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Thread: '67 twin reverb power tube cherry red

  1. #36
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    no i dont have the parts yet. im going in to the studio in a few hours to take a couple measurements though. ill be swapping the socket on thursday sometime.

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    So with no power tubes installed and the amp in standby, pin 5 on the power tube sockets: v1 is -44.45, v2 is -44.45, v3 is -44.5, v4 is -44.5. Still thinking a bad socket?

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    Now with no power tubes turn off the standby switch. Do the voltages at pin 5 stay the same or does the voltage on the affected sockets drop?

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by gromrock View Post
    So with no power tubes installed and the amp in standby, pin 5 on the power tube sockets: v1 is -44.45, v2 is -44.45, v3 is -44.5, v4 is -44.5. Still thinking a bad socket?
    Actually. I'm thinking along the lines of my post #28 but I wouldn't totally rule out the socket. Maybe there is a leakage path from pin 3 or 4 to pin 5.
    Whatever the problem is it is affecting both the V3 & V4 positions. The results of the test suggested by 52 Bill will help. If the bias voltage changes when you switch the high voltage on with the tubes removed then I'd lift the coupling caps from the phase inverter and re-measure the bias voltages.

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  5. #40
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    Quote Originally Posted by 52 Bill View Post
    Now with no power tubes turn off the standby switch. Do the voltages at pin 5 stay the same or does the voltage on the affected sockets drop?
    Turning the standby switch on and off makes no difference. The voltage stays the same, its around -45vdc across all pin 5's.

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  6. #41
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    sorry double post

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    As far as I can tell, you've proven that the bias supply is probably ok, the coupling caps are ok and the tube sockets are probably ok. That only leaves the tubes. Try installing one tube at a time and measure the voltage at pin 5 in the suspect socket. Does one tube cause the bias voltage to drop?

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  8. #43
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    yeah i will start swapping out tubes tomorrow night into that socket and have the meter leads clamped on pin5 and ground and see what happens. I think though it still could be the socket. if swapping tubes around doesnt prove out then i will change the socket and hopefully thats it.

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  9. #44
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by gromrock View Post
    ... I think though it still could be the socket.
    Ask yourself why you think that. Can you explain the reasoning to yourself?
    Quote Originally Posted by gromrock View Post
    ...if swapping tubes around doesnt prove out then i will change the socket and hopefully thats it.
    Re: "hopefully that's it". We can always hope but it would be better to have some proof.
    As you know a common reason for redplating is under bias or the total loss of bias (which is an extreme under biased condition)
    Consider the evidence that BOTH tubes on the same side were reading -25 V on pin 5. So what would cause that? Assuming that the problem is not intermittent then I think it's more likely one of the tubes. I hope you marked the tubes so you know which socket they were previousl installed in.
    I hope to hear good news from you soon.
    Good luck,
    Tom

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  10. #45
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    Sometimes if I suspect a tube(s) I'll label them with a sharpie 1,2,3,4 or A,B,C,D so later I'm not confused which tube was which. Make a note what socket they are in and the test results, and then swap one tube at a time to the other side and see if the low socket changes side.

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  11. #46
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    (Partial) SUCCESS!!
    It must have been a cold or bad solder joint where the 0.1 cap and 220k resistor meet. I sucked out all of the solder out of that eyelet, pulled the cap out to test if it was good (it is), then filled the eyelet back up with new solder. Filled it back up with tubes, even with the one ive been red plating, and the bias voltage was the same across all pin 5's. Wish it was something less embarrassing. And when i said partial, now the reverb effect does not work at all. no change in sound whatsoever no matter how high i turn the dial. i did try switching the red and white RCA plugs at the amp side to see if i plugged it backwards, but i only heard a very faint, delayed reverb sound, so it must not be plugged backwards.

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  12. #47
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    Quote Originally Posted by gromrock View Post
    (Partial) SUCCESS!!
    It must have been a cold or bad solder joint where the 0.1 cap and 220k resistor meet. I sucked out all of the solder out of that eyelet, pulled the cap out to test if it was good (it is), then filled the eyelet back up with new solder. Filled it back up with tubes, even with the one ive been red plating, and the bias voltage was the same across all pin 5's. Wish it was something less embarrassing. And when i said partial, now the reverb effect does not work at all. no change in sound whatsoever no matter how high i turn the dial. i did try switching the red and white RCA plugs at the amp side to see if i plugged it backwards, but i only heard a very faint, delayed reverb sound, so it must not be plugged backwards.
    When ever you have an amplifier gutted you can do a highly sophisticated procedure mostly referred to as the chopstick technique. Have a non conductor and surgically poke around in the amp, solder joints in particular. Some of the most puzzling issues I've come across have turned out the been bad solder joints to ground.

    I don't quite get the reverb thing you started now..? Was the reverb busted all the time or did it go south during your work on the amp? For starters I prescribe the same procedure as mentioned above. Target the reverb circuit.

    Cheers!

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  13. #48
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    Quote Originally Posted by gromrock View Post
    (Partial) SUCCESS!!
    It must have been a cold or bad solder joint where the 0.1 cap and 220k resistor meet. I sucked out all of the solder out of that eyelet, pulled the cap out to test if it was good (it is), then filled the eyelet back up with new solder. Filled it back up with tubes, even with the one ive been red plating, and the bias voltage was the same across all pin 5's. Wish it was something less embarrassing. And when i said partial, now the reverb effect does not work at all. no change in sound whatsoever no matter how high i turn the dial. i did try switching the red and white RCA plugs at the amp side to see if i plugged it backwards, but i only heard a very faint, delayed reverb sound, so it must not be plugged backwards.
    When ever you have an amplifier gutted you can do a highly sophisticated procedure mostly referred to as the chopstick technique. Have a non conductor and surgically poke around in the amp, solder joints in particular. Some of the most puzzling issues I've come across have turned out the been bad solder joints to ground.

    I don't quite get the reverb thing you started now..? Was the reverb%2

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    Last edited by überfuzz; 06-13-2012 at 02:05 PM.
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  14. #49
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    The reverb was working fine before I dug into the power section to try to get it to stop eating fuses. Im pretty sure it was because of a diode that had fallen appart. then it started red plating, then that got fixed. In the midst of all of that i was working on a reissue twin reverb with a serious voltage/bias issue that had always had a nasty distorted reverb section. i swapped pans to cross check it in the reissue and I think in the process I killed both pans. The reissue now has the same lack of reverb as the 67BF does. I metered the tank from the reissue and one half meters fine to around 200ohms and the other half has no connection between the chassis of the tank and the pin of the RCA ( i forget which is which color/input/output-wise). So im off to jrfonds shop here in town between shows to get a new tank. oh tube amps.

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    A standard Fender tank has an isolated input jack, so there will be no reading from chassis to hot pin of the RCA jack. Read directly across the RCA jack. It should read somewhere around 2 ohms.

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  16. #51
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    Quote Originally Posted by 52 Bill View Post
    A standard Fender tank has an isolated input jack, so there will be no reading from chassis to hot pin of the RCA jack. Read directly across the RCA jack. It should read somewhere around 2 ohms.
    Damn, literally just picked up a new tank. Now that I have it I will try it anyway. Side question: Why is the plate voltage on the reverb drive tube over 400vdc when spec for an 12at7 is supposed to max out at 300?

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  17. #52
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    I don't really get the reverb situation. Was it this amp that had a distorted reverb section..?

    Still, a wild guess is that it's not a bad reverb pan... What's been tested so far, more then the 67BF pan? Did you pull tubes to possibly narrow down the search? Have you probed any thing? Are you sure sure it's not going south before the 12AT7 tube. You could check the plate resistor on v2.

    Also, you might start a new thread regarding this issue (you'll probably get more attention than just mediocre me): Twin Reverb 65' - Distorted Tremolo channel

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    Shocker. Wasn't the pan in the reissue. Didnt have time to test it in the 67. I did try swappimg tubes and all of the basic checks without removing the chassis. I will start a new thread in a couple weeks if i dont get it solved myself before then. Its just baffling that both the reissue and the 67 are having the exact same problem. And to clerify uber, the reissue had an extreme distorted reverb EFFECT. The vibrato channel itself sounds fantastic. Still does, just nlw thdre is no distortion or any effect coming from the reverb effect in the reissue. Thanks to all for helping me through the red plating issue!

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  19. #54
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    This sounds all too familiar. If the socket fails to fix it do you have a scope? I had a thread on here back in March or April with a tweed twin and similar symptoms. Drove me nuts for days and I went through the same part replacement as you (I'm not a professional). My issue turned out to be an oscillation in the PI circuit. After banging my head against the wall for days I finally scoped it. I had replaced the OT and tried routing the leads without lifting the board. Obviously a lead dress problem. You may have an oscillation due to something else but I had the same symptoms and only on one side. I don't know if the tube would have red plated because I never gave it more than a few seconds with the standby off once I saw the voltage difference. May not be an oscillation but if you have a scope it's quick and easy to find out.

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  20. #55
    Old Timer J M Fahey's Avatar
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    I read this thread (54 posts) and slowly felt I was getting crazy.
    Why not stick to the original problem and point all guns to it until solved (or junked) instead of stopping and messing with reverb (a very different problem, and fully unrelated to the original) and even messing with *anpther* amp, with a *different* problem?.
    jm2c

    PS: and labelling it "partial" success is demeaning or at least wildly inaccurate.
    The original problem was *fully* solved.
    The amp has *another* problem, in another separate area?
    Unrelated.

    And ... the solution was suggested as early as post #4 !!!
    The socket itself could be bad, or a connection leading to pin 5.

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    Yes JM and now someone as unprofessional as I can learn a great deal from this thread and save some time and money. It was a learning experience. Money is not a giant concern of mine (i am not wealthy, btw) and every minute i had my hands in a piece of technology much older than I, I am thankful for what I learned from it. Sorry you wasted your time reading it all.

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    Quote Originally Posted by gromrock View Post
    Yes JM and now someone as unprofessional as I can learn a great deal from this thread and save some time and money. It was a learning experience. Money is not a giant concern of mine (i am not wealthy, btw) and every minute i had my hands in a piece of technology much older than I, I am thankful for what I learned from it. Sorry you wasted your time reading it all.
    Dude, don't worry about it. At least you didn't use crazy language to get your message through. ;-)

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