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Hickok test settings for a Chinese tube (6A2)?

DrJAB

DrJAB

New Member
Recently picked up a nice little Fosi headphone amp which uses two Chinese 6A2 tubes. Tube rolling choices for these little amps include GE 5654s and 6AK5s. I've also rolled in Soviet 6J1Ps with nice results. When I've tested the 6A2s on my Hickok 800 using settings for either the 5654s or the 6AK5s, I get no reading at all. I've searched hi and lo for proper test settings for the 6A2s but to no avail... any help would be appreciated, thanks. DrJAB
 
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I'm a little confused on your bias setting. 1.8??? My scale for bias is 0-100. And did you omit the English setting (which I presume I could set to 86.5 (red dot at 6000) to read in the 0-6000 scale). And what's the "C"? thanks in advance. What tube designation are these settings for (other than the 6A2)?
 
DrJAB said:
I'm a little confused on your bias setting. 1.8??? My scale for bias is 0-100. And did you omit the English setting (which I presume I could set to 86.5 (red dot at 6000) to read in the 0-6000 scale). And what's the "C"? thanks in advance. What tube designation are these settings for (other than the 6A2)?
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Oops that's my mistake, I was looking at my Hickok 539b.

Everything including bias should be the same except where on the 6AK5 they have the suppressor switch set to "0" you want to set it to "7". That's the only difference.
 
Got it! Thanks. I'm reading somewhere around 0.6 mA/V. This is quite lower than the 6AK5 or the 5654, not sure what to make of it. On the Chinese language datasheet I see two values reported in mA/V: 4.5 and 0.3. But without translation I have no idea which one (if either) refer to Gm. In one case the tube is fine, in the other case it's trash! It's working in the amp, so I'm thinking Gm might be 0.3.
 
So according to my Chinese co-worker, the Chinese datasheet for the 6A2 reports "oscillator transconductance" as 4.5 mA/V and "inverter transconductance" as 0.3 mA/V. I don't know how to distinguish between these two, and I've never seen two different values for transconductance reported for other tubes before. Since these tubes are testing at 0.6 mA/V, and are considered new, I think it's more reasonable to assume that the transconductance that is reported with the Hickok 800 is inverter transconductance, and that the two tubes I have are testing well above spec.
 
Testing for gm is fine but only half the story. In circuit testing is better because it actually shows how the tube will behave under load which will show the other half of the story which is plate impedance. I'd just compare in circuit gain between it and the 6AK5.

Try changing the suppressor switch setting from "7" to "2" and see if that changes the gm readings.
 
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