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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have an application that will need a variable an anode supply from 750 to 1250 volts. The cathode will vary from -60 to 0. I was planning on using a rectified step-up transformer output for the anode and a step-down transformer's rectified output for the cathode? I was then thinking of using power resistors to set up a voltage divider with a pot for the cathode side, but I was hoping there is a better way. I was going to use a variac to vary the input to the the step-up transformer to give me the 750-1250 range that I need.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I think one variac for the Anode supply and one for the Cathode biasing (both with transformer, rectifiers and capacitor) would be the safest/simplest approach, unless you are very confident of your switchmode power supply skills.
Low power variacs aren't cheap but they are very reliable and have a multitude of other uses.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I suggest this should be merged with your 211 tube tester thread since it looks like this is the 211 tube tester.
Why do you need a negative supply for the cathode? It's more usual to ground the cathode and apply a variable negative voltage to the control grid. This is called fixed bias. The reason being that the grid draws practically no current, so you just need a tiny power supply and a say 10k pot for the grid bias voltage. On the other hand, the cathode sees the whole plate current.
An alternative is to use cathode bias: the signs of the currents and bias voltages are such that, with the control grid grounded, connecting a power resistor between the cathode and ground will make the tube bias itself. You adjust the resistor value to get the required plate current.
Neither of these methods works if you need to bias the control grid positive wrt. the cathode, because the grid then starts to draw considerable current. But I've never seen a tube that idles in this condition. It usually makes the tube draw fuse blowing levels of plate current. So I very much doubt you need it in a test rig: I think you got your minus signs mixed up.
Even if you were testing the tube for a grounded grid application, a bit of thought should convince you that grounding the cathode and biasing the grid amounts to more or less the same thing as grounding the grid and biasing the cathode, give or take a minus sign here and there.
P.S. If it is the 211, then a look at the datasheet shows that it wants to bias up with the grid about 60v more negative than the cathode, and does not need the grid driven positive to get full output. You could cathode bias this tube by connecting the grid to ground and putting a resistor of about 1k between cathode and ground, which would give roughly the datasheet conditions with a 1kV plate voltage.
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setting up proper biasing
