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Old 15th February 2008, 08:20 PM   #1
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Default High voltage problem

I was wondering if someone could help me out.
I'm building a tube preamp (first project) and I'm having problems with the hight voltage after the rectifier tube (EZ81) . I don't measure any dc voltage at pin 3 (cathode). From the documentation I have the following setup: pin 1 and 7 (anode) 270 VAC, pin 4 and 5 6.3 VAC. pin 3 should contain the DC.
Anode voltage is there, as well as low voltage.
I have tested the rectifier tube in an old radio and it seems ok. Also i tested with another tubesocket, still nothing.
Am I doing something wrong here?
thanks in advance
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Old 15th February 2008, 08:47 PM   #2
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Are you absolutely certain that your wiring is correct? And to the correct pins? (They are read clockwise from the bottom of the tube starting with pin one after gap in the pins.)

Check to see if you can see the tube's heater glowing.

You say you have AC between the plates. I hope you are using a center tapped transformer (three secondary leads instead of two). The two ends of the transformer winding go to the tube plates and the center tap becomes ground. Your output DC voltage will be between ground and the rectifier cathode. If you don't have a center tapped transformer, this will not work.
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Old 15th February 2008, 09:07 PM   #3
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Hi Duo, thanks for reaction,
I think this must be my problem I have the following set up from the transformer: 250, 0, 250.
Right now I have the following going to the tube : 0, 250.
From your reaction I understand I have to connect the two 250 connections to the tube and the center (0) to ground. Am I right?
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Old 15th February 2008, 09:18 PM   #4
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I'm sorry Duo, I hit the enter button, which apparently posts the message, so I'm continuing here. I was already wondering why there were 2 250 VAC taps on that secondary.
I'm new at this so maybe I'm saying stupid things here, but am I not connecting 500 Volts to the tube then, which is above the margin of the tub
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Old 15th February 2008, 09:22 PM   #5
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This is exactly right. Both 250 wires go to the anodes of the tube and the 0 wire to ground.

This is a full wave rectifier using two diodes, either 250V wire will go positive for half of the AC wave and thus each half gets rectified by either one or the other diode to rectify the full wave.

The way you had it set up, I would expect you to still have read a DC voltage on the cathode (higher than normal as well.) This might be a clue that one side of the tube isn't working?

First try wiring it up as normal, with 250 wire to each anode and then the center wire on ground and see what you get at the cathode.

If this works, it might be a good idea to test with either anode disconnected just to see if one side of the tube is bad? (If one side is bad, the circuit will still work, but it will have a lot more AC ripple due to only have of the wave being used.)

Take care during testing, these voltages hurt, and not in a good way!
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Old 15th February 2008, 09:24 PM   #6
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Hello again, I've accidentally posted half way through as well. It happens.


No, you're not going to hurt the tube. You are putting 500V between anodes, but this is not what's important. You're putting only half that between cathode and one of the anodes at a time. (Both anodes are 180° out of phase and thus one is high while the other's low.)

Hopefully this helps.
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Old 15th February 2008, 10:24 PM   #7
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Thanks Duo, I will trie and test things the way you suggested. This was very helpfull. I'll get here and post some pictures when I'm done (or scream for more help)
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Old 15th February 2008, 10:29 PM   #8
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No problem.

It would be nice to see your project.

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Old 16th February 2008, 06:28 PM   #9
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Hello trompetter,

The first thing I would do is disconnect the two sec wires, then measure the AC voltage between them. If that's OK, reconnect the wires. Disconnect the filter capacitors then recheck pin 3 of the tube. If its still low, try replacing the EZ81 with a pair of diodes (caps still disconnected). If you get a nice high voltage then, it's a bad tube. If not you have a short somewhere.

~John~
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