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Old 15th April 2012, 03:25 PM   #21
DF96 is online now DF96  England
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If you are sharing one secondary winding between an audio valve and a rectifier then you should not connect the winding to the rectifier cathode. Rectifiers are designed with good heater-cathode insulation to cope with a big voltage difference. Ordinary valves are not.
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Old 15th April 2012, 04:16 PM   #22
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They are not sharing a secondary windings, but, as I said earlier, it is 6-0-6V or 12V CT, so there are 2x6V windings but one of each windings taps are tied together.
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Old 15th April 2012, 06:02 PM   #23
DF96 is online now DF96  England
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Yes, so it is one secondary winding. Don't connect it to the rectifier cathode. I would start by grounding the CT. If that gives too much hum then try raising it up to a DC level.
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Old 16th April 2012, 06:19 AM   #24
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The reason for recommending regulated DC for the filaments is that in my experience some of the concerns like stable filament voltage from a low transformer or biasing to minimize hum, are no longer a problem with DC filaments. You don't have to bias DC filaments to reduce leakage hum, because it doesn't exist.

However if you do stick with AC the voltage might be near the ideal 6.3V if the transformer isn't 100% loaded. Since your rectifier tube can handle 500V from cathode to filament, I don't see why you can't use one half of your 6-0-6V to power the rectifier. But, if you do get too much hum with the CT connected to ground, biasing might get a little tricky with the common connection, because there will be some leakage from the rectifier cathode, and that noise is not a nice sinewave. It will couple through the transformer secondary to your audio tubes.
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Old 16th April 2012, 08:40 AM   #25
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Thanks guys for the advice.

So I'll bias the heater on the amplifier tube as described and leave the heater on the rectifier alone.

Do I have to connect the CT to ground?

The transformer is 24VA, so 4A at 6V. The EZ81 needs 1A, the 6H30 needs 0.85A. The mains here seems to always stay above the rated 230V, so I think I should be getting above 6V fine.

If I have to, I can also get a separate transformer for each tube, or even go DC.
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Old 16th April 2012, 12:33 PM   #26
DF96 is online now DF96  England
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Quote:
Originally Posted by Navyblue
So I'll bias the heater on the amplifier tube as described and leave the heater on the rectifier alone.

Do I have to connect the CT to ground?
One of us is confused. You have a single secondary winding, giving 6-0-6V. You intend using one half of the winding to power the signal valve heater, and the other half to power the rectifier heater. Therefore whatever bias you put on it will necessarily apply to both valves. You can't make an independent choice.

You have to connect the winding to something, as a heater circuit must always have a voltage reference. The simplest option is to ground the CT. This is also the best way of reducing any capacitive feedthrough from the rectifier cathode to the signal cathode.

Why does such a simple issue get so complicated?
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Old 16th April 2012, 01:11 PM   #27
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Quote:
Originally Posted by DF96 View Post
One of us is confused. You have a single secondary winding, giving 6-0-6V. You intend using one half of the winding to power the signal valve heater, and the other half to power the rectifier heater. Therefore whatever bias you put on it will necessarily apply to both valves. You can't make an independent choice.

You have to connect the winding to something, as a heater circuit must always have a voltage reference. The simplest option is to ground the CT. This is also the best way of reducing any capacitive feedthrough from the rectifier cathode to the signal cathode.
Please pardon me if this seems rudimentary to you. This is the part that I don't understand. When the heater is biased, and then grounded, wouldn't it get unbiased?

Quote:
Originally Posted by DF96 View Post
Why does such a simple issue get so complicated?
I think you are overestimating me. As I said earlier, I am new with this thing.
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Old 16th April 2012, 05:53 PM   #28
DF96 is online now DF96  England
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Quote:
When the heater is biased, and then grounded, wouldn't it get unbiased?
Yes, you can't connect one point to two voltages. So choose which one.
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