12 volts filament supply

[denitronik]

Are there any advantages/disadvantages to using a 12.6 Vac supply for the tube filaments instead of 6.3Vac. I have all my 12ax7s and 12at7 filaments in series and the two 6L6s in series also. The amp is a 1968 Silverface Fender Bandmaster with a AB763 circuit and it's running very well. I'm just curious about the 6l6s, is there a difference in the push/pull operation if the tubes are "heated" in series instead of in parallel?
I guess the only advantage is that the amp is consuming less current to heat the filaments!

[Enzo]

Your heaters require the same amount of power regardless of how you get it to them.

[loudthud]

I've seen in tube manuals that the 6.3V connection is recommended for low noise on 12A?7s. I'm not sure why. Maybe one reason is the lower AC voltage means less crosstalk to adjacient wires. There is certainly no reason not to use the 12.6V connection if the performance meets your requirements. Shouldn't be any problem with running 6L6s in series.

[Old Tele man]

Originally Posted by loudthud

I've seen in tube manuals that the 6.3V connection is recommended for low noise on 12A?7s. I'm not sure why. Maybe one reason is the lower AC voltage means less crosstalk to adjacient wires.

...recall reading samething for 7025A "low-noise" operation, the reduced filament voltage and the helix-wound filaments yielding greatly reduced noise.

[HTH]

afaik, the major point with 12vac heaters is that the current draw is 150mA whereas it's 300mA with 6.3vac heaters. the lower current will give less radiated hum via the AC wires, right?

[Enzo]

Well, that is still .15A running through each heater. SO that wouldn;t change inside the tubes. And the book suggests the 6V wiring as the less noisy.

[guitician]

I always thought the center tap(pin 9) helped to cancel hum. Like using two 100 ohm resistors.

[mooreamps]

Originally Posted by loudthud

I've seen in tube manuals that the 6.3V connection is recommended for low noise on 12A?7s. I'm not sure why. Maybe one reason is the lower AC voltage means less crosstalk to adjacient wires. .

No. It is because the filament inside the 12ax tubes are center tapped to pin 9. The static electromagnetic field generated between pins 4 and 9 is supposed to be nulled by the static field generated between pins 5 and 9. This splits the filament in two, and they run in parallel with each other. Thus, running the filament at 12.6 volts AC, between pins 4 and 5, you don't get that cancellation effect ; with AC filaments. Now, having said that, running dc filaments gives you even better hum rejection at 6.3 volts.

-g

[GainFreak]

Now, having said that, running dc filaments gives you even better hum rejection at 6.3 volts.

I wonder what's the situation with a 12DC filament where pin 9 is connected to ground

[denitronik]

Thank you all for your insight.

I did this change because a friend of mine gave me an old power transformer to replace the one in my amp which was broken. After measuring the voltages in the old transformer, I noticed that it had 12.6 volts for filaments.
I installed the old transformer and made the change with the filament wiring.
I fired up the amp and wow!, it worked and I haven't noticed any increase in hum.
Actually I had to do other changes because the old transformer was also missing the 70Vac tap for the fixed bias but that's another story.

[mooreamps]

Originally Posted by GainFreak

I wonder what's the situation with a 12DC filament where pin 9 is connected to ground

You can't do it that way. Pin 4 to pin 9 {ground} is 6 volts, and pin 5 to pin 9 is 6 volts. It's only from pin 4 to pin 5 is 12 volts, and either pin 4 or pin 5 must be at ground.


-g

[GainFreak]

You can't do it that way. Pin 4 to pin 9 {ground} is 6 volts, and pin 5 to pin 9 is 6 volts. It's only from pin 4 to pin 5 is 12 volts, and either pin 4 or pin 5 must be at ground.

Exactly. DC filament supply positive and negative ends are "floating" and you connect all pins 9 to ground. I did it several times without any problems. This is when you have separate filament winding or supply.