Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on January 08, 2014, 12:06:20 am
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Im posting this because i have been buying a lot of power tubes of all different makers latley. Needless to say god only knows for sure where they are coming from. I am finding a good amount of tubes that are only giving me an 80-90% emissions and struggling to do that. I have two different testers and all of my NOS tubes blast my testers meter from the 100% to even 115% in emissions. I just tested 16 new in the box and receieved Sovtek 6L6GWB tubes and every one of them struggled to hit 90%. I even had 2 of them show a short in a non shorting position on my tester. I know my tester and it has been good to me. It is calibrated and i keep it that way. Are any of you experiencing this?
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... I am finding a good amount of tubes that are only giving me an 80-90% emissions ...
Are you actually using an emission tester?
I'd strongly discourage it (Langford-Smith also recommends against it in RDH4), as it doesn't tell you much of anything and can permanently damage the tube. A tube can also register good emission and have no Gm or power output in use.
The best test of any tube is as it will be used. Slap a preamp tube into a circuit with a known gain (with a known good tube), apply a test signal and measure the output at the plate (you can calculate gain, or some old VTVMs had their ranges additionally marked in dB so you could compare readings on 2 different ranges and know how many dB of gain there was directly).
For power tubes the best measure is power output. RDH4 shows a parallel-feed SE circuit for testing one tube at a time. But I bet you could again start with a known-good set of output tubes, and set the bias to a specific voltage that will be used for all tubes. Drive the output tubes with a test signal so that the input signal peak doesn't exceed the bias of the tubes (doesn't drive the grids positive). Maybe monitor with an o'scope to determine a good low-distortion drive setting). Measure volts output across a dummy load and calculate clean power output.
- Now, put in your new set of tubes and leave the bias voltage exactly where it was for the "standard set" you used earlier. Drive with the same clean test signal. Measure volts across the dummy load, and compare calculated power output to the standard pair.
The power output test won't tell you Gm of the output tubes, but will give a real-world measure of the health of the tubes.
Anyway, the guy who wrote the book on tube testers (literally; Alan Douglas wrote a book mainly about vintage tube testers of all types) will tell you that even the best only give an indication of the health of any tube, and that even "calibrated" testers of the same brand and type may give differing readings from one to the next. Since an emissions test has the least correlation with how a tube really functions in an amp circuit, it also correlates the least to telling you if a tube is good or bad (other than showing open heaters, shorts, maybe gas or a completely dead cathode).