Jim, As usual, a spot-on summary from Jim. I’m running much the same setup here: K3 directly feeding the input of my homebrew 6m dual 3-500ZG amplifier withMessage 1 of 11 , Mar 5, 2012View SourceJim,As usual, a spot-on summary from Jim. I’m running much the same setup here: K3 directly feeding the input of my homebrew 6m dual 3-500ZG amplifier with vacuum relays in the amp for RF switching, and with all T/R switching controlled by VOX on CW and WSJT modes. I have full QSK on CW. (See note at end with respect to VOX and SSB.)Before making a commitment to this design, however, I wanted to make very certain that the Elecraft guys incorporated a path through the RF Delay circuit in ALL modes potentially controllable by VOX. I was assured that they did. :-)Having been thus reassured, I went ahead and designed the T/R section of the 6m amp to use vacuum RF-rated relays. I then checked the RF delay on the K3, found that 8 ms was both the lowest it would go and the default setting – so I set it there. With the old amp configuration and its bulky frame T/R relay, it had been necessary to set RF Delay to the max (20 ms.) No more!With this configuration, you do lose the first few milliseconds of whatever you are transmitting in whatever mode. But 8 ms isn’t enough of a clip even to be noticed.I emailed Wayne Burdick and asked him why he couldn’t provide for faster RF Delay values, as most vacuum relays are rated at 4 ms or less, and are usually even faster than that – and of course if you use PIN diodes, switching is almost instantaneous. He responded with a most convincing argument for leaving it the way it was designed. Unfortunately, I can’t remember his train of thought. But I believe him. :-)In addition, because the K3 incorporates its own audio isolation connected to its Line In and Line Out ports, all that is required for digital modes is a couple of cheap Radio Shack jumper cables for the audio between the computer and the K3. Another cheap jumper between the K3’s Key Out port and the amp’s Key In port completes the setup. That’s it! It’s been working perfectly since I built it that way several years ago. Never a complaint from anybody – though I don’t tend to use VOX on SSB. The K3’s VOX is outstanding (the best I’ve ever used), but there’s just something about VOX I don’t like on that mode unless SSB contesting – then it’s potentially useful. Oh well, just me. Love it on QSK CW and for digital modes.Bill W5WVO
On 3/1/2012 7:09 AM, Lance Collister, W7GJ wrote:
> This built in sequencing is a very useful feature and ESSENTIAL
> if you are running an amplifier and don't want to burn up coaxial
> relays and/or amplifiers (especially solid state amps).
That depends on the transceiver. The better rigs have a built-in delay,
typically 8-12 msec, between the time they assert TX to the power amp
and the time that they begin generating RF. And in the best of those,
the delay can be adjusted via a menu. This is, for example, true of the
FT1000MP and its family, and the Elecraft K3. I remember that my TS850s
had a built-in delay, not adjustable, on the order of 10 msec.
> With an amplifier, it is never good practice to operate the station
> in VOX mode, which is what some cheaper interfaces require.
Again, that depends on the rig, and to some extent, the amp. I've never
used anything but VOX, driving Ten Tec Titan 425 and Hercules II amps,
and also the Elecraft KPA500. I use this setup for CW, SSB, RTTY, and
WSJT modes. In all cases, I've always switched the amp into TX from the
transceiver's TX control line. There are more than 120,000 Qs in my log
since 2004 using this setup, and the vacuum relays in my amps typically
last for 5 years (a lot less when I used full QSK on CW). In a RTTY
contest a few weeks ago I made 1,600 Qs. Ten Tec amps are designed so
that they sense the position of the T/R relay, and will not go into
transmit if the T/R relay has not pulled in, nor will they let the relay
drop until RF drive is no longer present. The Ten Tec amps I own are all
30 year old designs. Perhaps I'm spoiled -- I've never used junk amps
(like the mighty fine junk) or cheap transceivers to drive them.
I do strongly agree that hot-switching a T/R relay is a recipe for
disaster, but with decent gear there's nothing wrong with letting VOX
key the transceiver.
73, Jim K9YC