Push Pull Bias for a 6B4G

[battradio]

I going to build a couple Push Pull 6B4G amps , and a bit confused on the biasing of them , A single tube show a 750 ohm cathode resistor , and PP shows a 780 ohm cathode resistor .
Have a Acrosound circuit and it shows and 850 ohm cathode circuit .
I was thinking that if one tube had a 750 cathode resistor 2 tube would use half the resistance .




Mark

[kheper]

I going to build a couple Push Pull 6B4G amps , and a bit confused on the biasing of them , A single tube show a 750 ohm cathode resistor , and PP shows a 780 ohm cathode resistor .
Have a Acrosound circuit and it shows and 850 ohm cathode circuit .
I was thinking that if one tube had a 750 cathode resistor 2 tube would use half the resistance .

Mark

I think it is implied that 2, identical cathode bias resistors be used in a PP, cathode biased amp.

See page 2 below: (Triode Connection. Push Pull. Class AB1. Cathode bias. "* It is essential to use two separate cathode bias resistors.")

http://tdsl.duncanamps.com/link.php?target=4725A897

[EWBrown]

The 6B4Gs would be biased differently for Class A operation, than for Class AB1. In either case, the 15Watts plate dissipation should not be exceeded.

For Class A operation, the plate current through a single 6B4G would be approx 60 mA, at V P-K of 250VDC, and for AB1 operqtion, closer to 40mA idle, and 50 mA max, for each tube with a V P-K of 300VDC.. \The 80 and 100 mA specified in the Class AB1 ratings is for both tubes combined.

The B+ voltage for the PP 6B4G amp is 375 VDC, as compared to the usual 300 VDC for Class A SET operation.

Class A PP would work, and sound very good, it's just a lot more inefficient in power output vs DC power consumption. Also, the maximum RMS power output would be approx 6-7 Watts, vs 10-14 Watts for AB1 operation.

With the values given, for Class A operation, the voltage across the "cathode" resistor would be approx 45 VDC (60 mA X 750 ohms) and for AB1 PP operation, it would be approx 62 VDC (80 mA X 780 ohms).
In the Acrosound design, the cathode resistor is 850 ohms, so this would allow for using a higher B+ with slightly lower cathode current.

/ed B in NC

[kheper]

From the wayback's copy of the old fi-primer:

http://web.archive.org/web/200602142055 ... b4g-pp.gif

Above, I wasn't sure what the question being asked was. ED got it.

[battradio]

I settled on 750 ohm resistors for bias resistors , will be using a seperate divider net work and sense resistor for each output tube filament so the total resistance will add up to 780 ohms

http://www.ebay.com/itm/170938641326?ss ... 1497.l2649