done that. My U2 usually drives an outboard 20W 475 kHz PA in WSPR mode,
and at the moment is driving a 180W PA on 137.500.
leave the U2 PA in circuit, and feed it into a 50 attenuator to reduce the
drive to the outboard PA. (My outboard PA has it's own driver, and only needs
about 10dBm drive)
can buy a 630m or 2200m plug in LPF from Hans Summers, or make your own, as
I did. For a long time, the only LPF I had for my U2 was a 20m filter. I left
this in circuit, and relied on the filter in the PA to remove any harmonics. The
20m LPF is completely transparent at 630m.
also rigged Send-Receive switching for my U2. The U2 processor keys a BS170,
which is in series with the output BS170. You can use the same keying voltage to
drive another BS170 that controls a Send-Receive or antenna changeover relay. It
also switches the PA from "Standby" to "Send". The relay switches over with each
of the QRSS symbols, which gets a bit noisy, but is fine for
added a slight delay to my outboard PA so that the PA driver doesn't turn on for
a few milliseconds after it is switched to "Send" by the U2. This removes the
risk of the Send-Receive relay hot switching when there is RF present.
630m PA uses a single IRF540 in Class E, running from 13.6VDC, and is loosely
based on one of Roger's designs. (GW3UEP). Efficiency and output could be
improved by running it from 24VDC, but it's fine when running WSPR2 at 25W
There's a couple of pics of the U2 on my QRZ.com
Dave G3WCB IO91RM.
If anyone has successfully used a Ultimate 2 or 3 to drive an
external 500kHz class D PA direct from the DDS outpins or via the BS170
(say into a TC4426 driving a IRF640) Id like to hear about any pit
falls/expereinces before I let smoke escape :-)
One lonely decode last night WG2XXM 475.712 -31
@1006Z on L400B/25ft @ 4600Kms - sure was a noisy night. Nothing
decoded non the NNE/SSW 1/4Y resonant loop - sometimes small is
Laurence KL7L WE2XPQ