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Author Topic: Can I xmitt on 20m on this 80, 40, 15 meter rig?  (Read 5819 times)
KC9KEP
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« on: September 08, 2010, 11:44:23 AM »

Hello all,

A ways back, I built an ARRL "Three-Band One-Tube Novice Transmitter" (Dec 1957 QST)

A link to my project is here: http://www.bignick.net/Morgan_Radio/Radio_11.htm

I'd like to operate this unit on 20 meters.

The transmitter is specified to operate on 80, 40 and 15 meters.

Why do you suppose the author skipped past 20m and included frequencies above and below 20m?

Could I get a 40m crystal to resonate at 20m?  Would I need an additional tap to the PI network
in order to achive this resonance?

Thank you!

--KC9KEP AKA Tom Nickel
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WB6BYU
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« Reply #1 on: September 08, 2010, 11:59:05 AM »

It doesn't include 20m because that band wasn't available to novices.

The rig already doubles on 15m (the schematic shows a 10.5 MHz crystal for 15m.)  It probably would work
with a 7 MHz crystal and tripling as well, but then there is more danger of spurious outputs at 14 and/or
28 MHz if the output tank isn't tuned properly.

Adding an extra tap on the output tank coil should give you 20m output from a 40m crystal.  A couple of
turns onto the larger output coil should get you pretty close, and you can experiment from there for
best results.

A spectrum analyzer would be handy for tune-up, as you want to find the point that gives best results
on 20m with minimum outputs on 40 or 15m - mark the right place on the plate tuning control, as it
probably has enough range to tune on at least one other band.  However, if you put a series-tuned
20m circuit between your SWR meter and dummy load, you'll see a lot more reflected power when the
rig isn't tuned for the right band, and that probably is adequate.  (Running the output through a tuner
should do the trick as well, but make sure that the tuner is adjusted first using a rig with low harmonic
content.  If you have stray RF on other bands you'll never get the reflected power down to zero.)
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WB6THE
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« Reply #2 on: September 08, 2010, 04:01:40 PM »

Hi, Tom...

20M operation should be possible.
You'd need to place a tap on the pi-network coil
someplace between the 40M and the 15M tap.

A 40M xtal operating at one-half the desired 20M
frequency should work. You would be tuning the
output circuit to the second harmonic of your 40M
xtal. It isn't that the 40M xtal is resonating on 20M,
just that you are tuning on its 2nd harmonic. You
could possibly use harmonics of xtals cut for other
frequencies, too. Power output might be slightly
lower if you tune the harmonics.

You would need some means of checking that the transmitter
is not radiating excessive energy on unintended harmonics or
sub-harmonics of your xtal.

Alan,
WB6THE
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W8JI
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« Reply #3 on: September 08, 2010, 06:44:39 PM »

Hello all,

A ways back, I built an ARRL "Three-Band One-Tube Novice Transmitter" (Dec 1957 QST)

A link to my project is here: http://www.bignick.net/Morgan_Radio/Radio_11.htm

I'd like to operate this unit on 20 meters.

The transmitter is specified to operate on 80, 40 and 15 meters.

Why do you suppose the author skipped past 20m and included frequencies above and below 20m?

Could I get a 40m crystal to resonate at 20m?  Would I need an additional tap to the PI network
in order to achive this resonance?

Thank you!

--KC9KEP AKA Tom Nickel

I think you are going to have problems Tom.

20 meter crystals are rare, and I would NEVER multiply in a single stage rig. I would never multiply to 15 meters using the 10 MHz rocks.

I would not multiply because the fundamental frequency will be so much stronger in the tube than the harmonics, and the output tank is a low-pass. You better be darned careful with that thing on 15 meters.

The reason they don't triple is efficiency. The third harmonic energy is even less, so you would cook tubes. Also 14 is closer to 21, and you would likely have a spurious signal on 20 meters if you tripled a 7 MHz crystal.

If that rig were mine, I'd add a 6AG7, 6F6, 6K6, or similar beam tetrode or pentode oscillator. Then you could safely double or triple in the oscillator.

My actual first novice rig was a 6V6 PA stage like yours, just lower power. I converted it to a 6L6, and then moved the 6V6 to an oscillator role.  Later I converted the 6L6 to an 807.

I think it is an extremely bad idea to multiply in a single stage, especially with a pi network on the output.

73 Tom
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KC9KEP
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« Reply #4 on: September 09, 2010, 05:54:22 AM »

Thank you for your replies, gentlemen :-)

It all makes sense.  I can see what I'd be up against.

That's one of the reasons I love homebrewing .. One learns stuff!

73

--Tom Nickel AKA KC9KEP
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W8JI
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« Reply #5 on: September 09, 2010, 08:05:38 AM »

Thank you for your replies, gentlemen :-)

It all makes sense.  I can see what I'd be up against.

That's one of the reasons I love homebrewing .. One learns stuff!

73

--Tom Nickel AKA KC9KEP

Throw a 6AG7, 6F6, or something similar in there as an oscillator and you would have a safe rig to use on 160-15 meters. It might even make 10 meters.

Look at what Globe Scout did:

http://www.w8ji.com/globe_scout_modifications.htm

73 Tom
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KQ6Q
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« Reply #6 on: September 11, 2010, 08:00:06 PM »

Doubling and tripling was routinely done. The Heath VF1 VFO had two outputs - 160m and 40m  -the dial was calibrated for 160-80-40 for the 160m output, and for 40-20-15-10 for 7 mhz output. The simple rigs - Knight T-50 (mine) or Heath DX 20, DX35, etc doubled from 40 to 20, and tripled to 15m. On 10m the oscillator buffer doubled 7 to 14 mhz, and the final doubled 14 mhz to 28mhz.  There were harmonic issues  -I met a terrific Elmer when he sent me an OO notice that he was hearing my 40 meter CW signal on 15 phone! But he was less than half a mile away.
So, yes you can double or triple, but be careful of tuning and reduce unwanted harmonics. Requirements for spurious outputs are a lot tighter than they used to be.

Fred KQ6Q (KN6VVD back in 1957-58)
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N2EY
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« Reply #7 on: September 12, 2010, 04:16:47 AM »

As noted elsewhere, the rig skips 20 because it wasn't a Novice band. (In 1957, Novices had parts of 80, 40, 15 and 2 meters only, with crystal control and no more than 75 watts DC *input*.)

In those days there were many designs for Novice transmitters in amateur publications. Simple one-stage oscillator transmitters were often described because:

1) they gave the most watts-per-dollar
2) they had the lowest parts count
3) they did not need neutralization or other stability measures
4) they had fairly simple tuneup

But such rigs often had problems:

1) Some stressed the crystal too much
2) Some chirped, clicked, or both
3) Some had excessive harmonics or sub-harmonics
4) None could be easily converted to VFO and/or 'phone operation when the General license was earned.
5) Some, including this rig, used the grid-plate oscillator circuit with beam-power tubes which have the BF plates connected to the cathode. Which can cause all kinds of odd effects due to unwanted coupling. (the grid-plate oscillator circuit is best implemented using tubes where the suppressor grid or beam-forming plates can be grounded for RF, which requires that they not be connected to the cathode).

Still, a lot of hams got started with such rigs and learned a lot about radio from them. Not many survive because they were usually taken apart for the components once the Novice upgraded.

You can modify that rig to do 20 and even some of the WARC bands. Doubling in a rig like that isn't impossible, or even difficult, but you need more selectivity in the output than just the pi-net, to keep the fundamental out. A classic split-coil Transmatch is one solution, or you could use a high-pass filter with cutoff below 14 MHz.

The 6DQ6A has a plate dissipation rating of 18  watts, so the input should be determined accordingly. The design only runs 30 watts input, which gives a lot of room for low efficiency and mistuning without tube damage.

If you can get xtals in the 10.1 to 10.15 MHz range, the rig could do a good job on 30 meters. Conversion to 160 is also possible, but would require a redesign of the pi-net with bigger L and C.

I would not modify the rig to add another stage. Better to build a new MOPA from the ground up.

20 meter crystal were made; I have several from before WW2 in round holders, but haven't tried them out. Unfortunately they're all in the 'phone band!

73 de Jim, N2EY
« Last Edit: September 12, 2010, 04:22:36 AM by James Miccolis » Logged
W8JI
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« Reply #8 on: September 18, 2010, 06:30:02 PM »

You guys must not have looked at the circuit Tom posted.

It is a single stage transmitter using a Clapp oscillator. The Clapp circuit always oscillates at the fundamental series resonant point of the crystal. It is technically IMPOSSIBLE to come anywhere close to meeting safe spurious levels when doubling in a transmitter like that.

The stage would have around several dB more fundamental power than 2nd harmonic power. It's a low-pass pi network, and so it has minimal fundamental frequency rejection. It will also have lower efficiency on the second harmonic because most energy in the anode circuit is at the crystal fundamental.

If you read the text, they did not double with the crystal. They tripled. There is actually more third harmonic energy in a system like that then 2nd harmonic, although I bet it still was not very clean.

Tom, I would not modify that transmitter to multiply in the single stage. It just isn't a good idea. Run it on 80 and 40 and if you really want to use 20 or higher just add an oscillator stage to your chassis. Then you can multiply in the plate of the oscillator and have a much better working transmitter. It really is not a good idea to multiply in the stage driving the antenna when that stage is a Clapp oscillator.

73 Tom






Tom really needs to add an oscillator circuit to drive the PA tube of he is going to multiply a crystal. I'm sure in 1950




 

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N2EY
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« Reply #9 on: September 18, 2010, 08:55:33 PM »

You guys must not have looked at the circuit Tom posted.

I did.

It is a single stage transmitter using a Clapp oscillator. The Clapp circuit always oscillates at the fundamental series resonant point of the crystal. It is technically IMPOSSIBLE to come anywhere close to meeting safe spurious levels when doubling in a transmitter like that.

The circuit is more commonly known as the "grid-plate" oscillator.

As to the ability to get an acceptable signal from it, I pointed out that the pi-net, by itself, is inadequate. I pointed out that additional external filtering would be needed to insure that the unwanted harmonics are sufficiently attenuated.

I also mentioned that 14 MHz fundamental-mode transmitting crystals were made, to avoid the problem.

The stage would have around several dB more fundamental power than 2nd harmonic power. It's a low-pass pi network, and so it has minimal fundamental frequency rejection. It will also have lower efficiency on the second harmonic because most energy in the anode circuit is at the crystal fundamental..

Which was also pointed out.

If you read the text, they did not double with the crystal. They tripled. There is actually more third harmonic energy in a system like that then 2nd harmonic, although I bet it still was not very clean.

The schematic shows 10.5 MHz crystals being used on 15.


Tom, I would not modify that transmitter to multiply in the single stage. It just isn't a good idea. Run it on 80 and 40 and if you really want to use 20 or higher just add an oscillator stage to your chassis. Then you can multiply in the plate of the oscillator and have a much better working transmitter. It really is not a good idea to multiply in the stage driving the antenna when that stage is a Clapp oscillator.

Adding an oscillator stage isn't simple, it results in a redesign of the transmitter to the point where it becomes a whole different animal. Might as well build a MOPA from the ground up.

IMHO the design would be better all around if a different oscillator circuit were used. A simple straight-through screen-grid crystal oscillator would be simpler and avoid those problems. The only reason to use ECO-type circuits such as the grid-plate is to permit multiplication in the oscillator stage - which isn't a good idea in this case.

Why the original design insisted on including 15 meters is a puzzle to me. The harmonic problem is obvious, and the use of a tube with the beam-forming plates connected to the cathode just makes it more troublesome. In those days the frequencies available to Novices (the prime audience for such an article) were not harmonically related, so being able to use 80 meter xtals on 40 wasn't much of an advantage. Better to make it an 80-40 rig and be done with it.

It would be really interesting to run some of these classics through modern lab tests and see how good or bad they really are, in various ways.

73 de Jim, N2EY
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W8JI
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« Reply #10 on: September 23, 2010, 03:07:39 PM »

McCoy wrote many articles, and was a big asset to ham radio, but a few of the articles were not quite such sound ideas technically.

The problem with doubling or tripling in the plate circuit of a stage feeding an antenna is that the fundamental and all of the harmonics of the fundamental feed through. This pretty much means to be clean, the rig needs a *bandpass* filter. A second issue is efficiency.

I'd go along with the suggestion of a 20 meter crystal to work 20, but an oscillator stage is a very simple change that would make everything better. It actually is about as simple to add an oscillator as it is to built a couple bandpass filters.

Of course Tom is the one in the driver's seat, but it is interesting to learn about how old rigs work. The stuff we got away with amazes me.  :-)



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W5DC
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« Reply #11 on: September 23, 2010, 03:25:06 PM »

Of course Tom is the one in the driver's seat, but it is interesting to learn about how old rigs work. The stuff we got away with amazes me.  :-)  - W8JI

My novice transmitter had two or three stages ending with an 826 uhf triode.  It originally had a twisted wire neutralizing capacitor that allowed it to work on 80 except for one 40 meter contact before the 826 turned back into an oscillator.  The day that I passed my General Class exam at the FCC office in New Orleans, I bought a genuine, commercially made, neutralizing capacitor and moved to 40 and above. 

Dunc, W5DC
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