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Old 17th Apr 2016, 6:49 pm   #1
anikimtiaz1990
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Default Wideband HF amplifier

hello,
Is it possible to build a wideband HF power amplifier that can cover a range between 100khz-10mhz range? I searched all over internet but haven't find anything except solid state device. Is it possible to use a grounded grid amplifier with various tuning setup for various frequency?

I really need your help on this matter...

thanks
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Old 17th Apr 2016, 8:52 pm   #2
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Default Re: wideband HF amplifier

100kHz will be a problem. The impedance transformation from a typical anode load to (presumably) 50 Ohms is the problem. High Z and wide bandwidths don't go together.

For transistors and ferrite cored transformers, it is an awful lot easier.

If you only want a few frequencies and can switch tuning arrangements by bands, then there are valve answers to the problem. Wideband is usually used to mean something which can handle any frequency in a range without needing re-tuning.. or can possibly handle multiple frequency components at once.

For transistor power amp designs look for the old Motorola applications noted written by Helge O Granberg.

David
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Old 18th Apr 2016, 7:20 am   #3
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Default

yes transistor can be used,but i think it can not handle kw level power....valve amplifier can give more power.so i need to build a valve amplifier...

thanks for your reply....but i think transistor can not handle much power like 600w to 1kw.
i want to use a rf signal generator as a source and then amplifiy the voltage of the signal source...valve amplifier can do the job very well...i think.
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Old 18th Apr 2016, 9:13 am   #4
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Default Re: Wideband HF amplifier

Why do you want to build what is effectively a 1kW transmitter?
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Old 18th Apr 2016, 10:02 am   #5
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Default Re: Wideband HF amplifier

a kilowatt amplifier can give high voltage,at high frequency range.i need at least 1kv at high frequency range...a kw amplifier can deliver such voltage i think....and transmitting is not my desire i want to drive cyclotron and some tesla coil for various experiment...where high voltage and high frequency is required.........
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Old 18th Apr 2016, 12:15 pm   #6
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Default Re: Wideband HF amplifier

A 1kW HF amplifier in the hands of a novice can be a dangerous thing, both to him and bystanders.

If someone asks specific questions about some detail of design then I assume he basically knows what he is doing but just needs a little advice. If someone asks basic generic questions about high power equipment then I start to worry for his safety.
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Old 18th Apr 2016, 12:23 pm   #7
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Default Re: Wideband HF amplifier

On my web site you will find the details of a commercial wide band linear amplifier using valves - so it can be done. You could do something similar with less expensive valves for your application. There was an article in the VMARS magazine recently on the design of amplifiers of this type.

See Linear Amplifier Model 122C which gives 200W for 10kHz to 220MHz.
On http://home.btconnect.com/gmb/
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Old 18th Apr 2016, 3:42 pm   #8
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Default Re: Wideband HF amplifier

@G8HQP Dave dont need to worry....i have experience on working with high voltage...vacuum tube technology is new to me and i am trying to learn as much as i can..

@GMB thanks for your direction.it is a distributed amplifier isnt it ? yeah it is great for wide range of coverage...the problem is the cost..purchasing 12-13 tube at once is very difficult for me....thats why i am trying to find alternative..
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Old 18th Apr 2016, 4:10 pm   #9
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Default Re: Wideband HF amplifier

That design relies on the number of valves and the sum of their Gm figures to drive 50 Ohms. 11 is an awful lot of 250W devices to give 200W of output, and it is because they are being run off much lower HT voltage than 4CX250Bs routinely use in narrowband amplifiers. There is also power going to the output line terminator rather than to the main output. It does make a wonderfully broadband amplifier, but you'll need a lot more to get into the kilowatt league.

There are bigger devices than the 4CX250B, such as the 3CX1500 ( also known as 8877) but these things rely on greater voltages rather than just greater currents, and getting to 50 Ohms (if you want that impedance) is harder.

But what impedance are you really going to drive? Things which have flat Z across a wide frequency range aren't very common.

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Old 18th Apr 2016, 4:26 pm   #10
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Arrow Re: Wideband HF amplifier

The BLF184XR looks like a device worthy of consideration: you will probably need two, each feeding into a combiner network and an impedance matching network at the output. For 1 kW output, you'll need a PSU capable of 50 v.d.c. at 20 amps minimum. You'll also need some protection circuitry, since these types of transistors are expensive - and in my experience are easily destroyed.

The alternative - which I would choose - would be a valve design, probably based on a QY4-400. With 4kv. on the anode plus forced air cooling, you should get near to 1 kW R.F. output. Again, an impedance matching network will be required which will be frequency-sensitive. Again, protection circuitry is required: for you (4 kv. with the required current capability is unquestionably lethal) and for the valve - especially for failure of the forced air cooling. Valve power R.F. amplifiers are more tolerant of load mis-matches than corresponding semi-conductors.

Whatever route you take, this a project that will be substantially technically demanding, expensive, and certainly hazardous in the case of the valve version.

Al.
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Old 18th Apr 2016, 5:08 pm   #11
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Default Re: Wideband HF amplifier

Quote:
@G8HQP Dave dont need to worry....i have experience on working with high voltage...vacuum tube technology is new to me and i am trying to learn as much as i can..
Do you have experience of working with high power RF too?
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Old 18th Apr 2016, 6:18 pm   #12
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Default Re: Wideband HF amplifier

There are many websites for people interested in extremely high voltage generators, Tesla coils, Cockroft-Walton style multipliers etc. A wideband amplifier is not really the best solution for EHV generators. I have seen designs for valve drivers which consist of a single high power TX valve of the 300 - 500 Watt class connected as a variable frequency self-excited oscillator driving the primary of a Tesla coil, all very hair-raising!

This forum is not really the best place to ask, we tend to deal mainly with equipment found in the home using maximum voltages of about 350, EHV is another field altogether.
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Old 18th Apr 2016, 6:30 pm   #13
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Default Re: Wideband HF amplifier

Quote:
Originally Posted by G8HQP Dave View Post
Quote:
@G8HQP Dave dont need to worry....i have experience on working with high voltage...vacuum tube technology is new to me and i am trying to learn as much as i can..
Do you have experience of working with high power RF too?
no i dont have experience on high power rf....but i need to take that risk...
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Old 18th Apr 2016, 6:47 pm   #14
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Default Re: Wideband HF amplifier

I have found a schematics where there is an rf amplifier unit in the middle with multiple tuning...

Is it possible to use this schematics with more powerful tube?
Attached Files
File Type: pdf rf amplifier.pdf (359.5 KB, 137 views)
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Old 18th Apr 2016, 6:56 pm   #15
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Default Re: Wideband HF amplifier

Quote:
Originally Posted by RF Burn View Post
There are many websites for people interested in extremely high voltage generators, Tesla coils, Cockroft-Walton style multipliers etc. A wideband amplifier is not really the best solution for EHV generators. I have seen designs for valve drivers which consist of a single high power TX valve of the 300 - 500 Watt class connected as a variable frequency self-excited oscillator driving the primary of a Tesla coil, all very hair-raising!

This forum is not really the best place to ask, we tend to deal mainly with equipment found in the home using maximum voltages of about 350, EHV is another field altogether.
i am sorry that i came in to wrong place....but i dont have other place to ask...
can you suggest any website that can help ? i want to use rf amplifier because it can give clean signal with high power...unlike spark gap,and many people like mr eric dollard also suggested to use an amplifier to amplify the signal and then drive the coil...
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Old 18th Apr 2016, 7:32 pm   #16
anikimtiaz1990
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Default Re: Wideband HF amplifier

Quote:
Originally Posted by RF Burn View Post
There are many websites for people interested in extremely high voltage generators, Tesla coils, Cockroft-Walton style multipliers etc. A wideband amplifier is not really the best solution for EHV generators. I have seen designs for valve drivers which consist of a single high power TX valve of the 300 - 500 Watt class connected as a variable frequency self-excited oscillator driving the primary of a Tesla coil, all very hair-raising!

This forum is not really the best place to ask, we tend to deal mainly with equipment found in the home using maximum voltages of about 350, EHV is another field altogether.
i am sorry that i came in to wrong place....but i dont have other place to ask...
can you suggest any website that can help ? i want to use rf amplifier because it can give clean signal with high power...unlike spark gap,and many people like mr eric dollard also suggested to use an amplifier to amplify the signal and then drive the coil...
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Old 18th Apr 2016, 9:47 pm   #17
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Exclamation Re: Wideband HF amplifier

Quote:
Originally Posted by G8HQP Dave View Post
Do you have experience of working with high power RF too?
Quote:
Originally Posted by anikimtiaz1990 View Post
No: I don't have experience on high power rf....but I need to take that risk...
Quote: I need to take that risk. But do you? There is a substantial difference between 'need' and 'want'.

For the inexperienced in this field of electronics, the risks here are daunting: 1 kw. in 50 Ω gives over 300 v. peak - and that's R.F., too. A shock of that voltage at mains frequency can be dangerous enough, but at R.F. such a shock - plus the inevitable R.F. burn - can be very serious. In a past employment, in a moment of distraction, I dropped my guard and received a R.F. shock from the filter coils in an 250-watt 100 MHz R.F. amplifier feeding a 50 Ω load: the peak voltage was about 150 v. That really hurt: fortunately it was only across a finger, but I still have the scar from the R.F. burn to this day. So if you do decide to "take that risk", please be aware that it might be the last thing you ever do.

Experimenting with H.V. and high power R.F is not a suitable pastime for beginners. You say that you have come to the wrong place. On the basis of the safety advice you have received here, I would say that your visit here has been potentially beneficial to you: so, perhaps, not the "wrong place", although that advice is perhaps not what you were initially seeking.

Al.

Last edited by Skywave; 18th Apr 2016 at 9:53 pm.
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Old 18th Apr 2016, 10:58 pm   #18
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Default Re: Wideband HF amplifier

Most of the people doing serious high power RF design (My day job involves designing radar transmitters) served an apprenticeshp, starting with an amateur radio licence and low powers. You make mistakes which are survivable, and you learn from them. It's also a LOT cheaper in terms of the prices of burned out things. It takes time, but you get to live to be able to do things with the knowledge gained.

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Old 19th Apr 2016, 12:39 pm   #19
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Default Re: Wideband HF amplifier

Quote:
no i dont have experience on high power rf.
That is what I suspected.

I only have nearly 50 years experience of DIY electronics, including mid-power RF (10W), so I would not wish to attempt what the OP is considering. Having received wise warnings from us I suggest that the Mods now close this thread on the grounds of safety.
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