On 12/01/2011 01:54 AM, Geoffrey S. Mendelson wrote:
Edward Knobloch wrote:

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.

Not really. It was designed as a cheap tube tester for home use or the amateur
TV/radio repair person.

If you consider that most failures of vaccuum tubes are caused by the heaters
then a simple tapped transformer would do for the average guy fixing your
TV in your home, or his collection of TV's or radios. Probably 6.3 volts
alone would do for TV's and maybe a handful of other voltages for radios e.g.
5, 6.3, 12.6, 35, 50 and 70.

Any conductance out of the tube would be good enough to declare it "working".

Beyond that would require some skill in diagnosing failures, or a good tube
tester which was beyond the means of an amateur "fixer" or junior tech, so the
set would have to be hauled off to the shop for repair by someone with real
skill and understanding.

In plain English, you could make a good living in the 1950's and 1960's
carrying around a cheater cord, a similar tube tester and a suitcase full
of replacement tubes.

You could make an arrangment with the local TV repairman with a real shop
and real skills to fix the ones you could not and give you a reduced price
if you brought it in, or a finders fee if he had to go out to it.

I made a fair amount of pocket money in the late 1960's just opening sets
and looking for lit heaters. The ones that did not light up or were not warm
went to the drugstore for their tube tester.

It's hard to imagine, but many people still had tube TV's and radios well
into the 1980s. It's impossible to tell when US TV watchers got rid of the
last of them, as they continued to work with over the air broadcasts
until the digital switch, and the BBC ran a 405 line system until 1985, but
I'm sure there were plenty of 625 line tube TV's around afterward.

Note that I am not talking about collectors, special purpose (e.g. ham/swl)
radios etc. I'm talking about your average consumer who at least into
the 1980's bought things because the old one wore out, not because the
90 day warranty expired. :-)

Geoff.



Those "drug store" tube testers had a gazillion different sockets
(mostly octal IIRC) to make them simpler for the average Joe to use.
Rather than having a bank of switches to connect the 4 test leads
(heater, heater, cathode, plate) to the various elements the tester just
had different sockets for each possible base connection. For a tube
that had multiple elements (IE: 6SN7GT) the tester either just put them
all in parallel or used TWO sockets and tested each half independently
They were emission testers and probably calibrated on the conservative
side to sell tubes. Still if you knew how to read them you could get a
good idea of the actual condition of the bottle. If you held down the
test button while the tube was warming up you got some more information.
If the tube suddenly jumped in emission or took a very long time to
gradually reach a good reading (or very much higher than good), that
could be a bad sign. Tapping the tube while watching the meter could
show microphonics or shorts. The testers also had short and gas
indicators, but I didn't put too much faith in them.