5F1 Power Transformer - Low Heater Voltage?

Discussion in 'Shock Brother's DIY Amps' started by decsnspace, Oct 15, 2019.

  1. decsnspace

    decsnspace Tele-Meister

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    Today I broke out my first build, a 5f1 Mojotone amp that I put together several years ago and decided I'd check out the voltages and bias, when I noticed that the 6.3 VAC heaters only measured @ 5 VAC with 120 VAC applied.

    I figured there must have been an issue with my wiring but I didn't see anything obvious so I disconnected both green and yellow filaments to test the filament lines out of circuit.

    I first measured resistance and got .3 ohms for both green and yellow filaments and when powered with 120 VAC, both filaments were near identical voltages.

    Green: 5.53 VAC
    Yellow: 5.49 VAC

    So I guess my question, is this normal for these champ power transformers (Mojo 760) to have these kinds of similar readings on both heaters? And what effect would running it at this low voltage for the heaters have on the circuit? Considering pretty much all other voltages in the amp are much higher than normal with modern 120 VAC?

    The amp does work alright but it has always had a slight bit too much hum for my liking, which I have previously tried to resolve through increased power filtering, shielded cable, and bi-passing the volume pot switch to no real resolution.
     
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Did you take voltage readings when you originally built the amp? It's possible that the PT was wound incorrectly from the start but that seems unusual.
     
  3. decsnspace

    decsnspace Tele-Meister

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    Yeah, I was very much a noob when I first built the amp. Reviewing my original build post today, I notice that I did mention the issue at the time, but no one really commented on it and since the amp eventually fired up ok, I figured it wasn't an issue and forgot about it until now. Actually when I later did the further mods to the amp a few years later, I must have just assumed the heaters voltages were good and likely overlooked them since the tubes lighted up and were functional.
     
  4. Dacious

    Dacious Poster Extraordinaire

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    Wouldn't worry about it. All specs on vintage amps are +/- 15%. Tubes aren't that critical.

    I'd just check your first socket from the trans heater connections.
     
  5. shortcircuit

    shortcircuit Tele-Meister

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    As for your hum issue, the Mojo760 looks like it doesn't have a center tap on the heaters. Try running a pair of 100 ohm resistor from each leg of the heaters of the 6V6 the cathode (pin 8) for an elevated virtual center tap.
     
  6. decsnspace

    decsnspace Tele-Meister

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    Thanks for the suggestions. I did elevate the filaments with an artificial center tap during the initial build. I'll need to get this one on the bench again and spend some more time with it. A few years back I almost completely rewired it but still couldn't get rid of the hum. The other two SE amps I built both have diode rectification but are otherwise super quiet so not sure why this one gave me such a hard time.

    Sent from my LM-Q610(FGN) using Tapatalk
     
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