JJ Tube Test Results

[dcgillespie]

Thought I would pass along some data I just generated on six new JJ EL34 tubes that a local repair shop asked me to verify the performance of.

They had purchased them as a matched sextet and wanted to know just how well they were matched.

With real world voltages applied, they all required within .7 volt of the same bias voltage to draw a set 50 ma of plate current. And their Gm was matched within 3% as well.

What interested me was the power output results. Here they were within 10% of each other (ranging from 74.7% to 81.8%), but importantly, only one of them could just reach into the "Good" range of power output (defined as 81% or greater).

The Average New Power Output (ANPO) established for the EL34 in my PO tester is based on more tests using classic Mullard, Telefunken, and Sylvania 6CA7/EL34 tubes than I care to remember. Therefore, the important take away point is that the JJ tubes basically start life as "WEAK" from the get go. For designs that were based on real McCoy, that can be significant then.

On a positive note, many JJ output tubes that I have tested in the past display a notable sinking power output level from the point that the test was initiated, which I attribute to a poorly activated cathode. None of these tubes displayed this characteristic, nor did any them light their screens up during full power tests either like so many of their tubes have also done in the past.

So, maximum power output is still notably limited from the JJ EL34s as compared to the genuine article. But with the other all too typical observed JJ characteristics seemingly improved in these tubes, could it be that JJ is starting to get the message? Time will tell, but let's hope so!!

Dave

 

[gadget73]

I wish them the best. It would be nice to have viable competition in the tube world, rather than just a couple brands that each have major but different problems. Seems like half the time you have to pick your fault and go with it.

 

[dcgillespie]

Agreed, and well said!

Dave

 

[dcgillespie]

Posi -- In my tester, the tube operates in a fixed bias circuit from regulated voltages and load values capable of allowing the tube to pull maximum rated cathode current. During the test, the tube is driven to the onset of clipping as viewed on a small scope, at which point the peak voltage developed across the plate load resistor is measured, from which power output is calculated. Thus, mu has little to do with the test result.

From my experience in testing other JJ tubes, they do not measure any better with time.

Now 4CX250s -- those are real tubes for ya!

Dave

 

[GordonW]

One wonders if they "deliberately" decreased cathode output on the JJ EL34- to alleviate the screen problems they've been known for in the past? Less emission = less screen current.

IIRC, the EL34 and 4CX250s are really apples-and-oranges, for the following reason. The EL34 has a coated cathode, which should be "max output" out-of-the-box. OTOH, the 4CX250 probably has a "naked" tungsten cathode- that not only can be repeatedly rejuvenated by elevated heater voltage- it also takes some "activation" to get it to run at full emission, upon installation.

In short- a coated cathode will never get "better" than when it's fully activated, that first time, when it's new. OTOH, a bare tungsten cathode can be brought back, many times... this may be the cause of the results described above...

EDIT: Well, so much for that theory. The 4CX250 also has an "oxide coated" cathode. Though, I can't find what KIND of oxide coating (barium? strontium? calcium?) is on it. May well be that the oxide coating on the 4CX250 is enough thicker/heavier than that on most EL34s, that it can be re-activated by heat (which, in effect, "boils" up some more oxide from the inner regions to the surface of the cathode, to replace that which is depleted from the surface, in use), several times, while the EL34 simply "runs out" of oxide, very quickly...

Regards,
Gordon.