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Old February 23rd, 2013, 09:55 PM   #1 (permalink)
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Full Wave Bridge - Elevate Heaters?

I'm trying to reduce noise in a recent SE amp I built. I'm using a cheap PT that requires a full-wave bridge rectifier (in other words, the PT's HV center tap is NOT grounded).

Will this matter if I choose to elevate the heaters by putting the 100z 6.3v artificial center tap resistors to the power tube's cathode?

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Old February 23rd, 2013, 10:46 PM   #2 (permalink)
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I've been Googling like mad... kind find a thing.

I may have built a lot of amps, but I am definitely still an amateur :)

Time to keep searching...
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Old February 23rd, 2013, 11:09 PM   #3 (permalink)
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With this transformer, I'm going with AC heaters and dropping resistors (not DC heaters as shown) . I am planning to install the artificial center tap on the heaters (to the power tube cathode) and ground the negative side of the full wave rectifier (HT section) as shown.
I have done this before and it worked well.

http://site.tubedepot.com/pdf/tr-pw-879s.pdf
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Old February 23rd, 2013, 11:15 PM   #4 (permalink)
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That's not quite my question... perhaps I'll rephrase it...

My PT's HV center tap does not go to ground (and it doesn't have a heater CT)... instead, the full-wave bridge rectifier goes to ground. In reducing heater noise, would referencing the artificial heater CT to the power tube's cathode actually cause more noise/headaches?
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Old February 23rd, 2013, 11:42 PM   #5 (permalink)
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If you're connecting each end of the secondary to the cathode of a separate diode and
these go to ground.Then that's the same as a center tap. So adding the two resistors isn't needed, I guess. There'd be no harm in actually trying it would there?
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Old February 24th, 2013, 02:22 AM   #6 (permalink)
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Quote:
Originally Posted by JohnnyCrash View Post
That's not quite my question... perhaps I'll rephrase it...

My PT's HV center tap does not go to ground (and it doesn't have a heater CT)... instead, the full-wave bridge rectifier goes to ground. In reducing heater noise, would referencing the artificial heater CT to the power tube's cathode actually cause more noise/headaches?
No it should be fine. You can either:

run the 100Rs to the output tube's cathode resistor, or

use a 180R-250R pot with each pot lug end connected to an end of the heater winding and the pot wiper connected to the output tubes cathode resistor (i.e. a hum-dinger), or

do either of the above but make a voltage divider from the HT with (say) 180k and 22k 2W resistors, and decouple the 180k/22k knee to ground with a 10uF-47uF 100V cap, and hook the ground reference point up to that. With this in parallel with the first (reservoir) filter cap, this heater elevation circuit doubles as a bleeder for the main filter caps.
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Old February 24th, 2013, 05:08 AM   #7 (permalink)
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No it should be fine. You can either:

run the 100Rs to the output tube's cathode resistor, or

use a 180R-250R pot with each pot lug end connected to an end of the heater winding and the pot wiper connected to the output tubes cathode resistor (i.e. a hum-dinger), or

do either of the above but make a voltage divider from the HT with (say) 180k and 22k 2W resistors, and decouple the 180k/22k knee to ground with a 10uF-47uF 100V cap, and hook the ground reference point up to that. With this in parallel with the first (reservoir) filter cap, this heater elevation circuit doubles as a bleeder for the main filter caps.


Thanks, I haven't tried the third option before... perhaps now is the time to give it a shot :)
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Old February 24th, 2013, 06:02 AM   #8 (permalink)
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I use the elevation to cathode of the power tube in my SP6, and it indeed reduced a lot the hum, but in SE, a filtering choke can be useful too.
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Old February 24th, 2013, 03:14 PM   #9 (permalink)
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It's been a while since I built an amp (school kept me busy)... I need to put my thinking cap on.

The noise level is just unacceptable. Time to consider a filter choke (thanks kleuck!), my lead dress (though it's all point-to-point! O_o), and my heater situation.
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Old February 24th, 2013, 03:31 PM   #10 (permalink)
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My last amp build used a cheap PT with full wave bridge rectifier, a filter choke, and very tightly wound AC heater wiring.....dead quiet!
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Old February 24th, 2013, 05:17 PM   #11 (permalink)
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What size are your filter caps?
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Old February 24th, 2013, 07:59 PM   #12 (permalink)
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SE amps definitely benefit from a choke in the main filter (and a whole-of-supply choke at that). The choke needs to be DC-rated for the peak DC current that will be drawn.

The benefit of a choke is that is smoothes out changes in current (i.e.: ripple current the power supply) without losing voltage (whereas a resistor will always drop voltage).

(In a PP amp, you don't need a choke filter for the whole of supply, because any hum that is riding on the signal gets otherwise cancelled out in the opposing sides of the OT primary winding, but this doesn't happen in a SE OT)
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Old February 25th, 2013, 12:11 AM   #13 (permalink)
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Johnny,

You might find this useful:

http://www.tubecad.com/april99/page2.html

'Could save you buying a big choke to handle the power tube current...

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Old February 25th, 2013, 04:20 AM   #14 (permalink)
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I think I found my main problem - it was a third gainstage. I had a single cap for the first power node, then a dual cap can for the next two nodes. I swapped it so the dual can was for the first two nodes, and the last node used a single cap. The third stage was feeding low freq hum through the shared can ground into the earlier two preamp stages.
.
Now I think I will go back and reconnect my NFB loop. I like how it added highs and smoothed the mids and bass.
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Old February 25th, 2013, 05:32 AM   #15 (permalink)
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A great calculator :http://www.ampbooks.com/home/amplifi...ripple-filter/
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