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Author Topic: Easy way to lower filament voltage  (Read 2287 times)
K8AXW
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Posts: 6994




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« on: August 26, 2010, 01:08:39 PM »

Greetings all:

Well, after about 6 months downtime, my SB-200 is almost ready to go back online.  One last thing I would like to address is the filament voltage.

The 572-B tubes specs the filament voltage at 6.3VAC @ 4A/tube.  Mine are reading, at the tube sockets, 6.51VAC, measured with a Fluke digital meter.  This is 0.21VAC over the specs and from what I have read down through the years its good to run slightly under the filament voltage specs.  While my operating time is very low, I'm thinking that while the amplifier is still out of the case, this is the good time to play with it.

My questions is, what is a good (read: simple, easy, cheap; not necessarily in that order) way to drop the filament voltages down, say 1/2 a volt?  I understand Ohm's Law but would like to know the method used by you guys.

73

Al - K8AXW
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WB2WIK
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Posts: 21809




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« Reply #1 on: August 26, 2010, 02:23:46 PM »

~3 feet of #18AWG copper wire formed into an open-air choke?  Might get warm at 8A but if it's in the tube cooling air stream it should be okay.

(#18AWG copper is .0075 Ohms per foot, and you want about .02625 Ohms.)

Frankly, I wouldn't bother. Cheesy
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K7ZRZ
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Posts: 279




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« Reply #2 on: August 26, 2010, 05:53:43 PM »

Al,

I brought my filament voltage on the Clipperton L down from about 6.9 volts to 6.3 with a piece of #16 solid copper insulated electrical wire of about 12 feet.  I just coiled it up into a 4 to 5 inch coil and pushed it into the area above the power supply component board... which has much shorter filter capacitors with the new Harbach board.  I used the trial and cut method. Tubes do seem to be holding up better. But the Chinese tubes still aren't all that good. I just stuck in a set of Cetron tubes I was lucky enough to find and in good shape. I think they will be fine for a long time now (hopefully).

Brian K7ZRZ
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Brian K7ZRZ
KM3F
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« Reply #3 on: August 26, 2010, 07:16:42 PM »

Not familiar with either of these amplifiers but for example; on  an AL80B, the input line voltage is adjustable off the AC power line.
I adjust for the filament voltage as close as it will come (at the time) because the line voltage can change as well.
In some instances with a poor AC power circuit feed that loads down under key up, the average heater voltage will drop with it so is sort of self regulating to some degree.
If your in the 6.3 +/-  .2 or so don't be to concerned.
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AA4PB
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« Reply #4 on: August 26, 2010, 07:45:37 PM »

You probably measured it with no load on the amp. When you start drawing current from the AC line the line voltage will likely drop a little due to the IR loss in the circuit between the house panel and the amp. A drop in line voltage will cause the filiment voltage to drop. The line voltage can also vary from day to day as well. I don't think I'd worry about that little bit of difference. Its only about 3% high.
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Bob  AA4PB
Garrisonville, VA
K0BT
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« Reply #5 on: August 27, 2010, 03:18:17 PM »

Tom Rauch, W8JI, offered some thoughts in a previous series of posts on filament voltage.  He has probably forgotten more than most of us will ever know on the subject.

http://www.eham.net/ehamforum/smf/index.php/topic,1585.0.html
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K8AXW
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« Reply #6 on: August 27, 2010, 08:01:53 PM »

I defer to Tom's analysis on filament voltages.  Thanks for the input.  I put the amp in the box and I'm now ready to play once again.

73

Al - K8AXW
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W8JI
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« Reply #7 on: August 28, 2010, 06:07:29 PM »

Power transformer voltage drops as the transformer gets hotter, and it drops as the HV supply current loads the transformer.

6.5 volts at the pins on a cold unloaded transformer is not worth messing with.
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