Richard -
First, there are two major types of tube testers. The least expensive
is the "Emission" type which merely applies power to the filament and
plate of a tube and measures the current on a "Good-?-Bad" meter scale.
They are slightly better than using an ohmmeter on the filament pins to
see if the filament is intact! The more expensive type, the "Mutual
Conductance" type, applies filament and plate voltage and then applies a
known AC signal to the grid to measure the actual "gain" of the tube.
The Emission tester will tell you if the cathode is still emitting
electrons, and the Mutual Conductance tester will tell you how the tube
amplifies _at the single test point and level_ chosen by the tester
manufacturer. Neither tells you much of anything about operation at RF.
Typically only the "latest" tube testers will test the "Compactron" type
of tubes (6JB6) without some sort of adapter.
One of the better tube testers available is the military TV-7/U family.
It doesn't have sockets to test the Compactrons, but someone has made a
set of adapters for it to allow it to test the common sweep tubes, like
the 6JB6.
All the rest of the tubes in the Drake gear are 7 and 9 pin "miniature"
types, which just about any tester made after about 1940 will test.
These tubes are all the same size pins and pin circle.
All that said, a tube tester isn't all that much help anyway. The only
REAL test for a tube is in the circuit you want it to work in. Your
best approach to maintaining your Drake gear is to get a couple of each
tube type, "test" them by plugging into a working radio and if they
work, put them in a safe place!
This is especially true for 6JB6 tubes used at RF frequencies. These
tubes were designed for use as horizontal output tubes for TV sets.
They operated at almost 16 kHz in that service!! Sylvania, in the early
60's, characterized (tested) some of their "TV Sweep" tubes such as the
6JB6 for linear amplifier service from 2-30 MHz. The resulting data was
used by Drake and several other Amateur equipment makers to develop
transmitters that would run considerably more power than those using the
6146, and do it at a lower plate voltage! Add that the sweep tubes were
about 1/3 to 1/2 the cost of the 6146, and that a lower plate voltage
could be used for higher power output and it was a done deal. The
pertinent characteristic was a high "perveance", or high currents at
lower plate voltages. They were designed to operate 24/7, inside a TV
set with 25 other tubes and keep working, so they were able to serve in
CW or SSB service at considerably higher power levels than their
published specifications..
The catch here is that since the tube was designed to operate at 16 kHz,
NOT 30 MHz, not all brands of the same tube type were interchangeable.
Operation at 30 MHz requires considerably more attention to internal
tube structure such as lead lengths, interelement capacitance, etc. The
result was that one manufacturer might make a 6JB6 with considerably
higher reactances that worked just fine at 16 kHz, but would make it
impossible to neutralize in an HF transmitter without changes in the
neutralization circuitry. Add the fact the some of the smaller, even
well known companies that put their name on a tube may well have had
that tube made by more than one manufacturer, and it gets difficult to
say which tubes will work in a T-4X and those that won't.
Further complicating the situation is that the RF gain of a tube falls
off with decreasing emission, and falls off first at the higher
frequencies. 6JB6 tubes in T-4X service will fall off first on 10M,
then 15, then 20, then ... Most will continue to put out SOME power on
80 and 40M after 10M output is just about zero.
My experience has shown that Sylvania, Zenith, RCA and GE are the ONLY
brands of 6JB6 that I have found to work _consistently_. Raytheon,
Westinghouse, Standard, and most others are potential trouble. Some
work, some don't, depending upon who _really_ made them.
By the way, it is NOT a good idea to replace ALL the tubes in a radio at
once. "Retubing" is somehow seen as similar to "replacing all your
tires". This is not only wasteful of a finite source of vacuum tubes,
but can turn a working radio into one that doesn't! Even New Old Stock
tubes, all manufactured over 30 years ago, are not all "good".
So get a few spares for each type, _check them in a currently working
radio_, and if they are "good" put them in your tube stash for a rainy day.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Richard Palmer wrote:
I hope this is not off topic. With the use of "vintage" being used
for all Drakes and tube testers, many from the 1940's and before, up
to the last ones made, it has become impossible for me to determine
what will service my early TR-4.
I have spent hours and hours looking up tube testers by type, make and
model. I have spent hours squinting at pictures trying to count socket
pins. I am burnt out and am looking for help in finding just where to
look. I have no idea if post WWII testers even test 9 pins, or if all
7 pins are of the same size.
Richard Palmer
KB8NXO
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