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Old February 9th 05, 12:03 AM
Antonio Vernucci
 
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Default Question on I-177 tube tester

The I-177 tube tester has a very unusual non-linear wire-wound =
potentiometer.

Does anyone have an idea on whether any surplus store could have it on =
sale?

Thanks and 73

Tony, I0JX


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Old February 9th 05, 01:34 AM
Scott Dorsey
 
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Default

Antonio Vernucci wrote:
The I-177 tube tester has a very unusual non-linear wire-wound =
potentiometer.

Does anyone have an idea on whether any surplus store could have it on =
sale?


How unusual is it? What taper is it?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3   Report Post  
Old February 9th 05, 09:45 PM
Antonio Vernucci
 
Posts: n/a
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Once I went to a friend of mine and measured the potentiometer of the =
I-177 after fully disconnecting it from the circuit, and wrote down a =
table of ohms vs. degrees. I should have that table somewhere in the =
shack. That potentiometer, though wire wound, is highly non linear. I =
guess I would need the original spare part. I tried with a normal wire =
wound 3,000 ohm potentiometer but readings are obviously greatly offset. =
I also tried out to figure out a way of emulating the non linearity =
putting fixed resistors in parallel and in series, but unfortunately =
whatever one does the obtained non-linearity goes in the opposite =
direction w.r.t. it should be.

Do you have a 3,000 ohm non linear potentiometer to check?

Tony I0JX

"Scott Dorsey" ha scritto nel messaggio =
...
Antonio Vernucci wrote:
The I-177 tube tester has a very unusual non-linear wire-wound =3D
potentiometer.

Does anyone have an idea on whether any surplus store could have it =

on =3D
sale?

=20
How unusual is it? What taper is it?
--scott
--=20
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Old February 10th 05, 12:29 AM
Scott Dorsey
 
Posts: n/a
Default

Antonio Vernucci wrote:
Once I went to a friend of mine and measured the potentiometer of the =
I-177 after fully disconnecting it from the circuit, and wrote down a =
table of ohms vs. degrees. I should have that table somewhere in the =
shack. That potentiometer, though wire wound, is highly non linear. I =
guess I would need the original spare part. I tried with a normal wire =
wound 3,000 ohm potentiometer but readings are obviously greatly offset. =
I also tried out to figure out a way of emulating the non linearity =
putting fixed resistors in parallel and in series, but unfortunately =
whatever one does the obtained non-linearity goes in the opposite =
direction w.r.t. it should be.


So it's an antilog taper and not a log taper? How far off from a simple
reverse log is it?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Old February 10th 05, 03:40 AM
Alan Douglas
 
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Default

Hi,
The pot is nonlinear and the dial scale is also, so the resulting
curve defies logic. But all Hickok bias pots have been the same,
pretty much, since the 1930s. There is -40V across the whole pot, and
-3V at a dial setting of 20 (22 for the military TV-7).

73, Alan


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Old February 10th 05, 05:16 AM
Edward Knobloch
 
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Default

Hello, Tony

Sorry, I can't help you find that tube tester bias pot.

One thing, though:
I know the TV-7 manual gives voltmeter readings
versus bias pot settings, set up using a military DC voltmeter
of only 1K Ohm/volt sensitivity.
The available voltmeter ranges were 0 to 10V
and 0 to 50V, so the load across the bias pot
was either 10K or 50K depending on the voltage
to be measured during calibration of the bias pot.

I assume the I-177 was also intended to be calibrated
using a DC voltmeter of that sensitivity.

73,
Ed Knobloch


depending Antonio Vernucci wrote:
The I-177 tube tester has a very unusual non-linear wire-wound potentiometer.

Does anyone have an idea on whether any surplus store could have it on sale?

Thanks and 73

Tony, I0JX

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Old February 10th 05, 05:03 PM
Scott Dorsey
 
Posts: n/a
Default

Alan Douglas adouglasatgis.net wrote:
The pot is nonlinear and the dial scale is also, so the resulting
curve defies logic. But all Hickok bias pots have been the same,
pretty much, since the 1930s. There is -40V across the whole pot, and
-3V at a dial setting of 20 (22 for the military TV-7).


I am thinking that if you could use a standard antilog or log pot
(would a 5K audio taper be close enough?) you could then change the dial
scale accordingly. If you are willing to change the dial, you only have
to get reasonably close to the original taper.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #8   Report Post  
Old February 10th 05, 07:32 PM
Antonio Vernucci
 
Posts: n/a
Default

I found the table. The original potentiometer behaves as follows:

Reading on scale Ohm
10 55
20 175
30 360
40 825
50 1200
60 1700
70 2190
80 2900

The scale on the panel is LINEAR. Of course I could paste a non-linear =
paper scale on the panel, thus emulating the non-linear potentiometer. =
But if one tries to do so, he will immediately realize that an important =
part of the scale gets highly compressed, with difficulties in =
accurately setting the knob to where indicated by the book.

Thanks for the suggestion to use a standard potentiometer that =
approximately has the same behaviour and to paste a paper scale to =
compensate for the difference. However, I will have to verify two =
things:

- whether potentiometers having a similar behaviour can really be easily =
found
- how much power the original potentiometer was supposed to dissipate. =
It may not be easier at all to find high-power carbon potentiometers=20

73

Tony, I0JX

"Scott Dorsey" ha scritto nel messaggio =
...
Alan Douglas adouglasatgis.net wrote:
The pot is nonlinear and the dial scale is also, so the resulting
curve defies logic. But all Hickok bias pots have been the same,
pretty much, since the 1930s. There is -40V across the whole pot, =

and
-3V at a dial setting of 20 (22 for the military TV-7).

=20
I am thinking that if you could use a standard antilog or log pot
(would a 5K audio taper be close enough?) you could then change the =

dial
scale accordingly. If you are willing to change the dial, you only =

have
to get reasonably close to the original taper.
--scott
--=20
"C'est un Nagra. C'est suisse, et tres, tres precis."

  #9   Report Post  
Old February 10th 05, 07:36 PM
Antonio Vernucci
 
Posts: n/a
Default

Thanks for the info. However, for the I-177 I have seen no calibration =
procedure. I guess everything relies on the fact that the potentiometer =
has the specified response.

73

Tony, I0JX

"Edward Knobloch" ha scritto nel messaggio =
news:[email protected]
Hello, Tony
=20
Sorry, I can't help you find that tube tester bias pot.
=20
One thing, though:
I know the TV-7 manual gives voltmeter readings
versus bias pot settings, set up using a military DC voltmeter
of only 1K Ohm/volt sensitivity.
The available voltmeter ranges were 0 to 10V
and 0 to 50V, so the load across the bias pot
was either 10K or 50K depending on the voltage
to be measured during calibration of the bias pot.
=20
I assume the I-177 was also intended to be calibrated
using a DC voltmeter of that sensitivity.
=20
73,
Ed Knobloch
=20
=20
depending Antonio Vernucci wrote:
The I-177 tube tester has a very unusual non-linear wire-wound =

potentiometer.
=20
Does anyone have an idea on whether any surplus store could have it =

on sale?
=20
Thanks and 73
=20
Tony, I0JX

  #10   Report Post  
Old February 25th 05, 12:47 AM
Scott Dorsey
 
Posts: n/a
Default

Antonio Vernucci wrote:
I found the table. The original potentiometer behaves as follows:

Reading on scale Ohm
10 55
20 175
30 360
40 825
50 1200
60 1700
70 2190
80 2900


Okay, if you used a standard "audio taper" 3K pot, you would get:

Full left 0
25% 189
50% 450
75% 1500
100% 3000

This looks like a slightly different taper, but probably close enough
that you could get away with it by slightly altering the markings.

Thanks for the suggestion to use a standard potentiometer that =
approximately has the same behaviour and to paste a paper scale to =
compensate for the difference. However, I will have to verify two =
things:

- whether potentiometers having a similar behaviour can really be easily =
found


You can get audio taper pots easily, which have a standard log scale.
Getting antilog pots is harder.

- how much power the original potentiometer was supposed to dissipate. =
It may not be easier at all to find high-power carbon potentiometers=20


Now THAT may be the rub. BUT you can still get audio-taper wirewound
pots for speaker level applications. They're getting harder and harder
to find, though.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


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