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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

5F2A Weirdness...Troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by keithb7, Nov 24, 2017.

  1. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Hey folks, I was recently jamming through my 5F2A. I noticed that when cranked up to 10, (ya that happens a lot) I was loosing some volume. I pulled the upper rear panel off and laid the amp speaker down on the floor and watched the tubes while I hit the front end hard with my ES-335. Lights out now, in the dark, I could see the 6V6 put out a little white glow when played a hard E, then fade dark again as the note faded off. Hmm.. Fireworks in tubes. I estimated I had a bad 6V6.

    I swapped it out and saw this happening again with another new tube. I shut things down for further inspection. My 5F2A was a very early amp build for me. Pretty green. I noticed that the 6V6 cathode cap and resistor were touching. I pulled them both and put in new. Just in case. I cleaned up some other lead dress and a few ugly solder joints. Turned the amp on. No sound. A slight buzzing from the speaker that's it. Weird.

    I pulled out my DMM and measured all resistor values. I checked every single connection against my Weber layout sheet. I triple checked it all again. All good. No ground problems. All seems well.

    I hooked up my DMM , when I turn the amp on I watch my B+ climb to 500V DC or so, then there's a slight pop in the speaker and down she goes to about 350 and stays there. Again I re-inspected everything, all connections look good. Swap in a different 6V6 again. Same thing. Swap in a different NOS 5Y3. Same results.

    Folks I think I may have a failed output transformer. The B+ goes to the primary side. Seems that it builds up, then a slight pop and B+ drops. The buzzing in the speaker I am hearing is the short in the OT maybe? I'm thinking so. I am not sure how to test an OT, however maybe I should swap a new one in. See what happens? I have not had this happen before, but can a failing 6V6, take out a OT? Or can a failing OT show fireworks in the power tube as I saw? Thoughts?
     

  2. Nickfl

    Nickfl Tele-Holic

    710
    May 24, 2016
    Florida
    How loud is the buzzing? I had an intermittent OT short on a F51 and it sounds similar to what you are describing except the buzzing when it shorted was loud. Plug it into a lightbulb limiter and see if it lights up when you get the fault condition and that will at least confirm you have a short somewhere, probably in the OT. In fact, make sure you use a limiter from here on in while you are diagnosing or you might end up smoking the PT as well, that is what happened to me when I was trying to diagnose the OT short on that champ...
     

  3. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Here's what I distill that down to…
    Amp was working; you were playing it loud and noticing some volume loss. But -- amp was working.
    Then you replaced cath resistor & cap. You redo some lead dress and some solder joints.
    Now amp doesn't work.
    I'd examine the rework you did closely. (If my synopsis is correct.)

    It's possible that when reheating solder joints, one of the under-board wires popped loose...?
     

  4. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    Where are you measuring the B+, on the plate of the output tube? If so a bad output transformer isn't likely. If you're measuring it across the first filter cap then the B+ will be high until the preamp and PA tubes start conducting.
     
    King Fan likes this.

  5. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    "Amp was working; you were playing it loud and noticing some volume loss. But -- amp was working.
    Then you replaced cath resistor & cap
    ." - Not quite yet. Inspected amp further to see flashing in 6V6 while playing it.
    Saw problem in the tube. Shut things down and started inspecting. Then replaced cathode cap and resistor.

    "You redo some lead dress and some solder joints. Now amp doesn't work.
    I'd examine the rework you did closely" -
    I re-examined the work I did a few times and traced everything with a hi-liter on layout diagram.
    Tested everything I could aside from OT.

    "It's possible that when reheating solder joints, one of the under-board wires popped loose...?" I lifted the board and inspected everything under there. Nothing is loose. Board was reinstalled, it is elevated with plastic hardware. Nothing on the board is touching the chassis. All here appears good.

    I was measuring B+ voltage at rectifier tube pin 8, also checked it at positive end of a B+ cap. Same measurements at both points as described in post 1.
     

  6. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    Measure the B+ at the plate of the PA tube. If it's high then drops off it's because the tube isn't conducting as soon as you're expecting it to. If that isn't the case, meter across the primary of the output transformer. If you're seeing a high voltage that then drops to a low voltage, then the output transformer is open... I don't know why that would cure itself so reliably, so I doubt that's what's going on.
     

  7. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Measuring on 6V6 pin 3, the plate, the voltage quickly reaches 500V or so, then starts dropping down. There is little pop and then it drops faster. I said to about 350V or so above in post #1. This time I left the amp on longer and watched go as low as 320V.

    Measure across primary of OT? To be sure, a positive and a negative lead on each of the two PT input wires? That's ok for the meter?
     
    Last edited: Nov 24, 2017

  8. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    You're just measuring voltage, why wouldn't it be okay?

    In any case, the measurement you already made rules out the power transformer.
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Couple questions:

    Are you running the stock fifties 5F2-A schematic?

    Have you checked the screen supply and grid leak path on your power tube socket?

    A white glow from within a power tube can be a screen grid overdissipating under extreme overdrive conditions as screen current flow skyrockets. I run big 1k, 3W screen resistors on EL84s just for that reason. Get the screen grid too hot, it melts down and shorts the tube.
     
    King Fan likes this.

  10. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Agreed @dsutton24, the OT appears fine. I scratch wired in a temporary spare one to test. Same result. Checked speaker and cable with another amp. Good . Tried swapping out all tubes. No improvement. Checked and re-checked my wiring a couple more times, nothing found.

    Pulled rectifier tube to check voltages. Red wires 396.5 VAC each. Yellow wires: Pin 2 5.7 VDC, Pin 8, 0 VDC.

    I need a beer and go to bed. Lol. Been at this too long tonite. I will lift the board again tomorrow and take another look.

    Running this one below @clintj. I'd love a pointer how to check grid leak path and supply screen. I have swapped in other known good tubes with no improvement. Thx.

    [​IMG]
     

  11. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Make sure you have a good, solid connection from pin 5 to ground through the 220k resistor.

    The screen grid circuit is that filter cap after the 10k dropping resistor and its wire to the power tube, pin 4. I don't see a screen resistor in that layout. 470r/3W or 5W on pin 4 is a good idea for longevity on a 6V6 without affecting tone. Without it, there's not a practical limit on screen current under overdrive conditions - it can pull as much current as it wants from its filter cap until something gives out.
     

  12. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Great stuff, Clint. I recall Merlin suggests even 1k, although I suspect 470R would be fine on a 5f2a. OTOH, maybe class A pushes the tube harder? In any case, I do know a certain Canadian who definitely pushes tubes harder. Wasn't it you, Keith, who tested an amp wearing earplugs under industrial hearing protectors? :)

    As far as B+ going high and then dropping, I was happy to see the point from @dsutton24 about tubes not conducting at first. I had noticed this on my most recent amp and hoped it was that, since I'd just gone through the higher B+ on no-tubes startup testing. But I also have a humble idea -- I wonder if the 'pop' we hear when measuring is due to the MM, since I only hear it when measuring.
     
    Last edited: Nov 25, 2017

  13. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    @King Fan yes, I l do like the tweeds dirty. I will experiment with a screen resistor and report back.

    On my bench this 5F2A pops upon start up with or without a multi meter hooked up. I agree with the 6V6 not conducting at first, then maybe when it does, this one is making a pop.

    I plugged the amp in through my light bulb limiter. Turning the amp on, the bulb stays dim, as soon as the pop is heard through the speaker, the light bulb lights up. Looks to be another clue leading to a short. I compared the light bulb test results to other amps I have here. Bulb comes on bright as soon as the power switch is turned on , and dims down as the amp warms up.

    With 6V6 out B+ stays stable at about 500V.

    I will further inspect for a possible short. Its such a simple little circuit. I am humbled by it today. :rolleyes:
     
    Last edited: Nov 25, 2017

  14. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Ok an update: Progress, and a video.

    I pulled the board again and found no problem. Decided to clean up some more. Damn near replaced every wire I put under there, or connected to the board. I know, I'm adding more variables to the problem. However I built this amp 6 years ago, early in my amp adventures. There was some rookie stuff in there. I did find a problem. Tucked up under the chrome lip by the tabs of the volume and tone pots. A connecting wire sheathed in plastic, had been kissed by my soldering iron. Wire was exposed touching the solder contact on centre lug of volume pot. Hard to see up in there. I can't blame anyone for this but me. No man's hands except my own have ever been in this amp.

    Reassembled all. Amp works again as it should. It also resembles something I'd be happy with my name on today.
    However I see the 6V6 arc flashing. I took a a video. See it here. It only happens when amp is cranked and NFB is switched off.
    I have not yet installed a screen grid resistor off 6V6 pin 4. That'll be next. I'd like a little clarity. Pin 4 has a wire to B+ as seen in diagram above. Am I to install the 470R 3W or 5W, from pin 4 to chassis ground? Thanks, Keith

     

  15. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    image.jpeg Interesting light show

    Plate Voltage rising up to 500 VDC is normal.

    I expect a little glow on the plate in class A, a little white stripe in the dark, but I've not seen the flashing.



    For the screen resistor, move the wire from pin 4 to the unused pin 6, then complete the connection by connecting the resistor between pins 4 and 6. Keep the resistor away from directly above the tube for heat reasons.
     
    Last edited: Nov 25, 2017
    Thin69 likes this.

  16. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Thanks @peteb. Resistor is now in place. 6V6 plate voltage, while playing is hovers between 414 to 419V. Arcing has vanished.
    Thanks for all your input folks. There was a couple of things going on here. Not the least, I have been running this amp mostly cranked and dirty for years.
    This experience was a new one for me, and interesting.
     
    dr_tom, Thin69 and King Fan like this.

  17. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'm kind of wondering what was going on here now. Couple of questions.

    What are your B+ node voltages?

    What OT are you running, and what primary impedance is it? And what speaker impedance? (ClassicTone has a multi impedance option, so I'm curious)

    I just want to sketch a quick load line for the 6V6 and see where it lies, and make sure it's not in a bad place as far as above or below the "knee" in the graph. Your supply voltage is a bit high for a SE tweed, it seems to me.
     
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  18. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm curious about this, too, Keith. I don't know much, but those plate voltages on the 6V6 do sound really high. What kind of 6V6 is that you're running? Is it perhaps an RCA 6V6GT gray glass? Or is it some kind of black glass? Those RCA 6V6GT gray glass tubes from the 50's can pop and arc under those kinds of voltages. @muchxs has noted this a few times on the forum. I ran one of those in a SF Champ running at about 400V on the plates when I had it, and that RCA 6V6GT gray glass popped and arced. The gray glass Sylvania 6V6GT that I ended up with in there did not. Like others, however, I've never seen a tube flash in the same way that one did in your video when the NFB was disengaged.
     
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  19. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    OT is a Allen T08C. 8 ohm tap used. Speaker is a Weber 10A100 8 ohm.
    I have not heard the term node voltages before. Is that anode, and cathode? Is there a certain spot in the power supply you'd like me to take measurements?
    Tube is a vintage GE 6V6 made in the USA. Dark glass.
     
    Last edited: Nov 25, 2017

  20. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Node just means a connection point between two or more circuit elements. So B+ nodes would mean the voltages across all the supply filter caps.
     

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