One nice thing about home brewing a grid driven tetrode amp with the
GU74B is that you don't need a tuned input, just a 50 Ohm resistor.
With 2 tubes, 50-60W drive gives you 1500W out. With suitable cooling
the extra heat is not an issue. The NOS tubes are still available for
around $150.
The biggest issue with GU74B is the failure due to grid to cathode
shorts, and even that can often be fixed (at least for a while) by
zapping the short away.
- Paul
At 08:06 AM 10/30/2014, Jim Thomson wrote:
Date: Thu, 30 Oct 2014 10:51:07 +1030
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?
Indeed Peter.
And high plate bias current and associated low efficiency is part and parcel
of achieving high linearity and low intermod from AB1 tetrodes.
Low plate idle current and low IMD are mutually exclusive objectives. There
are no free lunches here :-(
At least in an SSB linear amplifier one can significantly offset the impact
of high idle current with a competently implemented EBS system. When EBS is
done properly there are no audible or spectral artefacts stemming from the
tri-state transitional bias switching.
73
Leigh
VK5KLT
## Unless your tri-state EBS is being used on PTT, it buys you
nothing when using VOX.
Emtron EBS wont switch fast enough between syllables, much less words.
No free lunch there..and ditto with ameritron, et all. On a GG amp,
you can run the idle current
a lot lower on ssb mode. On CW mode, you can easily reduce the idle
current to just above zero.
## 2 x 4CX-1000s vs a single 8877 ? No brainer there
folks. The 8877 wins hands down.
2 expensive breech block sockets, 2 tubes, 2 chimneys, 2 parasitic
suppressors, regulated screen supply,
zero watts grid diss, no thanks. The list goes on and on. Then to
top is all off, the 4CX-1000 is no longer available.
Its yet another of alphas silly tube lineups.
Jim VE7RF
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