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Old 7th February 2007, 11:39 PM   #1
nhuwar is offline nhuwar  United States
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Default series connection of filament in push pull output stage

Have any of you ever heard of any problems of connecting the filaments in the output stage of a push pull amp in seires? You know to run twice the voltage but half the current I figuare that cut down any possible noise or hum from the filaments.

Oh ya this is dc to.
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Old 8th February 2007, 12:07 AM   #2
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Hi,

This doesn't work so good with AC because of hum problems.

With Regulated DC it works fine but any tube that you put in series has to have the same currents so they get the correct filament voltage.

I made a guitar amp once using a transformer that had a 12 volt filament winding. I wired 2 6550's in series but could never get the hum low enough for my liking. I ended up regulating to 12 VDC with a LM338. This worked well and the amp was very quiet in regards to hum.

Daniel
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Old 8th February 2007, 12:54 AM   #3
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I do it routinely. Good attention to lead dress and grounding is necessary as usual. The fact that this is an output stage and hum is common mode (if you wired things correctly) are all helpful.
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Old 8th February 2007, 01:05 AM   #4
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Well the whole reason for doing this is I have 2 nice big 20vdc linear regulated supplies. Ie so I dont have to build the filament supply and save weight.

Nick
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Old 8th February 2007, 02:40 AM   #5
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Wow, DC? Clearly no problem, but take precautions to prevent common mode noise from coupling to the cathodes via the heaters.
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Old 8th February 2007, 03:49 AM   #6
ArtG is offline ArtG  United States
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Sy,
Could you clarify this some? What specific precautions did you have in mind, other than perhaps a positive bias of the heater supply? Also, I am under the impression that differential amplifier stages are very good at canceling common mode noise. Isn't a push-pull output stage in the same category? Or am I assuming, while you are thinking single ended? I have under construction a large linear DC supply, that should output close to zero ripple and noise, but your post makes me think I'm overlooking something. Thanks!
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Old 8th February 2007, 05:25 AM   #7
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Default Beware of RF

You would probably want to filter out the possibility of RF hash getting into the filament supply through your DC PS. The RF could jump the gap to the cathode through the h-k capacitance (I may have run into this problem with a disastrous cascode I tried to build).

Elevating the heaters will certainly help, though. Using a common mode choke with some ceramic caps right at the heater terminals to chassis will do a lot to help reduce any RF. I use shielded twisted pair for my heaters, now, as a precaution.
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Old 8th February 2007, 06:02 AM   #8
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I had couple of power transformers from tube transmitters and used them in power amps. They had couple of 12.6V filament windings each.
In one of them I connected 8 tubes 6P3S, 2 each in series. 6E1P indicators I connected in series as well (one per channel).
Also, rectified it and used regulated voltage to power 2 x 6N2P and 2 x 6N1P tubes, each pair in series.
The second one I use to power GU-50 tubes, they have 12.6V filaments, but the rest of tubes I powered the same way.
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Old 8th February 2007, 01:35 PM   #9
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Yes, elevate heaters. Common mode noise should be taken care of with a common-mode choke in series with the heater feed and bypass caps from each side of the heaters to ground. Also, experiment with the relative "polarity" of the heater connections from one tube to the other in the p-p pair.
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Old 8th February 2007, 01:39 PM   #10
nhuwar is offline nhuwar  United States
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Oh The tubes are directly heated.

Nick
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