Hi guys, I'm a new member asking if you can please help? I want to balance the current through two vacuum tubes in this circuit below. Currently I am measuring the voltage drop across the 1 ohm resistors with a multimeter but I want to build in to the amp housing some sort of comparator feeding an analogue meter or LEDs. The LM317HV is being used as a constant current sink of around 120mA. Voltages available are 5V and 380V.

Thanks in anticipation.

Fizzer

 

So where are the valves connected?

 

Here's a more complete picture of the circuit at this end:

 

You could use a op amp connected as a differential amp, or a differential instrumentation amp to amplify the voltage across each 1 ohm resistor. The output of these amps could drive a meter or an LED bar-graph using an LM3914.

If you wanted just the difference between the two measurements, then you could have the output from the two 1 ohm resistor amps go to a third differential amp. That could drive a center zero meter to display the unbalance.

 

Thanks Carl, I like all those ideas but the last one seems best. Once the current source has been calibrated the main requirement is to ensure both valves conduct the same amount. The valves tend to drift with heat and age so the zerometer option seems best. I'm afraid its a long time since I did any electronics - do you have a circuit in mind and a way to connect it?

 

What is the DC voltage at the tubes' cathodes?

 

The voltage is about 50 volts at the cathodes.

 

I'm sorry, disregard the above. The voltage at the cathode is 20-25 volts

 

What current resolution do you need? Is 1mA or so adequate?

 

Sorry about the delay in replying but I am away on business. Yes, 1mA resolution would be fine. Thanks.

 

Below is a circuit that will give you the output difference between the two bias currents. To minimize the problem of amplifier offsets and allow the use of a low cost standard op amp, I increased the value of the cathode resistors to 100 ohms. That's still only a drop of about 6V across each resistor so I don't think that's a problem.

The nominal output at balance is about 0.6V, This can be fed to an LM3914 which will give a mid scale reading. Plus or minus 5mA offset of the bias currents will give a full scale reading.

Does that work for you?

Note: I didn't show any decoupling caps. A 0.1uF cap should be added between the V+ pin and ground on the LM324.

 

I wonder what the transconductance of this tube is. Adding a total of 200 ohms between the cathodes may reduce the gain significantly.

 

I wonder what the transconductance of this tube is. Adding a total of 200 ohms between the cathodes may reduce the gain significantly.

If that's a problem, then the resistors could be bypassed with a capacitor between the two cathodes.

 

Back from my travels - temporarily!

Thanks very much, Carl, your work is appreciated. The value of the resistors you mention shouldn't matter as the cathodes are connected to a constant current sink. The sink current would be adjusted at setup and then the current drain from each valve would be adjusted to balance as the tubes warm up/age.

I shall give it a go. Thanks for all your help.

Regards
F