> From: yo3ctk <yo3ctk@alltrom.ro>
To: <amps@contesting.com>
> Date: Tue, 22 Jul 97 14:48:13 +0000
> Oh boy...I really hope I will not regret this.....
:-)
> For instance, EIMAC 4CX1500B. According to the manufacturer,
> this tube is "specifically designed for exceptionally low
> intermodulation distortion". Its typical operation is class AB2,
> grid-driven service for frequencies below 30 MHz. Indeed, performance is
> good: -40 dB 3rd order IMD, for 2750 V DC plate voltage and 1100W power
> output. In this conditions, DC grid current is 0.95 mA for -34 V grid
> bias.
We can add ANY tube that can tolerate grid current to that list.
Some tubes are better than others, but by careful design virtually
all tubes that allow grid dissipation can be used in AB2 grid driven
service.
For example, the 4-1000A had an extensive article describing either
AB2 or AB1 RF PA operation.
Just because the typical operating guidelines do NOT list a certain
class, it does NOT necessarily exclude that class.
> Maybe this example will emphasize the point
> that class AB1 and AB2 can be used with equally good results for
> grid-driven amplifiers, depending on the tube selected.
Or cathode driven class A, AB1 or AB2, and class C for many tubes.
> Perhaps I missed something essential here, but several messages
> on this subject have stated that distortion is due to the "bending" in
> constant-current curve above zero volts grid to cathode voltage. May I
> point that this happens in both GG or grid-driven configurations, so
> distortion must be the same, all other things being equal (grid voltage
> excursion, ZSAC and so on).
Right, and the correct SLOPE of a bend causes no odd order
distortion products. Only odd order products cause harmful IMD!
The idea transfer characteristic is a straight line turning into
a properly curved slope at the limit, instead of abruptly ending.
You can find that in most IMD related technical descriptions.
>There is one thing, however: GG stage
> configuration has an inherent negative feedback, thus lowering the
> distortion of the amplifier. IMHO, this prove only that it is possible
> to obtain good IMD performance from an inferior tube in GG service and
> nothing else. Mixing issues, such as class AB1 versus AB2 and GG versus
> grid-driven configuration, is somewhat confusing.
True, but negative feedback is easy to add in most grid driven PA's
by adding an un-bypassed cathode resistor.
Thank's for your input. Perhaps it will help clean up some of the
obvious distortion.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|