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Voltage on Heaters but no Lamp or Filament glow...

Discussion in 'Amp Tech Center' started by James Knox, Feb 24, 2021.

  1. James Knox

    James Knox Tele-Holic Silver Supporter

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    This is a new one for me!

    I have voltage on the Lamp Assembly Tabs, Power Tube Socket and Preamp Tube Socket, but NO Lamp Bulb or Filament Glow on either tube.

    Ive taken continuity readings on Lamp Assembly, Tube Sockets and Tabs, switched out Bulbs and Tubes, etc.

    Any ideas where to check next?

    50BE6AD7-586C-4EC5-869B-F39E4D2B9812.jpeg 417D12FA-672A-4E55-8502-88A903AAA5F5.jpeg
     
  2. Whatizitman

    Whatizitman Friend of Leo's

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    What PT are you using? Does the 6.3V have a CT?
     
  3. Jon Snell

    Jon Snell Tele-Holic

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    If you have 6.3volts AC across the bulb tags, then the bulb is blown. I expect you don't have AC there.
    Check again.
     
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  4. James Knox

    James Knox Tele-Holic Silver Supporter

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    Hammnd 290AX with CT lifted at the Power Tube Cathode Tab.
     
  5. Bill Moore

    Bill Moore Tele-Afflicted

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    Check the resistance of the bulb to verify if it's open as well as the heater circuits on the tunes.
     
    Last edited: Feb 24, 2021
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  6. James Knox

    James Knox Tele-Holic Silver Supporter

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    I tried 3 different bulbs. I’ve got his when measuring from either Lamp tag to ground... 1C105A06-4A55-4FF7-B2DB-2C3592074298.jpeg


    And then this when measuring from one Lamp tag to the other...

    AD0737A3-66F9-4D88-A804-9D646FED2468.jpeg

    Am I measuring them wrong?
     
  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    EDIT: Just saw your DMM pics. Note the instruction to check *resistance* across the bulb.

    Measure heater voltage VAC pin-to-pin not pin-to-chassis. What do you get? What continuity are you testing at the lamp and sockets? Could there be a short somewhere in the heater wire chain?

    I assume the amp passed light bulb limiter startup? Bulb goes dim with 40-100W bulb? You might do Paul Ruby's no-tube heater startup step; I use an LBL instead of his 250mA fuse, and of course you won't have or need standby:

    "With no tubes installed and switched to standby, install your pilot lamp and use a 250mA slo-blo mains fuse. Click on the power for 1 second just to see the pilot come on nice and bright. This is primarily checking the 6.3VAC supply line is not shorted and is properly connected to the lamp. If the lamp did not come on, check to see if the fuse blew. If not, try another lamp and do the 1-second power thing again. If the fuse blows, there is a short on the PT or mains. If the fuse survives, then it's likely that the 6.3VAC is not properly connected to the pilot lamp. Use your AC voltmeter to check for 6.3VAC (actually more like 7VAC with no tubes installed) at the lamp and all the tube sockets. If you have a recto tube powered by 5VAC, then also check that you have 5VAC at the recto tube socket now. This will be up around 6VAC without the recto tube installed as yet."

    If no lamp light with the heater chain intact, I'd "lift" (disconnect) the downstream heater wires from the lamp and make sure it can light when the sockets are offline.
     
    Last edited: Feb 24, 2021
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  8. James Knox

    James Knox Tele-Holic Silver Supporter

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    Yes, I have continuity between tip and sleeve
     
  9. James Knox

    James Knox Tele-Holic Silver Supporter

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    Thank you for this good detailed post @King Fan!

    I have run out of bench time for today, but will step-by-step it as son as I am able. I suspect here is a short somewhere in the heater line.
     
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    ...and you have 6.6 VAC between the two lamp tags?
     
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  11. D'tar

    D'tar Friend of Leo's

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    In short...... You have a short!
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    A short reply indeed. :)

    Yeah, that's kinda what I wonder. If you get to the step of lifting the downstream wiring off the lamp (and the lamp lights at that point) I'd also hitch your ohmmeter across the downstream heater wires you took off the lamp. With no tubes, and without the lamp and PT inline, you should get 'open' -- 0.L, no continuity, no ability to measure resistance -- across the heater wires.
     
    Last edited: Feb 24, 2021
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  13. James Knox

    James Knox Tele-Holic Silver Supporter

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    Ok, I disconnected the Green Heater wire from the lamp to the tubes. Fired it up, no lamp. No AC voltage. Turned off, unplugged, drained Filter caps.

    Unsoldered green heater leads from lamp assembly. Put alligator clips on DMM leads coming directly out of the PT. Fired up; NO AC voltage. Ran a jumper from the Green/Yellow Heater Center Tap That was “Elevated” at the 6V6 Cathode Tube lug, just to make sure that the CT was indeed grounded. Still, no AC volts.

    Switched DMM to Ohms and got nice long beeeeep. Continuity. The Ohms reads an “auto ranged” 0.3 ohms.

    Does this mean have a faulty Transformer?
     
  14. Jon Snell

    Jon Snell Tele-Holic

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    If, you lift the green wires from the transformer and get a beep, or low resistance across it (transformer side that is) then the transformer is probably OK. Next, watch your fingers for HT coming up and measure the AC voltage across the green wires from the transformer. Across those twisted wires only, not to the chassis.
    What have you got? ..... 6 volts AC?
     
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  15. goldguitarguy

    goldguitarguy Tele-Meister

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    Did you check to see if there is voltage on any of the other secondaries of the PT? Like 5vac on the rectifier heaters or B+?
     
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  16. James Knox

    James Knox Tele-Holic Silver Supporter

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    No VAC at all.
     
  17. James Knox

    James Knox Tele-Holic Silver Supporter

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    Yes, I have DC Primary voltage and Rectifier voltage.
     
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Is there any resistance or continuity with the other wires of the PT? Is there any resistance or continuity with the body of the PT?

    Do you still get a 3.3 VAC from the green heater wires to the heater CT wire?
     
  19. D'tar

    D'tar Friend of Leo's

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    I take this back. You have quite the opposite.

    I would be looking for broken wire or poor connection to the 6.3 windings. Ever take the bells off a PT? Lol

    3.3vac suggest at least half the winding is conducting
     
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  20. dan40

    dan40 Friend of Leo's

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    This issue happened to me years ago on my first build. I must have been to rough with the wires when twisting them and ended up with a broken wire inside the PT. Was a bit of a pain to fix but I eventually got it.
     
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