rm1 wrote:
>
>
>>
>>> In the 2 meter amplifier after a 3 minute CW transmission at about 20
>>>wpm
>>>into a dummy load, the average anode exhaust temperature went from 205
>>>degrees F
>>>to just under 100 degrees F when using the EBS.
>>
>>This is the aim.
>>
>>EBS seems to have a poor reputation because of either:
>>
>>1. Being the only means of keying the amplifier, which is guaranteed to
>>cause hot-switching. Competent EBS works along with the hard-wired PTT
>>line - it cannot replace it.
>>
>>2. Being RF-actuated at too high a level (ON4UN suggests around 0.5W in
>>a kW amplifier, related to the dynamic range of he speech processing for
>>SSB).
>>
>>3. Being too slow to turn on.
>>
>>Any or all of the above can cause a poor signal, and then of course
>>there's:
>>
>>4. Being unreliable and easily damaged.
>>
>>... but it doesn't have to be that way.
>>
>>Like many other features in amplifiers, EBS can be very useful if it's
>>competently done - or a disaster if not.
>>
>? Good points, Ian. The solution is to allow the transceiver to control
>the EBS with its +DC-V on Tx signal. This gives the RF relay contacts in
>the amplifier at least a 5mS headstart so they can be closed Before RF
>arrives. When the EBS is controlled by RF arriving from the transceiver,
>this is not the case. In other words, when the transceiver DC-controls
>the T/R switching in the amplifier, all will be well
The logic would be to kick the EBS on immediately the PTT line goes low
(TX). When the PTT goes high, release the EBS after a short delay
(because transceivers typically output RF for a few ms after the PTT
goes high). While the PTT remains low, it can be left to the RF detector
to control the breaks during the transmission.
That would work, but also requires that the EBS operates very quickly...
so no relay!
>-- provided that the
>amplifier T/R switches faster than the transceiver.
>
The PTT needs to switch to TX *before* the RF appears. Most modern
transceivers do this for CW... but not if you use the relay-switched PTT
option. The timing for SSB VOX is anybody's guess.
Ideally the PTT should also hang low for a few ms *after* the end of a
transmission (or end of each dot /dash in full QSK). I don't believe any
transceiver does this - more typically the PTT goes high and then
there's a 'tail' of RF for a few more milliseconds.
These should all be transceiver problems, but in practice the amp logic
has to be prepared for all kinds of weird timings between the PTT line
and the RF.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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