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1968 Ampeg J12 - Rectifier Diode Replacement

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  • #31
    No thats fair. Really, my example was more relevant to C26. But Im a bit of a compliance dink, and best practice is caps on or across main lines voltage should be safety rated class X.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    • #32
      Your numbers look fine. Remember, this is just a guitar amp, not precision lab equipment. More or less is close enough. We can look at numbers and decided getting everything within 1% of some imagined standard, but what is better about that? You might like the sound better with some misbalance. And none of the rest of the circuit is all balanced, so why forus on just bias? Getting closer to hi fidelity is great for a home stereo amp, but this isn't a hifi.
      Education is what you're left with after you have forgotten what you have learned.

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      • #33
        More important is if you like the way it sounds. I had a 1960 Ampeg Rocket for years. It was the 6v6 version. Cool that it was made in NYC.. but it was never one of my favorite amps. Definitely didn’t sound like a Fender. I eventually sold it as an unmolested vintage piece.

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        • #34
          thank you guys!
          yes, that's the problem of being an engineer professionally and also a musician. somehow what i hear needs to match something i can measure.
          ultimately with all these old amps what I am looking for is to make sure they are working as intended back in the day they were manufactured and, only if my ears feel there's something missing, I will modify them.
          After all your support through the different posts I feel pretty confident to be able to go backwards to a previous state if at some point the sound is not going in the right direction.
          You are right, the ultimate goal is to please my ears and annoy my neighbors

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          • #35
            Originally posted by Enzo View Post
            Is there any DC on your grids, pins 6?
            yes! there is!

            I've measured about 60 mV DC to ground and after about a minute it went up to 200 mV
            similar values on both tubes, turned the amp off after I've reached the 200 mV

            are the caps leaking??? C17 and C18 are new Panasonic Orange Drops, C19 is also new

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            • #36
              are the caps leaking???
              Probably not. 0.2V is small enough to be a normal grid leak effect.
              - Own Opinions Only -

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              • #37
                Originally posted by Helmholtz View Post
                Probably not. 0.2V is small enough to be a normal grid leak effect.
                thanks! is there a “limit” to normal leak?
                i mean, I stopped measuring after 200 mV but don’t really know how much higher woud it have gone


                @G1, put back the .047 uF at the switch to prevent the arc and the noise when turning the amp off is gone. thx!
                Last edited by TelRay; 05-09-2020, 08:59 PM.

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                • #38
                  thanks! is there a “limit” to normal leak?
                  I've not found any published numbers but think that anything below 1V should be non-critical.
                  - Own Opinions Only -

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                  • #39
                    Originally posted by Helmholtz View Post
                    I've not found any published numbers but think that anything below 1V should be non-critical.
                    got it
                    thanks!

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