Transistor VTVM made common parts.

[Smithdoor]

8Update schematic
This uses command switches off the shelf and will fit in ½" pvc conduit [16mm ID].

Dave

[Smithdoor]

For 500 volt change Resistors
R20 to 4.7 Meg and R31 , R34 and R23 to 1 Meg
All Pot are 500K for fine tuning.

R21, R24, R32 and R35 not used.

Voltage Source (VS)
500
Volts (V)
Resistance 1 (R1)
4.95

megaohms (mΩ)
Resistance 2 (R2)
1.25

megaohms (mΩ)
Output Voltage (VOUT)
100.806

Dave

[Smithdoor]

The problem I am have probe is finding a switch the will fit in ½ [12.7mm] tube.
I am now going back to drawing board. It is looking like need to build 3 probes and skip the switch.

Dave

[Smithdoor]

Here is probe that fits a small switch.
The switch is only use to determine AC from DC and works upto 150 volts.
It can change to higher voltage.
I stop at 150 volts as I did not do work over 120 volts and give a finer read of scale.

Dave Dave

Note R20 is shown 2 time will change on the drawing later today.

[Smithdoor]

Here the other way I thinking about just finished the drawing tonight

Dave

[Smithdoor]

This a update to high voltage probe.
The voltage can be increased to 250, 500 and 1,000
Note the pair 1N34A will change to a 1N270 not shown on schematic.

Dave

[Smithdoor]

Probe to house the new high voltage equipment.
The pvc is rated to 600 volts max probe voltage is 500 volts. The 500 volt size can also be 250 by switching to 50 volt setting

Dave

[Smithdoor]

If do not have lathe use caps for pvc pipe do not glue.

[Smithdoor]

Here is a article on VTVM vs others

VTVM and TVM's
Posted by Dawn on 16 Jun, 2012 22:22
With the recent threads on multimeters and voms, you just know that there had to be an equivalent on electronic analog voltmeters. Back in my early days during the mid 60's and early 70's when I first started in this hobby, a VTVM kit was among the 2nd or 3rd pieces of test equipment that would have been sought for a basic bench after initially acquiring a hobby grade VOM. The 70's brought in FET and transistor based VOM's at the kit/hobbly level. Many of them nightmares in the short run and unable to deliver the reliability of an 20 y/o second hand VTVM. There were many things a VOM just couldn't do and nor could a latter DMM with respect to a VTVM. To this day, a VTVM does have uses that are not duplicated in a DMM. It probably would be fair to say that they are irrelevant nowadays with the current crop of electronics and certainly no place in a development laboratory. Yet in service, there are still applications they are used for occasionally and not easily substituted and merit being kept on a shelf if needed. Some applications that they excelled in were:

Most all could bias the movement for a zero center.
High frequency response up to several meg AC
Capable of reading very low ohm values right down to emitter resistors that regular DMM's cannot.
Higher end volt ranges often to several thousand volts directly.
Isolation resistor in the probe and tube amplifier gave a very high impedance that didn't disrupt circuit operation or detune it.
Like 20k/ohm voltmeters, were standardized instruments that procedures and reference voltages were documented in older service literature.

There are others, but suffice to say that if you're working in a service vocation, inevitably a VTVM would make your life easier at times.

Regarding the TVM's that never seemed to succeed the VTVM's. One of their worst malady's was the presence of RF. Sometime even probing around and oscillator would drive the meter full scale. That forcible "thwack" any time a transmitter was energized or around powerful oscillator circuits made many of them undesirable for service. While there were many TVM's that were designed to be RF proof, most were service instruments designed by and unique to the trade. Alignment was touchy and could vary with battery or supply voltage and short time ageing. Thankfully, DMM's at hobby and service prices began to make their debut shortly afterwards.
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[Smithdoor]

Here better view of the high voltage probe housing. Sorry I left in inches.

Dave

[Smithdoor]

It taking time to make a decision on the last probe.
I decided it would better to use 250 volt and 500 volt range.
Om the probe by switching the range switch from 50 v and 100 volt range

So ranges are
0.5v
1.0v
5.0v
10v
50v
100v
250v
500v.
Giving a wide range for a Transistor VTVM.

See calculations below
This the very last to do

Dave

4.7m
.25m variable resistor
======
4.95m
‐----------------------------------#
1.0m
+ .25m variable resistor
====
1. 25m
≈==============
voltage divider calcs
Voltage Source (VS)
500
Volts (V)
Resistance 1 (R1)
4.95

megaohms (mΩ)
Resistance 2 (R2)
1.25

megaohms (mΩ)
Output Voltage (VOUT)
100.806
Volts (V)

[Smithdoor]

Here is drawings for 250 / 500 volt probe.
Note the can be used for bench work 5 volts and 50 volts .
Can be very handy as the AC DC switch in in the probe. It can only be calibrated for one setting the rest is 1% to 5% accuracy.
The PDF file has two pages

Dave

[Smithdoor]

Reduce the number of resistors used in this schematic easier to fit on pc board.

Dave

[Smithdoor]

Update the drawing to include a way to build the without a lathe.
The PDF file includes both new schematic and drawing on probe.

Dave

[Smithdoor]

Photo of new high voltage probe.
The 1/2" conduct is rated for 600 volts.
Dave

[Smithdoor]

FYI
When first start the build only had plans to 100 volts DC and 70 volts AF and RF.

You do not need to build the high voltage probe.
I only built the probe just in case I every need to go over 100 volts.

Dave

[Smithdoor]

For any one planing on building a New Vacuum Tube Voltmeter using transistors. I posting a set of 11 drawings for the building.
It has just any note need for build.

I have used the one built since April of this year and work better than ever could believe. It is less than 1% error

It is in a PDF file only.

Dave

[Smithdoor]

The front view

[Smithdoor]

Here a probe for 1.5 GHz
Did not build this probe.

[Smithdoor]

Adj radio