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Thread: Help Needed with Heater Wiring

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    Help Needed with Heater Wiring

    Hi all! First time posting here after years of browsing and taking notes. Thanks for all the help so far

    I'm wrapping up a custom build and trying to iron out an issue with my filament supply. The heaters for the first three preamp tubes will occasionally go out - not intermittently but just be completely off when I power on the amp.

    I first ran into the issue when I wasn't getting any sound through the amp and noticed that the first three 12AX7 tubes weren't glowing at all. Oddly enough, the fourth preamp tube(12AT7 LPT phase inverter) and the power amp tubes(4X 6L6GC) do glow. Then I go pop open the amp to measure the AC voltage on the heaters, and sometime thereafter the heaters for the trouble preamp tubes eventually come back. This has happened on a couple separate occasions. I've double and triple-checked my wiring but am totally stumped at this point! Also worth noting that I have an artificial center tap on my filament supply, 100ohm 1/2W resistors to ground.

    Anyone have ideas on what might be the issue here? I thought maybe it's a short in the filament supply, but I imagine this would affect all the tubes+lamp rather than just the three.

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    Supporting Member TomCarlos's Avatar
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    Welcome to MEF -

    Please post a diagram or schematic so we can see what you are working on.

    What do you have connected to pin 9 of the 12AX7 tubes?

    What voltage are you measuring at the 100 ohm resistors when the tubes are lit?

    So when the tubes go out, what is the voltage at each of the 100 ohm resistors?

    And the tubes go out, do you still have your B+ voltage (to make sure you have not lost that power too) and the pilot light is on?

    Tom

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    Last edited by TomCarlos; 06-24-2019 at 04:37 AM.

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    Hi Tom,

    I am having trouble posting any attachments at the moment. I think my account might be too new, so I'll circle back and edit this post with photos/schematics once I get those privileges.

    Pin 9 is connected to one side of the filament winding - I kept the phase consistent between preamp tubes. The tubes are daisy-chained together so all but V1 have two sets of wires soldered onto them. Pins 4 and 5 are shorted together and connected to the other side of the filament winding.

    I'm reading ~6.3VAC between the two HT filament wires and ~3.15VAC across each artificial center tap resistor. As far as I can tell, the that's the reading across those 100Ohm resistors regardless of whether the three 12AX7s are out or not.

    Interesting thought about the B+ voltage - I hadn't thought to check that but I will try that once I catch the amp in this state again.

    Thanks for your help!

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    Senior Member SoulFetish's Avatar
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    First, nice work on your screen name. (We see what you did there.)


    Second, and this is just a heads up and not a big deal,
    ~6.3VAC between the two HT filament wires.
    HT is the common designation for the high Voltage supply, along with the term "B+". This could might cause some confusion the way it is phrased above, but I know what you meant.

    So back to your issue. I know that you double checked your wiring, but I suspect that was just to confirm that you routed the wires to the correct terminals. I imagine you physically wired the heaters with the commonly used parallel scheme, and it sounds like you did. Then the fact that you lost heaver voltages in the last three tubes in that chain is a strong indication that you your problem lies in the wire connections at the heater pins of tube V3 or V4 (the phase inverter.)
    It only takes one of the two heater wires which run from the phase inverter socket to the V3 socket to have an intermittent connection to cut off the supply to the last three tubes.
    Therefore, I would check your solder connections at the phase inverter and V3 sockets to find your problem. Probably a good idea to reflow solder on all four of your preamp socket as well.
    look into that and report back.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by SoulFetish View Post
    Probably a good idea to reflow solder on all four of your preamp socket as well.
    look into that and report back.
    Completely agree. It's pretty sure there is a bad connection along the heater string somewhere. You can visually inspect and check resistance and everything might look good but still can't deliver the current.
    I had an amp once that had been to several shops over several years that had a very intermittent drop out. It turned out to be one of the heater wires stuffed into the lug never got it's solder. You couldn't directly see it because it was the underside one, and you could pull it with a plier and it would not pull out. The other wire was holding it tight in there, but it occasionally couldn't handle the current because it never got solder right from new.

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    My eyesight is shot (has been since I was 3) so I have to use a magnifying glass to find the cold solder joints.

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    No shame in that. I started using a stereo microscope for tiny surface mount work. Now I use the scope for everything, even turret boards! Mock me if you want, I'm just using the tools I need to do a good job.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Tony Bones View Post
    No shame in that. I started using a stereo microscope for tiny surface mount work. Now I use the scope for everything, even turret boards! Mock me if you want, I'm just using the tools I need to do a good job.
    While I never invested in a stereo microscope, I had used one a few times on SMD work, but quickly changed to surgical loupes and fiber-optic headlight, as seen in my avatar. Even the illuminated magnifying lamp was a PITA. While I haven't yet had the $500 for a 5X pair of surgical loupes, the 2.5X do really well, as the set I have are adjustable per eyeball, so you can set the working distance, and the headlight puts the high intensity light beam right where you're looking, and there's nothing in the way of your tools & soldering iron tip and the PCB/terminal strips. Still want to try an WHITE LED-based light source to unteather me from the halogen light source.

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    Okay everyone pretty much had it without even needing to look at the circuit. I reflowed solder on pin 9 of the phase inverter (V4) and everything is working consistently! So I guess the daisy-chain heater wire pair connection to V3 was intermittent. Thanks all

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    Senior Member SoulFetish's Avatar
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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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