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  • #16
    Originally posted by g-one View Post
    Did you try ch.B yet? Looks like plugging an open plug or any patch cord into the footswitch jack should make it switch. However, it looks like all that does is change the master pot, so it's not really a different channel.
    Absolutely correct and you can either use a simple 1/4 inch plug and touch tip and sleeve to change channels or a single footswitch t/s style and yes it just changes volumes pots not channels. Kinda useless and definitely a one trick pony amp but it really does sound very good and gets a nice bluesy distortion or full blown heavy thick distortion. I just use the power amp but it is very reliable and I also had to upgrade the rectifier on this one but it has been solid since then.
    KB

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    • #17
      Well I checked the voltages and everything seems fairly normal. 470 in the power amp, 320 on the PI, and 300 and down in the Pre-amp. all the biases seem good and the cathode followers cathodes are around 120V.
      Under a strong signal, low E on neck pickup, my B+ sags about 25V, is this a normal sag? I have replaced both IC's and they work without properly. I tried jumpering the send- return jacks and there was little improvement. Oh and now my volume reduction seems permanent. I'm kinda leaning towards a bad output transformer... but that seems extreme to me. I also tried jumpering from the output of the tone stack straight to the PI, but got no signal amplification at all...

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      • #18
        Did you try turning the effects loop pot back. It is in the loop so it's possible it's causing some feedback. Doubt it's the OT but you could try swapping the OT Secondary around. You did check for microphonic tubes right ?
        KB

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        • #19
          I guess I need to state that this is no longer a feedback problem, the amplifier used to cut and lose volume with a large signal and with either master volume up. after playing for a while the volume drop became permanent. I now have a 100 watt amplifier that gets about as loud, with poth pre- volumes and either master up to 80%, as maybe a 18 watt amp. it also is very grainy and gritty sounding, then again the preamp is cranked. My voltages at the tubes are all good. I also bypassed the effects loop and it made no difference. I ran a signal through the amp and tapped connections to see if I got volume back but I had no change. Jmaf you had said to "touch" the grids, I tried this with a chopstick but I got no audible effect, should I touch with something else? or try running my signal into the effects return?

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          • #20
            That's how mine sounds when I try to use the jack in the front when something is plugged into the return. Have you tried running a signal straight into the return. If it still does it then it's something in the output from the return to power amp. If you take the send out to another amp and all is well at least you know it's not in the preamp from the send back.
            KB

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            • #21
              Yeah I'll do that tonight to isolate it either to the preamp or the PI/power amp

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              • #22
                Yes, isolate whether the problem is pre or power amp first. If it is preamp, go back and check your DC heater supply for V1 thru V3, also (if you haven's already) resolder those sockets.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #23
                  WEll I know the heater supply is good, and actualy the pre-amp sockets had been replaced at one point, but I did touch up the wiring.

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                  • #24
                    Does turning down the treble and presence controls change anything?

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                    • #25
                      SO I recorded the problem into garage band, just to get an audio sample

                      JCM900SLX.band.zip - 0.62MB

                      Hopefully you can listen to this alright, and that is not the mic distorting, that is what the amp sounds like. now I'm almost certain I have a poor connection in the preamp as when I plugged into the return jack the distortion was not present, although the volume was very low.

                      This thing is driving me nuts, and I wish I could just scrap it, but it's my brothers and I told him I'd find the problem no one else has been able to.

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                      • #26
                        I listened to your sample and that sounds like either bias is set too negative or missing/too low screen voltage.

                        When you say bias is right, what do you mean by that? That amp can't be set by the transformer bypass method, the OT's DCR is too low to be bypassed..... How are you setting bias for it? Do you have a biasrite or something like that?
                        Valvulados

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                        • #27
                          You can check bias by removing the output tube cathode fuse and inserting your meter on milliamp setting. Screen voltage should be very close to plate voltage. Earlier you asked about touching grids with to get a popping sound, a chopstick will not work, try a meter probe.
                          Treat this loss of volume as a new condition. Any thing you checked before the volume dropped out should be rechecked. You know the volume loss is somewhere after the FX return jack. Forget about the preamp till the power amp is working right.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


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                          • #28
                            The return jack thing tells you a lot. I've seen maybe a dozen of these amps in the last year, and probably 3-4 of them had problems with the FX jack contacts. Sometimes you can clean then with contact cleaner, other times you need to replace the jacks entirely.

                            Another thing to check is the OT primary. There have been 4-5 cases in the last year where someone brought me a JCM900 100W head of one sort or another, complaining of "distortion and volume loss," and I would measure about 10-20W on the output and the waveform would look really ugly on the scope. In each case, the OT primary was partially shorted. All of them got JCM2000 100W OTs, which are basically the same, and were fine afterward. So now with JCM900s, one of the first things I check is the DCR across the primary terminals. It should be fairly equal, about 16-17 ohms from center tap to each end, about 34 from end to end. If it's twice as high on one side than the other, for example, that might indicate a problem. Easy to check and eliminates this is as a possible cause.

                            (disclaimer, I am NOT saying this is your problem, and I do NOT think the OT is something to suspect early on)

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                            • #29
                              well I finaly did a proper bias check with my bias right. # tubes were idling at 21mA and one a 27. so I dialled it up to about 33mA, the one tube that was out before kept climbing up to 42mA, I guess this might have been why some one biased it so low before, perhaps they were only monitoring this tube/pair? but once the bias was up I have a good volume level, although it still had a bit of a funny sound, that for now I'm blaming on the bad tube. so I'll put a new quad in and double check things.

                              I suppose the check the basics rule should never be forgotten...

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                              • #30
                                Never mind, new tubes, got rocking and zap, back to crappy sound and no volume. measured the output transformer imediatly, 17 ohms one side, 1 ohm on the other... there we have it.

                                SO classic tone or is there a better source for an OT?

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