Rich said:
"Eimac makes two tetrodes that are designed to draw grid current in linear
service --the 4CX1500B/8660 and the 4CX600J Typical grid current is
less than 0.5 mA in either. // Is the data sheet for the QBL5/3500 a
JPEG photocopy? Do you know the url? How much grid current does
Phillips recommend for linear service? What do they say about using
unregulated screen potential?"
The URL is www.pocab.se/amp/g2daf.htm
Look at the data sheet yourself so that I do not misquote it. It is larger
than .5 ma. The data sheet says nothing about unregulated screen potential.
That is not at issue in my note. My assumption was a "stabilized and
variable" screen supply.
The G2DAF circuit is not at issue here. It is the operation of a linear
amplifier with less than the "normal" screen voltage. One problem in some
tubes would be that grid dissipation would increase. In some tetrodes or
pentodes, this would exceed specified maximum permitted. But, in many
surplus tubes, there is plenty of grid dissipation to spare - which is why
the 813 works well as a grounded grid amplifier.
The matter was brought up, Rich, for the process of learning. I do not find
anything significant in your response to cause me to not want further
information. It seems you have closed your mind to learn. I am reminded of
the old P-47 warbird. When it first went into production, it had a 3 bladed
prop and was a dog. Soon, thereafter, someone tried a 4 bladed prop. -
which was just the thing needed to transfer all that horsepower. This
single change made a substantial improvement in not only speed but the
ability to turn, resulting in a kill ratio of 18-1. I suppose there were a
number of engineers out there who scoffed at the idea because it came with a
3 bladed prop. But, sometimes things are learned by trying them. I expect
you would have been arguing against even trying a 4 bladed prop. because
there is no 300 mpg carb.
Suppose, years ago, some "ham" had suggested to you that he could drive the
cathode instead of the grid. I expect you would have belittled him - since
the tube was originally never designed to be cathode driven.
When the cathode is driven and the grid draws substantial current, how does
the plate know it is cathode driven and therefore ok, yet distort if the
grid current comes while the cathode is grounded. I did not know that
tubes were so smart.
Does this mean that when I go to the local parts store and purchase a grid
current meter that I need to specify whether it is to be installed in a
grounded cathode or driven cathode because the grid current is different?
Colin K7FM
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