Hi, Tony

I looked at the schematics of the Lafayette TE-50 and TE-36B
tube checkers found on line. They are somewhat unusual in that they
have no high voltage transformer.

The usual emissions tester (Triplett, Heath, Knight Kit, Eico, etc.)
uses about a 40VAC plate voltage transformer.
The Lafayette uses full line voltage through a 470 Ohm fixed resistor
in series with the 10K adjustable "Load" pot as the plate supply.
The autotransformer in the Lafayette is used merely
to select the heater voltage.

The usual emissions tester has a "Circuit" switch that selects
the current limiting resistor, and the "Load" resistor is merely
to adjust plate current meter sensitivity.

The Lafayette has no selectable fixed load resistor: the Lafayette
"Load" pot really +is+ a load resistor - if you set it to zero
during a Quality test you may damage the tube
due to excessive current, as the manual warns.

The Lafayette circuit looks more like a constant current source,
using a higher voltage plus a higher series resistance
than the standard tube checker.
If the tube works at all, you should get a "good" meter reading.

Very odd.

73,
Ed Knobloch


On 11/23/2011 4:47 PM, Antonio I0JX wrote:
I have a Lafayette tube tester that works in a very simple manner: it
connects all the tube elements, but the plate, to the cathode. It then
measures the current flowing through the diode formed by the plate on
the one hand and by all the other elements on the other hand. Very simple.

The tube tester has a potentiometer that permits to adjust the current
meter sensitivity.

What is unclear to me is why ther tube tester manual instructs to set
that potentiometer at position "30" (over a 100 scale) for almost all
tubes. I would have expected that each tube requires a different setting
(like in the I-177 tube tester), but it is not so: the magic "30"
setting is valid for almost all tubes, big or small it does not seem to
matter.

Can any one please help me to understand the reason for that?

Thanks and 73

Tony I0JX
Rome, Italy