Bias check point question

[catalin gramada]

why don't we put in the test resistors in the plate to check the bias current please ? It is a general fear it can be broked to leave the circuit open please? or other reasons are concerned ? Thanks

[Helmholtz]

why don't we put in the test resistors in the plate to check the bias current please ? It is a general fear it can be broked to leave the circuit open please? or other reasons are concerned ? Thanks

That's simple:

Cathodes: Low voltage with ground reference.
Plates: High voltage on both leads. And connecting wires to plates may cause oscillation.

[catalin gramada]

That's simple:

Cathodes: Low voltage with ground reference.
Plates: High voltage on both leads. And connecting wires to plates may cause oscillation.

Cathode voltage meant plate+screen current
There is no a reason to mount plate resistors to avoid oscillations in plate circuit ...?
Despite the care for high voltages concern do you recommend a test point like this ?

edit: or you refer at oscillations caused by test circuit? Dmm, test probes.. ? but yeah any risk of oscillations will derate the original intention by any precise measurement

[Helmholtz]

Cathode voltage meant plate+screen current

Yes.

There is no a reason to mount plate resistors to avoid oscillations in plate circuit ...?

Not sure if I understand your question. To measure the voltage drop you need to connect your meter probes/leads. Might require to pull the PI to avoid oscillation.

Despite the care for high voltages concern do you recommend a test point like this ?

I do not recommend it. It's not necessary. Screen current at idle can be taken from the tube datasheet or calculated from the voltage drop across the screen resistor.
And it really doesn't matter if it's 4mA or 5mA.
Power tubes typically have a fixed ratio between plate and screen currents at idle.

[g1]

Some ampeg's have plate resistors. SVT and V4 are a couple. One, five and ten ohms are variants if I'm remembering correctly. I think they were using them primarily as fuses, they never suggested measuring idle current with them. Even SVT's that had them still put test points for idle current in the cathode circuits.

[catalin gramada]

Yes, I noticed as they put one in each half cycle, and not individual ones, better to use for quick DC balance. I wonder if mounting individuals on each cathode then grouping on a common rail for each half cycle will not be a useful toy to monitorise each cathode curent and easy balance ? But they can test individual current over the plate resistors , of course

[Enzo]

And in SVTs and such, they are not running wires over to test points.

[vintagekiki]

why don't we put in the test resistors in the plate to check the bias current please ? It is a general fear it can be broked to leave the circuit open please? or other reasons are concerned ? Thanks

The thing is simple.
On the anodes there is high voltage (300 - 500VDC) which dangerous for life, because it can lead to death.

[J M Fahey]

Knowing NON Techs will try their hand at masuring bias current, it is good practice to suggest the safest way possible.

Not kidding, I have seen tyros set multimeter to the correct 200mA current scale (so far so good), but connect one probe to plate .... and other to ground

Flashlight enough to take a picture, loud Bang!, destroyed Multimeter (the 200mA fuse is not enough under 450V DC coming from a capacitor situation, etc.
Surprisingly, I know of nobody killed or badly hurt.
Beginnerīs luck?

[The Dude]

I don't know, but I'm not sure it would even be legal (UL and all) to have lethal voltages readily available to the end user.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Test points with high voltage connected to circuits that can provide sufficient current to cause electrocution are generally frowned upon.