[Heathkit] Tube chart compatibility

[manualman at juno.com]

Obviously, it's just plain simpler to get the correct chart for your tube
tester and forgo the head scratching and thought process.

Pete, wa2cwa
http://www.manualman.com

On Thu, 25 Jul 2013 06:51:34 -0700 Chuck Penson <wa7zze at gmail.com>
writes:
> 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
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