Page 1 of 2 12 LastLast
Results 1 to 35 of 51

Thread: Weird problem or crappy PT?

  1. #1
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32

    Weird problem or crappy PT?

    I'm working on a Champ right now. The HV seemed to drag down too much with current so I checked everything in the circuit but found no joy. Then I checked the DCR on the HV secondary winding and read 450 ohms!?! 5V and 6.3V windings are fine. HV reads 480 without a load and 380 loaded. Which is about 40V below spec on the schematic. It doesn't run hot and the amp is "working" but the screen voltage is 8V higher than the plates and remains so until the output starts to compress (also weird). I'm scratching my head over how this could happen. Shouldn't it be low from some shorted windings or open with no continuity? Or might this just be how it is? What could cause high DCR on a fixed length of wire? Perhaps that winding was wound with the wrong gauge wire by some ninny and somehow passed inspection? Thoughts and experiences appreciated.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  2. #2
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,497
    Thumbs Up/Down
    Received: 2,543/7
    Given: 0/0
    Rep Power
    57
    Horses, not zebras...

    Fender doesn't have a guy sit down and wind a transformer as he builds a Champ. It is REAL unlikely they made a run of transformers with the wrong size wire. But anything is possible.

    Looking at a 5E1 for example, note the B+ goes to the screen, and then through the OT primary to the plate, so it will always sit there with higher screen voltage. Then compare the 5F1 where the B+ comes off two nodes, and the screen is 10k downstream from the plate, so it will be different there.

    If we assume a 1k screen node resistor, and maybe 5ma current through it, that will drop 5v from the B+ supply, but the 6V6 draws a lot more plate current than 5ma, and through that 450 ohms it drops more than 5v, and so will have a lower voltage.

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

  3. #3
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Thanks Enzo.

    This is a silver face amp. So no choke. I'll try to get my head around it, but it seemed to me that since the plate and screen are both downstream of the winding DCR the screen voltage could only START from there. The OT primary has all of the plate current through roughly 290 ohms. And the screen only 5(ish)mA through 1k rail and a 1k at the socket. I figure this is where the problem is.

    Still holding out for any speculation on the high HV secondary DCR.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  4. #4
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,107
    Thumbs Up/Down
    Received: 629/3
    Given: 332/0
    Rep Power
    28
    Quote" HV reads 480 without a load and 380 loaded. Which is about 40V below spec on the schematic"

    Which schematic are we refering to?
    The schematics that I have show 360V on the OT.

    0 Not allowed! Not allowed!

  5. #5
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Sorry... This one:

    Click image for larger version. 

Name:	fender_champ_cbs_schematic.gif 
Views:	311 
Size:	37.7 KB 
ID:	37908

    A bit illegible. The voltages on the plate says 420 and the screen says 410. I think that was wishful thinking. I do know the plate voltage is right there for the same era Vibro Champ.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  6. #6
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,497
    Thumbs Up/Down
    Received: 2,543/7
    Given: 0/0
    Rep Power
    57
    I am probably senile, but don't follow you quite.

    5ma through 1k to the screen drops 5v, so 420 becomes 415. Or correct for lower voltage loaded. Now what is the plate current through that 290 ohm winding? And so what is the drop across the transformer. Guessing say 30ma, gives me like 9v, so the plates will sit lower. No choke enters into it.

    I have no idea what the resistance of that HV winding ought to be. If it is way high, I wouldn't think the wire itself changed resistance, but the winding wire has to be connected to the outside wires, and that connection could be faulty.

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

  7. #7
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    3,455
    Thumbs Up/Down
    Received: 856/6
    Given: 1,765/10
    Rep Power
    19
    SF Champ, converted to Vibro. Mine is 320 ohms, with the recto tube pulled. Just cuz I was bored & hadn't seen inside an amp in a while, I checked...
    Tranny # NSC125P1B, EIA606-030. Huh, I always heard mine was a 79... guess it's an 80, or the PT is replaced...

    Justin

    0 Not allowed! Not allowed!
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  8. #8
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by Enzo View Post
    I am probably senile, but don't follow you quite.

    5ma through 1k to the screen drops 5v, so 420 becomes 415. Or correct for lower voltage loaded. Now what is the plate current through that 290 ohm winding? And so what is the drop across the transformer. Guessing say 30ma, gives me like 9v, so the plates will sit lower. No choke enters into it.

    I have no idea what the resistance of that HV winding ought to be. If it is way high, I wouldn't think the wire itself changed resistance, but the winding wire has to be connected to the outside wires, and that connection could be faulty.
    Sorry Enzo. I hate trying to follow descriptions of circuits too. What I meant was that the plate and screen both follow the HV winding. So I figure the drop there is moot. So, from the 380V node there is the OT primary DCR at 290 ohms dropping 9V, so 30mA. from the same node the rail resistor is 1k between the OT/plate and the screen grid. That resistor is dropping 5V, so 5mA. I think that should make the screen grid 4V higher than the plate. In reality there is an 8V difference because I have an el34 in the power tube socket and the screen now has an additional 1k "screen resistor" off the HV node. I'm sure the math would bare it out If I had finite figures in front of me.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  9. #9
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Thanks for checking that Justin Since the PT is a replacement I think it's fair to find the DCR is a little different (better?). I'm thinking now about what Jazz mentioned. The Champ right before this CBS design does indeed indicate 360V with the same PT.?. So I'll speculate (entirely unsupported) that the spec for that part number changed at some point and this seemingly high HV wind DCR may be correct for that 360V model, and this amp may have been built during a transition and has that PT.

    It IS a class A amp so there isn't any appreciable sag when the amp is played clean. It doesn't seem that bad when the amp is clipping either actually. I've just never seen an HV winding with that much DCR. Usually MUCH less. So I was checking in on the matter. Your HV DCR is actually high by my experience too. So that's what leads me to think this may be "normal".

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  10. #10
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,882
    Thumbs Up/Down
    Received: 599/1
    Given: 556/2
    Rep Power
    19
    Hammond helpfully provide winding resistances; the Champ types seem to be around 330 ohms across the HT Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers

    0 Not allowed! Not allowed!

  11. #11
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by pdf64 View Post
    Hammond helpfully provide winding resistances; the Champ types seem to be around 330 ohms across the HT Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers
    Thanks. That supports Justin's finding to be sure. The only mystery now is whether the PT I have is a representation of the previous 360V spec for the same part number.?. I think it must be. When Hammond "clones" those transformers I doubt they sample a range of examples. So it's sort of a crap shoot as to which year/variant is represented by their "clone".

    Justin, did you measure your plate voltage when you had the amp open? I'd be interested to know if the example with the 320/330R HV winding equates to Champ models that have over 400Vp.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  12. #12
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,107
    Thumbs Up/Down
    Received: 629/3
    Given: 332/0
    Rep Power
    28
    Question: Does the amp work o/k?

    Not too sure what you are fixing.

    0 Not allowed! Not allowed!

  13. #13
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by Jazz P Bass View Post
    Question: Does the amp work o/k?

    Not too sure what you are fixing.
    Well, sort of "runs ok". I'm not fixing anything. I was asking about my findings to see if I needed to. My reference point is limited to bench testing and two previous SF Champs I worked on many years ago. I feel like the amp should be a little louder even though I measure 5W output. I replaced the speaker with an efficient 10" and even with that it's not matching my memory for volume. But memories (and ears) have a way of changing with time The screen voltage remains higher than the plate voltage all the way up until the wave form starts to compress and even played clean there is, what I perceive to be, more intermodulation distortion than I remember for these amps. The OT tests fine. Well, for the crappy little walnut sized thing that it is.

    More to the point of your inquiry. The power supply isn't giving me any trouble. PT runs warm, not hot, with a long burn in. All other voltages, 5V and 6.3V windings are a little high as should be expected.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  14. #14
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    3,455
    Thumbs Up/Down
    Received: 856/6
    Given: 1,765/10
    Rep Power
    19
    Not recently, but yes, it's about 425, last time I checked. I put.up a.thread here asking why so high, because the scheme said 360... then someone kindly pointed out that I was looking at a BF schem, and they posted the SF schem... mystery solved. What's the date code & part # on your PT? Maybe it was replaced with an old junker?

    Justin

    PS - Mine can hang with a polite drummer I n a sort of "punk jazz" way... never measured, but it's not a wuss!

    0 Not allowed! Not allowed!
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  15. #15
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,107
    Thumbs Up/Down
    Received: 629/3
    Given: 332/0
    Rep Power
    28
    Quote:"I replaced the speaker with an efficient 10""

    You did use a 4 ohm replacement, correct?

    0 Not allowed! Not allowed!

  16. #16
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,882
    Thumbs Up/Down
    Received: 599/1
    Given: 556/2
    Rep Power
    19
    The higher than normal winding resistance may facilitate a few more uF of reservoir capacitance, as it will act to restrict the peak plate current.
    Regarding the plate and screen grid voltages, what is the resistance of the OT primary?
    If that's higher than normal too, it would tend to have that effect.

    Maybe trying different brands of 5Y3 could help to tweak the HT voltage, or possibly a 5R4 if it can be squeezed in?

    0 Not allowed! Not allowed!

  17. #17
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,654
    Thumbs Up/Down
    Received: 2,189/24
    Given: 5,521/11
    Rep Power
    24
    I just picked up a JJ and an older sovtek 5Y3. Running in an amp with a pair of 6V6. Also compared a UOS. The sovtek put up a 40V increase in B+. The JJ agreed with the UOS.

    0 Not allowed! Not allowed!
    "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

  18. #18
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by Jazz P Bass View Post
    Quote:"I replaced the speaker with an efficient 10""

    You did use a 4 ohm replacement, correct?
    Yep.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  19. #19
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by pdf64 View Post
    The higher than normal winding resistance may facilitate a few more uF of reservoir capacitance, as it will act to restrict the peak plate current.
    Regarding the plate and screen grid voltages, what is the resistance of the OT primary?
    If that's higher than normal too, it would tend to have that effect.

    Maybe trying different brands of 5Y3 could help to tweak the HT voltage, or possibly a 5R4 if it can be squeezed in?
    I do have a 5V4 on hand... But I'm saving that for me and there's no budget in this project left after the speaker (which cost me $21 for shipping

    I don't see how the lower voltage is a deal breaker for volume. It's not THAT low. I think I'd replace that pathetic OT if I could though.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  20. #20
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by g1 View Post
    I just picked up a JJ and an older sovtek 5Y3. Running in an amp with a pair of 6V6. Also compared a UOS. The sovtek put up a 40V increase in B+. The JJ agreed with the UOS.
    I've read, here, long ago, about some Sovtek 5Y3's on the market that weren't really 5Y3's and folks were having some trouble with high plate volts because of them.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  21. #21
    Senior Member elipsey's Avatar
    Join Date
    May 2010
    Location
    Upstate
    Posts
    268
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    Quote Originally Posted by Chuck H View Post
    I've read, here, long ago, about some Sovtek 5Y3's on the market that weren't really 5Y3's and folks were having some trouble with high plate volts because of them.
    I have heard this as well. I have one here, and it gives me higher Vp then any of 5Y3 or 5U4 series recto tubes in my junk drawer. It seems have about the same voltage drop as 5AR4 in my small push pull amps. Sample size n=1, so this observation might be useless.

    I have found that building new clones of small fender designs seems to result in way less sag, and thus higher Vp than I expect, in a way that doesn't seem accounted for by wall voltage difference; that is the slope of voltage/load is shallower than I expect. I seem to recall reading (perhaps from Mark Baier of Victoria, but I can't find original source, so maybe it wasn't him?) that most new clones of those PT's usually sag much less then the originals. This seems to me to imply that the originals were higher impedance or DCR, and a bit inefficient. Sorry if I'm repeating unattributed BS here

    0 Not allowed! Not allowed!
    Last edited by elipsey; 02-22-2016 at 08:41 PM.

  22. #22
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,497
    Thumbs Up/Down
    Received: 2,543/7
    Given: 0/0
    Rep Power
    57
    Chuck, your plate and screen voltages come from the same supply node, yes, but you are ignoring the resistance of the OT winding. That is why the plate voltage is lower than the screen. The plate voltage does not come from teh B+ node directly it flows through that resistance. Yes, the voltage at the plate end of the 1k resistor will be highe3r than teh screen voltage, but not at the tube plate due to the widning resistance.

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

  23. #23
    Senior Member ThermionicScott's Avatar
    Join Date
    Dec 2009
    Location
    SW CR IA US NA PE
    Posts
    608
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    A silverface Champ running at 380V rather than 420+ is something to celebrate, not "fix."

    Similarly, it's no big deal if the screen idles a couple volts above the plate. Hardly any screen current flows at idle, so the screen dissipation is negligible. But if the spirit moves you, replacing the 1K/1W with a 1.5K would help filter out hum going to the screen and further down the line...

    - Scott

    0 Not allowed! Not allowed!
    Last edited by ThermionicScott; 02-23-2016 at 06:22 PM.

  24. #24
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by ThermionicScott View Post
    A silverface Champ running at 380V rather than 420+ is something to celebrate, not "fix."

    Similarly, it's no big deal if the screen idles a couple volts above the plate. Hardly any screen current flows at idle, so the screen dissipation is negligible. But if the spirit moves you, replacing the 1K/1W with a 1.5K would help filter out hum going to the screen and further down the line...

    - Scott
    Quote Originally Posted by Chuck H View Post
    The screen voltage remains higher than the plate voltage all the way up until the wave form starts to compress...
    This is my problem. The screen remains high to the point of clipping. By my figures it will take about 3.3k between the OT and screens in the HV rail to keep the screen below the plate. In a class AB amp I would be somewhat concerned about the sag. I don't know that this is a problem with class A.

    Because actually playing the amp seems to be distorting early in volume regardless of my 1k test putting out 5W I decided to sweep the frequency on the test. Distortion is increasing with frequency. The more I increase the frequency the quicker the onset of distortion. At 5kHz the amp can only put out about 3/4 watt undistorted. There's no way to get my 5W across the speakers range. So the amp is limited to a very low volume for clean tones. I've read about people having trouble with low volume regarding Champs. This is the first time I'm experiencing it for myself. Sweeping the OT proves it out to be a real loser for performance.

    I need to talk to my guy and convince him it needs to go. So far he's been alright but a mutual acquaintance tells me he's tighter than two coats of paint and I should get money in hand before handing the amp over. No problem... Been there.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  25. #25
    Senior Member ThermionicScott's Avatar
    Join Date
    Dec 2009
    Location
    SW CR IA US NA PE
    Posts
    608
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    Quote Originally Posted by Chuck H View Post
    This is my problem. The screen remains high to the point of clipping. By my figures it will take about 3.3k between the OT and screens in the HV rail to keep the screen below the plate. In a class AB amp I would be somewhat concerned about the sag. I don't know that this is a problem with class A.

    Because actually playing the amp seems to be distorting early in volume regardless of my 1k test putting out 5W I decided to sweep the frequency on the test. Distortion is increasing with frequency. The more I increase the frequency the quicker the onset of distortion. At 5kHz the amp can only put out about 3/4 watt undistorted. There's no way to get my 5W across the speakers range. So the amp is limited to a very low volume for clean tones. I've read about people having trouble with low volume regarding Champs. This is the first time I'm experiencing it for myself. Sweeping the OT proves it out to be a real loser for performance.

    I need to talk to my guy and convince him it needs to go. So far he's been alright but a mutual acquaintance tells me he's tighter than two coats of paint and I should get money in hand before handing the amp over. No problem... Been there.
    I'm not surprised that the DC component of the plate voltage would always be a couple volts lower than the screen voltage in this design, but so what? It's only when the plate voltage swings close to the diode line that the screen dissipates any meaningful power. If you're worried about that, then adding a screen stopper would limit the instantaneous dissipation nicely, just as Fender did on most amps bigger than a Champ or Princeton.

    I think you're on the right track with the OT, though. Got a spare you can wire in temporarily?

    0 Not allowed! Not allowed!

  26. #26
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by ThermionicScott View Post
    It's only when the plate voltage swings close to the diode line that the screen dissipates any meaningful power. If you're worried about that, then adding a screen stopper would limit the instantaneous dissipation nicely, just as Fender did on most amps bigger than a Champ or Princeton.

    I think you're on the right track with the OT, though. Got a spare you can wire in temporarily?
    I did add a 1k stopper to the circuit. Unfortunately I don't have a suitable OT on hand.

    Edcor makes a couple of models for just $19. A stellar price, but they have a really crappy shipping policy so it costs $21 for shipping!?! I called them about that and asked why they couldn't just ship USPS ground. The semi illiterate lady on the phone babbled about problems they've had and finished with "So now we just charge a three pound shipping weight minimum. Sorry we can't help you." Really? She never addressed why they won't ship at a level below "priority" or why their website automatically allocates one of three shipping "options" (all over $20) but doesn't let the buyer actually opt for their choice of the three. A really cumbersome affair overall that made me want to shop elsewhere. But I've historically been very happy with Edcor products and service. Those would have been bigger, more expensive iron where the shipping cost seemed appropriate. They even made a primary voltage modification to one of their models for me at no cost! Oh well. You take the good with the bad I spose. In the end $40 for the OT and shipping is still the best price for a 10W unit twice the size of any "Champ" OT so I'll probably buy it.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  27. #27
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Moving to a new post for this next matter...

    I had a funny thing happen. I'm not the most technical guy on the forum by a stretch, so I didn't see any immediate problem with trying a 12v/5W zener between the screen filter node and the screen stopper resistor. This seemed to work fine, until I drove the circuit into clipping. Then the plate current dropped and the screen current increased dramatically turning the 1k screen grid resistor a nice golden brown. Why did this happen?

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  28. #28
    Senior Member ThermionicScott's Avatar
    Join Date
    Dec 2009
    Location
    SW CR IA US NA PE
    Posts
    608
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    Quote Originally Posted by Chuck H View Post
    Edcor makes a couple of models for just $19. A stellar price, but they have a really crappy shipping policy so it costs $21 for shipping!?!
    That sucks!

    It occurred to me later on, while in a meeting, that you wouldn't need a "Champ" OT to tell if it's the problem. If you wire up some generic 4KΩ 40W OT and get more than a watt or two out of it, that should be evidence enough, right?

    0 Not allowed! Not allowed!

  29. #29
    Senior Member ThermionicScott's Avatar
    Join Date
    Dec 2009
    Location
    SW CR IA US NA PE
    Posts
    608
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    11
    Quote Originally Posted by Chuck H View Post
    Moving to a new post for this next matter...

    I had a funny thing happen. I'm not the most technical guy on the forum by a stretch, so I didn't see any immediate problem with trying a 12v/5W zener between the screen filter node and the screen stopper resistor. This seemed to work fine, until I drove the circuit into clipping. Then the plate current dropped and the screen current increased dramatically turning the 1k screen grid resistor a nice golden brown. Why did this happen?
    What's the rating of your 1kΩ screen stopper? An EL34 (I keep forgetting that that's what you have in the amp) can be expected to draw more screen current than a 6V6.

    P.S. I dug through some EL34 datasheets, and they expected a healthy voltage drop across the OT primary/ies. Supply voltage of 265V, and only 250V at the plate in their single-ended example. No wonder.

    0 Not allowed! Not allowed!
    Last edited by ThermionicScott; 02-24-2016 at 05:16 PM.

  30. #30
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by ThermionicScott View Post
    What's the rating of your 1kΩ screen stopper? An EL34 (I keep forgetting that that's what you have in the amp) can be expected to draw more screen current than a 6V6.
    The stopper is a 3W unit. 5W might be a better choice, but shouldn't be necessary. I don't think that was the problem as this resistor has been fine until the zener was placed behind it in the circuit. When clipping hard the plate rose 12V and the screen dropped 40V! I didn't have the opportunity for further testing because, seeing that this was a clearly dangerous circuit, I stopped experimenting with it.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  31. #31
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by ThermionicScott View Post
    That sucks!

    It occurred to me later on, while in a meeting, that you wouldn't need a "Champ" OT to tell if it's the problem. If you wire up some generic 4KΩ 40W OT and get more than a watt or two out of it, that should be evidence enough, right?
    I considered that. Maybe I'll try it. Maybe not. There's nothing about the circuit that should cause the distortion rising with frequency affect. Other than the failed zener circuit, which was just an experiment and is out now, it's all testing as it should and I can't see any reason not to pin this on the OT.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  32. #32
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,654
    Thumbs Up/Down
    Received: 2,189/24
    Given: 5,521/11
    Rep Power
    24
    I'll take that to mean you have verified that the signal getting to the power tubes is good and consistent with the frequency increase.

    0 Not allowed! Not allowed!
    "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

  33. #33
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,882
    Thumbs Up/Down
    Received: 599/1
    Given: 556/2
    Rep Power
    19
    Quote Originally Posted by Chuck H View Post
    .. trying a 12v/5W zener between the screen filter node and the screen stopper resistor. This seemed to work fine, until I drove the circuit into clipping. Then the plate current dropped and the screen current increased dramatically turning the 1k screen grid resistor a nice golden brown. Why did this happen?
    I think that it would be normal for the main HT voltage to rise when a class A / SE power tube is pushed into clipping, due to the regular bias excursion mechanism reducing the average plate current draw, thereby reducing HT sag.
    I don't see why that would cause the screen grid to draw more current though?
    Or why the zener diode would make things any worse?

    Are you sure this doesn't happen without the zener in place?

    0 Not allowed! Not allowed!

  34. #34
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by g1 View Post
    I'll take that to mean you have verified that the signal getting to the power tubes is good and consistent with the frequency increase.
    Nope. I sure didn't. But I will now.

    Like Enzo sez... Never invent a reason to NOT test something.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

  35. #35
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,096/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by pdf64 View Post
    Are you sure this doesn't happen without the zener in place?
    Certain. I ran the same tests before trying the zener and then again after I removed it just to be sure.

    0 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "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

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Weird, Weird stuff with the PT of a Twin, Secondary Voltage.
    By wizard333 in forum Maintenance, Troubleshooting & Repair
    Replies: 11
    Last Post: 07-11-2013, 07:58 AM
  2. Weird residual voltage problem...
    By EETStudent in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 04-07-2013, 06:11 PM
  3. Weird VOX problem
    By Valvehead in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 10-27-2011, 02:17 PM
  4. crappy coil winder
    By Mystic in forum Pickup Makers
    Replies: 8
    Last Post: 02-16-2008, 07:34 PM
  5. Weird OT problem
    By tubedogsmith in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 08-07-2007, 07:16 AM

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
  •