Working On My JCM 800

Discussion in 'The Workbench' started by kam4ff, Mar 5, 2013.

  1. damienbeale

    damienbeale Well-Known Member

    Joined:
    Mar 1, 2013
    Messages:
    3,798
    Likes Received:
    1,753
    Location:
    Oxford ish sorta...
    Sorry, I wasn't implying anything along those lines. Simply that a 2204 made me realize that my 4 or 5 months playing through a crappy SS Carlsbro combo with a ton of gain hadn't exactly aided my learning. I quickly learnt to get the best of my amp, and pedals definitely weren't part of the big picture for me.
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    23,295
    Likes Received:
    10,387
    Location:
    US of A
  3. john l

    john l Well-Known Member

    Joined:
    Oct 24, 2011
    Messages:
    1,060
    Likes Received:
    474
    Location:
    ORANGE COUNTY
    Totally man I said that very light heartedly
     
  4. damienbeale

    damienbeale Well-Known Member

    Joined:
    Mar 1, 2013
    Messages:
    3,798
    Likes Received:
    1,753
    Location:
    Oxford ish sorta...
    Perceived weakness in the sound between that and other 2203's, may be down to the difference in filtering.

    The lead dress is pretty horrid on the preamp tube sockets too. Try to shift things so they look a little more like this... [​IMG]
     
  5. Dimitar

    Dimitar Active Member

    Joined:
    Jun 10, 2010
    Messages:
    111
    Likes Received:
    55
    Location:
    Skopje, Macedonia
    @kam4ff Thank you for the pics. I have the very same amp. The 3 cap version. I want to know where is the black wire of the choke wired to? Mine is wired to the little pin next to the R29 resistor.
     
  6. RussBert

    RussBert Active Member

    Joined:
    Apr 8, 2012
    Messages:
    584
    Likes Received:
    248
    Location:
    connecticut

    That's where it belongs.


    [​IMG]
     
  7. RickyLee

    RickyLee Well-Known Member VIP Member

    Joined:
    May 22, 2010
    Messages:
    9,642
    Likes Received:
    3,124
    Location:
    Riverside CA U.S.A.
    I can only find 5% tolerance flameproof 2W 1 ohm resistors at the Electronics store I go to. I have used them for bias resistors in a few amps. I had some of the Dale precision 1 ohm resistors I ordered online a few years ago.

    So my question is about the tolerance. Is that the measured tolerance or value you will see that has to stay within that spec when measuring, or does that tolerance spec mean that it will stay within that spec while under load, or in circuit with current flow?

    The 5% flameproofs I bought measured right at 1 ohm on my Fluke 77 III while those precision Dales measured right at 1 ohm as well. But then, 5% of 1 is .05 and I will only see .1 increments on my meter anyway.

    :D
     
  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    23,295
    Likes Received:
    10,387
    Location:
    US of A
    Resistor tolerance is the allowed deviation of the manufactured value from the stated value under standard conditions which includes free range. :) You know, resistors allowed to roam free around the workbench.

    Small tolerance percentages are not a big deal. But 10% can be a bit much if you are shooting for 39 and really end up with 43 on the average Marshall type amplifier.
    1% can be out up to about 0.4mA
    2% can be out up to about 0.8mA
    5% can be out up to about 2.0mA
    10% can be out up to almost 4.0mA

    If possible it is best to measure the resistor with a fairly accurate meter so you know the ohms value being worked with.

    But the thing is most current resistors will tend to be practically dead on or very close to the correct value, unless it was abused.

    So anyway if a less accurate meter reads the 1.0 resistor at 1.0 use that value but keep in mind to adhere to a slightly lower bias, maybe calculate at 65% instead of the normal 70%. And if the same meter reads the 1.0 resistor at 1.1 use the 1.1 value to calculate and the normal 70% should suffice.

    I hope that makes sense.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

    Joined:
    May 22, 2010
    Messages:
    9,642
    Likes Received:
    3,124
    Location:
    Riverside CA U.S.A.
    Makes good sense to me Mickey, as you always explain this stuff very well.


    :cheers:

    I just ended up using those flameproof 5% 1 ohm'ers some time ago, as I figured even a few mA off on the bias was no big deal to me anyway. I can find those in the local part stores here. Plus I always bias closer to 60% dissipation/idle and leave the screen current figured in as part of the cathode reading just to be even safer - which means I can aim for 65%.

    Shoot High . . .

    Aim Low

    :D
     
  10. damienbeale

    damienbeale Well-Known Member

    Joined:
    Mar 1, 2013
    Messages:
    3,798
    Likes Received:
    1,753
    Location:
    Oxford ish sorta...
    Unfortunately most multimeters wander like crazy as resistance that low. They're not really best cut out for measuring only 1 ohm. This is where decent analogue meters come good. But really, special low ohms meters are really the only thing that works properly down that low. If only I'd had the foresight to pinch the CSE workshop one before they closed down. :(
     
  11. kam4ff

    kam4ff Active Member

    Joined:
    May 21, 2010
    Messages:
    223
    Likes Received:
    36
    problem is I dont really want to order from online as its kind of a rip to pay 4 for parts and 5 for shipping and Im having a bitch of a time sourcing locally
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice