[Amps] Reducing heat in tetrode amps with EBS

[Jim Garland]

	Some people don't like tri-state EBS to cut down on heat in tetrode
amps, but I found it works very well in my homebrew 3-tube GU74B amplfier
and is easy to implement. The three bias levels are -120V(cutoff), -62V(CW),
and -50V(SSB). In the SSB mode, an RF detector at the amplifier input
switches the bias from -62V to  -50V when the RF input reaches 1/2 W.
Between words and pauses in speech, the bias is at -62V, where the quescent
plate current is only about 50mA per tube. The criticism of this scheme is
the possibility of clicks on the transmitted signal when the bias swtiches,
but I've had no reports of that. I set the time delay for the circuit to be
long enough that it doesn't trip between syllables, but does trip between
words.
	The EBS circuit is implemented on a 3x5 inch PCB which holds the
bias regulator circuitry. A small solid state relay (in a 4-pin DIP) toggles
the bias between the two settings. The tubes run at 2600V plate voltage and
325V screen voltage.
73,
Jim W8ZR

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Paul Baldock
> Sent: Thursday, October 30, 2014 10:48 AM
> To: Jim Thomson; amps at contesting.com
> Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?
> 
> 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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