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Old 30th August 2011, 02:17 PM   #1
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Default Tube Rectifier with transformer without Center tap

Hi. I have a tube amplifier to repair. The problem is with the High voltage supply, there is no HV...

The rectifier tube is an EZ81.

I opened the amp and came across a strange HV section connection. The power transformer has no center tap for the tube rectifier (as I'm used to) but use a pair of silicone diodes to GND (1N4007) to simulate a full rectifier bridge (half tube and half solid-state). Both diodes were dead. I replaced the diodes (cathode to the HV transfo wires, both anode shorted to gnd), and tested the EZ81. It has good emission on the tube tester.

I connected the EZ81 and the problem is the tube rectifier is becoming really hot and glows a nice red cherry color (nice and frightening...). There is no HV produces and I suspect the SS diodes short the tube to gnd.

Any idea, suggestion? Is the problem the tube rectifier? Should that kind of circuit been working in the first place (it looks fine in theory). Is it a safe practice (I guest not)?

Please help.
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Old 30th August 2011, 02:34 PM   #2
TheGimp is offline TheGimp  United States
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Verify your diode connections, that they have not shorted again, and measure the transformer for secondary short to ground.
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Old 30th August 2011, 02:35 PM   #3
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Diodes checked ok last night.
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Old 30th August 2011, 02:49 PM   #4
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A strange configuration for sure. I would make certain that each of the SS diodes is bypassed by a suitable cap to take care of any transients as the power transformer is turned off. Normally, the first filter cap would fulfill that duty, but with half tube, half SS, the possibility for damage is pretty high. The circuit should work. If the diodes are good and correctly installed, and there are no shorts in the B+ distribution system, then the tube should work properly as well -- unless one section of it is damaged as well.

Dave
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Old 30th August 2011, 03:15 PM   #5
DF96 is offline DF96  England
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Probably a short in the amp, which then blew the diodes. Find the short, otherwise the new diodes will blow too.

This half-silicon, half-vacuum rectifier bridge is becoming quite common now, because CT secondaries can cost more.
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Old 30th August 2011, 03:47 PM   #6
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There is no short on the B+. I disconnect it and installed a temporary cap, and same problem with the tube eZ81.
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Old 30th August 2011, 04:03 PM   #7
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I think this is common 'hybrid' circuit, half SS, half tube, to setup a full wave bridge rectifier without CT...

But the problem why the diodes keep blowing is quite strange...

Have you check whether the circuit are OK? Disconnect the PSU from the circuit to check whether you have some short which sucks the current above normal...

Thanks.
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Old 30th August 2011, 04:08 PM   #8
kevinkr is offline kevinkr  United States
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I suspect you should replace the EZ81 before going any further.. I would also add 1M/1W flame proof metal oxide resistors and 0.01uF/1kV ceramic caps across each 1N4007..
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Old 30th August 2011, 04:20 PM   #9
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Quote:
Any idea, suggestion? Is the problem the tube rectifier? Should that kind of circuit been working in the first place (it looks fine in theory). Is it a safe practice (I guest not)?
Nothing wrong with a hybrid graetz bridge. It is also safe. But I would add 2 or even three ss diodes in series per hv wire. Just to be on the safe side. As you can see the previous ones died prematurely.

What is the value of the first capacitor after the EZ81. And how much current is being drawn by the circuit.

Did you check the pinout and connections? Maybe the previous owner tried to fix it himself. And instead of replacing the ss diodes he rewired the rectifier tube?
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Old 30th August 2011, 05:27 PM   #10
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It was bought by my customer directly from the guy that built it, so I guest it is as built. Yes, the pinout is ok.

This is the kind of safety recommandation I was looking for. I'll certainly install the RC and some more diodes.

This is what I think that the tube is defective. I'll order a replacement tube and check.

And thanks Bas for the name of the circuit. This I was looking for too. Here a schematic of this connection, for those curious as me

It is connected exactly like this in the amp, so the diode polarity is correct too...
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File Type: gif hybrid graetz bridge.gif (22.7 KB, 741 views)
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