Re: [Amps] QSK on the cheap

["Carl" <km1h@jeremy.mv.com>]

Yes the proper reed relays can work fine at 1200W for QSK but you have to 
characterise them. The Johnson Thunderbolt II  used them which became the 
Henry Tempo 2000. However that was with 3-400Z triodes.
For a bit over 2 decades I ran a NCL-2000 with its very touchy 8122 tetrodes 
at 1200W out on CW running VOX CW in  heavy DXing and contesting. That was 
with its original small open frame antenna relay and never a problem from 
hot switching and/or blowing screens. That relay also controlled the screen 
supply center tap. The amp is still in use using VOX SSB as part of the 
vintage SSB station with its 1964 relay and 67 date code tubes.
With your amp and true QSK Id employ a multi channel scope and fully test 
all phases of the switching. You may need to use a sequencer since the 
exciter is also part of the system. These are commonly used by VHF and above 
operators who have multiple steps between generating RF and getting it to 
the antenna and sometimes around a tower mounted preamp.
Carl
KM1H




----- Original Message ----- 
From: "Bob" <bobt@iafrica.com>
To: <amps@contesting.com>
Sent: Saturday, October 06, 2012 12:05 PM
Subject: [Amps] QSK on the cheap


> Hi to the gang.
> I have been reading the mail about doing QSK by switching the screen 
> voltage
> with interest. I have a home brew 1200 Watt GU43 based amp which has had a
> hard life.before it got to me. It has several types of switching relays
> mixed in and stabilizer tubes to control the screen supply and it looks as
> if a lot of the bits came from an R140 setup(It was built by a Russian I
> think). It has a 3kV anode supply @ ~ 1 amp
> My question of Carl and the rest of the guru's is, that can reed relays be
> used for the input and output switching if switching the screen supply 
> fast
> shuts the amp RF output down quickly.
> I have a card of the Crydom frd 13501 RF rated reed relays with 6 amp 1A
> contacts and 8500 volts peak voltage rating (these are normally used for 
> QRO
> auto tuners). I'm thinking of two in parallel to switch the output (one 
> will
> handle 1kW maybe but only if the SWR is good) and one or two in series to
> switch the input. The spec on the relays is 3 mS on time (including 
> bounce)
> and 1 mS off.
>
> The issue I think is that these relays will carry the current and do the
> isolation but won't be able to do any hot switching so "killing" the 
> output
> before switching is critical. If I use one of the power mosfet screen
> regulators available, and switch it quickly, how quickly will the RF shut
> off ? In other words I can easily get the screen volts down low (say from
> 350 volts down to < 20 volts ) in under 100 uS, but will the RF be gone
> before the 1 mS  delay of the relay falling out has gone by. If this is
> possible then a very fast simple QSK amp circuit is do-able.
>
> Comments and flames please
>
> Regards Bob ZS6BXI
>
>
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