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Old 13th June 2005, 10:14 AM   #1
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Default 6sn7gtb Filament Voltage

Hi, me again.

This time, it's overvoltage on the driver tubes. I have my hammond 300BX wired for 100vac on both primaries and get my 800vac and 5vac (rectifier) correctly. I am, however, getting 6.9vac out of the 6.3vac lines (loaded and with a true rms meter). I don't get it!

My questions are:

1. long term effect on the 6SN7's at 10% over.
2. how to reduce the 6.9 to 6.3.
3. if the 800vac and 5vac/3a lines are on the same windings and the other filament secondaries are on the other set of windings. If so, I can wire the filament primary for 110vac and get the 6.3 out (Japan - house voltage is 100vac).
4. would this problem cause the loss of treble that I am currently experiencing. I've tried removing the grid stopper resistor only to get shrill, annoying treble.

Seems like I'm not having a lot of luck with my power transformers.

I've fixed my 300B filament supply by driving a 115vac/6.3vac transformer at 90vac in.
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Old 13th June 2005, 10:34 AM   #2
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You could always install a series resistor on the 6SN7 filaments and drop the voltage to 6.3 volts.
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Old 13th June 2005, 10:35 AM   #3
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If the transformer has the center tap grounded, will I need to run a resistor on both legs?
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Old 13th June 2005, 10:42 AM   #4
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To minimize hum, I would put a resistor on each leg.
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Old 13th June 2005, 10:56 AM   #5
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How does that work? Is it because if I place a resistor on one side only of a CT xformer, it would cause a DC flow due to the imbalance? Kinda like an unbalanced p-p amp?
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Old 13th June 2005, 11:02 AM   #6
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Not exactly. The grounded center tap reduces (cancels) the ac hum when the load (filaments) are balanced. Placing a resistor on only one leg will disturb the balance and the hum cancellation.
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Old 13th June 2005, 11:04 AM   #7
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Cool, thanks Frank!
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Old 13th June 2005, 11:15 AM   #8
Giaime is offline Giaime  Italy
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Why not a Zener on the filament's center tap? Doing so will increase hum? I ask that because it's common practice when you want to lower the B+ to put a power zener on the HT windings center tap, but I don't know what will happen if used on the filament's center tap...
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Old 13th June 2005, 11:16 AM   #9
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Giaime,

Thanks for the input, but I"m using AC filament voltage.
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Old 13th June 2005, 11:53 AM   #10
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Frank Berry
Not exactly. The grounded center tap reduces (cancels) the ac hum when the load (filaments) are balanced. Placing a resistor on only one leg will disturb the balance and the hum cancellation.
A resistor will also limit cold current for the heaters, which is an added benefit.

Quote:
Originally posted by Giaime
Why not a Zener on the filament's center tap? Doing so will increase hum? I ask that because it's common practice when you want to lower the B+ to put a power zener on the HT windings center tap, but I don't know what will happen if used on the filament's center tap...
Since the heater current is not running through the ground, and thus thecenter tap, a zener there would not drop the voltage at all. In theory, you could put two diodes in antiparallel, in series with the heater supply, and thus rop about 0.6V, but the waveform would not be sinusoidal, rather like a sine with very high crossover distortion. Such a waveform has a lot more high order harmonics, and these will more easily spill into the circuit, so you will get more hum, andit will be more objectionable. A simple resistor offers best price to performance ratio...
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