Capsule polarization question

[LevinGuitar]

34v can be enough, try a lower ratio transformer for higher gain. What voltage do you have on cathode?

 

[vmanj]

1.1V at the cathode.
I measured the voltage when turned on, the voltage gradually increases to 75 volts and then decreases to 32 volts.

 

[Khron]

Must be some measurement "artifact", with the meter input impedance being relatively "low".

If you get 65v on anode, grid's grounded, so unless the capsule is internally conductive (which would show in the cathode voltage), you're getting that 65v across the capsule.

For a sanity check, you could connect the other capsule terminal to a fixed 60-65v and check for sensitivity differences.

 

[LevinGuitar]

1.1V at the cathode.
I measured the voltage when turned on, the voltage gradually increases to 75 volts and then decreases to 32 volts.

As far I know there would be unlikely 30v difference between cathode and grid in a working mic. Tubes tend to stabilize themselves to a certain point. But I can be wrong


Do you also get +31v on the grid measuring relative to ground?

 

[Biula]

I tried to make the polarization close to that of the Lomo 19-a9 microphone, but the voltage on the capsule was too low.
Maybe I'm measuring incorrectly?

What is your measurement rig?

 

[vmanj]

I took measurements with another multimeter.
I get an increase from 0 to 115 volts (as the tube warms up), then a decrease to 32-36 volts.

In order not to spoil the capsule, I made measurements with a capacitor in place of the capsule, as well as without a capacitor (capsule) at all.

I also changed the values of the grid and cathode resistors, and turned off the bypass cathode capacitor.
This does not produce significant changes.

Perhaps these are features of the "creative" Soviet design, or I'm doing something wrong..

 

[LevinGuitar]

Without any connection at all in the grid, with the cathode at 1v, you should have near 0v or slightly negative voltage at grid (respective to ground). Measure anode voltage with that configuration. If connecting a capacitor from grid to anode, the voltage of the anode nor cathode does not change, the grid surely can't measure different voltage from before. Cathode, grid and anode influence each other, you can't have a different value in one of them without changing the rest of them

Does your tube pinout is properly connected btw?
Is the tube ok? Does the mic works?

 

[Khron]

I took measurements with another multimeter.
I get an increase from 0 to 115 volts (as the tube warms up), then a decrease to 32-36 volts.

So that's between anode and grid, right?

Keep in mind that your multimeter's input impedance is maybe 10Mohm. On one hand, that forms a divider with the 510Mohm from grid to ground. Adding to that, it's also a DC conduction path (where the capsule itself would not be).

So when you perform that measurement, the tube conducts as much as it possibly can, which drags down the anode voltage to the 30V-ish you're reading.

 

[vmanj]

Without any connection at all in the grid, with the cathode at 1v, you should have near 0v or slightly negative voltage at grid (respective to ground). Measure anode voltage with that configuration. If connecting a capacitor from grid to anode, the voltage of the anode nor cathode does not change, the grid surely can't measure different voltage from before. Cathode, grid and anode influence each other, you can't have a different value in one of them without changing the rest of them

Does your tube pinout is properly connected btw?
Is the tube ok? Does the mic works?

Yes the pinout is correct.
I tried different options, incl. cathode resistor jumper...
All voltages do not change much.

 

[vmanj]

I drew in red what I tried to change in the diagram.

If there were 34 volts left on the capsule, I wouldn't worry too much.
The most important thing is that the voltage increases above 100 Volts when the microphone is turned on, which can damage the capsule.
This is why I cannot start and test the microphone.

 

[LevinGuitar]

Reply #28 by Khron explains your issue. Measure grid respective to ground only, and you should have your problem about 34v polarization solved. But the original one, the 120v for some seconds before the tube gets warm is worrying. You can reduce your B+ to 80-90v for tests if you don't want to use a divider

 

[Khron]

Or delaying the B+ by 5-10sec would also likely solve the issue.

If both the heater and B+ power up at the same time, it takes a few seconds for the heater to warm up, and subsequently the cathode to start boiling off electrons and the tube to start conducting, thus dragging down the anode voltage.

Heater warms up first, then the cathode's "ready to go" whenever you DO end up opening the taps on the B+.

Measure grid respective to ground only,

That won't solve anything, the grid's already grounded Maybe anode voltage respective to ground only?

 

[LevinGuitar]

That way he can have 65v difference between two points without measuring that two points respect each other

 

[vmanj]

Everything is correct, in the original circuit the power supply has a delay in the anode voltage, I did not take this into account.
Thank you all for your participation.