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Is 4.5 Vdc Heater Enough????

3/23/2005 1:45 AM
Yannick Is 4.5 Vdc Heater Enough????
I've got a transformer able to give me 6.3VAC at 1A but once rectified and filtered it gives me 4.5Vdc when feeding 3 12AX7. Those tube are supposed to pull 300mA each:  
How can I get that low? Is this a problem?  
3/23/2005 2:05 AM
Don Symes

It ain't gonna work.  
Running AC, you've got no headroom (900mA of a rated 1000?) as it is. There's another thread that has the math about why doing the DC thing requires more current than you think, but that's basically the deal.  
You could maybe get away with two tubes on AC and one on DC with that tranny, but I'd really watch for overheating.  
Unless you're going completely nuts with the gain, you shouldn't need DC on any but the first tube anyway.  
3/23/2005 3:07 AM
Bob Ingram

That's really low, I'd get a filament transformer rated at least 2x the required AC current.
3/23/2005 3:17 AM
Bill C

the published spec for filament voltages is +/- 10% , so 4.5V is way too low....
3/23/2005 3:56 AM
Dr. Photon

how does it hold up with the tubes connected with standard AC? I haven't noticed much hum from AC heaters, provided I am careful with the lead dress of the heater wires and elevate the center tap to a positive potential of a few tens of volts. But you are at the limit of the transformer. You might want to consider simply getting an auxiliary filament transformer and mounting it in an out of the way location under the chassis. Any electronics store (even radio shack) will sell you transformers that put out 6.3 volts at an amp or two. You can also get a 12.6 volt center tapped transformer and either connect you 12AX7's as 12 volts, or use only two diodes and the center tap.
3/23/2005 6:03 AM

Wait a minute, 6.3VAC rectified and filtered yields 8.9VDC. If you get 4.5VDC, I am guessing you grounded the center tap and rectified it like a center tapped high voltage supply? You are then rectifying 3.15VAC, not 6.3.  
A 6.3VAC at one amp tranny should handle the three tubes easily.
3/23/2005 11:17 AM
Dave H

"Wait a minute, 6.3VAC rectified and filtered yields 8.9VDC."
In an perfect world it would :) In the real world with diode voltage drop, ripple voltage and the regulation of the transformer (1A isnít enough for a DC supply) you could end up with only 4.5V.  
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