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Old 15th July 2004, 06:52 AM   #1
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Default Negative Voltage with Tube Rectification?

can i obtain a -ve voltage through tube rectification? if yes, how to i do that?

hope it does not sound silly....
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Old 15th July 2004, 07:05 AM   #2
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As with any diode, vacuum tube rectifiers can be turned around.

But you will need to use seperate single rectifier diodes and use independent heater windings for each of the rectifier diodes.
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Old 15th July 2004, 07:59 AM   #3
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Even easier, connect the + i.e the cathode side of the tube rectifier to ground and take the negative voltage from - side, works perfectly as long as you have a separate winding on the transformer, otherwise you need to use separate filament windings or indirectly heated rectifier tubes.

Many old amplifiers with fixed bias had a single diode connected with cathode towards one of the windings on the FW transformer, for bias half wave rectification is good enough due to the small current.

Regards Hans
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Old 15th July 2004, 09:35 AM   #4
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Tubetvr,
i am totally confused......

Let's presume i need it for fixed bias for a tube....and i am going to use a single indirectly heated tube....eg. the EZ80 to do that......

Pinout is as below:
1. anode
2. no connection
3. cathode
4. heater
5. heater
6. no connection
7. anode
8. no connection
9. no connection

so i connect pin3 (cathode) to ground.......then where can i take the negative output from?
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Old 15th July 2004, 10:41 AM   #5
dhaen is offline dhaen  Europe
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For half wave rectification:
EZ80 anode to winding end "a"
EZ80 cathode to ground
winding end "b" has the negative voltage. Connect a smoothing cap between this point and ground.

If you are considering using this for a grid bias voltage, you must consider failue modes. If the EZ80 fails, what happens? Probably your (expensive) output valves conduct excessive current and glow red . You must employ protection of some form.
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Old 15th July 2004, 11:12 AM   #6
316a is offline 316a  England
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Default Diodes

Quote:
Originally posted by dhaen
For half wave rectification:
EZ80 anode to winding end "a"
EZ80 cathode to ground
winding end "b" has the negative voltage. Connect a smoothing cap between this point and ground.

If you are considering using this for a grid bias voltage, you must consider failue modes. If the EZ80 fails, what happens? Probably your (expensive) output valves conduct excessive current and glow red . You must employ protection of some form.
I would use schottkey diodes here , if the power amp HT warms up before the EZ80 there could also be trouble . If the output stage uses a current sink , it could be a way around this , but still not as recommended as solid state diodes

cheers

316a
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Old 15th July 2004, 12:05 PM   #7
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Quote:
so i connect pin3 (cathode) to ground.......then where can i take the negative output from?
Assuming as I wrote that this tube is connected to a separate winding you take the negative output from the centertap of the transformer.

On the other hand if you want to produce a negative bias voltage from the same winding as is giving the anode voltage you just connect the cathode side of a diode to one side of the transformer, assuming a center tapped transformer, and take negative voltage from the anode.

This scheme was often used with tube rectifier for the anode voltage and a selenium rectifier for the bias which was not a very good idea due to relaibility issues, using a tube or a silicon diode it is however practically fail safe. You dont need any extra protection scheme, it is much more likely that the anode voltage rectifier fail or that the output tubes fail due to other reasons.

Regards Hans
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Old 16th July 2004, 04:40 AM   #8
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Just a thought: is your power tranny a toroid? If so, don't use half-wave rectification because it will tend to cause DC saturation of the core.

As others have said, if you want a negative voltage just for bias, then it's better (and safer for your OP tubes) to use SS rectification.

If you want it to provide a negative return for a stage such as a differential amplifier or LTP splitter, be careful to ensure that you do not exceed the heater-to-cathode maximum voltage when switching on, i.e. before the B+ supply has reached its proper level. You can protect against this with back-to-back zener and ordinary diodes.
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