You'd have to install a hard-wired method of keying the relay.  The "key" jack is for a code key, not T-R keying.  You could re-wire that jack so it closes the coil connection to the T-R relay, and make that your "amp keying" line.

You'd also have to replace the input and output tank components, which are all set up for 10m (and 11m!) and won't cover 20m at all -- you need "bigger" parts to do that.  And you'd want to re-wire the "VFO" input jack to the cathode tank of the PA tubes so you can use a transmitter to directly drive the PA stage.  The way it's factory wired, if you transmit into the jack labeled "VFO," you're likely to blow up everything that follows that, since it's a low-level input.

What power level do you have it "set" to?

Plan to be at multiop station N6VI, Frazier Peak, CA (DM04) elevation 8012' asl; 6m is 500W to 6L at 55' above ground.

The KX-3 is an amazing piece of equipment but I wouldn't recommend it as a "first HF rig" for anyone.

The digital operation attributes aren't that handy for portable operation, where you still need a keyboard; and popular digital modes are 100% duty cycle modes that will drain the batteries quickly (compared with CW or SSB, which are low duty-cycle modes).  In bright sunlight, I can still work CW or SSB since I don't have to "look" at anything, whereas you may need a shaded area or one protected from direct sunlight to be able to read the screen.

I'd recommend cutting your teeth on something else, used at home; make a lot of contacts, figure out what you really like, and then go for something more appropriate for portable work -- which may indeed be a KX-3!

When you operate some from home, I think you'll find as all of us did that the operator, antenna system and propagation do all the work and the "rig" is pretty far down the list of important stuff.  With a $1000 budget, I'd normally spend $750 on antennas and what's left on station equipment, because that will generally result in the most contacts, the easiest.   Learning operating practices and about HF propagation is important, too, and doesn't cost anything but time.

It makes sense only at higher frequencies.  Obviously 3.9 pF across a 50 Ohm line at 3.5 MHz is doing absolutely nothing.  But as the stripline and its connective wiring becomes less perfect (at higher frequencies) it can require some reactive compensation to try to pull it back to 50 Ohms Zo.  I'd leave them there.

It's odd the problem would occur only on 20m, considering its design; but anyway, sounds like a bad connection at the 20m loading coil, or the "L20" tuning spokes (4 of them), or the coil strap at the 20m coil connection.

I'd take it down, carefully inspect, make sure all connections (hardware) is very tight, including the tubing clamps.  I hope you used an anti-oxide compound like NoAlOx or similar during assembly, otherwise intermittencies can occur where the tubing sections join and you can't visually see that.

I built a DX-60A back in the mid-60s and used it on AM a fair amount for a while; the D-104 seems a good choice (it's what I used, also) because it has a light of "brightness" to it (high frequency response), and the DX-60 unmodified is a little bit "dull" sounding (too many lows) -- the D-104 "brightens it up" a bit and sounds pretty good.

Of course, there are modifications that can be made to almost anything including the DX-60 to make it sound a bit better than "stock."

Why not try the D-104 and see how it works?  The AM guys who are rabid AM enthusiasts (quite a lot of them!) are all into "modulation," and will give you some critical reports.

I've been active on VHF weak signal modes for about 45 years and surely agree that when the 6m band is "open," a lot can be done with very little.

However, for the other 95% of the time when the band is not open, small antennas -- especially vertical ones -- typically yield a lot of "nothing heard here."

An interesting and somewhat eye-opening test which I've demonstrated hundreds of times is this one I run at home:

6m loop at 30 feet; Cushcraft Ringo vertical antenna at 30 feet; 7 element horizontally polarized beam on telescoping tower whose height I can change from about 28 feet to 65 feet by telescoping or nesting the tower.  All low-loss feedlines brought to a 4-position antenna switch, so antenna changes can be made in less than a second.

Tune around with the beam at 65 feet, and with various beam headings, I can hear six 6m beacons 24 hours a day, regardless of propagation.  (When the band's open, of course, there are many more in various directions depending on conditions at the moment.)  But those six vary in distance from about 60 miles to about 300 miles, and they're always readable.

Now, switch to the loop:  Two beacons can be heard, weakly, not nearly as well as with the beam.

Now, switch to the vertical:  One beacon can be heard, very weakly.

Works this way every single day.

The antennas all "work," but the beam's advantage is so obvious that many are astounded.

So, on an average day with no special conditions (no sporadic-E, no any kind of ionospheric propagation), just tropo, I can "work" stations in about a 300 mile radius (about 283,000 square mile area).  On the same average day, I can "work" stations in about a 60 mile radius using the loop (about 11,000 square mile area).  The difference in the number of stations that might be workable is a ratio of 283/11, or nearly 26 to one, in favor of the beam.

That's why if 6m is "pretty quiet" using the beam, it can sound "abolustely dead" with a loop.

 

I think whoever told you that didn't know much about antennas or propagation.  Half the HF ham radio world uses vertical polarization, and it's predominant on the lower bands, especially 80m and 160m, where verticals are used extensively for transmitting.  The statement you quote begins to "sort of" apply to cross-polarization for direct-wave or tropospheric paths, as we mostly use on VHF and UHF (and SHF, and EHF, etc).  But almost all our HF work is ionospheric; the signal is refracted by the ionosphere and returns with different polarization, largely circular, and changing.

The design is not by CCI, it's by Motorola (Engineering Bulletin #63).

However, after reading your writeup, I'd strongly recommend you package the amplifier properly and that alone may make the feedback path causing the oscillation to completely disappear.

I've built EB63 amps before and never had any problems with them; however, they're built inside aluminum enclosures with short leads to the input and output coaxial receptacles, bypassing and ferrites on the DC power (+) lead, panel-mounted fuseholder and 20A fuse, etc.  The aluminum enclosure (with tightly fitting cover, held in place by 8 screws) helps -- a lot.

If you don't have an enclosure, I'm guessing you have a heatsink fitted to the two output transistors.  Since the mounting surface for this amp board is very close to the backside of the PC board, make sure not a single component lead protruding through the bottom of the board can come in contact with (or arc to!) the heatsink (or chassis/enclosure when you start using one).

Although the highest DC voltage in the amplifier is 14V or so, a sharply pointed component lead protruding through the board can possibly have much higher RF voltage on it than that and create an intermittent arc to anything grounded that's close by.

Birds may even try to land (and successfully) on the Hexbeam; if not the wires themselves, then the spreaders.

I've found a large (full sized) model Owl installed atop the mast supporting beams seems to discourage them.  I don't know why, but it works pretty well.  So well that some ham radio stores sell the model Owls which are made for mast mounting.

If you power it up with all covers installed, and plugged into a 3-wire grounded outlet as it should be, it can't bite you.  The switch is well insulated and there's nothing exposed that is dangerous.  Worst that can happen is blown fuses or circuit breakers.

Neither the choke nor the HV filter cap(s), or the resonating cap for the choke, are easy to test with simple equipment.  Often they would test "just fine" with low voltage DC applied from a multi-meter, but can fail when slammed with over 3000V, as will happen in operation.  Also no reason to expect they're bad.

Tuning is a reiterative process, and assuming the amp's components are all intact, it "sounds" to me like you're simply underloading it.

I've had an AL-80B for over 12 years and for it to run full power, the Ip is always well over 400mA; it can get up to 600mA, and for intermittent service, that's fine.  I keep Ig down to about 150mA max using the ALC.

But the first peak you hit while tuning is never the "final" peak; after the first peak in output is achieved (using both controls), increase LOAD more (clockwise) and repeak PLATE.  Then increase LOAD more, again, and repeak PLATE, etc.

The "final" adjustment should achieve 150mA Ig and at least 500mA Ip, and when both controls are re-peaked for MAX, turn the LOAD control slightly more clockwise to reduce Ig about 10%, and then leave it that way.

Regardless of 3-500Z tube ratings, I've checked linearity and IM3, 5, 7 and 9 with the amp loaded to almost 600mA Ip (takes about 75-85 drive power, usually) and Ig = 150mA, and it's still linear (delta input vs. delta output remains the same as at lower power levels, with about 10-11 dB uniform gain; it starts to flatten out beginning at about 800W carrier power output, and is in compression at 1kW output) and worst case IM3 is about -33 dB.  IM5, 7 and 9 are lower, in the -39 to -45 dB range.

Might be something else, but first guess is you're just not loading it heavily enough.

Is the amp generating any measurable RF output power when it's doing this, or is the "hash" just something you observe on a receiver?

That's an oldie with tube rectifiers and such, but they were good amps and well made.

After a thorough cleaning and inspection, I'd request the assistance of a local ham who is highly experienced with amplifiers (best would be someone who has built amplifiers himself) to help you bring it to life.

If the 3-400Zs were stored unused for a long time, pretty good chance they'll be "bad;" although we get lucky sometimes, and maybe they won't be.  3-400Zs haven't been made in over 40 years, but 3-500Zs can be used with small modifications.

That model had a HV choke and oil-filled capacitors that can and sometimes do go bad with age; a failure in operation can be quite exciting, which is why I recommended you do the first power-up with some assistance if possible.