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Post by mikegp on Feb 7, 2017 19:53:38 GMT -7
Hi,
I purchased a supposedly matched set of Gold Lion KT66 tubes, however, when I bias them there is a 8 mA difference in their cathode currents. I measured the screen currents and they appear to be the same. I tried switching the tubes around to see if it was an issue with the tube sockets but the difference moved with the tubes. Is it normal to have this much of a difference in a matched set of tubes? I should also mention that when I measure the plate voltage I get 436 volts for both sockets with the tubes installed. I would have thought that the tube drawing the larger current would have a lower plate voltage.
Thanks for the help and advice!
Mike
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Post by Paul (TRANE) on Feb 7, 2017 20:03:52 GMT -7
I have seen that myself on various "matched" sets. If memory serves "matched tubes" can be up to 20% different from an industrial perspective. Now some sets are matched closer than others of course. But according to my notes, from when I bias my amps, there is a wide range of deltas....and all were matched duets or matched quads (KT66, EL34, 6L6).
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Post by mikegp on Feb 8, 2017 13:48:18 GMT -7
Thanks for the reply Paul. I'm no electronics expert but if one tube is drawing more current than the other, as in my case, shouldn't there be a difference in the plate voltages? I would think that the tube with the larger current would have a lower plate voltage.
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Post by perryr on Feb 8, 2017 20:14:18 GMT -7
My buddy dropped off an amp, sounds like poo. I pulled the tubes and they are all toast, power tubes are way apart (Gm & Current). I just now put all the tubes back in and measured the power tubes, voltage is within 2v side to side. Current measured 15ma and 8ma, thats 50%, and very low! No wonder it sounds like poo.
I would think voltage is a "supply" that is impacted by total current demands (and issues if any).
I match power tubes using a Maximatcher that measures both Current and Transconductance (Gm). I talked to the guy who designed it and asked what is better to match by, Gm or Current? He said Gm for certain, much better indicator of a tubes character. He said current is just easier and has been adopted. But when the amp leaves an idle state, it is Gm that will be more of an indicator as to how well the tubes perform together (being played!). I have watched my Compu-bias readings when playing and the numbers go all over the place, definitely performing widely apart from side to side at times (current).
Now, when measuring tubes, current and Gm do correlate, but not precisely, and it wouldn't surprise me to see 2 tubes with the same Gm have a 20% variance in current. With all that said, its possible your KT66 tubes were matched by only Gm, or by a more dynamic testing/matching process.
Does the amp hum (60cycle) with the new tubes, sound good? Maybe get some feedback from the seller on your measurements.
Perry
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Post by mikegp on Feb 9, 2017 7:42:39 GMT -7
Hi Perry,
Thanks for the detailed explanation. I do notice on one of the boxes that the tubes came in there is a label with Ip: 52 and Gm: 4,500. So maybe you're right maybe they used this Gm measurement when matching the tubes. I think Ip is a measure of the plate current. I don't really understand transconductance. The amp sounds good when being played. There is a small about of 60 cycle hum but it's not very noticeable. The white noise sound is louder when the amp is just idling.
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