Kofi Annan in: "300beee!" or "Kofi Gets a Toy"

[kevinkr]

300B based amplifiers of that time (late 1930's onward) would have used either the 80/280 fullwave rectifier or the Western Electric 274A/B full wave rectifier. Later still a 5U4 might have been employed. Any of these types would have provided sufficient current for a single 300B in SE or even a pair in pushpull.

Solid state rectifiers as you understand them (silicon) were not invented until the 1950's. There were copper oxide rectifiers generally used for low voltage high current applications, and selenium rectifiers generally used for low to moderate currents at low to moderately high voltages. Both these types have very low piv and consequently many were stacked in series to get sufficient piv rating. They were also quite lossy, so high voltage stacks ran hot and had lots of surface area to dissipate the heat. Voltage drops were often far higher than for small vt rectifiers at anything above a few 10's of mA. Selenium is extremely toxic. Both are subject to serious deterioration over time, becoming highly resistive to the point of becoming essentially open circuit as far as the load connected to it is concerned.. LOL

A (fullwave) bridge rectifier whether tube, hybrid or ss based does not require a center tap for operation, although one will be used obviously in a bipolar supply application.

A two diode fullwave rectifier whether tube or SS always requires a center tap. (Not talking about fullwave doublers where this is NOT true.)

The advantage of a fullwave bridge is that for a given output voltage only half the number of winding turns are required for a given voltage as compared to a fullwave center tap set up.

[gnomus]

What part was used for the common mode choke on the 300B filiment supply?

[gnomus]

Thanks Kevinkr.

I was not clear. I ment on the 300B filiment supply which has low voltage but high current when compared to the B+. Was a/c used? Or another tube rectifier(s)?

[Tweeker]

Where really high DC currents were needed in the 1930s, as for large transmitter filaments, motor generator generator sets were sometimes used. Things like 300B filaments would have got AC.

A bridge also gives better transformer utilization, as fullwave is only using half the transformer at a time. VT rectified ciruits didnt use bridges often as the tubes were expensive, the heaters eat power. If directly heated diodes (ei 274A) are used it requires three filament windings, and three separate filaments, so 3 tubes rather than 1 fullwave rectifier tube.

[gnomus]

I found some common mode chokes. They are for line noise EMI suppression. My question is which to choose. Triad makes them in a range of mh values. The highest mH model is 16 mH 2.6 amps .16 DCR . Would that be good? Or one with a lower value?

I also know of A/C rated caps that go across the mains input. That are also for EMI supression. They are small in size. It's a cheaper way to do it. Don't know if it works better than a common mode inductor.

[Tweeker]

They are often used in conjuction with a cmc for a pi filter.