The 2A3 can be almost as quiet, but ONLY in the dual-section paralleled types. I adopted a split balance technique. It only works with cathode bias due to the topology but it is effective. You also need a good quality filament transformer with an accurate center-tap. As usual, the filament transformer drives the filament of the 2A3. The center-tap is used for the biasing resistor (ONLY) to ground. This becomes a "fixed DC balance" for the directly-heated cathode. The next part is the AC balance which is adjustable. I used a 100-ohm wirewound potentiometer with the ends in parallel with the filament. The wiper of the balance potentiometer goes to ground thru the cathode bypass capacitor. I also pad the potentiometer with a pair of 12-ohm resistors, one from each end to the wiper. This reduces the effective resistance of the 100-ohm pot to less than 4 ohms (from the "cathode" to the bypass cap) and increases the wiper range.
Some information regarding some other DHTs I've used... namely the single-plate RCA 2A3 and the WE 300B. Both of these tube designs use what is known as a "center-tapped" filament. Yes, there are only two pins for the filament, but electrically there are two halves to the filament arrangement and they are strapped in parallel. If you look closely inside the WE300B, you'll note the filament takes the form of a double "M" arrangement. The bottom center where the two "M"s meet is the center-tap and goes to one of the filament pins. The two far ends of the filament are tied together and goes to the other filament pin. Because you effectively have two filaments operated out of phase within a single grid and plate structure, it's impossible to balance the hum out. In the 2A3, the arrangement is somewhat different as there are 8 runs of the filament wire and a single bar tensioned by two coil springs up top to maintain wire tension. It suffers from the same basic problem as the WE300B tube, but it's actually worse (and easily demonstrated). While the WE300B has a double-"M" filament, each loop is tensioned by a separate "fishing rod" tension wire. The single-plate 2A3 has 8 loops all on a common tension bar. As the filament heats up and each run conducts current, minor variations cause unequal expansion of the wire loops. This results in some wire loops being loose enough to to exited by small mechanical vibrations.
I also have much feedback regarding 45 and 2A3 brands and internal mechanical structures but will save that for another post. |
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