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Old 19th February 2013, 12:49 AM   #1
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Default AC Filament Voltage Question ?

I have 115V to 6.3VCT filament transformer.
The main voltage at my home is 124V AC.

When the filament transformer is connected to the main 124V AC and 6.3V filament tube, I get 6.8V AC.

How can I adjust the voltage ?
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Old 19th February 2013, 01:23 AM   #2
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6.3 V. is nominal. 6.8 V. is well within 10% of nominal. Unless you are dealing with a very finicky type, you are OK with things as they are.
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Old 19th February 2013, 01:25 AM   #3
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That is good to know.

My amp uses AC filament for 6CJ3 rectifier and 7788 driver.

Both are 6.8V

Last edited by HP8903B; 19th February 2013 at 01:51 AM.
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Old 19th February 2013, 01:39 AM   #4
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reduce the secondary of about 5 turn..i hope it will exact.
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Old 19th February 2013, 01:41 AM   #5
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I didn't make these transformers. I can not adjust the winding of transformer.
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Old 19th February 2013, 01:44 AM   #6
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The reason that I was concerned is the 7788 datasheet specifies 6.6V max.
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Old 19th February 2013, 01:45 AM   #7
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For a voltage drop of 0.7V you can use two 6-8 Amp diodes connected in parallel (cathode to anode) in series with the filament circuit. But be warned they will generate a considerable amount of heat.
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Old 19th February 2013, 03:03 AM   #8
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Quote:
Originally Posted by manp111 View Post
The reason that I was concerned is the 7788 datasheet specifies 6.6V max.

That sounds finicky to me. However, you still be OK. If you measured 6.8 VAC, with the filament winding unloaded, the voltage will come down, under load.
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Old 19th February 2013, 03:13 AM   #9
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6.8V is loaded
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Old 19th February 2013, 03:27 AM   #10
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Sounds like the P/N junction of a diode drop will clear this up just fine getting you 6.1V..........get a healthy current rating & PIV rating..lots of designers fine tune voltages this way.
PS, At 6,8V.....just about any PIV rating is gonna do.

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