I'm building a VTVM based on a BK Precision model 177I'm..

I have a 50uamp meter from a kirby flyback tester.

I'm gutting the kirby it has a 6w6gta tube I'm not going to use the 6w6.

Using a 12sn7gt dual triode instead of a 12au7 so I can make use of the existing tube socket in the kirby.

and a string of resistors to get to 11meg ohms 1%


I'll read the control grid, screen grid and plate currents, and HV on the 15 volt scale only.

Only trouble I see is how does one protect the meter needle from bending?
If there is a plate current surge, a glitch resistor might save the rest of the electronics but if the VTVM gets hit with 150vdc on the 15vdc scale It will rap the needle a few times around the end post.

tyvm in advance
de n8zu

"raypsi" wrote in message
...
I'm building a VTVM based on a BK Precision model 177I'm..

I have a 50uamp meter from a kirby flyback tester.

I'm gutting the kirby it has a 6w6gta tube I'm not going to use the 6w6.

Using a 12sn7gt dual triode instead of a 12au7 so I can make use of the
existing tube socket in the kirby.

and a string of resistors to get to 11meg ohms 1%


I'll read the control grid, screen grid and plate currents, and HV on the
15 volt scale only.

Only trouble I see is how does one protect the meter needle from bending?
If there is a plate current surge, a glitch resistor might save the rest
of the electronics but if the VTVM gets hit with 150vdc on the 15vdc scale
It will rap the needle a few times around the end post.

tyvm in advance
de n8zu


You pay attention to what you are doing so that does not hapen.

One other way to protect meters is to place a pair of diodes back to back
across the meter. Not sure what the voltage would be across the meter in
what you are building. The diodes will conduct at about .6 volts if silicon
and .3 if germanium.

I'll read the control grid, screen grid and plate currents, and HV on the 15 volt scale only.

Using resistor dividers to get 15vdc full scale for those four readings.

73 n8zu

I forgot the spec on the meter is 1800 ohms at 50uamp which is like 90mv full scale.

73 de n8zu

Looks like an auto ranging circuit would be needed to switch the VTVM to a higher voltage.

That means like a lm3914 in dot mode with one dot set to like 15.05 vdc that dot driving an cd4066 analog switch to the 1,500vdc scale.
I'll have to figure the input Z of the lm3914 circuit to fit it in the HV voltage divider. Just hope the 50ua meter movement is slower than the lm3914 and cd4066.

73 de n8zu

On Thu, 5 Sep 2013, raypsi wrote:

I'm building a VTVM based on a BK Precision model 177I'm..

I have a 50uamp meter from a kirby flyback tester.

I'm gutting the kirby it has a 6w6gta tube I'm not going to use the 6w6.

Using a 12sn7gt dual triode instead of a 12au7 so I can make use of the existing tube socket in the kirby.

and a string of resistors to get to 11meg ohms 1%


I'll read the control grid, screen grid and plate currents, and HV on
the 15 volt scale only.

Only trouble I see is how does one protect the meter needle from
bending? If there is a plate current surge, a glitch resistor might save
the rest of the electronics but if the VTVM gets hit with 150vdc on the
15vdc scale It will rap the needle a few times around the end post.


50uA is pretty small, do you really need a tube to buffer it? IN the old
days, one might just have the test points accessible, and use a VOM. By
the time you've dropped the voltage enough for the meter, surely it's not
loading the circuit much.

Michael

On Thu, 5 Sep 2013, n8zu wrote:

Looks like an auto ranging circuit would be needed to switch the VTVM to
a higher voltage.

No. You just arrange the "function switch" to put in the right divider as
the function changes. For that matter, just put the divider across the
circuit being measured, and then the meter just sees the dropped voltage.

This isn't a general purpose layout. A test meter is being used for
multiple things and often you arent' sure what you will be seeing.

But this is a fixed arrangement. A voltage divider there won't cost you
anything but a couple of resistors, just leave them there, and over there
for that other voltage.

Michael

That means like a lm3914 in dot mode with one dot set to like 15.05 vdc that dot driving an cd4066 analog switch to the 1,500vdc scale.
I'll have to figure the input Z of the lm3914 circuit to fit it in the HV voltage divider. Just hope the 50ua meter movement is slower than the lm3914 and cd4066.

73 de n8zu

In article ,
raypsi wrote:

Only trouble I see is how does one protect the meter needle from bending?
If there is a plate current surge, a glitch resistor might save the rest of
the electronics but if the VTVM gets hit with 150vdc on the 15vdc scale It
will rap the needle a few times around the end post.

N8ZU-

Two thoughts:

1. The VTVM may work with a less sensitive meter movement. You could
connect a resistor in parallel to lower its sensitivity. Perhaps one
milliamp full scale would work?? (Another approach might be to use a
current sampling resistor in place of the meter. You would connect a
series resistor to the meter movement making it a voltmeter reading
voltage drop in the current sampling resistor.)

2. I think one side of the tube would go into saturation, and the other
side into cut-off at some point. Then meter current would be limited by
plate and cathode resistors. In other words, it is a non-linear circuit
when you over-drive it. You may not have as big a problem as you think.

Fred
K4DII

On Thu, 05 Sep 2013 09:52:20 -0700, raypsi wrote:

I'm building a VTVM based on a BK Precision model 177I'm..

I have a 50uamp meter from a kirby flyback tester.

I'm gutting the kirby it has a 6w6gta tube I'm not going to use the 6w6.

Using a 12sn7gt dual triode instead of a 12au7 so I can make use of the
existing tube socket in the kirby.

and a string of resistors to get to 11meg ohms 1%


I'll read the control grid, screen grid and plate currents, and HV on
the 15 volt scale only.

Only trouble I see is how does one protect the meter needle from
bending?
If there is a plate current surge, a glitch resistor might save the rest
of the electronics but if the VTVM gets hit with 150vdc on the 15vdc
scale It will rap the needle a few times around the end post.

tyvm in advance de n8zu

Are you using the power transformer from the flyback tester? If so,
since you said it had a 6W6, the heater winding is probably 6.3V. You
would need a 6SN7, not a 12SN7.

Since you are going to use this to monitor a transmitter, don't forget to
include adequate RF filtering, shielding, and bypassing. Otherwise, the
meter readings may have little relationship to the values being
measured. Service type VTVMs don't have these since they generally are
used to test receivers. You will have to add them to the circuit you are
following.

Use resistor values in the 6SN7 circuit that limit the current through
the tube to values that are safe for the meter.

Many of the other posts on this topic have good thoughts.

Good luck,

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
Then replace nospam with fastmail. Lastly, replace com with us.

On Thursday, September 5, 2013 12:52:20 PM UTC-4, n8zu wrote:
Thanks for all the great replies

Well I'm running the filament on DC so a doublier of 6.3vac with a tl431 programmable zener to get 12vdc only requires 3 cheap parts, with the doublier 7 cheap parts. 12sn7 was the cheapest I could find for a dual triode. Though I could have used a tl072 fet op amp, I like tubes and there's a tube socket that wanted a tube, instead of a tl072 ic mounted in a octal plug.

And the poster may be right about the plate and cathode circuits limiting the current a 12sn7 has like 3000umhos and the more current you run through a cathode circuit the more negative feedback you have, thus limiting the current.

When I used RCA VTVM's when I worked for Sears service back in the day. Sort of like band switching and you forget to change the band switch.
Alas I don't remember wrapping a needle around the movement post.
73
de n8zu