[Amps] QSK on the cheap AND Parasitic

[Jim W7RY]

The biggest issue for QSK is the exciter. Does it produce RF before the 
relay line is switched? (The Icom IC-7000 is one offender).
How long is the relay line active before the rig starts to put our RF AND if 
the rig stops RF before the relay control line drops out.
What kind of an amplifier keying relay does the rig have?

The only way to know is measure it.  If it's a Ten-Tec Omni-5, it has a huge 
slow clunky amplifier relay.  Some rigs, like the K3 (so I'm told) and the 
Icom IC-751A (which I know)  have an adjustment to make an allotment for 
this timing. Several Icoms like the IC-7600, IC-751, IC-751A have very fast 
(<1ms) reed relays for the amplifier keying line. The IC-7200 has a slower 
(2ms) mechanical relay. I've bypassed the relay in my IC-7200 and use a 
simple transistor switch from the T8 (transmit 8 volts) line in the radio. 
As you can see from the link below, the IC7200 has 5.5ms before RF after T8 
is active. With the internal relay, its 3.6ms. As you can see, by not using 
the internal relay, it's 2ms faster!  That can be huge in the scheme of 
things.

Here is a link to a page that demonstrates this with scope traces:
http://www.w0qe.com/amplifier_timing.html

Once you understand this, you can make designs using relays that will NOT 
hot switch which is the goal! No hot switching, no relay arcing.

***********************************
Oh.......... And while we're on the subject... I think that the major cause 
of arcing in amplifiers is bad relay timing and RF BEFORE the amplifier is 
ready for it!
NOT PARASITICS!

Since all of my engineering and design work on QSK circuitry, and hours and 
hours of testing this is the conclusion that I have made. After I have 
installed my QSK circuitry and relays into SB-220s, TL-922s, SB-200, and an 
Ameritron AL-82. I also understand the timing on the exciter PTT and relay 
circuits so I know how to measure an exciter to see if it's fast enough.

My amplifiers simply DON’T ARC ANYMORE!

73
Jim W7RY



-----Original Message----- 
From: Bob
Sent: Saturday, October 06, 2012 9:05 AM
To: amps at contesting.com
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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