DIY Audio Projects Forum

2a3 single ended
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Author:  shannon69 [ 29 Oct 2019, 01:53 ]
Post subject:  2a3 single ended

I want to build this ... ematic.htm
How do i power the second channel, from the same point of channel one, or how!?

Author:  tjj226 [ 23 Nov 2019, 14:26 ]
Post subject:  Re: 2a3 single ended

This is clearly intended to be a stereo amplifier because the power supply shows 2 individual DC heater supplies for 2 2a3 tubes and the EF86 heater supply shows 2 heaters in parallel.

I believe the intent is that you make the second channel and simply attach the B+ to the same pole of the switch S2 as the first channel.

Also note that the switch S2 is important. The idea is that you flip the main power switch and allow the heaters to warm up first. And then after a minute or two, you flip the main B+ switch. This helps prevent cathode stripping and helps your tubes last longer.

I would also change this design a bit.

1: The power supply transformer IMO is a bit weak. A full bridge rectifier with a capacitor input will only provide ~62% of the AC current rating of the transformer. This is just enough current for all the bias currents of the tubes. In a class A SET amp, the average currents will be close enough to the bias current that this kind of approach would work. But usually what happens is that if your amp tries to draw large peak current, the PSU struggles to keep up and your B+ voltage will sag. It isn't the end of the world if you build a PSU like this, but I think upping the current spec on the transformer will help the amplifier's transient response. Thats my 2 cents at least.

2: You will also have a fair amount of cross talk with a design like this. Both the right and left channel will share the same PSU cap for the 2a3. I would probably add a RC filter after C8 and split the PSU rails so that you have individual PSU rails for right and left channel. This will give you much better channel separation.

3: I highly doubt its necessary to DC heat the EF86 tubes. I would try AC heating them first. You can always DC heat later on if its necessary.

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