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Old 29th Dec 2022, 11:38 am   #21
Aub
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Default Re: Some times I start something without any real plans.

Very good. Keep going. I wouldn't worry too much about the loading into a bulb being seemingly better than the dummy load.

Cheers

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Old 29th Dec 2022, 6:02 pm   #22
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Default Re: Some times I start something without any real plans.

That build so far looks very good indeed. My only concern would be local heat from mainly the 6V6 near to the vfo.
In my teens, a 15W bulb was my only dummy load in my pre and post licence days!
Very useful and a good visual indication as well.
Over 55 years later I have had (for over 30 years now) a Bird Termaline 0-15W, 0-150W, 30-500MHz I think.
Plus a couple of Racal-Dana 1MHz to 1GHz loads with built in power meter for either 0-1-3W or 0-30-100W
Rob
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Old 29th Dec 2022, 7:27 pm   #23
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Default Re: Some times I start something without any real plans.

A few more tests and I think for 50 Ohm I need a few more turns on the inductor for 160m. Shorting a few turns out makes it worse. I could get 6 Watts into 50 Ohm on the meter.

In this picture I have a 35 watt halogen bulb connected, this loads up OK, I seem to have 10 Watts input, not sure how good the 6V6G is as I have not tested it.

As to the heat affecting the VFO, there is now a screen in between the driver and 6V6 and the VFO and OA2. The next stage would be to play with the mod stage?
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I have some metal mesh I hope to use to provide the rear and top.

A bit at a time.

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Old 29th Dec 2022, 8:04 pm   #24
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Default Re: Some times I start something without any real plans.

Quote:
Originally Posted by AdrianH View Post
A few more tests and I think for 50 Ohm I need a few more turns on the inductor for 160m. Shorting a few turns out makes it worse. I could get 6 Watts into 50 Ohm on the meter.
I would expect to see a larger loading (output) capacitor for 160m, generally at least a paralleled 3-gang 500 or 530 pF to provide 1500pF or more. Alternatively some switchable mica capacitors in parallel with the dual 500pF.
The calculations for pi-networks can be found in any of the old classic radio handbooks (RSGB, ARRL, "Radio"), and no doubt in online articles too.
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Old 30th Dec 2022, 1:05 am   #25
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Default Re: Some times I start something without any real plans.

Before retiring for the night I tried a Silver Mica cap of 340pF across the output, so in parallel with the double tuning cap. That seemed to load better into 50 Ohm, no more output but seemed easier to get the dip in the anode current. Still Max of 6 Watts for 10 input,

Not sure how to work out or find from any spec sheets what the Ra of the valve would be?

Also need to make the PSU safer.

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Old 30th Dec 2022, 9:19 am   #26
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Default Re: Some times I start something without any real plans.

To add a bit more to the above comment on the valve Ra. The Radcom manual in 68 quotes the following: -
6BW6 At 250 Volts HT and 10 watts input (40mA), the RF impedance is approx 2900 Ohms, .... However with a Voltage of 300 Volts and 10 watts input the impedance is 4100 Ohms.

It can not be just a straight Volts over current for the impedance as: -
250 V / 40 mA is 6250 Ohm.
300 V / 33 mA is 9000 Ohm.

Looking at the spec sheet for the 6V6 or 6BW6 says the optimum load is around 5,500 Ohms, I had always assumed a load that matched to Ra would provide optimum output, and that is in the region of 50+ KOhm.

So my knowledge is definitely missing something?

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Old 30th Dec 2022, 2:09 pm   #27
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Default Re: Some times I start something without any real plans.

Your calculation would be right for a Class-A audio amplifier, where there is anode current flowing at all times through the signal-cycle but in the RF amplifier case you are talking Class-C where the vallve is biased to spend much of its time cut-off and so the anode current only flows in brief pulses rather than continuously, therefore you need to take into account the conduction-angle when calculating output impedance.

In some old Eitel-McCulloch [Eimac] design-notes I have here they specify the output impedance for a Class-C RF amp to - simplistically - be:

460 x (Vht/Ia)

[Anode- or anode-and-screen modulation of such a Class-C audio amplifier can lead to variations in the '460' term in the above equation]
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Old 30th Dec 2022, 6:51 pm   #28
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Default Re: Some times I start something without any real plans.

Quote:
Originally Posted by G6Tanuki View Post
simplistically - be:

460 x (Vht/Ia)

[Anode- or anode-and-screen modulation of such a Class-C audio amplifier can lead to variations in the '460' term in the above equation]
Should that be 0.460 or 460, just checking as if I use 460 then the impedance would be 287K. 0.460 would give 2875 which is closer to the RSGB article?

It could be 460 * Vht/Ia if Ia is in milliamps I guess.

And again this is where I need to gain more working knowledge on biasing.

Looking at the circuit with basically the cathode strapped to ground and the grid to ground via a 22K, it looks like the valve is kept hard on, but I am assuming is it the RF driving it biasing it negative, some articles say there should be around 2 mA grid current, which would indicate around 24 Volts negative bias from grid rectification. I only learn via building and then asking questions as to why and could it be because of: -.

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Old 30th Dec 2022, 8:07 pm   #29
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Default Re: Some times I start something without any real plans.

Quote:
Originally Posted by AdrianH View Post
[Should that be 0.460 or 460, just checking as if I use 460 then the impedance would be 287K. 0.460 would give 2875 which is closer to the RSGB article?

It could be 460 * Vht/Ia if Ia is in milliamps I guess.
Yes, using this formula the multiplier is 460 and the anode current is in Milliamps.

Quote:
Originally Posted by AdrianH View Post
And again this is where I need to gain more working knowledge on biasing.

Looking at the circuit with basically the cathode strapped to ground and the grid to ground via a 22K, it looks like the valve is kept hard on, but I am assuming is it the RF driving it biasing it negative, some articles say there should be around 2 mA grid current, which would indicate around 24 Volts negative bias from grid rectification. I only learn via building and then asking questions as to why and could it be because of: -.
Adrian
It's always good to ask questions!! And to experiment.

Yes the RF drive from the driver stage is rectified by the output-valve's 'diode' consisting of the control-grid and the cathode; this generates a negative DC component which appears across the grid resistor and this biases the valve towards the cut-off condition. As you note, the negative bias causes current to flow in the grid resistor which is the 2mA you mention.

The more RF drive you apply to the grid the greater the rectified DC voltage becomes, and the more the valve is pushed towards cut-off.

Usually you aim for something in the range of 1.5 to 5mA of grid-current.

In the absence of the RF drive to the grid, there is no rectified grid-bias, and the output valve will turn fully-on, possibly destructively! In times-past some people wired a small low-voltage bulb [6v 60mA] in series with the HT supply to the output-valve to act as a fuse in the event that the grid-drive failed and the output-valve started to draw excess current.

Under normal operation the bulb would glow dimly, and present a small resistance, but in an overcurrent situation the bulb's resistance went up, it dropped more volts, so it dissipated more power, heating its filament more, making its resistance go higher still - until it burned out.

[The other protective approach is to use a 'clamp valve' - wired with its anode to the output-valve's screen grid and its grid to the -ve DC voltage produced by the grid-drive to the output valve: provided there is negative grid-voltage there [indicating the presence of RF drive] this clamp-valve is cut off and doesn't influence the rest of the circuit's operation, nut if the RF drive is lost there's no grid-voltage so the clamp-valve turns on and pulls the output-valve's screen-grid voltage down, which in turn reduces the output-valve's anode-current to a safe level]
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Old 30th Dec 2022, 11:53 pm   #30
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Default Re: Some times I start something without any real plans.

A quick calculation gives the following values for the pi network, assuming a loaded Q of 12 (the usual recommended value):

300V HT, 10W input, 7W output, 50 ohm load:
Tuning C = 214 pF, Inductor = 36 uH, Loading C = 1966 pF

250V HT, 10W input, 7W output, 50 ohm load:
Tuning C = 273 pF, Inductor = 26 uH, Loading C = 2340 pF

So in either case:
The "Tune" variable capacitor can be 365 or 500 pF (one section of a standard broadcast variable)

The "Load" capacitor will need a fixed 1500 pF silver-mica plus both halves of a 2 - gang 500 pF broadcast variable, all in parallel.

The inductor can be tapped for both calculated values, and maybe one or two more in between to allow for HT voltages somewhere between the two assumed values.

-----

When using a light bulb as a dummy load, bear in mind that the resistance is low when cold, and increases as it warms up - don't assume it will always be the value calculated from the rated voltage and power (unless it's actually running at the rated power).

Last edited by m0cemdave; 30th Dec 2022 at 11:59 pm.
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Old 30th Dec 2022, 11:53 pm   #31
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Default Re: Some times I start something without any real plans.

OK thanks for the explanation, now I see the point of the bulb in the Codar PSU, that seems a simple thing to add as there is a brief seond where the anode current rises above 50 mA just before the VFO and driver starts passing RF to the PA.

Many thanks

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Old 31st Dec 2022, 12:29 am   #32
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Default Re: Some times I start something without any real plans.

Hello Dave;

Thanks for the post, I used what the original Codar coil is said to be at 45 uH so that is what I wound for on mine, just using a thicker wire and larger dia, to make it easier for myself. The Codar PA tank coils is said to be is 64 turns, 2 inches long, with a diameter of 1 inch.

Can you point me to a calculator or article you are using for this and if the L can be a fixed value?

One thing I do not know is if it ever goes on air, what I will use as an aerial, I can certainly not get a half wave up for top band, so would probably have to try some form of loaded mobile whip and small ground plane, so it would be interesting to see what I can actually load it u into.

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Old 31st Dec 2022, 11:00 am   #33
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Default Re: Some times I start something without any real plans.

I had a sort of revelation moment trying to get to sleep last night. In trying to understand how one could change impedance between the Valve and load without loosing power and how it was done.

If I treat it like a PI resistor network, I can work out the resistances to get from 2900 Ohms to 50 Ohms and get the three resistance values.

The revelation part was that at a given frequency these values can be obtained from L and C, and in theory these are lossless in power (barring a bit of DC resistance of the coil).

Now I may be on the wrong idea, but something my brain clicked on to.

Adrian
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Old 1st Jan 2023, 4:14 pm   #34
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Default Re: Some times I start something without any real plans.

Moving on a stage further has me thinking of the modulation stages and although crystal microphones are expensive to pick up now, and their performance is not guaranteed, I would like to go the same way as the Codar and others. with the ECC83 and 6BW6.
But what to use s the microphone, I am initially thinking along the lines of an electeret mic insert and a small set up transformer the type 600 Ohm to 10 K high impedance, this may well require a single transistor driver stage as well.

The other thing to consider is the audio transformer/choke/balanced driver to the PA.

In the Codar a 15 VA 240/110 Volt autotransformer, in another circuit the HT side of a 250-0-250 HT. So this seems to have a wide range of possibilities even down to the primary side of a split primary mains transformer neglecting the secondary side.

Having a quick read of an old publication it suggests that a small autotransformer could potentially be have the size of a normal mains transformer, i.e. approx equivalent size to a 8 to 10 VA mains transformer with secondary could be used for the 15 VA autotransformer.

So would appreciate comments on these thoughts.

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Old 1st Jan 2023, 4:43 pm   #35
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Default Re: Some times I start something without any real plans.

For the transformer, in the past it was common to use a push pull output transformer rated at around 5 Watts. I even remember a transceiver design where the modulator was also used as the receiver audio output.

Microphone, an Electret type with around 10V bias will deliver enough audio to substitute for the old Acos type crystal mic without needing a transformer.
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Old 1st Jan 2023, 4:53 pm   #36
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Default Re: Some times I start something without any real plans.

I do not have any push pull transformers to hand, but on Ebay I note they are around £50 plus when delivered, so I think I may try to wind an auto transformer and give it a shot. Good to know about the electret.

I have missed the edit time on my previous post but one sentence would be better trying to say: -

Having a quick read of an old publication 'Wolpert' it suggests that a small autotransformer could potentially be half the size of a normal mains transformer, i.e. approx equivalent size to a 8 to 10 VA mains transformer with secondary could be used for the 15 VA autotransformer with no secondary?

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Old 5th Jan 2023, 12:27 am   #37
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Default Re: Some times I start something without any real plans.

I have wound what I hope would work, I had a EI-62 stack that was for a 12V mains transformer and used that.

If I got the equation correct I have worked it for a low frequency of 100Hz and taken the voltage across each leg as 120 Volts giving 0-120-240 Volts considering the original autotransformer was 0-110-240 Volts. When tested for inductance with my TC1 the total inductance is 7.8H.

I have received a few spare valves, I have two 6BW6's one to use as the modulator one spare. And a spare 6V6GT for the 6V6G PA.

I will I think have a go at wiring the modulator on copper clad and trying it before I drill holes in the chassis, I had considered putting the modulator on the PSU chassis but decided against it as that would stop me ever doing a separate 12 Volt to HT power supply inverter.

Slow progress.

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Old 5th Jan 2023, 10:26 am   #38
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Default Re: Some times I start something without any real plans.

Nice build job.
That coil does look like it may need a few more turns. If you can find one of the old tube marine radios that are obsolete, the coil out of one of them should work.
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Old 8th Jan 2023, 1:19 am   #39
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Default Re: Some times I start something without any real plans.

Quote:
Originally Posted by AdrianH View Post
Can you point me to a calculator or article you are using for this and if the L can be a fixed value?
I used the formulae in the 3rd edition of the RSGB Handbook (pp 187-189 in my copy).

Bear in mind that the AT5 and similar UK designs were normally designed for 75 ohm loads. This uses a smaller output capacitor and larger inductor than the now-standard 50 ohm circuit, all else being equal.

I didn't mean adding a switch for coil adjustment. My suggestion of taps was just to allow selecting the optimum value without having to rewind it, given the uncertainty over your HT voltage.
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Old 11th Jan 2023, 8:25 pm   #40
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Default Re: Some times I start something without any real plans.

Thanks for that Dave, I have found the section in the 68 version.

I have managed to get further with the project having installed the modulation stage with the transformer I wound, it seems to work quite well, to me at least.

Have done a couple of basic videos on the project to date. The next stage would be to complete a case for it and add any necessary switching. I am awaiting a quote on a mains transformer to give me 280 Volts at 120 to 40 mA to allow some volts drop with an EZ81, the antenna switching may well be in the PSU, it depends on what relays i can find around. Have still to add an 80 metre coil so perhaps that should be the next part?

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