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Power Tube load sharing

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  • Helmholtz
    replied
    Well, a bias balance pot like in a SF Twin would be nice here...
    What model of Twin is it?

    Leave a comment:


  • nevetslab
    replied
    This morning, I checked my notes to see which tube I found having the silk screen discolored. It was one of the two tubes having the higher plate current. I left off yesterday with mixing the lower current tubes with the higher current tubes. I plugged in my 4 ohm 200W dummy load, with AC Voltmeter across it, as well as the distortion analyzer/scope, and turned up the output as Fender states for full power measurements: 1khz 5% THD into 4 ohm resistive load, Normal Ch, tone controls fully CW, with Vib ch controls all CCW.

    With this idle plate current balance being:

    V7 28mA
    V8 32mA
    V9 25mA
    V10 34mA

    I turned up the output to where I found 5% THD. 5.9VAC, 8.7W! Forget the 5% THD figure. I reset the output to where I see Visible Clip on both sides of the waveform, which was about 10% THD. That was 17.1VAC, 73W. Just to see the difference, I powered down, then swapped the tubes back to how they were positioned when the amp came in (though I had increased the bias from where it was):

    V7 27mA
    V8 27mA
    V9 20mA
    V10 23mA

    I turned up the output to visible clip, which was 15.1VAC, 57W. So, that current imbalance on the Primary of the Output Xfmr does somewhat limit the output under similar distortion figures. I don't have a fresh matched quad set of 6L6GC's here on hand to verify my suspicions of only getting full power with all four tubes matched.

    I watched the tubes for around an hour between half power and full power, though never saw any evidence of red-plating.

    I restored the tubes to the first balance shown, each half having one higher current tube and one lower current tube.

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  • Helmholtz
    replied
    See if any of the tubes redplates between half and full power.
    Otherwise I'd go for equal sum currents on both sides of the primary.

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  • Mick Bailey
    replied
    I also wonder how a static match shifts under dynamic conditions. The only way to check is to run the amp into a dummy load and measure the respective dissipations at (say) 25, 50, 75 and 100% output with a fixed frequency input signal. That would be interesting because the shifting screen voltages would have more influence than at idle.

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  • Enzo
    replied
    What is your goal? Hum balance at idle? This isn't a hifi, so absolute balance for signal might not yield the "best tone". And I wonder how much of that most of th work occurs at audio, rather than DC idle.

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  • nevetslab
    started a topic Power Tube load sharing

    Power Tube load sharing

    Question: In the case of a Fender Twin Reverb, who's Sovtek 5881WXT Power Tubes measure 27mA/11.9W for V7 & V8, and 20mA/8.8W for V9, and 23mA/10.2W for V10, would you leave this as is, or swap them, moving the two 27mA tubes to both sides, to mix with the lower current tubes? I have one on the bench, and after swapped V7 & V10, I then bumped the bias up for reading 28mA/12.8W for V7, 32mA/14W for V8, 25mA/11W for V9 and 34mA/15W for V10. While this averages out to around 60mA per pair, it also places the higher current tube doing most of the work, and the load sharing isn't as good. With the original balance, the two tubes running 27mA vs the other two running 23ma & 20mA, the load current was better, though the balance of the two halves wasn't equal. What makes more sense? Besides replacing the set with a new matched set. The date codes on these tubes would warrant that, being 0601 and 0801 (2006, 1st 2k, 2008, 1st wk).
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