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Old 9th September 2010, 01:21 AM   #1
cjkpkg is offline cjkpkg  United States
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Default 300B filament winding center tapped?

Thinking about a 300B SE amp.

I am leaning towards the circuit John Broskie posted with the Aikido front end 12AX7 and 12AU7 and SS rectification for B+ and the driver heaters.

What would be considered the quietest method of handling the 300b filaments assuming they stay as A/C?
1. Find a transformer with a 5V CT winding?
2. Find a transformer with a 5V winding (5V-0) no CT?
3. Separate 5V xformer for the 300b's? (CT or non CT?)
4. Individual 5V xformers for the 300b's? (CT or non CT?)

I plan to reference the driver heaters to B+/4. I have not seen consistent methods of handing the heaters for the 300b's.

I am just a little confused over the whole CT versus non CT concept.


Last edited by cjkpkg; 9th September 2010 at 01:43 AM.
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Old 9th September 2010, 01:58 AM   #2
BZed is offline BZed  United States
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By using a center tap transformer winding and using the CT as the reference for the cathode the average hum voltage will be 0. As one end of the cathode/heater is + the other end will be - compaired to the reference. I would use a seperate transformer for each 300b tube.


Last edited by BZed; 9th September 2010 at 02:01 AM.
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Old 9th September 2010, 03:45 AM   #3
tomchr is offline tomchr  Canada
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For no hum induced by the 300B filament, ground one end of the filament and use a 5.0 V DC supply for it. DC heating of a directly heated triode is as quiet as it gets.
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Old 9th September 2010, 11:57 AM   #4
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In my 300B SE amp, The 300B are heat on 5VDC (rect.bridge + RC filtering) symmetrically balanced to ground. No CT on the Xfo, which is a 6.3VAC winding.

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Old 12th September 2010, 06:48 AM   #5
BZed is offline BZed  United States
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In general for directly heated cathode tubes like the 300b DC filiment voltage can cause asymetrical electron emission from cathode due to the difference in cathode to grid bias from one end of the filiment/cathode to the other. That is one reason I would use AC filiment voltage, longer tube life. The other reason is that when rectifying to get the DC voltage you get high current spikes in the diodes. The noise caused by these spikes can cause a buzzing sound that can get into the gourding system and is very hard to get out of the system.

I just think that using DC filiments is more trouble than its worth. Correctly done AC will perform just as well, is simpler to do and the tubes will last longer.

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Old 12th September 2010, 11:05 AM   #6
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Hi BZed,

My experimentations about AC heating of 300B, 50, 45, 10Y, 211, 845, 814 all showed that it was impossible to have no AC hum in the speakers, even when "correcty done" (short and balanced lenght of the wires, quality pot wirewound or molded carbon AB, etc...).

With my DC balanced non-regulated (RC filtered) heating, I have no hum (AC or spikes) in the speaker even with the ear on the diaphragm (98dB 15" woofer in a 232L bass-reflex), and nothing appears on the scope (less than 0,3mV visible ripple, which can be due to the measurement cables...) at the speaker output.

So you may have been more lucky than me in your AC heating experimentations, then...

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Old 12th September 2010, 01:36 PM   #7
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If going SE, I think your best bet would be separate, dedicated, shielded 5V transformers for each triode, and using a resistor / hum pot arrangement to null the 60Hz. It will not be possible to remove the residual 120Hz hum with the nulling procedure, though there are some more advanced and fancy methods of using injection or cancellation of the ripple voltage. No matter what you do, there will be a small amount that gets through.

You may find yourself more satisfied with a robust DC heating design. Ultimately it may prove simpler with better results.

PP is another story; you can get pretty respectable low hum with DHT's.
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