AC Heater (Filament) Supply

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AC Heater (Filament) Supply

Postby ChrisK » Mon Jul 06, 2009 8:39 pm

Hi,

Just to make sure I understand: I have a 6.3V AC supply with CT and a 5V AC supply with CT (one for each 5AR4/GZ34 in a two-channel Mark III amp).

What's the preferred way of referencing the CT? The MK III schematic shows a .02 cap connected between CT and ground (my star ground bus in this case).

In other circuits, I've seen resistors used: one from each side of the supply connected together on the grounded side. This method seems to be used when a CT isn't available.

Your thoughts would be appreciated. Thanks,

Chris
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Postby battradio » Tue Jul 07, 2009 9:06 am

Hi Chris ,

With the 5 volt winding the B+ is presant at the center tap . So the only safe way is to use a cap like the .02 at 1000 volts , if a 5U4 was used instead of a 5AR4 the center tap could be used as the B+ take of point instead of pin 8 . On the 6 volt winding grounding the center tap is the best way .

Mark
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Postby ChrisK » Tue Jul 07, 2009 11:00 am

battradio wrote:Hi Chris ,

With the 5 volt winding the B+ is present at the center tap . So the only safe way is to use a cap like the .02 at 1000 volts , if a 5U4 was used instead of a 5AR4 the center tap could be used as the B+ take of point instead of pin 8 . On the 6 volt winding grounding the center tap is the best way .

Mark


Thanks for the response, Mark. I appreciate it.

Just to clarify:

I have a single toroid that provides 2x400V B+ (no CT) and also has taps for 6.3V with CT. So, if I understand correctly, the 6.3V CT should be grounded with a .02 cap (I have some 2KV units on hand).

Each of the two 5AR4/GZ34 rectifiers have their own, independent 5V filament transformer with CT. Again, if I understand, these CT's can/should be grounded directly.

Hope this makes sense,

Chris
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Postby TomMcNally » Tue Jul 07, 2009 11:24 am

Mark is saying that there is B+ at the 5 volt center tap, so
if you ground it you will short the high voltage power supply
right to ground. You can either IGNORE the 5 volt center
tap, and cap it off with tape, OR if you use a 5U4 tube,
use the 5 volt center tap as the high voltage output.
If you are using a 5AR4/GZ-34, take the output from Pin 8
of the tube, and cap off the center tap.

Like he says, on the 6 volt winding, GROUNDING the center tap
is the way to go, no capacitors.

Hi Chris ,

With the 5 volt winding the B+ is present at the center tap . So the only safe way is to use a cap like the .02 at 1000 volts , if a 5U4 was used instead of a 5AR4 the center tap could be used as the B+ take of point instead of pin 8 . On the 6 volt winding grounding the center tap is the best way .

Mark
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Postby kt88pppamp » Mon Jul 11, 2011 6:29 pm

Would adding a cap from the center tap on the 6.3 volt CT winding to ground elevate the heaters above ground to take care of the heater to cathode voltage limitation?
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Postby EWBrown » Tue Jul 12, 2011 12:50 pm

That is why Dynaco did that, it worked well, and it was a ten cent solution to letting the filaments "float" at around halfway between the PI cathode and the VA cathode voltages, perhaps infuenced by the optupt tubes' cathode voltages.

I use the same approach with the 13EM7 SETs, with the two filaments connected in series, and the junction between them thru a 0.1 uF / 200V cap to ground. This makes for a totaly hum-free, noisless SET, along with good single point grounding.

Re the 5VAC rectifier filament center tap, the best thing iis to not use it, cut it short and cap if of woth some shrink tubing or a wire nut.

The 5VAC Center Tap could be OK for use with a DH rectifier like 5U4 or 5Y3GT, but with a 5AR4/GZ34, that would inject a 2.5VAC 60Hz hum on top of the B+, and that would mix in with its pre-filtering 120Hz ripple, which could make for a messy situation.

Best to just use pin 8 as the "cathode" B+ source, and that makes it easy to roll in different rectifier tube types.

/ed B
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Postby kt88pppamp » Sat Mar 17, 2012 11:28 pm

On my PPP amp I am using AC filaments. I debated whether to build DC heater supply, as with Dynaco MKIII's and the ST70 I built had a faint but audible hum due to AC heaters.

How come my PPP amp has absolute zero hum even though I am using AC, even on the driver tubes.

The drivers are a 12AY7 and a 12BH7. Low gain, or the low noise and microphonics testing option from TubeDepot?
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Postby battradio » Sun Mar 18, 2012 12:12 am

Alot has to do with the wiring of the filament strings on the tubes , and , the transformer leakage .The more of the auctual circuit that is push pull the more canceling of the hum there will be .
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