KT66 Testing?

[fredtroy]

I have some Genalex KT66's that I am testing in preparation of selling. I tested with Hickok 6000a transconductance tube tester and by installing in Quad II amps. I have a couple of tubes that test low on the Hickok 6000a, but look fine in the Quad II amp. The Hickok tester seems to be testing with plate at 170 pulsing DC, the Quad II, 400 DCV plate, -33.6V bias, 67ma.

Most of the other KT66's I have tested, Hickok tests very good, with almost identical measurements of the suspect tubes.

Can anyone explain what I am seeing?

Thank-you.

 

[Pio1980]

With no other obvious faults, the differences will be relative sustained peak power in the amps, and relative life tests on the checker.

 

[maxhifi]

The tubes which check low on the hickok are weak, simply. The QUAD II employs self bias, so even weak tubes should bias okay in it.

 

[fredtroy]

The tubes which check low on the hickok are weak, simply. The QUAD II employs self bias, so even weak tubes should bias okay in it.

In the Quad II, 400 DCV plate, -33.6V bias, 67ma, it is biasing within 1.5% of the tubes that test good in both the tester and the amplifier. It is also exactly on the tube data graph line. The only thing that I can come up with is that these tubes don't do well at low plate voltages.

 

[maxhifi]

In the Quad II, 400 DCV plate, -33.6V bias, 67ma, it is biasing within 1.5% of the tubes that test good in both the tester and the amplifier. It is also exactly on the tube data graph line. The only thing that I can come up with is that these tubes don't do well at low plate voltages.

Lower plate voltages are the point in the AC load line where the tubes are called on to deliver the highest current. If they can't do this due to worn our cathodes, power output will suffer and it is fair to call the tubes weak.

Plenty of weak tubes appear to bias fine in an amplifier but can't deliver power well. The best test of output tubes though is always to measure power output of a properly operating amplifier.

 

[fredtroy]

Lower plate voltages are the point in the AC load line where the tubes are called on to deliver the highest current. If they can't do this due to worn our cathodes, power output will suffer and it is fair to call the tubes weak.

Plenty of weak tubes appear to bias fine in an amplifier but can't deliver power well. The best test of output tubes though is always to measure power output of a properly operating amplifier.

I tested power output on the tubes that test NOS and the tubes testing low on the Hickok and they are almost exactly the same. I used a sine wave into the amp and a 6 ohm load resister, increased input until it clipped and measured the output.

 

[maxhifi]

I tested power output on the tubes that test NOS and the tubes testing low on the Hickok and they are almost exactly the same. I used a sine wave into the amp and a 6 ohm load resister, increased input until it clipped and measured the output.

How much power from each set?

 

[fredtroy]

16.83 watts, and thank-you for the dialog about this. I'm beginning to think tube testers intentionally test at these lower plate voltages because it is a predictor of tube life.

 

[Pio1980]

16.83 watts, and thank-you for the dialog about this. I'm beginning to think tube testers intentionally test at these lower plate voltages because it is a predictor of tube life.

Possibly. Life test of an indirectly heated tube is how fast emission drops when heater voltage is cut, or how much it drops with reduced heater voltage.

 

[maxhifi]

16.83 watts, and thank-you for the dialog about this. I'm beginning to think tube testers intentionally test at these lower plate voltages because it is a predictor of tube life.

Also the tube tester checks all tubes at the same plate voltage, and not all tubes are able to withstand a high voltage. I think that's good output for a QUAD II, seems to me the tubes are still good in the QUAD, and I certainly wouldn't hesitate to keep using them, but hard to say if it is ethical to sell them as good versus weak since the going rate for a real KT66 is so high now.