I am interested in tinkering a little bit with mods that can be easily reversed.

    Here are the things I want to try:

    First Attenuator/Peaker… replace with 68K resistor only (or run 2200pF or 1500pF parallel to stock R/C)

    Mid Pot replace with 50K linear or 100K audio

    Different cathode resistor on V1A (6.8K or 4.7K)

    Different cathode capacitor on V1B (.47uF, .33uF, or .1uF)

    Add cathode capacitor on V2A (.47uF, .68uF, or 1uF)

    Change presence capacitor to .68uF

    Change tone slope resistor and treble capacitor… maybe

    V1B 100K plate resistor replace with 150K or 220K (maybe add “fizz” capacitor… maybe not do this at all, not sure)

    How do I shop for individual resistors and capacitors?

    I assume I need all 1/2w resistors in the locations I am looking at, but do any need to be higher? Does metal film or carbon film matter?

    I see a couple of ceramic capacitors in the amp, but also a lot of rectangular capacitors, what are they? Do I need to worry about wattage or voltage rating when swapping these capacitors?

    If you read all that and can offer some advice, THANK YOU, I am trying to learn.
     
  2. South Park

    South Park Well-Known Member

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    Half watt resisters is all you need . You can get the parts on eBay . You don’t want to change the tone caps it will change the gain of the frequency’s . All the gain mods are in v1 if you don’t go to nuts with it. I would start with a gain mod only in v1 . The gain and tone work together on that amp take it in small steps so you know how far to go
     
  3. thetragichero

    thetragichero Well-Known Member

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    i use whatever's in the bin with appropriate power/voltage handling. i use a lot of 1w and 2w metal oxide but that's because of size and lead thickness, although less of a problem on pcb amps
     
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  4. junk notes

    junk notes Well-Known Member

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    bingo! Do that for now, and report back when able. Let us know where you found that sweet spot. ;-) That might change a few things.
     
  5. uncle sixer

    uncle sixer New Member

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    Just the 50K? I read somewhere that 100K audio will still function similarly over the first 50-75% and then ramp up at the end to kinda take the tone stack load away/leave more gain on the signal. Sorry, maybe I am saying that in a stupid way... any thoughts on 50K linear vs 100K audio?

    For guitar projects, I try to choose CTS... what is a good pot to use for this amp?

    Thanks
     
  6. uncle sixer

    uncle sixer New Member

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    Are plate resistors good at 1/2 watt, too? I have seen mixed things about that.

    What about the capacitors? I have rewired guitars and built effect pedals, but never saw capacitors like what is in the amp's circuit board.
     
  7. junk notes

    junk notes Well-Known Member

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    No, you are not saying anything stupid :cheers:
    There are no rules to what you want, 25k is the original spec for the circuit, but unless you know the desired affect, then 100k is complete overkill for that position; like a governor on an engine of a car or motorcycle. Does not matter or make a difference (actually worse).
    Again, listen for the sweet spot. Dial it in! It should start to come in after 1 O'clock..

    Yes, PEC or Alpha are fine to use.

    & Your choices in resistors are several to include carbon comp, carbon film, with metal film being used for stability reasons.
     
  8. CROWEMAG

    CROWEMAG Well-Known Member

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    I have learned very quickly that modding takes you down the rabbit hole, as you always lose something when changing something else. I had a bunch of mods going on and I was happy for a little bit.....until I started noticing my 800 did not sound like an 800 anymore. I am back to full stock and I am happy as hell. Also, the sounds you say you want can easily be had with a stick 800 and for the GnR a boost up front.
     
  9. uncle sixer

    uncle sixer New Member

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    Yeah, I get that. No time here to be lost in a rabbit hole....

    And I honestly don't know enough to work through this without a lot of trial and error for some of the mods. I know there are definitely things on my list that could interact with each other in a negative way

    The mid pot sounds pretty popular and idiot-proof, though...

    I kinda just want that feeling in my head that MY jcm 800 has a little something extra special under the hood. You know, it will make me play SOOO much better ;)
     

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