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December 06, 2022, 09:06:57 AM

Author Topic: Variations in measurements between tubes  (Read 87 times)

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Offline Larpy

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Variations in measurements between tubes
« on: September 24, 2022, 09:08:12 AM »
I finished building my Moreplay last week and have been enjoying it greatly: frankly, it's amazing to hear how good such a simple circuit can sound.  I was dubious of Doc's claim (earlier this summer) that the Moreplay sounded better than an Extended Foreplay III (which has been my preamp for the past 15 years), but he's right.

I have a question:  tube rolling 6V6s, I find that one pair (RCA 6V6GTs from the '50s) measured exactly 125 volts on each plate (to ground) and 8 volts on each cathode (again to ground).  But a pair of Sylvania 6v6Gs (from the '40s, I suspect) measured 135 and 143 volts on the plates and a bit less than 8v on the cathodes.

These readings are within 10-20% of the specified measurements, so I'm not concerned about anything burning up (and I don't hear a volume imbalance between channels), but I do wonder if there are sonic advantages to keeping more closely to 125 volts on the plates.  I could, for example, reduce the value of the cathode resistors and put 1K pots in series with them, and this would allow me to adjust the plate voltages very accurately.  Would this be worth doing?

I know matching power tubes is important when they're run push-pull and you want to run them hot but avoid excessive plate dissipation, but is there an advantage to matching tubes beyond that?  After all, circuit designers take care to choose particular operating points, but tubes are notoriously all over the place in terms of transconductance and how much current they draw at idle.  As I understand it, this is just an unavoidable consequence of tube manufacturing  A tube can test "good" but still measure "high"or "low" once put into a particular circuit.
Larry

Offline Paul Birkeland

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Re: Variations in measurements between tubes
« Reply #1 on: September 24, 2022, 11:07:57 AM »
I would run the Sylvanias on a tube tester to see how they test.  Because the stock Moreplay is both resistively cathode biased and resistively plate loaded, we would expect the plate voltages and bias voltages to be very stable until a tube is pretty dead.  Having said that, there can also be some operational differences between brands of tubes and samples separated in production by decades, so I wouldn't necessarily condemn the Sylvanias just yet.

Putting adjustable bias in may cause undesirable issues at the end of the day.  To pull the plate voltage down to 125V you may end up pushing the cathode bias voltage down further than would otherwise be considered desirable, so I'd just leave that alone.
« Last Edit: September 24, 2022, 04:55:30 PM by Paul Birkeland »
Paul "PB" Birkeland

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Offline Doc B.

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Re: Variations in measurements between tubes
« Reply #2 on: September 24, 2022, 12:08:26 PM »
Run the tubes in for a while and measure them again.
Dan "Doc B." Schmalle
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