Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Mon 17, 2018 3:47 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 4 posts ] 
Author Message
 Post subject: Bias potentiometer in TV-3C/U tube tester
PostPosted: Mar Sun 11, 2018 2:55 am 
New Member

Joined: Feb Sat 15, 2014 5:59 am
Posts: 16
Location: College Station, Texas
I recently acquired a TV-3C/U tuber tester. It seems to be in generally good condition, and the voltages on the plate, screen grid and grid match up pretty well with those in the manual. (For example, grid bias at 100% setting on the grid bias potentiometer is about -40V.) But when I measure the transconductance of various tubes, setting up according to settings given in the manual, I am typically getting figures that are way below what they should be. I am suspecting that the grid bias potentiometer has been replaced at some point by one that has an inappropriate taper, but I am puzzled by some things I have read in articles on the subject.

Typically, the TV-xx tube testers are said to have a non-linear taper that ensures the lower end of the grid bias range is stretched out. For example, in one article I found, 25% bias would give about -3V, rather than the -10V that would occur with a linear taper potentiometer. This makes sense, because for small signal tubes one would otherwise be having to set tiny bias percentages, which would be almost lost in the end-point errors,

Now comes my puzzle. According to articles I found, the TV-3C/U is exceptional among the TV-xx series testers in having instead a linear taper on the bias potentiometer. But, for example, the tube tables for the TV-3C/U give a bias setting of 22% for a 6SN7. With a linear taper, this would mean about -8.8V grid bias, which I think is implausibly too high (I mean too negative). And sure enough, sample 6SN7 tubes I test with the bias set to 22% measure implausibly low for their transconductance.

I cannot think of any reason why the TV-3C/U, as opposed to the other TV-xx testers, would call for a linear taper bias potentiometer, since as far as I can see its modus operandi is the same as all the others.

Can anyone shed any light on this?

Thank you in advance,

Chris


Top
 Profile  
 
 Post subject: Re: Bias potentiometer in TV-3C/U tube tester
PostPosted: Mar Sun 11, 2018 6:21 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 590
Location: Stafford, Texas USA
TV-3C/U tuber tester bias potentiometer is Special Taper.

http://www.mcmlv.org/Archive/TestEquipm ... -TV-3U.pdf

PDF page 74 table 8-4 page 8-10.

All Hickok design type tube tester bias potentiometer are Special Taper.

Jimmie Stewart


Top
 Profile  
 
 Post subject: Re: Bias potentiometer in TV-3C/U tube tester
PostPosted: Mar Sun 11, 2018 2:00 pm 
New Member

Joined: Feb Sat 15, 2014 5:59 am
Posts: 16
Location: College Station, Texas
Thanks. I had already been suspecting that the statement I had read in one Antique Radios forum posting (dating from 2008), to the effect that the TV-3C/U was exceptional among the TV-xx tube testers in not having a tapered bias potentiometer, must have been in error. For one thing, the bias potentiometer settings listed in the tube tables for the TV-3C/U are in general the same as those for the other TV-xx testers, so evidently the bias potentiometer must be tapered the same way as in all the other TV-xx testers.

In fact, I think the source of the confusion was that the poster who made the statement had mis-read the part description for the bias potentiometer in the TV-3C/U manual. The TV-3C/U manual makes no mention of the potentiometer being tapered, and furthermore it does say "not tapped." The poster incorrectly reported it as saying "not tapered," and then a few subsequent postings in the thread took up the (erroneous) meme of the TV-3C/U having a linear bias potentiometer.

The TV-3/U manual (the one you linked to), by contrast, not only mentions the fact that the bias potentiometer has a special taper, but it also gives its resistance values at the 35%, 50% and 65% settings (as well as at 100%). Now the TV-3/U was different in some respects from the slightly later TV-3C/U, but as I mentioned above, the bias setting listed in the tube tables in the two manuals agree, so the bias potentiometer should evidently be tapered in the same way in both testers.

My problem now is that I need either to acquire a replacement special-taper potentiometer intended for a TV-xx tuber tester (mine has evidently been replaced at some stage by a linear-taper potentiometer), or else I need much more detailed information about how the resistance of the properly tapered potentiometer varies with the percentage setting of the dial. With this information, I could at least make the necessary settings by adjusting the knob on the existing linear potentiometer to give the proper calculated bias voltage for the tube under test.

Does anybody by chance have detailed information for the resistance versus percentage setting for a tapered TV-xx bias potentiometer?

Thanks,

Chris


Top
 Profile  
 
 Post subject: Re: Bias potentiometer in TV-3C/U tube tester
PostPosted: Mar Mon 12, 2018 12:47 am 
New Member

Joined: Feb Sat 15, 2014 5:59 am
Posts: 16
Location: College Station, Texas
I've been hunting around for discussions of the bias potentiometers on TV-xx tube testers today, and I found various old threads on Antique Radios. The majority view seems to be that the resistance R(p) of the potentiometer, as a function of the percentage p of the full angular rotation, should increase in a quadratic fashion, since the former on which the resistance wire in the potentiometer is wound is like a wrapped-around wedge of linearly-increasing height.

With that information, and some of the datum-points given in the TV-3/U manual, I have constructed a table of grid bias voltage versus percentage of rotation of the bias potentiometer. This should enable me to enter reasonable bias settings when testing tubes on my TV-3C/U, which, unfortunately, has had the bias potentiometer replaced by a linear one. So to use it, I would look at the TV-3C/U tube tables to find the bias setting, and then the table I have constructed to find what bias voltage I should be setting. I would then ignore the percentage setting indicated on the dial of the linear-taper bias potentiometer in my tube tester, and instead measure the bias voltage and adjust it until the grid bias equals the value read off from my table. I've tried it for a few sample tubes and it seems to give sensible-looking measurements for the transconductance.

The copy of my interpolation table is attached. It has four pairs of columns, giving the translation of the percentage rotation of the grid bias potentiometer into corresponding grid-bias voltage, for 1% - 25%, 26% - 50%, 51% - 75% and 76% - 100% rotation, respectively.

The curve-fitting to the known TV-3/U datum points achieved with this quadratic fit is not perfect, but it seems to be roughly correct. Since the proper special-taper bias potentiometers seem to be hard to obtain, this seems to be a reasonable alternative. If anyone has any suggestions for improvements to the curve-fitting (or knows a good source for replacement special-taper potentiometers!), I'd be glad to hear more.

Chris


Attachments:
TV-7CU-Grid_Bias.pdf [12.35 KiB]
Downloaded 19 times
Top
 Profile  
 
Post New Topic Post Reply  [ 4 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 8 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB