[Heathkit] Tube chart compatibility

[Chuck Penson]

If this is old news, I apologize in advance.

There has been some discussion recently about the compatibility of tube
chart data between and amongst the TC-1, TC-2 and TC-3, and the IT-21 ,
IT-17, and IT-3117. As there was some difference of opinion, and since I
need to nail this down for my book, I thought I would dig into it a little
deeper.

Setting aside the TT-1(A), Heath's tube checkers fall into two groups. The
older group includes the TC-1, TC-2 and TC-3. The newer group includes the
IT-21, IT-17 and IT-3117.  Within each group the models are essentially
identical and chart data from one model will work on the other models
within the group. The primary difference between the two groups is that the
newer group (starting with the IT-21) has three additional selector levers.
The older group has 10 selectors (A-K, minus I) and the newer group has 13
(A-N, minus I). Heath added these three new selectors to accommodate a new
crop of tubes that had been introduced since the TC-1 was released in 1949.
Obviously then, there will be tubes listed on the IT-21 et al that are not
testable on the TC-1 et al. So backward compatibility will be limited.

What about forward compatibility? Will the settings on the TC-1, 2 and 3
work on the IT-21 and newer models? To answer this question I compared
chart data for the two groups. Only two charts were required. For the older
group I used a chart dated 1-1-65 and labeled specifically for use with the
TC-1 and 2 (but which also applies to the TC-3) and for the newer group a
chart dated 1979 and specifically labeled for use with the IT-21, 17 and
3117 (Both charts are available online as PDFs).

As I compared the charts I began to notice that the only differences
appeared to be related to the use of the "K" and "N" selector levers. In
every case tubes on the older chart that required use of the "K" lever were
listed on the newer chart as using the "N" lever. A short list of such
tubes includes (but is not limited to) 12A7, 12B6, 12B8, 12C8, 12F5, 12J7,
12K7, 12Q8 and  their counterparts using other filament voltages.

A little research revealed that what all these tubes have in common is a
cap. A large random sample of other tubes not requiring the "K" lever found
no capped tubes.  It is important to note that in all cases the other
settings (type, filament voltage, plate setting, up and down levers) were
identical between the older and newer charts.

This suggests that settings from charts in the older group will work on
testers in the newer group, and that tubes listed on charts in the newer
group that don't require the use of the L, M or N selector levers should be
fine when used on older testers.  It also suggests that if the tube you are
trying to check has a cap and you have an older chart but a newer tester,
all you need to do is use the "N" selector where the "K" selector is
specified.

One more thought: Forward chart compatibility makes perfect sense from  an
economic perspective. Figuring out all the settings for all the tubes must
have required a huge investment of time and effort. And as Einstein
observed, TIME + EFFORT = MONEY. Once Heath had spent the money to
accumulate all the data for their first tube tester it would not have made
economic sense to change everything and then have to figure it out all over
again for some newer model. At Heath, money ruled.

I only have an IT-21 so I am unable to do the tests required to prove some
of the compatibility questions, but the experiments would be easy enough
given the right sample of tubes. That said, my level of confidence is high.

Hope this sheds some clarity on the subject.

Chuck Penson / WA7ZZE