In a message dated 10/2/03 8:06:28 PM Central Daylight Time,
k7fm@teleport.com writes:
> Hi Dennis:
>
> I have had excellent results building testers, although the ceramic tubes
> present a problem without a blower system.
>
> The Eimac warning is out of context, as there is implied in the warning that
> the screen voltage is sufficient to draw enough current that the total power
> dissipated will exceed the screen power allowed dissipation. Using low
> voltage there is no possible way to blow out a screen.
Colin,
My problem was the 4cx120's and 350's I was trying to test were oscillating
for some reason. Rather then dig any deeper, I just pulled out a one tube grid
driven amp I use for testing and then tested them by substitution.
It was the path of least resistance and I was in a hurry to get the job
done.
>
> A few years ago, I tested a 6L6 with 24 volts on the plate and then use 0
> to 12 volts on the screen and 0 to -9 volts on the grid. I was able to
> calculate the transconductance and it was consistent with my expensive tube
> tester. The same technique was outlined in numerous books published prior
> to WWII, as that was often the only way amateurs could test their tubes.
I have a box of NOS tubes from that era that were designed to run with only
15 to 30 volts on their plates. I can't remember what number they were right
now, but I also seem to remember that there was the equivalent of two tubes in
the same envelope so the single bottle could be run push pull and 'share' the
filament, cathode, and screen grid. If I can remember the number and/or run
across those tubes again I will publish that tube number here.
>
> No doubt the placement of tubes in a real amplifier is the best method as it
> will test the tube under the limits for such things a parasitics.
I agree.
>
> But, for matching tubes, low voltage works fine.
OK, but I have had problems trying to do that on my B&K 747 tester.
>
> And, I have a home built regen receiver using an 807 using about 90 volts on
> the plate and about 30 volts on the screen. I have plans to build a 4-65A
> regen receiver just for novelty, but have not had time to build it yet.
Have you published the schematic of that receiver on a website?
The reason I ask is I have a few dozen NOS 807's still in boxes.
I believe they were all the GE brand, but I could be wrong about that.
Regards,
Dennis O.
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