Early beam tetrode output stages and screen voltages

[GregS]

The bulk of production amps and a fair amount of customs I've seen a sticking steadfastly to the 470 ohm and 1K or 1.5K resistor values for beam tetrodes and power pentodes, respectively, suggesting to me that too many designers have either buried their heads in the sand, or just aren't privy to the shortcomings of new tubes.

The re-issue of the 20W Marshall 2061, the 2061x, uses no screen resistors on its power tubes - just like the original. My point here is this supports what you're saying, not that this is an ok thing for tube life.

[pdf64]

To my way of thinking, the 2061 has a shared 470 ohm screen grid resistor.
Compared to a Vox, with choke (about 100 ohms) and individual screen grid resistor (100 ohms), 470 ohms is quite large. Pete.

[GregS]

There is a 470R dropping resistor between the plate and screen nodes of the power supply IIRC but there are no individual screen resistors. That's what I was thinking of by the term "screen resistors".

Greg

[pdf64]

Agreed, but the wider view may be to regard any additional resistance in the screen supply as beneficial to tube longevity. Pete.