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 Post subject: Tube amplifier for CW 4W input
PostPosted: Jun Sun 20, 2021 3:29 pm 
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Joined: Jun Wed 19, 2019 11:07 am
Posts: 68
Hi, I got a small 4 W solid state CW transmitter that has both low-Z and high-Z outputs. It operates on 80 through 30m.

I am looking for a small amplifier for it (simplest possible) so as to amplify the power to something like 20-100W.

I like grounded grid designs for their simplicity.

I have seen such CB linears http://rigpix.com/linears/kris_300m_schematic.gif http://www.bunkerofdoom.com/lit/cb/line ... 0_2082.jpg which could be converted but I would like a design with a standard transformer (220v-6v or 12v transformer) used for the inverter. I could also live with mains sets, but there is a big requirement for this. The anode voltage cannot be more than 350v, because I would then use a back to back standard transformer (220v-6v or 12v transformer) to power the heaters and the HV.

So I am looking for a simple design class C tube linear.

This one https://www.glowbugs.info/2010/06/6n7-t ... rainy.html seems great, but I would like a bit more juice.

Any ideas let me know.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 20, 2021 3:50 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7998
If you are using back to back mains transformers with a 220 volt secondary, then use a voltage doubler circuit on the 220V secondary which will provide around 600 volts which would be usable with a 6146 and excellent for a TV "sweep" tube. Take a look at the Heathkit HP-23 power supply schematic for a voltage doubler circuit.

With the higher voltage no longer an issue, you could use the final stage from any of the typical 100-200 watt class CW/AM transmitters as design inspiration for your amplifier.

Rodger WQ9E


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 20, 2021 5:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2983
Location: Westminster, CO, USA
I'm not sure why you specified a tube amplifier. I have had good luck with the IRF510 mosfet amplifiers and they are cheap and easy to build. They will work well over the bands you have mentioned. Something like this one:

https://www.g0kla.com/scpa/SimpleCheapPA.php

There are plenty of them out there. If tube is what you want then ignore this.

Tony
AD0VC

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Tony Casorso


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 20, 2021 7:16 pm 
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Joined: Jun Wed 19, 2019 11:07 am
Posts: 68
I do not like having to wind these relatively complex transformers.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 20, 2021 11:05 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1374
Location: Montreal, Quebec
Class C is never linear.

There were endless transistor transmitters described before much was done with broadband amplifiers. Most happened to be Class C, for CW and AM. And they had multiple stages to get up to power.

Even homebrew transistor SSB transmitters in the sixties were more traditional. Broadband came in the seventies with more than a few watt SSB transmitters, which also meant no retuning as the frequency was changed.

We can't live in the past. This stuff is fifty years old, yet people still want to avoid those "messy coils", let alone toroids and broadband transformers. So the past is just constantly rebuilt because somehow things that have been around for fifty years, like synthesizers and frequency counters and toroids and complicated superhets, are too complicated.

Twist some wire together, get the toroids or binocular cores, and get to work. Until yiu take that first step, it will always be more than you like. If I could do it 46 years go when I was 15, it can't be hard. Just the perceived high cliff.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Wed 23, 2021 1:46 pm 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 367
Location: Springwater, NY
sv3ora wrote:
....got a small 4 W solid state CW transmitter that has both low-Z and high-Z outputs. ....I am looking for a small amplifier for it (simplest possible) so as to amplify the power to something like 20-100W....I like grounded grid designs for their simplicity.....So I am looking for a simple design class C tube linear.


Simple, grounded grid, linear:

https://forums.qrz.com/index.php?threads/4x-6ag7-amplifier.646207/page-5

See post #45. The 6AG7 is not the only tube that can be used in this arrangement. 73 Dean


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Fri 25, 2021 11:24 am 
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Joined: Oct Thu 01, 2020 10:48 am
Posts: 90
Has anyone else noticed that when you log in, and click on "return to previous page," you don't get returned to the previous page?

Anyway, the 4 w. output of your rig is enough to drive the grid of a tube rig via a link coupled or pi input network. You can get some ideas on how to work this by looking at schematics for 100 w. input CW rigs from the pre-1950 days. A good place to start for a vacuum tube primer is the introductory pages of a RCA tube manual. The first 30 pages explains a lot about how tubes work. If the tube is a tetrode you can probably match the driver to the tube grid with a pi input network. You can use a triode, but it will have to be neutralized which complicates multi-band operation and would involve a balanced link coupled input.

"We can't live in the past. "

I'm puzzled by this attitude here on an antique radio forum. I can't speak for everyone but I can certainly "live in the past," and I delightedly do so, when it comes to radio at least. I'm not against modern technology, but I am against stupid modern technology (do we need to have everything on the internet?), and very much in favor of older designs if they do what I want to have done.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sat 26, 2021 1:09 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1562
When I saw your post, it made me smile because I once was confronted by the same question. Except that I solved the problem with transistors rather than tubes.

Back in the mid 1990's I wanted to amplify the RF output from a vintage transmitter (the famous NZ made ZC1 military radio) It put out about 3.5W over the frequency range of about 2 to 8 MHz.

I wanted to get the output up to well over 100W into a 50R load...How to do it ?

The task as it turned out was mostly an episode of Mechanical Engineering (hmmmm..... that doesn't sound right, but actually it was). The trick to getting a good result with any hand done mechanical engineering, is to try to get it to look like a machine did the work.

The required PCB's and Electronics were, and still are, readily available for the task. Their designs were done mainly at Motorola and they are transistorized broadband amplifiers that worked over the range 1.8 to 30 MHz, the kits of which can be obtained from CCI, Communications Concepts Inc, in he USA. The app notes are available from them too.

I have attached some photos.

I built an enclosure from scratch, using 10mm square aluminium bar (close to 3/8" square) and 3mm thick anodized satin, aluminium plate, this is close to 1/8" thick. And a large extruded heat sink from CCI.

The most difficult part (since it was all done by hand, including cutting the plates and smoothing their edges) was the correct positioning of the holes for the many countersunk 1/8" dia BSW screws. I used BSW because it has a similar thread pitch to 4-40 UNC but a slightly larger diameter. Coarse threads suit aluminium. But it was a long job tapping all the holes. Also, the rubber feet on the base have metal bushes placed inside them. You will be amazed how many people foolishly use stick on rubber feet and wonder why they fall off a few months later.

In any case, nothing good ever comes easy. Never underestimate the power of good mechanical engineering.

I added two relays, one to switch on the unit as the current is high for a small power switch. Another was added, driven by a circuit to detect the carrier wave output from the transmitter. When the ZC1 radio is in the receive mode, the antenna signal passes to the radios input. In transmit mode, the high carrier level is detected and the open frame switching relay routes the output of the RF power amplifier, to the antenna.


Attachments:
RFAMP1.jpg
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RFAMP2.jpg
RFAMP2.jpg [ 763.87 KiB | Viewed 806 times ]
RFamp3.jpg
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rfamp4.jpg
rfamp4.jpg [ 546.28 KiB | Viewed 806 times ]
RFamp5.jpg
RFamp5.jpg [ 652.68 KiB | Viewed 806 times ]
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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 27, 2021 5:45 am 
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Joined: Apr Sat 25, 2015 2:58 am
Posts: 90
Location: Bangalore
sv3ora wrote:
Hi, I got a small 4 W solid state CW transmitter that has both low-Z and high-Z outputs. It operates on 80 through 30m.

I am looking for a small amplifier for it (simplest possible) so as to amplify the power to something like 20-100W.

I like grounded grid designs for their simplicity.

I have seen such CB linears http://rigpix.com/linears/kris_300m_schematic.gif http://www.bunkerofdoom.com/lit/cb/line ... 0_2082.jpg which could be converted but I would like a design with a standard transformer (220v-6v or 12v transformer) used for the inverter. I could also live with mains sets, but there is a big requirement for this. The anode voltage cannot be more than 350v, because I would then use a back to back standard transformer (220v-6v or 12v transformer) to power the heaters and the HV.

So I am looking for a simple design class C tube linear.

This one https://www.glowbugs.info/2010/06/6n7-t ... rainy.html seems great, but I would like a bit more juice.

Any ideas let me know.


Hi OM Kostas,

Class C is not linear. A linear amplifier for SSB would be Class AB.

You'll find details of the Class C amplifier, which I built decades ago, here.

https://nandustips.blogspot.com/2011/02 ... ifier.html

73,

Nandu.

_________________
https://nandustips.blogspot.com


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Sun 27, 2021 2:20 pm 
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Joined: Oct Thu 01, 2020 10:48 am
Posts: 90
The key thing when it comes to integrating solid state circuits and vacuum tubes is impedance matching. Tubes are high voltage low current devices. Transistors are the other way around. The load impedances they present are very different from each other. So, getting a solid state driver to work into a tube grid and be happy and having efficient power transfer so the tube grid gets enough kick to work well is all about constructing a matching circuit that transforms the high grid impedance to the driver FET's typical load Z, which is often 50 ohms unbalanced. This is a grid input network. You basically can't have a transmitter without them. The exception are single tube rigs that have some limitations.

As an example to see how this works, a class C single 813 amplifier is a good choice for CW. The first thing to do is study the data sheet for the 813 which can be found with a google search of "813 tube". https://frank.pocnet.net/sheets/079/8/813.pdf In the section for class C RF power, you'll see some typical operation values. The ones of interest are the RF grid no. 1 v. (the control grid) at the peak of the RF cycle, which is 160 v. It's all Ohm's law now. They provide a grid leak resistor value of 6K, so the grid current is around 27 ma. You now need to construct an unbalanced matching network that transforms approximately 6K ohms down to 50 ohms. It helps to have a vswr analyzer at this point. You need a non-inductive resistor, carbon composition or metal oxide, that's 5K or 6K, and then you can start tinkering with coils and variable capacitors using clip leads on an insulated surface, your analyzer tuned to your operating frequency on one end, and your resistor on the other. This is low power, so receiving type air variable caps, and inductors made by winding enameled wire on toroidial cores will be okay. There are on-line calculators that will design something for you, but I rather enjoy messing around with parts and clip leads until I get into the ballpark and can start refining the circuit.


160 v. x .027 amperes is roughly 4 watts, just about right for your driver. The data sheet will give you an idea of the output power from the 813 for a plate v. of around 1.2 KV.

For other modes/conduction angles, employ the appropriate values from the data sheet and perform the same procedure.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Mon 28, 2021 5:41 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1562
For most RF power amplifiers over the years I have gone to class AB. The reason is, you never know when you want to feed it with a modulated carrier. Class C is great for efficiency and just CW transmission (unless you modulate the output stage requiring significant modulation power), but, sooner or later, you are going to want to put some modulation intelligence on that lower level carrier and linearly amplify it, it might just be a recording of your favorite song.

Going to class AB linear amp, means that you can feed the power amplifier, like the one I described above, with a modulated carrier and also, your output is relatively free from harmonics and retains the modulation well. Of course, it is less efficient than class C and uses much more power, but generally less than your hair dryer or a Bar heater that warms your feet in the winter, or an electric jug element that you use to heat water to make your morning coffee. So I never get that excited over efficiency issues for anything under a few hundred Watts. It is like those lovely old incandescent Xmas tree lamps with the painted and decorative glass bulbs, sure they use more power, but they look a heck of a lot better and more mellow than LED Xmas lamps that use less than 1/10 the power but are totally uninspiring and don't have a hope in Hell of capturing the magic of Xmas.

In Class A, the power consumption is relatively independent of the drive & output level. In class AB it tends to be proportional to the output power (ignoring the idle power) . For the amplifier circuit I posted, it is just a matter of setting the bias control, so the output carrier wave is just out of cross over distortion (much like you would for an audio amplifier) and the efficiency is actually pretty good, so it is a very hard system to beat.

In addition, you can modify the input and output impedances easily with similar types of broadband transformer cores that you can also get from CCI.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Mon 28, 2021 10:25 pm 
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Joined: Mar Sun 14, 2010 2:51 am
Posts: 611
Location: Newport, Oregon, 97365 U.S.A.
My feeling about low power rigs it, use them as-is, deal with them as they were originally used. Then why not have a separate higher powered
transmitter if and when you feel you need to have more power ?

Ramsey Electronics once had a range of 80, 40, 20 meter solidstate linear amp kits. I bought those shortly after Ramsey closed up shop. I have also taken apart ss ssb boat radios having a "brick" 100 - 150w PEP solid state linear amp.

I still own some hybrid ham rigs, tube RF output stage using sweep tubes, and i really dislike the idea of tube output stage in a transistor transceiver. I have thought about installing instead a lower powered transistor amplifier. The problem may be shoehorning in the bandpass circuits for each band.


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 Post subject: Re: Tube amplifier for CW 4W input
PostPosted: Jun Mon 28, 2021 10:34 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2806
I would think that any old boat anchor transmitter will work, either as an amplifier or on its own. Those can be had at swap meets or ham club meetings for low prices if you are patient.


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