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Thread: Deluxe Reverb strangeness

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    Deluxe Reverb strangeness

    Hello shut ins. I have a DR that came in for a loud buzz, turned out to be reversed reverb switch connector with Tremolo. That problem solved, I checked out the other functions and noticed the reverb and tremolo didn't work. That led me to the tremolo tube with the first triode shorted cathode to filament. Now here is where the oddness come in. When swapping out the bad tube, the reverb and trem both worked. I don't yet understand why, so I put the bad tube back in just to confirm my findings. It was then that the sound of the amp got quite low and gravelly, and a couple of times faded to nothing, until I put a probe on the reverb channel plate, which made a pop, and then the sound returned to normal. I probed and resoldered around that area, but found no change.

    This happened several times. I'm still collecting data, but here is what I think. The times it has done this is when the bad tube is in place when it is powered up after a long cool down period. Every time I touch a probe to a plate, it returns to normal. Once while it was in this condition I popped out the bad tube, and it stayed in this condition until I tried to measure with a probe. I even connected the probe to a few different plates before powering up, and in each of these cases it did not do it. As far as I know it has only done this condition when the bad tube was in place. Most of the time it powers up and sounds fine. Indeed, if it does power up in this condition, once I either probe it, or hit a solid guitar chord it will come back and as far as I can tell stay good.

    Now I know someone is going to tell me not to put the bad tube back in, but when it is not in the odd condition, everything looks and sounds pretty normal. I don't know what is the deal here, and can live with tossing the tube and just accepting that as the problem, but I would like to understand what is happening here. How does a bad trem tube make the reverb not work?

    http://ampwares.com/schematics/deluxe_reverb_ab763.pdf

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    It's weird, because it WAS working fine.....

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    When it was working without trem and reverb, it was normal sounding? Or always the 'low and gravelly'?

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Supporting Member Randall's Avatar
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    Through the fog of war, I am not certain of the timeline, but I think it was both, yes and sometimes no.

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    It's weird, because it WAS working fine.....

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    Well dang, I thought I had it beat, but alas, it did it again just now.

    At least now I can be more susinct about conditions, and hopefully less confusing. The bad tube in question has been sidelined before the most recent episode, so that rules it out. New tube in trem position, and two new plate resistors for V2. I left it on for about an hour and when I randomly checked it, it was doing the low fading out gravelly sound, but only in Channel 2, channel 1 was clear.

    I measured good filament voltages on both channel 2 and the PI, and as before, the second I touched my probe to measure plate voltage, it snapped back to normal. The entire area around V2 has been resoldered on both ends of the wires, prior to this. Also, A strong signal from the guitar will put it back to normal as well.

    This is a sticky and tricky one.

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    Touching a probe to the circuit completes a circuit with the impedance of your meter. It suggests to me perhaps and open grid return resistor. WHich plate? And by channel 2 we mean vibrato channel?

    Also, putting probe to plate pin pushed on the tube socket pin, which may be involved. Just for science, instead of measuring at the plate pin, measure at the end of the plate resistor or the coupling cap lead. Electrically the same but gets you away from the socket. Any difference?

    Or instead of looking at plate voltage, try checking cathode voltage instead. Does that also bring it back, or does the cathode voltage look right?

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    Don't forget the joker g1's Avatar
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    My only guess would be that it pulled down the heater voltage a bit, and one of the reverb tubes was more sensitive to low heater voltage than the other tubes.
    The heater is the only thing I see in common between those circuits, and you said it was a heater to cathode fault so who knows.

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    "WHich plate? And by channel 2 we mean vibrato channel?"

    Either one. And yes vibrato channel.

    " Just for science, instead of measuring at the plate pin, measure at the end of the plate resistor or the coupling cap lead."

    This is how I am measuring.

    "Or instead of looking at plate voltage, try checking cathode voltage instead. "

    Cathode voltage looks fine in normal operation. The next time I can get it to do it, I will put a probe on and see.

    "The heater is the only thing I see in common between those circuits, and you said it was a heater to cathode fault so who knows. "

    That's what I was thinking also, but remember the bad tube is removed from the picture when it failed this last time. And I measured good filament voltages on a couple of tube while in fault condition, so I don't think this is the issue.

    edit:

    OK, so I just turned it back on after a 40 minute cool down, and it went into failure mode, signal low and scratchy and fading out to almost nothing. Cathodes measure fine, and do not change anything when probed. Measuring the grids at the socket looked good, but made no change, but when I probed the resistors at the input jacks, it corrected. I do not know if this was from the inevitable pop this made. I hit them with freeze spray with no change. This is all at V2.

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    Last edited by Randall; 04-01-2020 at 05:14 AM.
    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    OK, how about apaply a signal, and when this happens, leave it that way. Use a signal tracer (listening amp) and see where the signal collapses. Hey, if you turn up the reverb, do you get reasonable reverb signal? Or is the reverb the same garbled stuff?

    You can probe grids without it resets? Fine, scope or listen to the grids of the two stages. SOund OK? Or do one or the other or both go bad?

    Did we do the whack test? Whack the chassis with a mallet, any reaction?

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    I may be wrong but I suspect that the amp is at the verge of instability and that the observed effects are caused by HF oscillation.

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    I'm not sure about the reverb, I missed it last time around. But have done the whack and probe test, no change.

    However, following Hemholtz lead, I do find what I think is a 45K wave at the speaker jack, plugged into the vibrato channel (the one in question), and channel vol turned up to 4. If my calculations are correct, I measure around 22 uSec for a complete wave. 1/0.000022 = 45, 454 Hz. Correct? No such wave appears with the normal channel plugged in and dialed up. Keep in mind, this is after the condition had reset, and the amp sounding normal. I will try to get the same measurements next time it fails. My problem here is when I touch a probe to some things, it resets, but not others.

    The wave disappears if the channel volume is turned full off, if the guitar is turned full off, or if either grid on V2 is grounded. So far moving leads has made no change.

    Am I barking up the wrong tree here, or is Hemholtz onto something?

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    Please always post scope settings with scope pics.

    A 45kHz signal at the output is clear evidence of oscillation. Prime suspects are typically filter caps.

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    Last edited by Helmholtz; 04-01-2020 at 10:39 PM.
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    Scope settings are 5 uSec/division and 0.1 v/div.

    I have more data. When I turned it on just now went into failure, and the reverb was also effected, scratchy and barely there. Also, in failure mode the 45K wave almost disappears into about 0.1v of hash. I probed the reverb send tube plates and it made no change, but it snapped back to life when I probed the B+ at the power supply side, and the 45K wave returned. I also swapped the preamp tubes before powering up to rule them out.

    If it is filter caps, why does it only effect the vibrato channel, but not the normal channel? And anyway they have been done fairly recently.

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    Last edited by Randall; 04-02-2020 at 01:46 AM.
    It's weird, because it WAS working fine.....

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    If it is filter caps, why does it only effect the vibrato channel, but not the normal channel?
    Oscillation often involves more than one amp stage and I think that the reverb circuit is involved.
    So especially check the screen filter cap and its ground connection. You could temporarily wire a known good cap in parallel.


    When I turned it on just now went into failure, and the reverb was also effected, scratchy and barely there. Also, in failure mode the 45K wave almost disappears into about 0.1v of hash. I probed the reverb send tube plates and it made no change, but it snapped back to life when I probed the B+ at the power supply side, and the 45K wave returned.
    One possibility is that the amp toggles between 2 different oscillation frequencies, the higher one being above the bandwidth limit of the OT.
    What is the bandwidth of your scope?

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    Last edited by Helmholtz; 04-02-2020 at 02:08 PM.
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    Bent Member Chuck H's Avatar
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    I'm going to second Helmholtz line of reasoning here. I was actually thinking it earlier, but wasn't quite sure enough to post.

    Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't. This would explain why you're only experiencing the problem with the vibrato channel and support the probability of bad filter caps.

    You can try paralleling a good cap with the last preamp filter cap. I'll bet it stops the problem. And...

    I know it's not your MO to change caps that aren't detected as bad. But let's try to rationalize this. If it turns out that replacing the preamp filter solves the problem I think you should just perform a full cap job on the amp. Unless there's some weirdness in there like THAT particular cap is the only original cap.?. If the caps are all of the same make it's entirely probable the other caps are not sound. They can test fine on a tester and still not be snuff in the higher voltage environment of that amp. The amp may even perform ok even if the other caps are failing or close to failing since the other nodes share antiphase circuits.

    It just seems counter productive to me to have an amp open with ONE of the filter caps is exhibiting failure and not replace them all. Including the bias supply caps since they're part of a critical circuit that prevents catastrophic failures!!! They ARE age sensitive parts. But it happens all the time. Someone finds 'a' bad cap and replaces 'the' bad cap. Amp "works" and away it goes. Chances are good the amp could "work" a lot better and more reliably if a full cap job were performed.

    JM2C

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    The bandwidth of my scope is 20MHz.

    And I stand corrected, the Sprague Atoms in the can are marked 9-99, so they are over 20 years old. I will lobby for replacing them. I already did the cathode caps, the bias cap is an F&T, I'm not too worried about it.

    I hope this gets it because this condition only happens sporadically, maybe once every 5 times it is powered up cold, so it is a time glutton, and is clogging up my bench. Not that I have all that much work right now anyway.

    I have only four 16mF/475v caps on hand, if you were to replace one of the Spragues with a 22uF, which one would you pick? Or does it really matter?

    "Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't. "

    I'm not sure what you are saying here. It looks to me that both channels as well as the reverb tube recovery are served by the last filter cap D.

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    Last edited by Randall; 04-02-2020 at 09:11 PM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Chuck H View Post
    Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't.
    Quote Originally Posted by Randall View Post
    I'm not sure what you are saying here. It looks to me that both channels as well as the reverb tube recovery are served by the last filter cap D.
    The reverb doesn't generally ascribe to the same phase considerations as the main (dry) signal path because there are multiple phase angles in the reverberated signal and it's rendered quite dissimilar from the main signal.

    Both channels are not used by the same instrument simultaneously in this design. We can ignore unused gain stages because there's no signal on them. If you follow the phase inversions in each channel you'll see that the normal channel only has one inversion before the phase inverter (which is supplied by the C node). The vibrato channel has two inversions of the dry signal, so there are like phase gain stages filtered at the same node. If that node isn't filtering there can be like phase interaction and that can cause oscillation. In fact like phase interaction is how oscillator circuits are made.

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    Last edited by Chuck H; 04-03-2020 at 01:50 AM.
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    I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.
    Sure. Why not.?. Though I don't think it matters. Back in the day those caps had a -20%/+80% value spec!!! Whatever modern part you're putting in will be spec with the original design regardless of where the 22uf goes. As to where it might benefit the circuit most? I dunno. I'd probably put the youngest cap in your collection in the D position in the hope that it will be the freshest and most viable. Not sure how long you store your electrolytics before using them.?.

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    Lifetime Member Enzo's Avatar
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    That^^^

    If I had a bunch of values, I'd put the largest one first. But you are talking 16 versus 22, and that is such a tiny difference as to not matter.

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    Supporting Member Randall's Avatar
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    Well, darn. I replaced the filter caps, and the oscillation is still there in channel 2. It hasn't gone in the weak snarly condition yet, but I expect it will.

    Now what?

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    Bent Member Chuck H's Avatar
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    Were all the filter cap ground lead terminations checked and/or redone? What year Deluxe Reverb is this?

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    Checking the filter cap grounds now, they all look good, less than 1 ohm to chassis. It is a 1973 DR.

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    Last edited by Randall; 04-04-2020 at 03:58 AM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Ok. Any modifications. Did it get "blacfaced" or have the small power tube grid caps been cut out, etc? The DR didn't have the same problems as other models, but they still added those grid to ground caps to the design. Probably to mitigate oscillation in shipped amps because the older wiring wasn't being duplicated.?. But I don't really know.

    I'm surprised the cap job didn't solve it. I still think it's an oscillation though. If the amp has been modified it's possible some grounds have been moved?

    I'll have to give this another read through tomorrow.

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    Quote Originally Posted by Randall View Post
    Well, darn. I replaced the filter caps, and the oscillation is still there in channel 2. It hasn't gone in the weak snarly condition yet, but I expect it will.

    Now what?
    Does pulling the reverb driver stop it?

    If yes, clean all reverb related contacts: cable jacks, plugs, jacks to chassis. Same for tank side.

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    Supporting Member Randall's Avatar
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    Not sure if has been modded, but indeed it does not have those output grid caps.

    Pulling the reverb driver makes no difference.

    I have powered it up and down between long waits maybe 8 times since I replaced the filter caps yesterday, and although I do see the wave on my scope in Vibrato channel only, it has not once done into failure mode. I need to get this thing figured out, it has been on my bench for a week now, and I have more jobs to do.

    edit: Well, no sooner had I typed this and it powered up in failure mode. In fact, I powered it up with a couple of 0.002uF caps clipped in from the output grids to ground, and it had barely any output. Thinking I had done something wrong I unclipped them, but it made no difference. Swapped V1 and V2 for good measure and that caused it to reset to normal. I however find that in normal mode the caps reduce the wave to about 1/5 in size. Strange how it didn't seem to effect the output? The wave is there regardless of which input tube is in V2.

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    Last edited by Randall; 04-04-2020 at 07:16 PM.
    It's weird, because it WAS working fine.....

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    Ok now use a 10:1 probe with your scope and trace back the oscillation proceeding from output to input.

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    Last edited by Helmholtz; 04-04-2020 at 08:09 PM.
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    Supporting Member Randall's Avatar
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    I'm sorry, I am an idiot. I am afraid I have led you guys down a primrose path. This "oscillation" is probably not the real issue. I have been testing with a strat lying on a table maybe 3 -4 feet away from the amp. I can reach over and brush the strings to tell if it is failure mode or not. I just figured out if I pick up the guitar and point it just right while touching the strings, the "oscillation" all but disappears. Why I don't see the same condition on the normal channel, I don't know, but I am now convinced this is not what is causing my problem.

    Interestingly, while I was realizing this, I only had a 12AX7 in the vibrato channel, none in the normal side. While in this configuration with the amp acting normally, I plugged in an RCA 12AX7 that the customer is bent on using into the empty normal channel. Once it came to temp, the failure condition happened in the vibrato channel. I pulled it back out, the amp popped, and the condition went away. I put this tube in my tester, and it showed one triode right at the minimum mutual conductance value allowed, and about half of the other triode. I'm starting to think this tube may be the culprit. Hard to prove without many power cycles to try to get it to go into failure mode with a different tube.

    If this is the fix, I am sorry to have wasted anyone's time.

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    Bent Member Chuck H's Avatar
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    I have had many conundrums on my bench just as a designer/builder. I actually like it in a masochistic way when things get tricky. But I don't know if I have the constitution for real working mans repair work!?! Hat's off to you (and others) that dedicate themselves and always figure it out.

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    Supporting Member Randall's Avatar
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    This thing is making me crazy!!

    I last had the wave on the scope with the vibrato channel sounding good, suspected faulty tube out, fresh replacement in. Turned it off for about an hour, switched it back on with nothing changed, and now it still sounds fine, but the wave is gone, and nothing I do gets it back. Not that I want it back, but how can this be? What is going on here? Am I going mad?

    Damn.

    edit: And, I power down, turn back on 12 minutes later, and the damn wave is back, but it sounds fine.

    edit again: I turned it off, waited another hour, and this time it powered up in fault mode, very little signal, but with NO WAVE on the scope! It stayed in fault mode until I put a probe on something, then it reset to normal, and still with no wave. ARRGG!!

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    Last edited by Randall; 04-05-2020 at 12:26 AM.
    It's weird, because it WAS working fine.....

  30. #30
    Bent Member Chuck H's Avatar
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    I hate to put you onto this... Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't.

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    "Take two placebos, works twice as well." Enzo

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    "Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't. "

    Yes. I have done that pretty early into this, on the top board. How do we check the bottom board?

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    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    "Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't. "

    Yes. I have done that pretty early into this, on the top board. How do we check the bottom board?
    I've never checked a bottom board. I was just observing that it's a possibility. I suppose you could try to wedge the two boards apart and jam some spacers in between. If the problem changes or goes away then that could be evidence, or circumstantial.

    Super great that you're not getting voltage reading on the component board. That would be a really big "Phew" (wipe forehead) for me because I've had to replace a few conductive boards and have learned to expect it as often as not. So good news there.

    I think it could still be something like a cold solder joint. I've had a couple of amps do really weird things that couldn't be entirely explained (by me) by cold joints I eventually found and fixed. You could try flexing the board a bit while the amp is operating. I know you've already poked at the components.

    Have you taken DC measurements when inducing the problem? Just to see if it's something happening in the power supply rather than the signal chain?

    Since you suspected that one tube as being the cause, but then the problem returned, did you reflow the joints on that tube socket? I would.

    You mentioned having a guitar on the bench when inducing the problem. You've also mentioned witnessing an anomalous waveform. I assume these are the same issue? One test audible with the guitar and the other visible on the scope?

    And you didn't mention if there's been any modification to the amp like blackfacing, etc.

    Just a thought... If touching a certain point in the circuit with your meter probe always fixes the problem you could just solder a 10M resistor parallel to a 10p cap from one of those points to ground and then see if you can't re instigate the problem. If you can't then the amp is fixed. Well, sort of. It's a band aid for an oscillation issue, but it might be fine and it would get the amp off your bench.

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    "Take two placebos, works twice as well." Enzo

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    Supporting Member Randall's Avatar
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    All good points. I have probed and prodded, and flexed the board. I have resoldered around V2, I have replaced the two V2 plate resistors. When the problem is happening, if I measure DC at either plate or red B+ point, it resets to normal. I don't see any mods except that the 120pF power tube caps are gone. Well, actually some of the tone caps have been changed.

    I know this has gotten confusing, but when Hemholtz suggested an oscillation issue, I dialed in what I thought was a 45K wave that was only there when the amp was normal. When the fault happened, the wave largely collapsed to about 1/5 of original size. This was pretty consistant for a while. Then the opposite happened. Since then I have had the fault happen with and without the wave present, and I have had it sound normal with and without the wave present. So at this time I think there is no correlation between the two. Also, I replaced the tube I suspected, with no change in results.

    I think the next thing I will do is just hit every solder joint. Can't hurt. This is such a beast because of the randomness of the fault condition. I have tried all day and some of last night to get it to do it again, but it hasn't done it once.

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    It's weird, because it WAS working fine.....

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    NO WAVE on the scope!
    No visible wave at the output doesn't mean the amp is not oscillating as explained above. The OT would hardly transfer something like 200kHz. So you need to look for oscillation before the OT.

    Freeze spray may help to identify a bad contact or component.

    I am sure the fault mode changes some cathode voltage. No need to touch a plate.

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    Last edited by Helmholtz; 04-06-2020 at 12:50 AM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    When the problem is happening, if I measure DC at either plate or red B+ point, it resets to normal.
    I'm just registering this detail now. That is weird. The impedance at that node should be virtually zero, so it's isolated from the signal path. That amp should have a grounded chassis, correct? So what sort of shift could your meter be making to the operating conditions when touching a B+ node???

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    "Take two placebos, works twice as well." Enzo

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

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