Page 1 of 2 12 LastLast
Results 1 to 35 of 50

Thread: Cathode Bias Red Plating Discussion

  1. #1
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0

    Cathode Bias Red Plating Discussion

    Hey,

    Was working on a Krank Rev Jr. recently, tubes are red plating. I would post a schematic but I can't find one anywhere. It is a cathode biased amp, seems pretty simple in terms of the OT tube stage, Cathode biased 6L6, can be subbed with 6v6 according to Krank.

    Increased the Cathode resistor from 500 Ohms to 1k (quick and dirty test...) no change there. Tried with (several) known working tubes, they all red plated. (These were crummy test tubes FYI).

    The Plate / Grid voltages are:

    WITH TUBES
    Pin 5 Control - 30mv
    Pin 4 - Screen - 267VDC
    Pin 3 Plate -243VDC

    WITHOUT TUBES:
    Pin 5 Control 0mv
    pin 4 Screen 411 VDC
    pin 3 Plate 411


    From my limited knowledge, I am under the impression that Screen voltage is kept slightly below plate voltage to help control the current in the output tubes. if this is true, could that be the source of redplating for the tubes?

    And further, can someone help explain the relationship between the screen and it's control circuit? Without a schematic (and with an untraceable, unremovable PCB) I'm at a bit of a loss.

    Thanks in advance,

    Mr. B

    0 Not allowed! Not allowed!
    Last edited by Mr_bibbles; 06-01-2018 at 09:59 PM.

  2. #2
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,399
    Thumbs Up/Down
    Received: 372/1
    Given: 312/0
    You didn't report the cathode voltage which, IMO, is the important thing. Measure voltage across the cathode resistor and calculate bias. Is there a cathode bypass cap? Maybe it's shorted. Measure the in circuit resistance across the cathode resistor. If you measure less than your cathode resistor, my bet would be a shorted cathode cap. Of course all conjecture without a schematic.

    2 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  3. #3
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Thanks Dude,


    Good guess! Measured 345 ohms across the resistor in circuit, measured 500 ohms with one leg up. There is no bypass cap connected to the resistor or near it, I did however bridge the resistor with a 100uf Cap in parallel to see if that would help, and it did not. The traces are on the underside and the sockets prevent me from removing the board without significant work. Bonus aggravation is polyester block caps/radial electrolytics who's leads I can't measure for continuity or anything else. I also already contacted Krank and they cannot get me a schematic...

    Cathode resistor voltage drop (Measuring at both sides of resistor with tubes in) 1.985VDC, which according to Weber Bias Calc puts the tubes at 5% of 6v6 dissipation at 290 Plate to ground voltage??? This is either crazy low or I am doing it wrong. Without access to a schematic, would there be a way to hook up a Bypass cap to correct the short? Or is there some way I could make an educated guess on the electrolytics around the OT pins??

    TL/DR
    Cathode resistor measures low in circuit, fine when lifted. Cathode bias seems low but I'm not 100% sure I calculated it correctly. Bypass cap possibly shorted, difficult to find out which one it is.

    0 Not allowed! Not allowed!

  4. #4
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,399
    Thumbs Up/Down
    Received: 372/1
    Given: 312/0
    Adding another cap won't remove the short (or low resistance). You'll have to hunt it down. Try removing the output tubes and checking the resistance again. You could have a tube with an H/K short. If that doesn't change things, you'll have to go deep diving. When you calculated bias current, did you use the measured resistor value or the value stamped on the resistor? The cathode cap, if there is one, would likely be in the 25-50V range. It wouldn't be one of the larger 350-500V caps. Since you don't have a schematic, can you upload a picture?

    0 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  5. #5
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Makes sense for the cap not making a difference, duh! 0 ohms (short) parallelled with anything will be 0.

    Measured, meaning in circuit value? 258 Ohms, plugged it in with same voltage drop and P --> C voltage, 9% per tube?

    I've tried multiple tubes, so I feel like I can rule out the 6v6's. What is an H / K short though?

    here are some pics:

    Click image for larger version. 

Name:	IMG_0410.JPG 
Views:	55 
Size:	120.9 KB 
ID:	49044Click image for larger version. 

Name:	IMG_0411.JPG 
Views:	58 
Size:	141.3 KB 
ID:	49045Click image for larger version. 

Name:	IMG_0413.JPG 
Views:	51 
Size:	93.1 KB 
ID:	49046Click image for larger version. 

Name:	IMG_0412.JPG 
Views:	53 
Size:	101.5 KB 
ID:	49047

    There are really only 3 appropriately sized polarized electrolytic caps for the bypass, I could just snip / top mount them all.... guessing it's one of the two on the right though. The one by the FX loop (left side) is 100uf, the other two are 22uf.

    0 Not allowed! Not allowed!

  6. #6
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,862
    Thumbs Up/Down
    Received: 120/0
    Given: 284/1
    my hypothesis: something (I can't say what) is shorting out the cathode resistor, or seriously reducing its ability to control tube current. You've tried replacing the cathode resistor? Good. Now unsolder it and remove it from circuit. Do the tubes still redplate or show the B+ being pulled down?

    edit: could a lead from the cathode resistor be touching the chassis?

    1 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  7. #7
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Well you said the cathode resistor measured 345 ohms in circuit and then you said it measured 258 ohms in circuit. If both of these are accurate then something other than the cathode resistor is kludging the resistance. I like escher's idea of lifting the resistor and rechecking. That would tell you if you need to hunt down a short real fast.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  8. #8
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,810
    Thumbs Up/Down
    Received: 728/4
    Given: 0/0
    Pull the tube, power up, any voltage on cathode pin?

    Lift 500 ohm resistor, now measure resistance to ground from cathode.

    Is cathode pin touching heater pin?

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Quote Originally Posted by eschertron View Post
    my hypothesis: something (I can't say what) is shorting out the cathode resistor, or seriously reducing its ability to control tube current. You've tried replacing the cathode resistor? Good. Now unsolder it and remove it from circuit. Do the tubes still redplate or show the B+ being pulled down?

    edit: could a lead from the cathode resistor be touching the chassis?
    Pulled the resistor, replated still. The B+ on pin 3 became 290, about 50 VDC higher than previous measurement. Does not look like the lead could touch the chassis on the pin. side.

    Quote Originally Posted by Chuck H View Post
    Well you said the cathode resistor measured 345 ohms in circuit and then you said it measured 258 ohms in circuit. If both of these are accurate then something other than the cathode resistor is kludging the resistance. I like escher's idea of lifting the resistor and rechecking. That would tell you if you need to hunt down a short real fast.
    I'm sorry, that 3 was supposed to be a 2! measured again, cathode resistor always around 250... When I removed it, i found 500 Ohms across cathode to ground.

    I poked around the circuit and found another 500 Ohm 5w Cement resistor near the filter caps, My guess is that the designer (or some previous tech) had used 2x 500 Ohm resistors in parallel, creating a combined 250 Ohm cathode resistor that would jump to 500 if one fails?

    Quote Originally Posted by Enzo View Post
    Pull the tube, power up, any voltage on cathode pin?

    Lift 500 ohm resistor, now measure resistance to ground from cathode.

    Is cathode pin touching heater pin?
    No Voltage on Cathode pin without tube.

    Resistance from cathode to ground becomes 500 when unsoldered, see above about two parallel resistors?

    Cathode shows OL to all pins except 1 ---> 8


    So, based on this information, we can rule out a short from Cathode to Ground? Leaky Coupling caps from the previous stage maybe?

    As always, thanks boys.

    0 Not allowed! Not allowed!

  10. #10
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,810
    Thumbs Up/Down
    Received: 728/4
    Given: 0/0
    Nothing to rule on, you found a parallel resistor.

    An amp designer would not use two different 500 ohm resistors in different places around the amp but wired together.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  11. #11
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Quote Originally Posted by Enzo View Post
    Nothing to rule on, you found a parallel resistor.

    An amp designer would not use two different 500 ohm resistors in different places around the amp but wired together.
    Maybe. We don't have a schematic and we've seen a few "things". I thought it was implied that the resistors were NOT together. So who knows why it was done. FWIW 250 ohms would be in keeping with most vintage Fender designs of the tweed era. Most of those amps ran pretty hot. Close to class A from what I've read here. I've never opened one.

    So... If 250R is a typical cathode resistor in similar conditions, why the red plating? And with the two 500R resistors located remote from each other, just what's really going on with the design? A schematic would really help and may be necessary.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  12. #12
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,810
    Thumbs Up/Down
    Received: 728/4
    Given: 0/0
    They were not placed together, but he said they were wired in parallel, that kind of together. Wired together in their separated placces.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  13. #13
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Quote Originally Posted by Enzo View Post
    They were not placed together, but he said they were wired in parallel, that kind of together. Wired together in their separated placces.
    Ok. Of course. Probably a heat thing.

    Still... 243Vp and 267Vg2? That's a wider than typical discrepancy and with an unloaded B+ of 411V along with red plating and the the cathode resistance (pretty much) checking out I think it's reasonable to suspect the plate loading. It could be a board short, the OT, a bad speaker cable, speaker, jack, etc. But I think something is loading the plates into too low an impedance rather than this being a bias issue.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  14. #14
    Senior Member
    Join Date
    Sep 2014
    Location
    se texas
    Posts
    172
    Thumbs Up/Down
    Received: 19/0
    Given: 7/0
    assuming this is a single tube amp... its no wonder tube is glowing, bias too hot!! should have 500 ohms R
    suggest remove one of those 500 ohm resistors... 250 ohms works for Two Tubes!

    the cathode voltage on pin8 would answer the question.. methinks

    0 Not allowed! Not allowed!

  15. #15
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Quote Originally Posted by dstrat View Post
    assuming this is a single tube amp... its no wonder tube is glowing, bias too hot!! should have 500 ohms R
    suggest remove one of those 500 ohm resistors... 250 ohms works for Two Tubes!

    the cathode voltage on pin8 would answer the question.. methinks
    Oh, hey, right! I was thinking two tubes. Indeed 250R for a single tube seems really hot. Voltage across the cathode resistor is reported at less than 2V??? I think there must be a mistake or a problem there. So the mystery continues.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  16. #16
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,315
    Thumbs Up/Down
    Received: 468/3
    Given: 1,006/2
    Can you lift both those cathode resistors and check resistance from pin 8 to ground?
    Also power up and see that it is not red-plating (with both resistors lifted) ?
    If you still have a problem, then I think there is a board issue. If no more problem, probably plate loading as Chuck mentioned in post #13.

    0 Not allowed! Not allowed!
    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

  17. #17
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Thanks everyone, to clarify there are 2 x 6v6 in the power section. I think another tech was working on this before me, it's possibly that he added the second resistor- it's wired straight from the pin to ground instead of to an appropriate spot on the PCB. Not sure on this though.

    It seems like the two resistors are in fact wired in parallel, when I removed both of them from the circuit and powered it on I still get red plating. Resistance from cathode pin 1 and 8 to ground with both removed became OL. (I measured it a few times, sometimes it showed OL, sometimes it shows 7Mohms, sometimes it shows 2MOhms..)

    So in summation-

    There are two Resistors wired in parallel, Possibly a modification, possibly intentional. There is no short from cathode to ground, but the tubes are still red-plating even with both resistors removed from the circuit.

    Possible next steps- Something is pulling B+? Can we rule out a bypass capacitor short with these measurements?

    0 Not allowed! Not allowed!
    Last edited by Mr_bibbles; 06-05-2018 at 07:20 PM.

  18. #18
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,810
    Thumbs Up/Down
    Received: 728/4
    Given: 0/0
    Red plating tubes means there is a current path through them. If you measure open to ground at the cathodes (2M or 7M or any other really high reading doesn't matter - that is open for us), then look to see if a tube is shorted internally. ANything shorting to heater?

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  19. #19
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,862
    Thumbs Up/Down
    Received: 120/0
    Given: 284/1
    How about a low resistance to ground from grid 2? Seems unlikely, but with 'surprise' resistors, anything else is possible. Is there something special about the screen circuit in this amp? The red-plating is on both tubes, correct?

    edit: I guess this is really a question: can grounded screens cause the plates to glow? Or do the screens melt first?

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  20. #20
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,862
    Thumbs Up/Down
    Received: 120/0
    Given: 284/1
    Quote Originally Posted by Mr_bibbles View Post
    Tried with (several) known working tubes, they all red plated. (These were crummy test tubes FYI).
    Did you check the test devices to see if they still worked in other amps after they red-plated? Just wondering if the screens could have been damaged, given that now we know the cathode isn't the source of low potential.
    Also, have you checked with different tubes, after removing both resistors and verifying no continuity to ground from the cathode pin?

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  21. #21
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,810
    Thumbs Up/Down
    Received: 728/4
    Given: 0/0
    I think grounding the screens is a good way to shut off current in the tube.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  22. #22
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,862
    Thumbs Up/Down
    Received: 120/0
    Given: 284/1
    Which is why I asked, not sure if the cathodes floating makes a difference. Now there's not a ground-potential 'target' the size of the side of a barn, and the screens could be exposed. I'd expect screen dissipation could work both ways

    1 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  23. #23
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,315
    Thumbs Up/Down
    Received: 468/3
    Given: 1,006/2
    I'm wondering if there may be a pc board leakage resulting in cathode short to ground only when powered up.
    If I read correctly, the voltage measured at the cathode was always close to 0 VDC.

    0 Not allowed! Not allowed!
    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

  24. #24
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    B+ is dragged down hard
    Red plating with an OL on the cathode
    When cathode resistor is in there is less than 2V standing
    Amp does this with several subbed tubes

    This one is a corker. But I'me going out on a limb and saying the answer is in the measurements, and likely how those measurements were taken. I'll guess that your cathode bypass cap is shorted, but only when the amp is on. Your meters test conditions may not be sufficient to induce the short, which may be internal arcing. The very next thing I would try is lifting the cathode bypass cap.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  25. #25
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    I'm thinking the cathode bypass cap must be shorting with operating conditions and the DMM test parameters don't induce the short. The next thing I'd try would be to lift the cathode bypass cap OR just replace it if it's a PITA taking the board out and putting things back together. A new cap won't hurt if that saves labor.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  26. #26
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Quote Originally Posted by Enzo View Post
    Red plating tubes means there is a current path through them. If you measure open to ground at the cathodes (2M or 7M or any other really high reading doesn't matter - that is open for us), then look to see if a tube is shorted internally. ANything shorting to heater?
    Nope! Tube shows OL to all pins from heater.

    Quote Originally Posted by g1 View Post
    I'm wondering if there may be a pc board leakage resulting in cathode short to ground only when powered up.
    If I read correctly, the voltage measured at the cathode was always close to 0 VDC.
    This sounds like the next step- can you explain a little more what you mean? Im guessing you mean a short somewhere in the PCB, but wouldn't it be limited in this case to the cathode bias circuit, which is only the two resistors? In this case, that would mean that the pins / traces around the resistor are damaged yes? If that's the case I'll poke around with a meter...

    Quote Originally Posted by eschertron View Post
    Did you check the test devices to see if they still worked in other amps after they red-plated? Just wondering if the screens could have been damaged, given that now we know the cathode isn't the source of low potential.
    Also, have you checked with different tubes, after removing both resistors and verifying no continuity to ground from the cathode pin?

    I repeated the experiment with another set of crappy,known working from another amp, test tubes (I have a whole box of 6l6's...) They both redplated, but one of them started much faster (I'll call this tube A). When I swapped sockets, they both redplated again, and tube A started first again in the other socket. Guessing that just means they're off by a few MA..

    0 Not allowed! Not allowed!

  27. #27
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,862
    Thumbs Up/Down
    Received: 120/0
    Given: 284/1
    Have you taken pin voltages with tubes in, under power, since you removed all components in the cathode network?
    Did you say you were going to remove the cathode bypass cap? Is that out as of the last reported test?

    ...I think I mis-spoke when I asserted the cathode pin was not the source of the ground potential. Forgive me if I got ahead of you.

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  28. #28
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Cathode bypass cap.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  29. #29
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Quote Originally Posted by Chuck H View Post
    Cathode bypass cap.
    That was my thought too, but I snipped both caps that could potentially be the bypass capacitor, one by one. each time I started it up, still redplated. Replaced both with 22uf new caps, still red plating.

    BUTTTTTTTT

    There are two diodes in the circuit, d3 and d4. I reconnected the cathode resistors, lifted the leg of one of the diodes, taking a guess that it might be a flyback / snubber / whatever you call it. Red plating has stopped, sound is good, a little buzzy. cathode of the Diode is tied to ground, anode tied to both pins 1 and 8. It's a IN4007, so I replaced it with a known good one. The red plating has come back!

    So what is the function of that diode, and how could it be tied to the bias? Could it be possible that there is an underboard short to this diode, forcing current through ground from the cathode, effectively shorting the cathode ---> ground

    0 Not allowed! Not allowed!
    Last edited by Mr_bibbles; 06-08-2018 at 11:34 PM.

  30. #30
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,315
    Thumbs Up/Down
    Received: 468/3
    Given: 1,006/2
    What is the other diode, and what is it connected to? I was wondering about maybe a zener somewhere and if it could be shorted.

    0 Not allowed! Not allowed!
    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

  31. #31
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    I thought it might be another flyback, but it's not connected to cathode. Also, I don't even know if you can use a flyback for cathode bias.

    The anode goes to x2 paralleled 1k resistors, which goes to the grid. The cathode is tied to the legs of two resistors, one 24k and one 10k, but I cannot find where the path goes after the resistors. Both are close to what I assume are the Coupling capacitors for the PI.... part of a long tail inverter???

    Note- this Diode DOES measure well in/out of circuit. Numbers are a bit scratched from removing it, but looks like 400-X- . In4007?


    Any thoughts on the other IN4007 tied to cathode? Considering that removing it stopped the red plating but otherwise sounded passable at the very least, that seems to be a prime suspect?

    0 Not allowed! Not allowed!

  32. #32
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    I'm afraid a schematic is in order as it seems there is an unusual circuit here. Trying to analyze it based on typical circuits (ie: Rk, Ck) is useless now and, I'm sorry, but the diode implementation is unclear.

    A diode with it's cathode to ground parallel to the cathode resistor pretty much moots the cathode resistance and the power tube cathodes are grounded. Without a negative voltage on the grids there is almost no bias. Ergo, red plating. The amp wouldn't have been designed like this.

    Maybe it would help to know some back story on this amp.?. Do you know if it ever worked? Do you know more certainly that another person has been modifying it or trying to "fix" it? It's a contemporary design, was it beyond warranty when it failed initially? Did someone void the warranty by poking around in it or is it possibly stolen ?

    Working from inside a box (knowledge of typical designs) trying to diagnose outside the box while waiting for Easter eggs of circuit info is becoming frustrating.

    1 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  33. #33
    Senior Member
    Join Date
    Mar 2016
    Posts
    133
    Thumbs Up/Down
    Received: 2/0
    Given: 10/0
    Quote Originally Posted by Chuck H View Post
    I'm afraid a schematic is in order as it seems there is an unusual circuit here. Trying to analyze it based on typical circuits (ie: Rk, Ck) is useless now and, I'm sorry, but the diode implementation is unclear.

    A diode with it's cathode to ground parallel to the cathode resistor pretty much moots the cathode resistance and the power tube cathodes are grounded. Without a negative voltage on the grids there is almost no bias. Ergo, red plating. The amp wouldn't have been designed like this.

    Maybe it would help to know some back story on this amp.?. Do you know if it ever worked? Do you know more certainly that another person has been modifying it or trying to "fix" it? It's a contemporary design, was it beyond warranty when it failed initially? Did someone void the warranty by poking around in it or is it possibly stolen ?

    Working from inside a box (knowledge of typical designs) trying to diagnose outside the box while waiting for Easter eggs of circuit info is becoming frustrating.
    Haha, here's the backstory. Not stolen, someone brought it to me after having it worked on at another shop. They had a bad experience, couldn't get it fixed there, and I'm guessing it's because there's no schematic available for these and it's a bit of a funkier problem.

    Called Tony from Krank, he was not able to help besides telling me "it's cathode biased", and no schematics exist of this online. Another caveat is that the board cannot be removed unless you unsolder the tube sockets from the board- they were soldered in after the board was set in place. I think unsoldering them is my next move if we cannot determine what this diode is. Everything looks original from the outset, but it's possible another tech lifter / added a parallel cathode resistor. because it's wired directly from pin 1 to a connector pin tied to ground.

    Chuck, I think I understand what you mean about the diode- if it's parallel to the resistor, current would flow with no resistance to ground, so no tech in their right mind would ever use one here. Perhaps there is a failed cap between cathode and this diode?? Or, as G1 astutely guessed a while ago, a board short.

    Assuming it's SUPPOSED to be there, would anyone else have ideas about what the purpose of the diode to ground is?

    0 Not allowed! Not allowed!

  34. #34
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,642
    Thumbs Up/Down
    Received: 659/3
    Given: 693/0
    Here's a question, that diode that seems to be across the cathode resistor has it's cathode terminated at ground, so... I'm sure you tested that termination as ground, but is it supposed to be? Is it possible that diodes cathode terminates on a trace that shouldn't be ground? You can probably determine this by looking at other circuits connected to that trace. If it seems the trace shouldn't be grounded then you can look for the board short that g1 suggested or maybe another shorted component.

    1 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

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

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  35. #35
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,315
    Thumbs Up/Down
    Received: 468/3
    Given: 1,006/2
    Agree with Chuck and was also going to suggest you verify the diode connections by following the traces if you have not done that.
    If you were just going by resistance readings, it's possible there is something shorted between tube cathode and diode anode. Or something shorted between diode cathode and ground. Trace it out visually.
    Also, does it look stock or like someone may have added it in?

    1 Not allowed! Not allowed!
    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. First Post - Bias voltage dropping until red-plating - follows the tube
    By ThermionicEmissions in forum Maintenance, Troubleshooting & Repair
    Replies: 27
    Last Post: 05-29-2018, 12:52 AM
  2. 12ax7a cathode bias resistor variance discussion
    By hewo in forum Theory & Design
    Replies: 17
    Last Post: 12-29-2015, 06:20 PM
  3. JVM 205H red plating V3 blowing 500ma ht fuse and VERY low Bias voltage on v3
    By Smitty02 in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 11-22-2015, 04:03 AM
  4. red-plating 6l6's?
    By chuckb in forum Maintenance, Troubleshooting & Repair
    Replies: 10
    Last Post: 06-24-2014, 03:13 AM
  5. red plating?
    By goober in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 03-20-2010, 11:53 AM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •