Page 2 of 2 FirstFirst 12
Results 36 to 51 of 51

Thread: Weird problem or crappy PT?

  1. #36
    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
    When we need a simple OT test of the go/no-go type, then it matters little what we sub in place. Who cares if the impedance matches, all we want to know is if the OT was the problem or not. Single ended OTs need a gap, but for a quick test, it doesn't matter. I have an old Fender Bassman OT I use for just everything, including using half of the primary for a quick single ended test.

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

  2. #37
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    Thank you Enzo and g1 for inspiring the testing and OT substitution. More on results in a sec.

    And my apologies to pdf64. I was "certain" that the screen overcurrent during clipping was caused by the zener. That was wrong. The screen overcurrent during clipping does happen without the zener as well. I must not have been paying attention to the right things at the right time during testing. I'm pretty embarrassed to be "that guy" already so please be gentle.

    I'm here with my hat in my hand. This problem is beyond me. Thank you to those who have been kind enough to follow along. And if you're still with me I have some test results to share. I'll try to be concise and brief.

    This is a schematic of how the power supply has been modified. My intent was to reduce hum. There is no hum at this time.

    *NOTE: I have changed the HV rail resistor feeding the screen and preamp from 1K (as it has been in the previous discussion) to 5k which is keeping the screen voltage well below the plate.

    Click image for larger version. 

Name:	chmpps1.jpg 
Views:	164 
Size:	79.0 KB 
ID:	37988

    Performance remains poor. With a 1kHz signal @ 70mV I get roughly 5W with the wave form just barely clipping the cutoff (figure a). However, compression on saturation and then an HF peaking rise steadily as the frequency is increased. With a 5kHz signal it is quite pronounced (figure b). Actual clean output with a frequency sweep is limited to about 1W.

    That is problem #1. Perhaps/likely related to problem #2. Under overdrive conditions with a 1kHz signal there is a steady and dramatic increase in the spike to easily twice the cutoff or saturation levels (figure c). This is actually much worse as clipping or frequency is increased. As clipping is increased from 'a little' to 'a lot' there is an analogous decrease in current at the plate of the el34 from 50mA to 42mA and a dramatic increase in screen current from 3mA to 25mA.

    Click image for larger version. 

Name:	chmpwf1.jpg 
Views:	115 
Size:	55.5 KB 
ID:	37993

    The preamp signal is clean at the power tube grid to well beyond where the problems manifest. Full preamp output shows some clipping (not a lot) on the bottom of the wave form and no indication of oscillation.

    Substituting the power tube for both a 6v6 and another el34 made no difference. Substituting the OT made no difference.

    The amp was pitifully thin and quiet before modification but no extensive testing was done. The modifications were to upgrade the speaker and the power supply as shown. It's fuller sounding now, but not a lot louder. Not as loud as I know it should be and suffering the dangerous operating conditions under clipping.

    I'm open to run any requested tests just as if I were a total noob.

    I have a hair brained theory about what might be happening. I'm putting in the next post to keep this one on point and spare anyone that doesn't want to go there.

    TIA gents

    0 Not allowed! Not allowed!
    Last edited by Chuck H; 02-26-2016 at 04:11 PM.
    "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

  3. #38
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    This is such a simple circuit that I'm baffled. There are only a couple of things I can think of. Perhaps with the higher than average resistance on the HV winding adding the 100R resistor and additional capacitance to the power supply is slowing it down even more and choking the tube? and/or

    The tube is biased cooler than 100% at about 19W. Not sure how this might relate since the wave form does clip in saturation before cutoff with an input signal above 1kHz. and/or

    The dreaded conductive circuit board. I can read .5V to 3V at any point on the board by stabbing my probe into it. The preamp cathode bias looks about right though at 1.65V and 1.75V even though I can read fully 1V on the board near those eyelets. Perhaps a conductive condition is worsening or modulating when the amp is conducting signal? I haven't checked for that yet.

    That's all my (perhaps useless) observations, aside from actual test results.

    0 Not allowed! Not allowed!
    Last edited by Chuck H; 02-26-2016 at 04:13 PM.
    "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. #39
    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
    This amp has no NFB, correct?
    Any chance of extreme HF oscillation you are not seeing on scope?

    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

  5. #40
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    This amp does have NFB but testing with and without showed no change in the problem. I will get a good close look at the preamp and power amp wave forms and report back.

    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. #41
    Professional Lurker eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    2,367
    Thumbs Up/Down
    Received: 522/0
    Given: 1,369/1
    Rep Power
    11
    Quote Originally Posted by Chuck H View Post
    I was "certain" that the screen overcurrent during clipping was caused by the zener. That was wrong. The screen overcurrent during clipping does happen without the zener as well. [snip] Under overdrive conditions with a 1kHz signal there is a steady and dramatic increase in the spike to easily twice the cutoff or saturation levels (figure c). This is actually much worse as clipping or frequency is increased. As clipping is increased from 'a little' to 'a lot' there is an analogous decrease in current at the plate of the el34 from 50mA to 42mA and a dramatic increase in screen current from 3mA to 25mA.
    The rise in screen current and the 'hook-shaped' waveform are consistent with what I've seen on champs (and any other SE amp, I presume). Part of their charm, I suppose.
    I'm curious about the waveform 'B' and what signal voltages you had going in and out of the power tube when the 5kHz signal started to clip. The drawing sure looks like cutoff to me, but that can't happen without full power from the tube (or some outrageous bias shift!). There is something strange going on there at the higher frequencies. Looking forward to seeing that you found it!

    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
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


  7. #42
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    As small, relaxed wiggle on the saturation half of the wave form when clipping (other than the hf spike). Nothing I wouldn't expect from any guitar amp. A close up look at the preamp and power amp shows no doubling of the trace or hair at the peaks or anything like that.

    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

  8. #43
    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
    Wow, still a problem with other OTs... I was so sure the OT would be the culprit!

    I like Enzo's heat gun/hairdryer solution if you suspect stray currents on the board -- might be worth a try.

    Have you tried a ~1.5K grid-stopper on the output tube? That 330pF cap between the grid and cathode is supposed to shunt HF garbage, but I have a growing preference for stopping resistors rather than capacitors for this job. Heck, if the cap is still there, you might try removing it and seeing how the amp behaves.

    0 Not allowed! Not allowed!

  9. #44
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    Quote Originally Posted by ThermionicScott View Post
    Wow, still a problem with other OTs... I was so sure the OT would be the culprit!

    I like Enzo's heat gun/hairdryer solution if you suspect stray currents on the board -- might be worth a try.

    Have you tried a ~1.5K grid-stopper on the output tube? That 330pF cap between the grid and cathode is supposed to shunt HF garbage, but I have a growing preference for stopping resistors rather than capacitors for this job. Heck, if the cap is still there, you might try removing it and seeing how the amp behaves.
    I might try removing that grid cathode cap. It's actually a grid to ground cap now as I did a little rearranging on the board. And I did add a 4.7k grid stop resistor to the power tube.

    As to the possibility of a conductive circuit board... I've heard that removing moisture can fix it, but I haven't seen that. I've heated and heated boards in the past for many minutes and all the wax was glossy for a half hour during and after the work (which does make the finish look better once it cools) with no resolution. I'm very glad it works some of the time for some people though. Since the preamp signal and bias voltages look ok and I didn't read any DC on the power tube grid I'm actually wondering what the affect might be of the voltages I can read on the board.?. My next test will be to float the power tube cathode connection to help eliminate the possibility that there is something about modulating voltages on the board pissing in my Cheerios.

    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. #45
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    Oh! Hey! I just reached 10,000 posts at 8:40am on #40. Can I get a WOOT! WOOT!

    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

  11. #46
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,815
    Thumbs Up/Down
    Received: 947/1
    Given: 694/1
    Rep Power
    14
    Quote Originally Posted by Chuck H View Post
    Oh! Hey! I just reached 10,000 posts at 8:40am on #40. Can I get a WOOT! WOOT!
    ..and you're STILL talking! Sheesh!

    Seriously... a big WOOT from me. I always enjoy reading what you have to say.

    0 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

  12. #47
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,890
    Thumbs Up/Down
    Received: 1,786/21
    Given: 1,462/0
    Rep Power
    14
    Quote Originally Posted by Chuck H View Post
    Oh! Hey! I just reached 10,000 posts at 8:40am on #40. Can I get a WOOT! WOOT!
    How's about a HARRRRUUUUUMPH! Hey can I get a HARRRRUMPH from that guy?

    Congrats on your 10,000th. Keep the fingers flying!

    0 Not allowed! Not allowed!

  13. #48
    Old Timer tedmich's Avatar
    Join Date
    Apr 2008
    Location
    Sonoma CA
    Posts
    3,476
    Thumbs Up/Down
    Received: 340/2
    Given: 126/3
    Rep Power
    16
    Congrats Chuck,

    But since RG and Leo appear in every one of your posts, they have more posts by the commutative property I believe...

    0 Not allowed! Not allowed!

  14. #49
    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
    10,000 posts? Hey, great, kid... DOn't get cocky.




    OK, so I am unclear, did we or did we not sub in a different OT?

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

  15. #50
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,482
    Thumbs Up/Down
    Received: 3,097/5
    Given: 3,732/0
    Rep Power
    32
    We did sub another OT. I have a few around. I used one half primary on a PP 20W unit that just happened to be the nearest hunk of iron. This presented a 3300 ohm primary and that did improve output a bit. The distortion increasing with frequency and the extreme screen dissipation remained. BUT...

    I need to confess that there is some dirt in my testing. Testing started with a 1k 70mV input signal. Like this the amp makes about 5W. When sweeping and watching the distortion increase with frequency I would then reduce volume until the distortion was gone at 5k then return the input frequency to 1k and THEN test the amps performance. My assumption being that for the amp to be clean at all audio frequencies THIS is as loud as it goes. Well... I decided to measure for max clean watts without returning the input signal to 1k. It turns out that with the same test, but leaving the frequency at 5k the amps output is about 9W. So, basically, with the amp volume adjusted to make 9W at 5k it is only making 2.5W at 1k. I was erroneously taking the lower number to be the maximum output across the sweep. But this is now looking like pretty normal behavior. Why the amp isn't as loud as it should be remains a mystery.

    I also experimented with a hotter bias and that did improve the plate and screen current behaviors a little. Reading about this sort of circuit on Merlin's site the potential for over dissipating the screens badly with clipping is discussed. This is apparently also normal behavior. As it happens, tuning a single ended el34 circuit requires a lot more idealizing than a typical AB1 guitar amp. Get one thing wrong and performance is spoiled. Screen current requires managing, bias is critical and if you plan to clip the amp then the screen voltage MUST be managed and kept below the plate voltage by a margin that doesn't allow it to be positive more than momentarily WRT the swing at the plate. Phwew!?! Learning a lot with this one. My usual low tech approach is just biting me on the ass I think.

    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

  16. #51
    Old Timer tedmich's Avatar
    Join Date
    Apr 2008
    Location
    Sonoma CA
    Posts
    3,476
    Thumbs Up/Down
    Received: 340/2
    Given: 126/3
    Rep Power
    16
    Quote Originally Posted by Enzo View Post
    10,000 posts? Hey, great, kid... DOn't get cocky.


    Enzo's posts per day is ~2.2 times Chucks so barring any finger injury King Enzo (Kingzo?) can never be caught...

    0 Not allowed! Not allowed!

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
  •