[Amps] Alpha 8410 using 4CX1500B - why?

[Paul Baldock]

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 at frontierisp.net.au>
>To: <amps at 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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