DIY Bias probe - advice needed

[spheikki]

Hi,

I am planning to make a bias probe for two tubes, with 2 LCDs indicating the plate voltage and current.

The probe would have a rotary switch (3x4) with positions OFF-TUBE1-TUBE2, for switching the probe off, measuring the tube 1 or tube 2.

Now my question is that what kind of specifications (voltage and amps) the switch shall comply with, and do you have any links for such? The currents going though switch connectors are quite small (below 100 mA), but some switch poles carry plate (anode) voltage max 500VDC and some the tube cathode, thus the potential difference between some poles of the switch is max 500VDC. What specifications should I be looking for from switch data sheet to be suitable to my probe? There are usually data something like e.g. 0.3A / 125VAC, Insulation resistance 10Mohm@500VDC min. Here's one example:

http://www.uraltone.com/kauppa/produ...0d951466cecc48

It seems harder to find switches with 500VDC rating. If you know any, I would be glad have dome references.

Also, I would like to hear other solutions for implementing the switch, if you have some.

One other thing: should the switch have some spark prevention in its contacts? If so, how should it be done?

Preliminary schematic (S1/S2 would be implemented with one rotary switch):

[stokes]

I used 2X 3a 250vac mini toggles in mine and it hasnt been a problem in ten years.I think you may want to put that 1ohm resistor in the cathode circuit.I've actually never seen it wired in the plate like that.

[tubeswell]

Why not make it so you can measure the current at the cathode, screen or plate?

[R.G.]

Even more pertinent - LCD voltmeters are cheap and switches are not. Why not do a voltmeter and current meter for each tube separately and not bother with switches?

[loudthud]

You can't use a common power supply between the LCD meters if one is in the plate circuit and the other is ground referenced. Some meter modules don't like common battery connections even of the input grounds are tied together. Check the specs carefully.

[spheikki]

You can't use a common power supply between the LCD meters if one is in the plate circuit and the other is ground referenced. Some meter modules don't like common battery connections even of the input grounds are tied together. Check the specs carefully.

loudthud, did I understood you correctly: I cannot use the same 9V battery to provide power for both LCDs? Measurement inputs are in different ground anyway?

[spheikki]

I used 2X 3a 250vac mini toggles in mine and it hasnt been a problem in ten years.I think you may want to put that 1ohm resistor in the cathode circuit.I've actually never seen it wired in the plate like that.

Stokes, if you are measuring the current from cathode (with 1 ohm resistor), the measurement includes also screen current, not just plate current, which I would like to measure, right? Because I need to measure also the plate voltage from the plate (anode), I need to take the plate wire to my meter anyway, so I can as well measure the actual plate current there.

Have you been using these 3A 250VAC switches with 500V terminals, or just in your cathode current measurement?

[loudthud]

loudthud, did I understood you correctly: I cannot use the same 9V battery to provide power for both LCDs? Measurement inputs are in different ground anyway?

That's correct unless the module is really something special. Most modules focus on low cost so I doubt it can handle inputs outside it's rails. One might go up in smoke of you try it. Even with a floating battery, the module might not like being 500V above ground and arc or leak to the case. Although measuring plate current independant of screen current is nice, it does present some significant problems.

[stokes]

Stokes, if you are measuring the current from cathode (with 1 ohm resistor), the measurement includes also screen current, not just plate current, which I would like to measure, right? Because I need to measure also the plate voltage from the plate (anode), I need to take the plate wire to my meter anyway, so I can as well measure the actual plate current there.

Have you been using these 3A 250VAC switches with 500V terminals, or just in your cathode current measurement?

Putting 500vdc on those switches is not a problem.The bias tester I made uses an external meter,I use a switch to alternate from current to voltage measurements,on each tube,I have seperate test points for each tube.I made mine before these things became popular and were so readily available,so it is kind of archaic compared to the one you are making.I just realized you need the rotary to switch between tubes.One of the same 3A 250vac rating should be fine.