Interesting way of generating negative bias

Max N · Post by **Max N** » Mon Aug 08, 2011 4:06 pm

While googling 2a3 push-pull amps, I came across this circuit:

It took me a while to figure out how the negative bias worked.

Post by **Nick** » Mon Aug 08, 2011 4:20 pm

Cunning. And also self regulating. as the current in the output stage increases, the bias becomes more -ve reducing the current. From a AC point of view I think its much the same as a bipassed cathode resistor.

JackOfAll · Post by **JackOfAll** » Mon Aug 08, 2011 4:57 pm

deleted

Post by **Nick** » Mon Aug 08, 2011 5:12 pm

Yep, if you redraw it and forget the field coils and tuner supply it gets very close to the western electric two caps across the bipass resistor idea, though with ground at the cathode, which can only be a good thing WRT noise on the fillament.

JackOfAll · Post by **JackOfAll** » Mon Aug 08, 2011 5:35 pm

deleted

Post by **Nick** » Mon Aug 08, 2011 5:44 pm

OK, I havent seen those docs, if you find the links it would be good to have a read. The thing that looks different to the circuits I had seen in the past was the actual ground point that the rest of the amp referenced to was the other end of the back bias resistor, in this case the first stage cathode is taken to the same ground as the output stage cathode, but the driver stage goes to the other end of the resistor. Gives the phase splitter more headroom I guess.

Max N · Post by **Max N** » Tue Aug 09, 2011 8:56 pm

Here's a link to the site where I found the article. There are several more articles, including some push-pull 807 amps.
http://greygum.net/pages/radiotronics

Cressy Snr · Post by **Cressy Snr** » Tue Aug 09, 2011 9:32 pm

Hmm

Nothing new under the sun
6AU6s driving 807s in the Radiotron A518 amplifier.
I'm in good company then.
Mind you they did triode strap the 6AU6s.

audio-talk

Interesting way of generating negative bias

#1 Interesting way of generating negative bias

#2

#3

#4

#5

#6

#7

#8