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RE: 300B OPERATION POINTS .. SURVEY!!!

24.18.240.124

In Reply to: RE: 300B OPERATION POINTS .. SURVEY!!! posted by drlowmu on December 01, 2007 at 08:21:22

A few additional points should be made here ...

1) We are talking about 300B tubes in this thread. Western Electric provides 36 different recommended operating points, from 6 watts to 36 watts plate dissipation; they rated the tube at 36 or 40 watts maximum at various times. The tube is most often specified at 300 or 350 volts, 60mA, which is 18 or 21 watts - roughly half the rated maximum plate dissipation. That's a big contrast with the 2A3, which is normally specified at 250v/60mA - equal to the maximum plate dissipation of 15 watts.

2) Voltage and current also kill tubes. Some 6DJ8s, for example, are notorious for having the voltage problem, and exceeding the rated current is a well-known cause of cathode poisoning.

3) If you simple reduce cathode current by increasing the cathode bias resistor, you will change the operating point in such a way that distortion will increase unless the load resistance is also increased. A bit of time spent studying the above-mentioned WE data table will show how this works. Of course, some folks like the extra second harmonic distortion of "starved" operation; I'm just saying that the two effects confound each other unless you are careful to separate them.

All that said, and with respect to 300Bs, I tend to agree - I don't like to operate them above 80% of maximum rated current or voltage, i.e. 360v and 80mA maximum. I do this because most modern 300Bs don't have a long history of manufacturing and are probably not as reliable as "golden-age" tubes would normally be. The exception of course is the WE 300Bs, and they are quite expensive - a different reason to operate them conservatively. I don't worry so much about 2A3s, which are more conservatively rated. Except for the delicate and fragile NOS single-plate and some modern "mesh" plate versions, they can operate at 15 watts dissipation without significant effect on life or performance. The modern Sovtek 2A3 is widely said to handle 20 or even 25 watts dissipation; a friend has operated his at 18 watts for many years.

Since the thread title still says "survey" I'll say that the Paramount runs nominally 368 volts plate to cathode at 72mA current, into a 3000 ohm load.

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