>
>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
I see no data. sheet on the QBL5/3500. The theme song from "The Good,
The Bad, and The Ugly" was a nice touch.
>
>Look at the data sheet yourself so that I do not misquote it.
Yeah, right.
>It is larger
>than .5 ma.
How much grid current does Phillips recommend for linear service?
>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.
ok
>
>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.
Another problem is that, on constant-current curves, linearity is
essentially lost when the zero volts grid potential line is crossed.
> 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.
This discussion is not about grounded-grid / cathode driven. It is about
grid-driven.
>
>The matter was brought up, Rich, for the process of learning.
One can not learn anything useful about grid-driven amplifiers by
learning about cathode-driven amplifiers.
The Misdirection Card:
> 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.
>
Not a good bet. One of the first amps I built was a 3CX100A5 in
cathode-driven config. for the 23cm band.
>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.
They aren't. RF-NFB reduces distortion in cathode-driven service because
the input is in series with the output. .
>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?
zzz
>
later, Colin
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|