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Fender Twin Amp - need help

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  • #16
    I know you knew that, Bob. I was pointing out the net results of the overall circuit for the less informed.


    yeah jason, we'll all see it with just one post.

    OK>
    1. the input tubes are more sensitive to microphonics and noise, so I suspect those tubes were selected from the choices Fender keeps as superior in that regard. Depending upon the tubes you select that difference may or may not remain.

    2. those descriptions don;t contradict any given here above already, so sure they work.

    3. WHo says it is not a "good tube?" You will find that there are numerous criteria upon which to judge a tube. It would be judged not only for its suitability for each task, but also for the sound it makes. One tube may be less microphonic than another, but it might also have a duller tone. Then again, the amp circuit may already be more bright than it could be, and the duller tube still is plenty bright. And so on. After several stages into the amp, signal levels are elevated, and microphonicity is much less of an issue.

    There is no magic formula, nor is it critical which tube you select for any stage. Feel free to try as many different 12AX7s as you can find in each position to see which ones make a difference (if any) you might prefer.
    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      V3's description sounded more "signal pathy" than the descriptions for V4-8, that's why I asked whether to use a "good tube" there. Sounds to me like V1-2 will be the most critical tubes- the mosy vertical signal ladders, if you will- between extremely low-level signal and output-level signal. So I'll put more money into V1-2 than I do into 3-8; is that naive? Thanks again.

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      • #18
        No, not naive. The first stages are the most delicate. Many amp owners use selected V1 tubes to tame amps and many stores offer selected tubes just for this matter. I just wouldn't expect some kind of voodoo. They're still tubes.

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        • #19
          hey guys, good news, i discovered V1 and V2 had 12au7s (rather than 12ax7s) plugged in. did a switcheroo, and it's about 3000 times louder now. shows what a difference an x makes. thanks for all your help(s). i have about 200 tubes on the way now, and i'm going to taste them all. will keep this conversation alive (much to all of your collective dismay). THANKS again. what a friggin' great amp, by the way. just absolutely stunning. can't wait to color it up with new tubes (by the way, did some late night reading, and settled on SEDs rather than the GT GE wannabes).

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          • #20
            Thank you, Enzo, reverb hum (not wooo) went away with a tube switch. Didn't think a tube could do that, but it did. Now I have a bad tube on the effects send/return (strange woo type hum, actually, at high return levels); need to try a new tube there. Thanks for the response. Excellent suggestions, by the way, regarding alternate hookups for troubleshooting- they make perfect sense.

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            • #21
              There you go.


              AMplification factor for 12AX7 is 100, for the 12AU7 it is 17.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                success (mostly)!!!

                thanks everyone; thanks enzo. i put some correct tubes in this thing and it's a led zeppelin reunion tour in my basement. now all i have to do is learn to play guitar. pulled some old philips miniwatt ecc83s (? 12ax7 equivalent) out of a farfisa organ and put 'em in slots 1 and 2; MAJOR difference from these russian garbage tubes. sheesh. so tragic that good tubes are out of production now.

                one thing i notice- no matter what i do, i get a little hum out of the reverb. moved the cord away from transformers, magnets, etc., no luck. zero reverb = zero hum, and 10 reverb = a little hum, but still annoying. is this normal?

                man, the amp just sounds so good. what a killer piece. thanks again everyone for your kind support/advice. jason

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                • #23
                  Yes, reverbs sometimes hum some. Keeping the OUTPUT end of the pan as far from the power transformer as possible helps. Cutting out a steel plate to cover the open side of hte reverb pan can help too. Sometimes your cables are not the best to the pan.
                  Education is what you're left with after you have forgotten what you have learned.

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