Hi Peter,
That's an interesting remark you make here given that Eimac on their Data
Sheet spec the 4CX3000A at a rather modest -35 dB for 3rd order and -40 dB
for 5th order intermodulation distortion. And that's at a zero-signal plate
idle current of 0.5 Amp, so that bias point represents a lot of plate
dissipation for such an unexceptional mediocre result. As most tetrodes do
well / better when backed-off in power you are probably quite right in your
assertion the attainable improved IMD for these two tubes could well be
similar when operating down at the circa 1 kW PEP output power level
compared with either tube's nominal rated power.
However tubes specifically designed for use in both analog and digital
television transmitter applications like the 7F71RA must without exception
have excellent linearity and very low intermodulation characteristics.
73
Leigh
VK5LT
-----Original Message-----
From: Peter Voelpel <dj7ww@t-online.de>
Sent: Saturday, 4 December 2021 6:40 PM
To: 'Leigh Turner' <invertech1@bigpond.com>; amps@contesting.com
Subject: RE: [Amps] FW: FW: Triode Amps / options / tetrodes
I don´t think that the 7F71RA is much cleaner then the 4CX3000A which is a
very similar tube also developed for low intermodulation.
What I see on the qrz page is the IMD at 1KW output and driven with the
super clean 4CX350FJ.
I am pretty sure that a 4CX3000A under this condition is doing equally well.
73
Peter, DJ7WW
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Leigh Turner
On the subject of capable HF linear amplifier tubes there is an interesting
ultra-linear tetrode made by NEC Japan designed specifically for low
intermodulation television broadcast service called the 7F71RA. This
substantial and not so commonly known tube with handles achieves impeccable
IMD performance in QRO linear SSB applications and I note that one such rare
NIB/NOS specimen is currently listed up on eBay as they do from time to
time.
The 7F71RA is in a similar class to the Eimac 4CX3000A / 8169 tetrode but
with significantly cleaner 3rd and 5th order IM specs. The NEC tube would
make a fine QRO linear for teaming with an SDR transceiver and might be of
prospective interest to linear amplifier homebrewers. JA1VKV has an
interesting qrz.com page outlining his neat homebrew amp using this tube
with just a modicum of NFB to attain superbly clean linearity performance.
And that?s without applying any SDR based Pure Signal adaptive predistortion
linearization techniques.
Leigh
VK5KLT / VK5LT
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