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Thread: Cathode follower without elevated heater voltage

  1. #36
    Bent Member Chuck H's Avatar
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    Well if the tubes were in place then there are problems other than filter caps. And the filters may not be part of what I'm seeing with those voltages. From the first to the fourth tube and half of the fifth tube you have no cathode voltage. With tubes in place you should have cathode voltage. I think you must have a ground fault for (at least) the cathode circuits for the first four and half of the fifth tubes.

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    That would be a new issue, right? I mean, if the amp at least partially worked before, this must be something that's happened while it's been inactive over the years? Oh...I should mention: My dmm isn't automatic. It was manually set on 1000 volts DC the whole time, and doesn't measure fractions of volts at that setting. Maybe that's why it read zero.

    Edit. Nah. Turned down to 200v range, where it measures to a tenth of a volt, and nothing on the cathode of the first stage. On 20v range and 2v still nothing (with 2 and 3 decimal places). I'm assuming I'm supposed to be measuring relative to ground, right? IE, the chassis?

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    Last edited by Dark Mavis; 03-09-2019 at 05:06 PM.

  3. #38
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Dark Mavis View Post
    It was manually set on 1000 volts DC the whole time. And all measurements were in reference to ground, where the common probe was clamped the whole time.
    Reference to ground is fine since the chassis is being used for all grounds in this amp with no apparently isolated circuits. But the manual 1000V range could be responsible for the zero (or virtually so) cathode readings. But you do show cathode voltage for half of tubes five and tube six.?. Still, maybe retest cathode pins on the preamp tubes with the meter range set lower and see if that doesn't change anything, AND/BUT, you're voltage drops are such that I still suspect a ground fault.

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    Quote Originally Posted by Chuck H View Post
    Reference to ground is fine since the chassis is being used for all grounds in this amp with no apparently isolated circuits. But the manual 1000V range could be responsible for the zero (or virtually so) cathode readings. But you do show cathode voltage for half of tubes five and tube six.?. Still, maybe retest cathode pins on the preamp tubes with the meter range set lower and see if that doesn't change anything, AND/BUT, you're voltage drops are such that I still suspect a ground fault.
    Already tried it. At least on the first stage. I'll check the others just to be thourough. Grids too?

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    Last edited by Dark Mavis; 03-09-2019 at 05:29 PM.

  5. #40
    Bent Member Chuck H's Avatar
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    Don't trouble with grids before we get cathode voltage. With no current through the tube there should be no voltage on the grid.

    With the amp off and voltage drained, please test resistance from the preamp tube cathode pins to ground (chassis).

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    Quote Originally Posted by Chuck H View Post
    Don't trouble with grids before we get cathode voltage. With no current through the tube there should be no voltage on the grid.

    With the amp off and voltage drained, please test resistance from the preamp tube cathode pins to ground (chassis).
    Okay.

    First tube
    Pin #3 806 ohms
    Pin #8 99.8k

    Second tube
    Pin #3 5.5k
    Pin #8 5.5k

    Third tube
    Pin #3 2.6k
    Pin #8 95.5k

    Fourth tube
    Pin #3 986 ohms
    Pin #8 1.47k

    Fifth tube
    Pin #3 1.47k
    Pin #8 1.47k.

    Sixth tube
    Both 22.5k (jumpered)

    Also, it's the first and third tubes that are wired as cathode followers. The first stage in the signal path is in the second tube. But that arrangement seems to make sense in terms of where they connect to the board. When I say "first tube", I mean the one physically farthest from the power supply side of the amp, and not neccessarily where they are in the signal path

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  7. #42
    Supporting Member eschertron's Avatar
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    If we take from your measurements that the cathode resistors are in place and wired as they should, then the reason for zero volts at the cathodes - and the reason for the high node and plate voltages - is that the tubes are not conducting. A fault in the heater wiring? If Chuck concurs, I'd suggest measuring heater pin voltages if not done already.

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  8. #43
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by eschertron View Post
    If we take from your measurements that the cathode resistors are in place and wired as they should, then the reason for zero volts at the cathodes - and the reason for the high node and plate voltages - is that the tubes are not conducting. A fault in the heater wiring? If Chuck concurs, I'd suggest measuring heater pin voltages if not done already.
    Concur. I hadn't thought of it before you mentioned it though. I thought it would be obvious if the tubes weren't lighting so I was looking for something else.

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    Quote Originally Posted by Chuck H View Post
    Concur. I hadn't thought of it before you mentioned it though. I thought it would be obvious if the tubes weren't lighting so I was looking for something else.
    They're under covers, so, I haven't looked. And the last thing I tried on this amp all those years ago was elevating the filaments on the preamp tubes. I may simply have forgotten to reconnect something. I'll take the covers off and see if the heaters are glowing, then check the voltage

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  10. #45
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    So...across the filament windings on the mains transformer, that powers the output tubes filaments - 6.3v. Across the separate transformer that powers the preamp tubes filaments - 7.2v. Also, the first four tubes ain't glowing. I dunno if there's a little short somewhere. The tube sockets are really badly wired, so maybe. I think I']m going to rewire the filaments anyways. The wire is in the way when I'm trying to measure stuff.
    Maybe the filament transformer is screwed, and that screwed the filaments? Or the other way around?
    And I really don't like that little circuit board with the channel switching relay on it, that's kind of hovering right over the fifth preamp tube. It's sitting right on the filament wire. In fact...looking at it now, it looks like the filament wire isn't even connected there, but the board hid that from me

    Edit. Nah. It's connected

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  11. #46
    Bent Member Chuck H's Avatar
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    We'll help where we can, but I don't envy you. I knew Dan Torres before he ever had the amp shop. He was just working out of his garage at his home. He was an early pioneer in the amp mod DIY genre in that he made efforts to bring it to the public rather than pretend it was wizardry And in truth I thought most of his circuits sounded good. He really only get's bad mouthed by other amp guys. Players typically like his stuff. And this from a guy who had somewhat of a falling out with him. I owned a couple of amps with his mods inside and another that he built in a Traynor chassis before he was making his own amps. I remember things getting a little crowded sometimes, but nothing like your amp. If you remember gigging with it then I have to assume it pretty much worked as it is (when repaired). Probably best to stay on track with that for now.

    The added filament transformer for the preamp was likely used because EL34 tubes draw more filament current than 6L6 tubes. So the existing PT filament winding wasn't up to the whole job by itself and needed a friend.

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  12. #47
    Supporting Member eschertron's Avatar
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    I thought I'd mention that the filament voltages at the transformer looks OK. I'd expect a broken connection somewhere. We now return you to your regular troubleshooting.

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    Quote Originally Posted by Chuck H View Post
    We'll help where we can, but I don't envy you. I knew Dan Torres before he ever had the amp shop. He was just working out of his garage at his home. He was an early pioneer in the amp mod DIY genre in that he made efforts to bring it to the public rather than pretend it was wizardry And in truth I thought most of his circuits sounded good. He really only get's bad mouthed by other amp guys. Players typically like his stuff. And this from a guy who had somewhat of a falling out with him. I owned a couple of amps with his mods inside and another that he built in a Traynor chassis before he was making his own amps. I remember things getting a little crowded sometimes, but nothing like your amp. If you remember gigging with it then I have to assume it pretty much worked as it is (when repaired). Probably best to stay on track with that for now.

    The added filament transformer for the preamp was likely used because EL34 tubes draw more filament current than 6L6 tubes. So the existing PT filament winding wasn't up to the whole job by itself and needed a friend.
    Most of the negative stuff I heard was more about the customer service at his store, and bad repair jobs and such that could have been done by some random employee for all I know. I like how the amp sounds, and I've not even been able to turn it up far enough to get it really singing.
    I dunno why I'm seeing 7.2 volts across that transformer. I just measured across the lugs where the resistors to ground are mounted. I never measured at each tube. It got too dark to be trying to poke the probes among all that mess. So...I gotta try to get it back to where it was, which was broken, but less broken than right now. Tomorrow, I'll check the pins for shorts. In some places, I could barely get a cigarette paper between one lug and another. I'll also swap out the 12ax7 in socket 5, with one of the four that have no glow, just to rule out 4 tubes with dead filaments, coincidentally ending up in the first 4 sockets. Since they're all wired in parallel, I guess it's the wiring rather than the tubes, unless 4 out of six tubes had the same problem and all ended up in the first 4 sockets, by sheer coincidence.

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    Quote Originally Posted by eschertron View Post
    I thought I'd mention that the filament voltages at the transformer looks OK. I'd expect a broken connection somewhere. We now return you to your regular troubleshooting.
    My thought exactly. The 7.2 volts seems a bit weird, but I'll worry about that later. I think there's a short or bad connection on that fourth or fifth tube. Maybe it's in the tube itself. I'll try subbing them, too. But that's gotta wait. it's too dark now

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  15. #50
    Bent Member Chuck H's Avatar
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    FWIW if there is a fault*** in the filament wiring then that could be the reason for the high voltage. Filament windings need to handle a relatively high current. The voltage will drop some when current is being pulled through the whole circuits resistance (winding, wire, tubes). I doubt it'll drop to 6.3V, but it'll drop some.

    *** I prefer to call an unintentionally open circuit a fault and an unintentionally closed circuit a short. I do this for clarity because a short is ONLY an unintentional connection and not an open circuit. Though it's common for people to call any circuit flaw a "short", it's technically wrong and confusing for troubleshooting and repair efforts.

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    Quote Originally Posted by Chuck H View Post
    FWIW if there is a fault*** in the filament wiring then that could be the reason for the high voltage. Filament windings need to handle a relatively high current. The voltage will drop some when current is being pulled through the whole circuits resistance (winding, wire, tubes). I doubt it'll drop to 6.3V, but it'll drop some.

    *** I prefer to call an unintentionally open circuit a fault and an unintentionally closed circuit a short. I do this for clarity because a short is ONLY an unintentional connection and not an open circuit. Though it's common for people to call any circuit flaw a "short", it's technically wrong and confusing for troubleshooting and repair efforts.
    I think I'll check continuity along the filament string, and check there isn't continuity where there shouldn't be. I only suspect a short 'cus ridiculously crowded and badly wired lugs shorting seems more likely than a complete break in pretty thick stranded wire. Or maybe something is up inside a tube, and filament current is going somewhere it shouldn't. In which case, I'd suspect the fourth tube, since the filament transformer connects at the sixth, and the fifth and sixth have working filaments? Just guessing here. I'll test the wiring first. If that seems okay, I'll sub some tubes. I don't know much about tube amps, so it's just educated guesses. Though...educated by what, I have no idea. All I remember from school, is what happened on the lunch breaks.

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    A short will cause excess current consumption, resulting in lowering of the winding's voltage and heat. A short will not make the voltage rise up at the winding. An open circuit will.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by g1 View Post
    ...make the voltage rise up at the winding. An open circuit will.
    I thought I was in the presence of Yoda for a second

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  19. #54
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    Quote Originally Posted by eschertron View Post
    I thought I was in the presence of Yoda for a second
    Yoda... John Candy. It's easy to get them confused.

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    Sooo...a new day. I'm thinking, it's gotta be the wires, right? A fault in one tube's heater, wouldn't stop the other tube's heater's working, surely?

    Hmmm...with all the tubes out, I get continuity the full length of both sides, BUT also from one side to the other. The resistance is 0.4ohms from one side to the other. Is that right? With the tubes out, he only things between those legs should be the 100 ohm resistors to ground, and the transformer. Think I'll disconnect the transformer and check again

    With the filament transformer disconnected, and no tubes in place, I'm getting 197 ohms between the 2 sides of the filament string. Kinda what I'd expect. So now I can check the wiring without that confusing me. Should the secondary on the transformer ven have such low resistance? I guess it's gonna be pretty low cus...stepping down from 240v to 6.3v, so maybe.

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    Last edited by Dark Mavis; 03-10-2019 at 02:17 PM.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Dark Mavis View Post
    With the filament transformer disconnected, and no tubes in place, I'm getting 197 ohms between the 2 sides of the filament string. Kinda what I'd expect. So now I can check the wiring without that confusing me. Should the secondary on the transformer ven have such low resistance? I guess it's gonna be pretty low cus...stepping down from 240v to 6.3v, so maybe.
    BING BING BING BING BING!!!

    That's what Enzo was talking about with "seeing" what's there to see. You nailed it. That filament secondary will be very low resistance. But do set your meter for a low range and see to it that there is some resistance. Maybe also check for continuity between the primary and the secondary (should be none) and each winding to the chassis (should be none).

    Does that transformer feed any tubes that ARE lighting up or just the tubes that aren't?

    Make sure the false CT (the two 100ohm resistors) is actually grounded. That 197 ohm reading would indicate either that they are not or that you lifted the ground before measuring.

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    Secondary resistance on that trans is .5 ohms. I can't be any more accurate, cus the meter doesn't do hundredths. There is no continuity between primary and secondary. Well, between the lugs of the power socket and the secondary. No continuity between either secondary and ground - They're disconnected, still, and sticking up in the air. Resistance to ground is about 100 ohm on every filament pin, as, I guess you'd expect. There's continuity to ground, from the false center tap. I also checked for continuity between every filament pin, and every adjacent pin. There was none, BUT only a resistance of around 1.5 ohms between the filament pins on tubes 4 and 5, and the cathode pins next to them. (again, with no tubes in the amp) The rest show no reading at all.
    Isn't around 200 ohms what you'd expect the resistance from one side of the filament windings to the other to be? Since you'd surely be measuring those two 100 ohm resistors in series, right? Form either side to ground, it measures 100 ohms.
    Maybe the filaments are working fine but just not glowing?

    Edit, yeah the extra transformer powers the filaments of all the preamp tubes, two of which were glowing. The last two. The driver and - I think - the stages before and after the reverb send and return.

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    Last edited by Dark Mavis; 03-10-2019 at 05:45 PM.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Dark Mavis View Post
    I also checked for continuity between every filament pin, and every adjacent pin. There was none, BUT only a resistance of around 1.5 ohms between the filament pins on tubes 4 and 5, and the cathode pins next to them. (again, with no tubes in the amp) The rest show no reading at all.
    I can't come up with any reason for that. It's odd so I thought I might be missing something obvious, but that's a problem.

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    Quote Originally Posted by Chuck H View Post
    I can't come up with any reason for that. It's odd so I thought I might be missing something obvious, but that's a problem.
    I see 1.5k resistors from the cathode to ground, on the schematic, and I think it's the right tubes. If they put in 1.5 ohm resistors by mistake...ah, no, cus then it would read 101.5ohms or more from the filament pin, down the filament wire, through the 100 ohm resistor to ground, through the chassis, through the cathode resistor, and to the cathode pin. I'll check those resistors anyways.

    My bad. It's 1.5K from the filament pin to the cathode. 1.57, actually.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Dark Mavis View Post
    It's 1.5K from the filament pin to the cathode. 1.57, actually.
    That's better.

    What's weird is that there's no current through half of V5 as well as the first four tubes.?. I would look for a wiring fault at pins 4 and 5 (typically joined) on V5 where the filament lead connects. I know you already tested for continuity across the sockets, but do it again and wiggle the probe on the pins. It might be an intermittent fault that you're closing when the you press the meter probe into it.?.

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    Lifetime Member Enzo's Avatar
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    You are missing one factor. Yes there were two 100 ohms resistors, but they are rally in parallel to your meter. One end they are connected together and to ground. The other ends go to the winding. But the winding has half an ohm resistance. That means the two resistors are essentially parallel.

    In fact when you say you measure 100 ohms to ground, I assume you have the transformer disconnected. Otherwise you'd get a reading of 50 ohms.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    You are missing one factor. Yes there were two 100 ohms resistors, but they are rally in parallel to your meter. One end they are connected together and to ground. The other ends go to the winding. But the winding has half an ohm resistance. That means the two resistors are essentially parallel.

    In fact when you say you measure 100 ohms to ground, I assume you have the transformer disconnected. Otherwise you'd get a reading of 50 ohms.
    Yes, there was some confusion. As it is the transformer is disconnected and the false CT is still grounded. So, 197R between the branches and 100R to ground from each sounds right.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  28. #63
    Don't forget the joker g1's Avatar
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    Dark Mavis: It is best not to make corrections via edits after there have been responses.
    If you must, just add the word 'edit:' and the corrections at the bottom of the post in question, or make a new post.
    Otherwise, people's comments no longer make sense and it can throw a wrench into the troubleshooting procedure.

    For example, Chuck's comment in post #56 about the 197 ohm reading. I know he put it there for a reason.
    Same for my deleted comment above.

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    My bad. I have the amp open next to me, and the screen open in front of me. If I find I mis-measured or made a mistake, I just changed the comment without noticing that there were any new posts.

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    So then...to be clear, the 200 (approx) ohms from side to side, with the transformer disconnected is as expected? And I have continuity the full length of each winding, so...maybe it's the tubes? Is there a way to check the filament of each tube? I know the DC resistance won't tell us much when the tube is cold.
    The thing that makes me think it's the winding, was just that it was the last two tubes that had glowing filaments. I have 8 preamp tubes here in total, so I can swap them around randomly a few times, and see if I get a different pattern? Or maybe just try a single tube in the last socket, then try every tube, by itself in that location?
    I also need to check the cathode and grid voltages with the dmm on a lower setting. I checked a few, but not all of them.
    I guess I should try plugging a guitar into the amp? I have no idea if this is part of the original problem, or something new. Remember: the last time I opened this amp, like 15 years ago, I got an accidental short with the probe, while reading the plate voltage, that caused the filament resistors to go up in smoke. BUT, that was on an output tube. The output tubes filaments have a different supply, and a different pair of resistors to ground, so I dunno how that could somehow burn out the filaments of 4 preamp tubes.
    But from what people are saying, we're looking for a broken connection, rather than a short, right? Like one or more filaments are broken completely, or just not allowing current through? The tubes were sitting loose in the bottom of the cabinet, so I'll give the pins a quick wipe with contact cleaner too, and push them in and out a few times.

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  31. #66
    Supporting Member eschertron's Avatar
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    You may try this: take a tube that 'glows' and try it in each socket. Some tubes have a heater glow that is more visible, so we can eliminate that variable with one tube only.

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    Quote Originally Posted by eschertron View Post
    You may try this: take a tube that 'glows' and try it in each socket. Some tubes have a heater glow that is more visible, so we can eliminate that variable with one tube only.
    I'll do that too. At the moment I'm just tidying the lead dress and totally rewiring the filaments, just so I can look at the amp without imagining noodles

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    Hmmm...I found one discrepancy between the amp and the wiring diagram. On tubes 1 and 3 - the ones wired as cathode followers, the drawing shows a small capacitor between pins 6 and 7, and a jumper between pins 7 and 1, but that's not what was on the actual socket in the amp, if you look at the pic

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    Last edited by Dark Mavis; 03-14-2019 at 02:05 AM.

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    Okay, so I decided to go ahead and clean up the wiring, and I'm glad I did. other than two preamp tube sockets wired up incorrectly, I found one cap that wasn't connected on one end, and a washer and piece of stray wire floating around between the circuit board and the blank board behind it that's supposedly there to prevent shorts...
    Also, I now feel that I know the circuit a lot better. Anyways...Here's the before and after pics.
    Click image for larger version. 

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    I'll post a diagram showing how tubes 1 and 3 were wired, vs how they were supposed to be wired, too. Then I'm gonna check and re-check, that everything is connected where it should be, then I'm just gonna plug a guitar in and see if the problem even still exists

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  35. #70
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    Here's how tubes 1 and 3 were supposed to be wired...

    Click image for larger version. 

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    What's in the amp was more like this.
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    I could be wrong about that jumper that shorts the plate resistor. It's hard to see on the photo. It might have gone from the grid of the first stage to the plate of the second. It definitely wasn't going from the plate of the first stage to the grid of the second, like it was supposed to. Also... The order of that .22 cap and 3.3m resistor is reversed in the amp. I don't think it makes a difference in this particular case, but...
    DOES it? Am i missing something there?

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