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80 meter "PW" amplifier




 
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k7mdo
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« on: June 01, 2017, 08:10:20 PM »

Now that I have a fully functioning "PW" AM transmitter it comes to mind I would like to have MORE WATTS!

The little transmitter (two 6AQ5's series modulating one 6AQ5) only puts out between 2 and 4 watts of decent RF but with the sun no longer cooperating with spots, I need some extra oomph.

I wonder if anyone has a schematic for a single band (80 meter) single tube amplifier that would boost my "PW" to about 10-15 watts?  

I would like as simple a circuit as can be imagined... some of the tubes that come to mind are the 2E26 to 4-400A....  I have plenty of transformers and a big parts warehouse just waiting to be used.

I like to build stuff so things like metal work are no issue.... any ideas?

73, Tom
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« Reply #1 on: June 01, 2017, 08:47:27 PM »

With that amount of drive, a grid driven tetrode or pentode is definitely the way to go, the first thought that comes to my mind is take a look at the schematic for a Central Electronics 600L, it was a 1950s amp using a single 813 with broadbanded input and output circuits that required no tuning. Since you're only interested in 80 meters, you could pretty much use any input or output circuit and just use the schematic as guide to follow on other components, voltages, etc. The 600L was made to go with CE's phasing type SSB exciters which were low power, so the amp only required 10 watts SSB to drive it, or 2 watts of carrier on AM.

http://bama.edebris.com/manuals/ce/600L/
The first two JPEGs are the schematic.
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« Reply #2 on: June 01, 2017, 09:37:58 PM »



I have a Retro75 80m transceiver hooked to a CE 600L amplifier. The drive at ~ 2 watts was too high, and i needed to pad the driver with a 50 ohm load resistor, and then vary the voltage from 10-13v to get the drive right. The 600L was able to fully modulate a 100 watt carrier, but the 813 was blushing. Something like 70 watts is better with room for some positive peaks above 100%.

The 600L is about the biggest and heaviest linear amplifier made, about 3 pounds per watt!  Grin

Jim
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K1JJ
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« Reply #3 on: June 01, 2017, 10:24:33 PM »

Hi Tom,

Yes, I agree that a grid-driven 813 would make an excellent linear final for the 6AQ5 PW rig.  A pair of 813s would have "AM" written all over it... :-)

To simplify, you could use a string of series connected diodes in the cathode to properly bias it into class AB1. Use a switch to short them for desired idle current.   The screen supply should draw normal current to be most linear, thus will require regulation. A simple string of zeners and power resistor circuit will do.   The 50 watt, 100V, 200V volt zeners, etc., are available.

Try to draw none, or as little grid current as possible to keep it cleanest. AB1 means no grid current, of course. Again, recommended screen current is best.

I would use a tuned input circuit because you are losing the cleanliness of grounded grid IMD NFB.  I believe a tuned grid will help IMD.
A 5-turn link wound around a tuned L/C input coil will do. More elaborate, use a reverse pi-network, 50 ohms to 2K for full adjustment and 1:1 on every band.

Be sure to load it heavily (less C2 loading - more unmeshed) to add more signal cleanliness.

End it with a stiff HV supply and you are golden. I know guys who have run 4KV on 813 linear amps, but 3KV might be a reasonable limit here.

Add some air or even a lantern chimney (search the web) to blow some air around the envelope. Air makes a HUGE difference when running AM.

Tom, K1JJ
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« Reply #4 on: June 01, 2017, 10:38:20 PM »

Wow, something in what I mentioned seems to have been pushed to the limit....  I kind of stuck the reference to a 4-400 in as overkill.... 

An 813 amp may be more of a burden on my parts bin than I hoped.

I think for the local rag chew I need only 15 watts carrier or so....

Is there any hope for something in that range.

The "essence" of QRP gets lost in the amplifier but if I keep the amplifier small too it will still represent the heart of QRP.

I have found a few transistor amps but had hoped to remain in the filament era for this one.

73, Tom

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K1JJ
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« Reply #5 on: June 01, 2017, 10:44:50 PM »

Tom,

In that case then maybe a single or pair of 6146s will work for you.

In general, take the tube plate dissipation and divide by 2 to get the AM maximum carrier level.  So, an 813 is 125 watts diss  /2 = 62 watts AM carrier.

A 6146B = 35 watts/2 = 18 watts of carrier AM.  A pair would be good insurance to run cooler.

Use the same ideas I wrote above for the 813 linear and the 6146(s) will work FB.

BTW, another excellent tube is the 4D32. Look it up. I use four in my AM PDM rig. A single one will really do you right!


T
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« Reply #6 on: June 01, 2017, 11:09:01 PM »

The 4D32 interests me as I have a small bunch of those.  I will look into starting point circuits for those. 

Tomorrow is the Seaside ham convention and ham fair here in Oregon.  Will look for parts!

Tom
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k7mdo
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« Reply #7 on: June 11, 2017, 06:12:14 AM »

A couple of weeks using the transmitter on 3885 has given me a new appreciation for QRP enthusiasts persistence!

A few things found:

The crystals I have seemed to get a touch warm after several minutes of use.  I solved this for now with a 27 ohm resistor in series with the crystal.

The tank tuning seems overly sensitive to slight antenna SWR problems.  But, with only 4-5 watts it is expected.

The power output is substantially higher with the full size FT-243 crystals as opposed to the smaller HC style, not sure why.

I wound a tank coil with a tap for 75 meters and enough turns to tune 160 but the band is not cooperating!

The REA monitor shows substantially higher positive % modulation possible when the cathode current is reduced.  My max current is about 45 ma but there the +/- % is equal and increasing the audio gain only provides distortion.  If I reduce the power output (by reducing the cathode current) I can get 150% positive or so easily without bumping the negative limits.
 
This turned out to be a substantially bigger build project than expected just to make a few contacts within 25 miles!

Tom

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K1JJ
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« Reply #8 on: June 11, 2017, 11:28:59 AM »

A couple of weeks using the transmitter on 3885 has given me a new appreciation for QRP enthusiasts persistence!

A few things found:

The crystals I have seemed to get a touch warm after several minutes of use.  I solved this for now with a 27 ohm resistor in series with the crystal.

The tank tuning seems overly sensitive to slight antenna SWR problems.  But, with only 4-5 watts it is expected.

The power output is substantially higher with the full size FT-243 crystals as opposed to the smaller HC style, not sure why.

I wound a tank coil with a tap for 75 meters and enough turns to tune 160 but the band is not cooperating!

The REA monitor shows substantially higher positive % modulation possible when the cathode current is reduced.  My max current is about 45 ma but there the +/- % is equal and increasing the audio gain only provides distortion.  If I reduce the power output (by reducing the cathode current) I can get 150% positive or so easily without bumping the negative limits.
 
This turned out to be a substantially bigger build project than expected just to make a few contacts within 25 miles!

Tom

Hi Tom,

Being a one tube RF power oscillator, the simplest transmitter possible, there is a fair amount of grid current required.  I use the full size FT-243 crystals and never had warming, but a lower mass crystal may warm up.

Yes, the lower the carrier plate current via biasing diodes or heavier plate loading , the more robust and higher the audio peaks will be due to headroom available.

The pi-net should match anything, but the antenna impedance and proper pi-network matching will have a big effect on audio peaks and cleanliness. The usual rig that has its own oscillator, buffer, driver, etc can get away with much more variance in the real world.

Experimenting with the 27 ohm resistor in the xtal circuit is a good idea, though it will eventually reduce the power oscillator's grid current until the audio peaks are flat topping due to low grid drive.

Yes, a good PW project often takes as long to build and troubleshoot as a bigger rig. Fine tuning usually takes as long as it took to build the rig in the first place...  Grin

All in all, you did a good job bringing it to life!

T
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« Reply #9 on: June 11, 2017, 01:30:30 PM »

Quote
I wonder if anyone has a schematic for a single band (80 meter) single tube amplifier that would boost my "PW" to about 10-15 watts? 

I would like as simple a circuit as can be imagined... some of the tubes that come to mind are the 2E26 to 4-400A....  I have plenty of transformers and a big parts warehouse just waiting to be used.

I like to build stuff so things like metal work are no issue.... any ideas?


You might try this circuit for starters:

If your QRP transmitter can tune to 120 ohms you're in good shape.

If it can't, you may want to put in a Pi-matching circuit after C1.

Phil - AC0OB

* 4D32 Linear.pdf (54.91 KB - downloaded 63 times.)
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« Reply #10 on: June 11, 2017, 03:14:24 PM »

Now that I have a fully functioning "PW" AM transmitter it comes to mind I would like to have MORE WATTS!

On a philosophical note, there's never enough watts. The last big rig I built is a 400 watt carrier class E rig. Satisfaction lasted for about a week, and I set my sights on a 1kw carrier version. I have most of the parts to build it, but quite frankly, I'm burned out on building for a while. The parts are stored in a box, so when I get the urge they're there.

So, my 2 cents... think big and make your next project double the size of that.

Jon

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« Reply #11 on: June 11, 2017, 04:39:10 PM »

While not the best choice, don't overlook the EFJ Courier.  If used with a pair of 572's one could get 50 to 75 watts easily.

I have one that works perfectly but because I tried to run it at 75 watts carrier with 811a's, it tended to eat the output tubes up.  However, with a carrier of 50 watts it might be an acceptable.

Probably any grid driven EFJ linear would do the trick better
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« Reply #12 on: June 12, 2017, 12:23:35 AM »

A lot of education in that pip squeak little transmitter!

Made a decent S-5 contact today in Astoria Oregon .... that is about 80  miles as the RF flies.  Pretty impressive.

DMOD, thanks for the diagram on the 4D32! Looks promising.

I am going to just use the transmitter for a while as it seems to be settled down enough that it is fully functional for rag chews if I don't run it wide open!  The grid wires start turning red if I press the output above 4 watts so I let it idle along fine at around 3. You can easily see the wires cool quickly at reduced power. Kelvin at work.

I did look up the 6BQ5 as a little higher power substitute but the tube prices on eBay are pretty high. I'll stick with the 6AQ5's for now.

Thanks for the thoughts, Tom
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« Reply #13 on: June 12, 2017, 08:47:24 AM »

The 4D32 idea is a great one.

Note in Class AB2 just 0.45 watts drive is needed for 125watts out. That'll get you a 30 watt carrier all day.

* 4D32 Tube Characteristics.pdf (2859.05 KB - downloaded 29 times.)
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« Reply #14 on: June 14, 2017, 10:43:50 PM »

I will have to poke around to find a filament transformer but otherwise I have all the parts needed for the amplifier....

I am still puzzling on the little transmitter.  It is REALLY sensitive to the antenna SWR....  I think that I have been pushing the single 6AQ5 a little hard because when I finally got around to inserting an antenna tuner in between the antenna and the set I am now easily producing 10 watts.  That is pretty hefty for a 6AQ5.  Sometimes I have clearly seen the grid wires inside the tube glowing red hot. 

I was unable to find enough diodes for the prescribed string Tom used on the original so I still have a 500 ohm resistor in series and I see I am dropping about 25 volts across it, the total cathode voltage being about 35 VDC.  I wonder if I couldn't go to some zener diodes to achieve the appropriate cathode voltage or not?  I am not certain that just a carefully selected value of resistor might not be sufficient?

Now that I have gotten great audio reports from some locals I am only trying to "solidify" the design a bit with an eye toward minimizing the burn out of 6AQ5's!


73, Tom

   
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« Reply #15 on: June 15, 2017, 01:32:00 AM »

Here is a fun idea for someone:

Get a BK Precision 3020 sweep generator.  It goes up to 2 MHz, and can be amplitude modulated 100% without much AF voltage.

Put it on 160, and run it into a pair of push-pull 6V6 in AB1.  Use a link couple series-parallel circuit on the 6V6 grid input to get the required RF voltage.  Something  to try.

:-)
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« Reply #16 on: June 15, 2017, 09:56:53 AM »

I see a Westinghouse MW-2 just a few miles from me for sale.... 3 KW with new tubes... wonder how I could incorporate the PW into that?

Tom
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« Reply #17 on: June 15, 2017, 01:06:48 PM »

I will have to poke around to find a filament transformer but otherwise I have all the parts needed for the amplifier....

I am still puzzling on the little transmitter.  It is REALLY sensitive to the antenna SWR....  I think that I have been pushing the single 6AQ5 a little hard because when I finally got around to inserting an antenna tuner in between the antenna and the set I am now easily producing 10 watts.  That is pretty hefty for a 6AQ5.  Sometimes I have clearly seen the grid wires inside the tube glowing red hot.  

I was unable to find enough diodes for the prescribed string Tom used on the original so I still have a 500 ohm resistor in series and I see I am dropping about 25 volts across it, the total cathode voltage being about 35 VDC.  I wonder if I couldn't go to some zener diodes to achieve the appropriate cathode voltage or not?  I am not certain that just a carefully selected value of resistor might not be sufficient?

Now that I have gotten great audio reports from some locals I am only trying to "solidify" the design a bit with an eye toward minimizing the burn out of 6AQ5's!

73, Tom


Hi Tom,

The built in pi-network should be able to match most antenna swr problems, just as any DX-60, etc rig would.

Being a one-tube power oscillator, the audio peaks will be sensitive to matching. If an external tuner helps, then it may mean the pi-network values need to be increased for a bigger range adjustment capability and /or the tank coil tap may need adjustment. Otherwise, check to see just what R and Z the antenna is presenting using an MFJ-259B ant analyzer. Maybe it's extremely low or high. The pi-net in the rig should match a wide range of R and Z.

Yes, zener diodes will work just as well as power supply diodes.   The resistor in the cathode circuit will also work, however, if there is any plate current/ cathode current shift during modulation, then there will be distortion. I know the current is reasonably stable there because the series modulator is in class A, but I would replace it anyway with some combo of zeners and diodes to insure rock solid bias stability.

The 4D32 linear amp will work well for you. Don't be afraid to load down the input if it is unstable. It will have a lot of gain, so you need to isolate the input from output circuitry with utmost care.  Neutralization may be needed, but not if you are careful with layout.  Be sure the antenna relay does not have the input and output leads "seeing" each other.  I use two relays for the ant switching job to separate them.

T
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« Reply #18 on: June 15, 2017, 01:32:49 PM »

Here is a fun idea for someone:

Get a BK Precision 3020 sweep generator.  It goes up to 2 MHz, and can be amplitude modulated 100% without much AF voltage.

Put it on 160, and run it into a pair of push-pull 6V6 in AB1.  Use a link couple series-parallel circuit on the 6V6 grid input to get the required RF voltage.  Something  to try.

:-)
I have a very stable Panasonic Sig Gen that I've been wanting to do this with for some time. I'd prefer to do the amp solid state. Wonder of anyone makes such an amp - maybe a kit?
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« Reply #19 on: June 19, 2017, 12:23:30 AM »

Today was interesting.  I acquired a 28 vdc zener and replaced the only resistor I had in the diode string with it.  If you remember I didn't have enough diodes available to limit the cathode current to acceptable limits.  Well, once I put in the zener I found I needed to short out some of the regular diodes to get back to about 4 watts rf output.

Then I noticed my cathode current now leaps up and down with modulation signal... before, the current changed a little slugishly with modulation. The REA shows far more achievable modulation with far less microphone gain. The only difference is the replacement of the resistor with the zener.

Not yet sure what to think.

Tom

It all looks good but I haven't had verification from neighborhood hams as to how it sounds. 

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« Reply #20 on: June 19, 2017, 11:30:25 AM »

Today was interesting.  I acquired a 28 vdc zener and replaced the only resistor I had in the diode string with it.  If you remember I didn't have enough diodes available to limit the cathode current to acceptable limits.  Well, once I put in the zener I found I needed to short out some of the regular diodes to get back to about 4 watts rf output.

Then I noticed my cathode current now leaps up and down with modulation signal... before, the current changed a little slugishly with modulation. The REA shows far more achievable modulation with far less microphone gain. The only difference is the replacement of the resistor with the zener.

Not yet sure what to think.
It all looks good but I haven't had verification from neighborhood hams as to how it sounds.  

Tom

Hi Tom,

Good - you are now seeing positive carrier shift under heavy modulation. When you had the resistor in place of the diodes, it was causing the bias to increase with modulation, thus, retarding (and adding distortion to) your natural large positive peaks.

You will now need to re-run some audio tones through and adjust all parameters for the cleanest and highest peaks.

This is a good thing and now gets rid of the shifting bias which is almost always a generator of distortion in transmitters.  Cathode bias via a resistor works only for a class A amplifier because there is no change in plate current to generate bias variation problems. If you run the rig with normal 100% positive peaks, the class A modulator would stay steady and a cathode resistor would work for steady biasing. But because you desire "super-modulation" with positive carrier shift, the zener and diodes, with their constant voltage drop regardless of current change, are the best choice for stable biasing in the cathode.

T



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« Reply #21 on: June 19, 2017, 07:36:25 PM »

Tom, YES, things are definitely better now...  I was able to record and listen to the demodulated audio today and watched the envelope on the IC-7600... the set is VERY good now and tonight I will try to drum up some hams locally to see that they are hearing.

As it turns out I was able to remove 20 of the power supply diodes and leave on the 28 volt zener and the twelve diodes on my rotary power switch.... works perfect, I can go from 1 watt to 8 watts with the selection of the number of diodes of the power switch.....

And, now the microphone gain can be turned way down while easily achieving 150% positive modulation while peaking at less than 100% negative.  Quite an improvement... also, with the lower microphone gain the cathode current does not fly around too dramatically....  I think it is finally working quite well.

Now I started looking at my transformer supply for the 4D32 tube....  I have two choices, but the physically smaller one just may not have enough grunt.  It has the filament supply amperes high enough but the HV on the transformer is 590 VAC with CT at 130 MA...  not sure the tube will be happy with that?

The other transformer has plenty of current capability but the plate rating is closer to 800 VAC with a CT.  I am still checking with the locals to see if anyone has a better choice but I am the "main" transformer" storehouse in the area because I have a concrete floor I guess.

Thanks again for the support, 73, Tom
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« Reply #22 on: June 19, 2017, 08:56:29 PM »

Sounds like you are making good progress, Tom -   That little rig will really push out the positive peaks once it is set up right.

As for the HV supply for the 4D32, I would put as much voltage on it as you can. At 800VAC, or  something over 1000 VDC under load, it will perform fine. The key is to have good HV regulation as the plate current swings up and down to insure the cleanest audio peaks.  Heck, I have 2200 VDC on my quad 4D32 PDM class C rig and it will do a 500 watt carrier with huge peaks. Your 4D32 linear requires maybe 1/2 the voltage because of the class B inefficiency. You want to keep the plate heat at a reasonable level.  25 watts of carrier and big peaks may be a good starting point for one 4D32 in linear service.  Put some air on it and you will be surprised.  I direct a muffin fan directly down from above to distribute air evenly all around the glass. Some holes around the socket will help the bottom pin seals if there is an air path thru the rig's bottom.

T

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« Reply #23 on: June 19, 2017, 10:26:28 PM »

I'd use the 800 volt xformer.

You'll be slightly less than 1.2kv unloaded, and probably sag a little closer to a kilo volt.

The 4D32 will show much love in the power out department.

Think of it this way:  that's 826 volts unloaded vs 1120.  That will net you double the pep capability, almost!  Increasing plate voltage 50 pct is a 3db increase in power out.   It lists 750 volts plate modulated,  so you should be good.

--Shane
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« Reply #24 on: June 20, 2017, 12:18:15 AM »

I knew the big transformer would be the choice, just have to find a real stout chassis for it.... it's heavy.

I will start setting out parts... I doubt I have a filter choke big enough so will probably look for capacitors.... Will study some of Orr's circuits. 

I have 3B28's but maybe solid state it for simplicity and room on the chassis.

That little transmitter went from a two evening project to 2 months... there is always something.

Thanks again, Tom
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