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'67 twin reverb power tube cherry red

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  • #16
    Is the bias control in this amp a BF type or the SF type that is actually a bias balance control? If the latter, re-wire the pot as the BF type.
    "In theory, there is no difference between theory and practice. In practice there is."
    - Yogi Berra

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    • #17
      Its a true ab763 1967 blackface twin reverb

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      • #18
        You checked the actual circuit, or simply looked at the tube chart? Does moving the bias pot increase or decrease the bias voltage equally on all pairs? Your bias voltages should be equal for all 4 power tubes.
        "In theory, there is no difference between theory and practice. In practice there is."
        - Yogi Berra

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        • #19
          i dont read tube charts i read schematics
          i left the studio. didnt touch the bias pot. i can only assume it will because the one wire coming off the wiper of the bias pot is feeding those 220k resistors/0.1 coupling caps. between there and the tube is the problem. the resistors meter just fine (as i originally said they are brand new) as are the coupling caps, but maybe i got a bad orange drop. i should probably order a couple more.

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          • #20
            Pull the power tubes and see what kind of measurement you get on pin 5 without the tubes in there.

            I have a similar problem right now with a Bassman 100 where one tube is red plating and the other on that side is not. Neither one has a negative voltage, but when I pull the tubes all sockets have pin 5 at the same negative voltage which is where it should be. So I did more checking to be sure. The grid resistor is fine when measured out of circuit. The coupling cap is fine when measured out of circuit. The wire has no breaks or shorts. So that leaves the tube socket. End result in this case is that pin 5 was not making contact with the tube pin. A retension is in order, and have to replace the red plating tube also as it reads just about completely dead on the tube tester.

            Greg

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            • #21
              Originally posted by gromrock View Post
              ...but maybe i got a bad orange drop. i should probably order a couple more.
              The cap can only "leak" voltage if the standby is off, sending high voltage to the driver circuit. If the reading you got was correct, then there has to be a problem with the wiring, the sockets, or the tubes.

              Try removing both tubes V3 and V4, what happens to the pin 5 reading? Pull out the end of the coupling cap at the junction with the 220K resistor, now what do pins 5 read? Take a resistance reading from pins 5 to ground (amp off). How do the readings compare side to side?

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              • #22
                Originally posted by soundmasterg View Post
                Pull the power tubes and see what kind of measurement you get on pin 5 without the tubes in there.

                I have a similar problem right now with a Bassman 100 where one tube is red plating and the other on that side is not. Neither one has a negative voltage, but when I pull the tubes all sockets have pin 5 at the same negative voltage which is where it should be. So I did more checking to be sure. The grid resistor is fine when measured out of circuit. The coupling cap is fine when measured out of circuit. The wire has no breaks or shorts. So that leaves the tube socket. End result in this case is that pin 5 was not making contact with the tube pin. A retension is in order, and have to replace the red plating tube also as it reads just about completely dead on the tube tester.

                Greg

                Originally posted by *Originally Posted by 52 Bill*
                Yes, red plating is very bad for the tube as well as the amp.




                If it is only happening on that one socket regardless of which tube is inserted in the socket, then you need to check the wiring going to pin 5 of that socket. It should be one of the 1K5 resistors. You also need to check for the negative bias voltage at that same pin. Also check the socket itself to be sure that the pin contact is good and tight.
                Originally posted by AmpKat
                Yup ^^^^^ monitor pin 5 at idle and make sure you are getting a good steady negative bias voltage for a couple minutes. Pin 8 should have a good solid connection to ground so verify it is grounded good. A loose contact inside the tube socket itself could also cause it so if it still redplates make sure the tube is pushed in well.
                Remember this !
                KB

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                • #23
                  Originally posted by gromrock View Post
                  i dont read tube charts i read schematics
                  ......
                  Absolutely. Not knowing your skill level and not having the actual circuit in front of me just wanted to eliminate another possibiity. Not all Fenders are perfectly true to schematic, and some have been modded or 'upgraded' over the years.
                  "In theory, there is no difference between theory and practice. In practice there is."
                  - Yogi Berra

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                  • #24
                    i did retension pin 5 on that one socket yesterday but it was still red plating. if that 82k resistor right before the power tubes was way low, say around 64k, is it possible that could be it? i only metered it while in the circuit so it could just be that. that was one of the ones i just replaced before the red plating started as well so it should be fine out of circuit. what about the filter caps? could they be a suspect? i'll try metering without any tubes in this weekend or monday and report back. i just like going in with a short list of very possible suspect parts. as always, everyone on this thread is pretty dang helpful and many thanks.

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                    • #25
                      The 82K (and 100K) resistors before the power tubes are for the phase inverter plates and have absolutely nothing to do with your issue. In addition, you are "shotgunning" and have replaced some perfectly good original parts in a vintage amp along the way (e.g. the tubular caps, which RARELY go bad).

                      Your bias voltage (-43V) is a bit low, but not so much as to cause red-plating.

                      You've replaced all of the resistors in the bias/grid network (1.5K, 220K).

                      You've replaced the screen grid resistors (470 ohms).

                      You've swapped tubes, and the problem follows the socket.

                      The OT isn't bad, or you'd have TWO tubes overheating.

                      My educated opinion says: It's time to replace the socket.
                      John R. Frondelli
                      dBm Pro Audio Services, New York, NY

                      "Mediocre is the new 'Good' "

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                      • #26
                        The shotgun parts replacement as listed in the original post and the guessing about what part could be bad is obviously not solving the problem. I thinks it's time to take a breath, re-read this thread and respond to the questions such as reading the bias voltage on pin 5 of each power tube socket with the tubes removed. The forum participants can then lead you through an organized troubleshooting process.

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                        • #27
                          i replaced the blue tubular caps before i ever fired the thing up when i got it along with the 2 prong power cord. it was part of "housekeeping/bringing up to code", and yes i keep all parts that were removed. and it aint a museum piece. the 82k and 100k's were on the low side so they were also replaced before this problem popped up, so i wouldnt say im completely pulling a dick cheney on this one

                          the new socket just got ordered. and i will certainly meter the unloaded sockets late this weekend/monday.

                          and if you want to talk about a shotgun approach, ill be happy to start a new thread about my '65 RI twin that has a seemingly unfixable issue. but im not touching that one again till after this one is done . that one is a doozy.

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                          • #28
                            Originally posted by jrfrond View Post
                            …You've swapped tubes, and the problem follows the socket…
                            We were responding in parallel. I don’t see a report that the problem follows the socket. Did I miss that? Which post?
                            Originally posted by gromrock View Post
                            With the amp in standby mode and all power tubes installed and amp powered on. V1 and v2 pin 5 = around -43vdc. V3 and v4 pin 5 = around -25vdc
                            EDIT: this -25vdc is also measured at the joint of the 220k resistor and 0.1 cap on the eyelet board.
                            The -25V reading on V3 & V4 being far out of expected range and so different than that on V1 & V2 is certainly a red flag. Do those readings change if you lift out the .1µF plate coupling caps? Or trade places if you swap the .1µF caps? There isn’t much in the AB763 circuit that can go wrong in this area. If it is wired correctly then it seems like
                            1. One of the coupling caps is leaking or
                            2. Something is loading down the bias voltage on the side that reads -25 V. Seems unlikely that it would be the socket causing the loading but I won’t say that’s not possible. I’ve had a defective tube load down the bias supply. If one tube of the pair does that then the bias voltage on the other tube on the same side of the push-pull pair will also be affected.

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                            • #29
                              some of the earlier replies made me think it was the socket, until i retensioned it and it still red plated. swapping the .1 caps was my next move but i had to leave to go to work yesterday. i also ordered new ones of those, this time 600v's. they were originally replaced with 400v's, which is what it said on the side of the original blue tubular ones. now also seeing that it regularly sees over 400 volts, why the hell not bump up to 600v. certainly could be a bad tube, though they were maybe 5 minutes old. still, could have a bad one, certainly possible. thanks tom, my gut was always thinking what you wrote.

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                              • #30
                                Have you made the tests posted above by 52 Bill in post #21? As he states a leaky .1 cap is only going to affect bias if B+ is on. (amp not on standby).
                                "In theory, there is no difference between theory and practice. In practice there is."
                                - Yogi Berra

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