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Oh boy...I really hope I will not regret this.....
-----Original Message-----
From: Rich Measures [SMTP:measures@vc.net]
Sent: None
To: yo3ctk@alltrom.ro
Subject: Re: [AMPS] re:4-250A
Rich wrote:
<snip>
- How many tubes does Eimac rate for Class AB2 grid-driven
linear
amplifier service?
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.
EIMAC says: "Because the 4CX1500B has very low grid interception
it is possible to drive the grid positive without any adverse effects
upon the distortion level or upon the driver. Class AB2 linear amplifier
operation is therefore possible and recommended".
It is not my intention to argue with the major contributors to
this list. Is just that I am slightly annoyed with words such as "never"
in a scientific discourse. 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.
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). 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.
73 de Mike, YO3CTK
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