[Amps] 572B Current Balance

[KA9S - 2]

I have an old Hammarlund HXL-1 coming to me and I have down loaded the manual for it. I have already decided to make some "improvements" to the amp beyond fresh electrolytic capacitors in the power supply.

The HXL-1 uses a pair of grounded grid 572B tubes in parallel. It uses capacitors to RF bypass the grids of the two 572B tubes. It then feeds a low voltage negative grid bias through 47-Ohm resistors to cut the tubes off during key up time. When the amplifier is keyed, these resistors are grounded.

The Heathkit SB-200 and the Yaesu FL-2100 do similar things. They use 33-Ohm resistors to each grid, and then combine them to a circuit, which provides the cutoff voltage during key up time and a low voltage negative operating bias during key down time.

Other amplifiers with parallel final tubes do not use this configuration. They mandate using "matched" pairs of tubes.

It would seem that the 47-Ohm resistors (or 33 Ohm) form a negative feed back function. The tube that is drawing more grid current will experience higher negative grid bias compared to the second tube. Does this help balance the current load on the tubes and thus elevate the need for matched pairs of tubes?

Is there some other reason this grid configuration is so popular?

What would be the draw back to directly grounding the grids of the two 572B tubes and provide the cutoff and operating bias through the cathode?

Jeff - KA9S