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volume control by pass power tube

audioland

Member
2015-04-22 1:13 pm
There is a volume control placed between the two big Pins power tube. Can I by pass it? What is the effect of turning it down (counter clockwise) and turning it up (clockwise) on the power tube it self? More or less power? Thank you for your help
 

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Parafeed813

Member
2012-08-15 8:24 am
This is not a volume control, but a humdinger pot.
It is used to minimize the hum when using ac to feed filaments/heaters.
What DHT are you using?
With low voltage types like the 45 and 2A3 you can get away with ac, higher voltage types like the 300B often are better off with regulated dc.
The minimum that you should do is properly twist the wires to the 'big pins'. The way it is now surely gives unnecessary high hum levels.
edit: read the sticky about heater wiring http://www.diyaudio.com/forums/tubes-valves/211731-heater-wiring-good-bad-ugly.html
 

audioland

Member
2015-04-22 1:13 pm
Hi, thank you very much. I am using 2A3. When i turn down all the way the hum goes up and when i turn up all the way up the hum goes up as well. I found the lowest hum at 10 o clock. The bass seems a bit punchier and lower, is it safe? I do not remember the correct or original set up but it is more than 10 o clock.
 
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That pot should be standard equipment for any DHT with AC filaments and should include the two resistors attached to the wiper. There is no "up" or "down" since it is in parallel with the filament transformer. It should be left in place and adjusted for lowest hum.

Your amp is cathode biased: the 880r resistor is the cathode resistor.
 

Parafeed813

Member
2012-08-15 8:24 am
Without schematic and measured voltages it is all guesswork, so keep in mind things could be different...
To get 10W (marketing talk for 8?) the voltage on the DHT should be pretty high. 400+
The datasheet of the 2A3 shows 250V for SE operation. Add the drop over the cathode resistor and you arrive to a little less than 300V.
My suggestions: use a good regulator board to drop the B+ from whatever it is to under 300.
There are plenty of people over here who know the 2A3 better than I and will tell you what voltage to aim for.
Next would be to change the cathode resistor. Check the datasheet and the people here.
Essential: improve the wiring and/or move to regulated dc on the filament.

On the other hand: why get rid of the JJ? I'm pretty happy with their 300B.
 
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The JJ 2A3-40 is rated at 40W plate dissipation. If you are running the tube at standard 2A3 operating conditions (the 880r cathode resistor adds verisimilitude to the assumption) the plate dissipation will be about 11.5W. Output power will be about 2.5W. To know the actual operating conditions will require measuring the plate to cathode voltage and the voltage dropped across the cathode resistor. The JJ 2A3-40 is a monoplate tube and as good or better than any NOS tube I have tried.