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Old 5th May 2017, 11:27 am   #1
Chris Wilson
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Default My first home brew RF valve amp, a couple of questions please?

I fancy having a go at my first home brew valve amp on 136kHz, a band which I believe the US will soon have without a Special License, but one we in the UK have had for some time. I have a variable voltage HT supply done, using a BIG Variac and a full bridge. I have a couple of GS35Bs, and a socket for one is no problem. My first question is my current Class D push pull FET based semiconductor amp has its output transformer feeding a low pass filter bank, then via co-ax to outside where a toroidal wound matching transformer matches the 50 Ohm output impedance to the impedance of my antenna, which is around 65 to 70 Ohms. The top hat loaded vertical antenna has a BIG loading coil which the matching transformer then feeds. Now, with a valve amp, can I make the Pi or Pi-L output so the matching transformer becomes redundant? In other words make it load direct into 65 to 70 Ohms?

My second question is the modes that I use are digital, and ones which do not need a linear amp. Do I still need to bias the valve?

Thanks for any replies, I have worked from known good designs so far, this is my first from scratch build, so treat me as an imbecile (but one aware of the dangers of high voltages...).
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Old 6th May 2017, 5:21 am   #2
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Default Re: My first home brew RF valve amp, a couple of questions please?

Is there a "digital" mode of RF ??
If its 136Khz, its LOW RF. or do waves travel square wave today ??

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Old 6th May 2017, 6:27 pm   #3
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Default Re: My first home brew RF valve amp, a couple of questions please?

Amateur bands apart, what about DTV, DAB and mobile phones? They all use digital modes.
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Old 6th May 2017, 8:44 pm   #4
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Default Re: My first home brew RF valve amp, a couple of questions please?

Quote:
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Is there a "digital" mode of RF ??
Morse?
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Old 6th May 2017, 11:15 pm   #5
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Arrow Re: My first home brew RF valve amp, a couple of questions please?

Quote:
Originally Posted by joebog1 View Post
Is there a "digital" mode of RF ??
Do waves travel square wave today ??
I'm sure someone will correct me if I'm wrong on this, but to put things simply, the necessary carrier wave is analogue: it is the modulation of that carrier wave that is digital.
(Such confusion often arises on account of misleading terms like 'digital aerials'; )

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Old 6th May 2017, 11:52 pm   #6
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Default Re: My first home brew RF valve amp, a couple of questions please?

No Al, EVERYTHING is digital nowadays...or so we are led to believe. You are right, it's the modulation method that makes it "digital". RF is still good old RF. Here starts the argument.
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Old 7th May 2017, 7:01 am   #7
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Default Re: My first home brew RF valve amp, a couple of questions please?

When giving a talk, one of my favourite tricks is to leap around like a loony (I find this rather easy) and cup both hands together in the air, look inside them, look disappointed and then repeat. This time I look inside and look happy. I pick a victim from the audience, announce that I've just caught an electron, show the hole in my cupped hands to him and ask in all seriousness whether it's an analogue one, or a digital one?

This always gets a good laugh, but once, just once, it got a great reply:

"It seems to be wearing a little pair of headphones, and, let's see, the jack plug is gold plated. It's a digital one!"

This got a bigger burst of laughter than my bit.

We have electrons and voltages and currents and electromagnetic fields. They are neither digital nor analogue. Digital and analogue relate to the meanings we attach to those things.

People buy "DIgital" cameras and are blissfully unaware that all of the important bits that affect the picture quality, and every single one of the zillions of pixels as as analogue as anything gets. Surely they have a case under the trades description act?

At one level of understanding everything looks analogue you really can have all the voltages, not just 'high' and 'low'. The next level made everything look digital -you either have an electron or a photon or you do not. Then quantum mechanics makes it all look analogue again! You might have an electron or photon, and might is a statistical variable!

I find digital aerials and headphones hilarious. Almost as funny as directional speaker cables and "Fast Bass".

Anyway, for 1366kHz, the OP obviously means he's interested in constant carrier modes where whatever information has to be conveyed in frequency or phase variations.

Valve transmitters are still used because over a significant frequency range suitable valves can be picked up more cheaply than semiconductors, and they can also prove to be somewhat more robust. However, 136kHz is comfortably in reach of the (relatively) cheap power MOSFETs used in switch-mode power supplies.

Choosing to use a valve here looks like a personal decision, just for the fun of it. The kV supplies seem more scary than the 100v supplies for power MOSFETs.

For a living, I design 300W 1GHz transmitters. Earlier ones were bipolar based, current ones ara silicon MOSFET based and the suppliers keep trying to talk me into shifting to higher-priced Gallium-Nitride parts. They run happily off nice, safe 50v supplies. I've blown up surprisingly few transistors. at >>£100 each, there's a bit of motivation.

David
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Old 7th May 2017, 4:45 pm   #8
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Default Re: My first home brew RF valve amp, a couple of questions please?

Strictly 'digital' implies an infinite range of frequencies so the rf cannot be as such. As said earlier the modulating signal can be digital but there will be a relationship with bandwidth of signal dependant on the modulation method used.
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Old 7th May 2017, 5:34 pm   #9
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Default Re: My first home brew RF valve amp, a couple of questions please?

Perhaps a general definition of "Digital" is that information being transported, stored or processed is numerical. What the numbers represent can be anything whatsoever.

The rate at which they are sent can also be anything subject to limitations imposed by the medium, the assigned channel bandwidth, or the required rate of moving the data.

Information theory routinely calculates the data flow capacity of a finite-bandwidth channel of finite resolution. Progressive developments have crammed more data into limited bandwidths and incorporated methods of handling multipath propagation, getting closer to the theoretical limit.

No, signals just because they are "digital" don't automatically imply infinite bandwidth.

Yes, lines carrying boolean information as fast switching between logic levels does fill a lot of bandwidth. Zero rise and fall times aren't mandatory and aren't wanted. Simple binary logic levels don't get applied to antennae or to long wire lines. some artful pracessing is needed first or else complaints will soon start.

David
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Old 7th May 2017, 10:30 pm   #10
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Default Re: My first home brew RF valve amp, a couple of questions please?

Can anyone answer the OP's questions?
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Old 7th May 2017, 10:52 pm   #11
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Default Re: My first home brew RF valve amp, a couple of questions please?

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Can anyone answer the OP...

Graham, it won't be me, but I think a diagram would help. Reading it, it isn't clear why the question arises instead of sticking to a reference design.
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Old 8th May 2017, 12:09 am   #12
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Default Re: My first home brew RF valve amp, a couple of questions please?

Hi Chris,
Think of your original arrangement as a system of four units, the transmitter, the filter, the coax feed to the outside, and the antenna as a sub unit comprising matching transformer, loading coil, and the antenna radiating element.

The antenna base equivalent impedance will be low resistance, much lower than 65 ohms, and in series with relatively large capacitance. The loading coil is designed to have an inductance of the same reactance as the antenna capacitance, and "cancels" it out, and the matching transformer matches the resultant low ohms of the radiator to the feeder coax.
This is the normal arrangement for MF. So you will have to retain the matching transformer.

Assuming all original units are impedance matched, probably to the value of your feeder coax cable, and probably 50 ohms.

Assuming you wanted to keep your filter and present coax feeder, your new valve transmitter output should match the filter input impedance of 50 ohms only.

Unless there are other considerations in your requirements, keep it all at a standard of 50 ohms if you can, so as to retain the use of test equipment.

The loss in the filter and coax feeder should be very low at 136 kc/s, so you can retain them.

I hope I have interpreted your question correctly.

Mike.
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Old 8th May 2017, 7:32 am   #13
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Default Re: My first home brew RF valve amp, a couple of questions please?

The OP's questions...

Can a pi network replace the output transformer? If it is CLC then yes it can. But an output transformer may be easier. Though the CLC pi network has the added advantage that it is also a low-pass filter and attenuates harmonics - and its flywheel effect is essential if the output stage is operating in anything other than Class A.

Does the valve need biasing if it is using digital modes? Don't understand the question! If the RF is being switched off and on to send digital data, then yes you will need bias. When the RF is on, how are you intending to run the valve - as a class A amplifier? Bias needed! Class B? Bias it to cut-off! Class C? Bias to beyond cutoff! Class D? I'd find this one hard to believe... Switching at some MHz using PWM to create a sine wave, but yes you need to bias beyond cutoff. Class E (effectively lossless Class C) - bias to beyond cutoff. Etc!
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Old 8th May 2017, 8:17 am   #14
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Default Re: My first home brew RF valve amp, a couple of questions please?

If you intend using your existing antenna match and a 50 Ohm feeder, then yout Pi network will have to transform 50 Ohms upwards to the load impedance you want to apply to your output valve.

Resonant matching arrangements need to run at increasing Q as the impedance ratio is increased. At 136kHz, this will make it a very narrowly tuned network. OK, you don't need to operate over much frequency range, but it's going to be ticklish.

Large ratio transformations also tend to be lossier however you do them, so going down to 50 Ohms and then back up to the impedance your antenna presents to the loading coil at its base counts double.

It all depends on where all the various items need to be and what sort of feeder you can run, but I'd think a higher-Z transmission line would be more efficient. They were certainly common in LF broadcast stations.

I too think an output transformer would be easier. You're in a part of the spectrum where there are some quite good ferrite materials though the sizes of cores you'll need may be difficult to find. I think the biggest problem you'll run into is getting good enough insulation.

To be honest I'd forget the valve and treat the thing as an SMPS design.

As far as digital modes go, plenty of them try to pack in more bits/second into every Hertz of bandwidth by making the final signal 'look' more analogue. Modulation formats such as QAM and QAM on each carrier of an OFDM set.

There is a theory which says that any channel carrying the maximum data rate will look like it is filled with white noise. If you're using any format which doesn't produce this sort of spectrum (and that means linear amplifiers are needed to handle the amplitude dimension) then you can be quite certain that you aren't getting the best data rate for a given bandwidth. The constant-amplitude modulation formats are a great convenience for transmitter design, and maybe you don't need the last iota of efficiency, or maybe you're fitting in with a legacy net.

If you can do it, a very good place for your power amp would be in a weatherproof box at the foot of your antenna.

David
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