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Old 17th Apr 2020, 10:59 am   #21
Herald1360
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Default Re: 1.8MHz A.M. bit of fun build.

Wasn't screen modulation referred to as "Series grate" (Series gate) because of its wonderful distortion properties?

On a different tack, my furkle has turned up 100pF and 2*500pF airspaced capacitors. A quick megger check showed the 100pF one could stand 1000V and the 500+500pF one flashed over somewhere between 500 and 1000V. Any use to you? Yours for a small parcel if you want.
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Old 17th Apr 2020, 11:38 am   #22
AdrianH
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Default Re: 1.8MHz A.M. bit of fun build.

The old HP scope is fixed, stupid beam finder switch! It's big, heavy but with 4 trace at 100MHz bandwidth it is a good unit when working.

Thanks Chris for digging, hang on to them the project may get a hammer yet! Got to ask why Herald1360 (Old Triumph) and you have an avatar of a land rover?

Tim and G6Tanuki, I could and may well change the circuits to be anything really, but it is the journey as to why it does not work what am I missing rather than just do this or the other, it may be something stupid I am missing but that is the point of learning whilst I still can. Not a lot else to do!

For instance at present the potential divider before the cathode follower has been removed and the 100pf Cap is driving the gate of the cathode follower, it makes the situation worse.

Back to the osc: The anode sits at 120 Volts with the osc running I have an 80 Vpp signal on the anode, so lots of signal from the first pentode.

Cathode follower: No osc. Sits at 67.5 Volts on the cathode and grid a few volts lower at 65 Volts with no osc signal. Let the osc run and the DC at the grid rises to 100 Volts with the ~80 pp signal centred at 100 Volts DC, the cathode has also risen in volts to a DC level of 125 V with a distorted waveform of 50Vpp with the bottom badly clipped? I have removed the connection to the final pentode so know that is not causing problems, but I cannot understand what is causing the DC shift?

What am I missing?

Adrian
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Old 17th Apr 2020, 1:52 pm   #23
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Default Re: 1.8MHz A.M. bit of fun build.

Hi series gate modulation (ie controled carrier ) will give distorted audio on a nearby receiver because the carrier is not constant and this upsets the agc of nearby receivers, screen (g2) modulation is not the same. screen mod is referred to as efficiency mod little audio is required to modulate a carrier but modulation depth will not be 100%. anode and screen gives the best audio and 100% modulation, but the audio needs to be 50% of the rf carrier to fully modulate. The circuit shown when setup gives good audio/modulation but overloads receivers close in. You can run valves beyond ratings due to the output not being constant hope this helps Mick
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Old 17th Apr 2020, 7:55 pm   #24
AdrianH
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Default Re: 1.8MHz A.M. bit of fun build.

Hello Mick, I saw that circuit in the RSGB VHF-UHF manual 3rd version, it's going to be a matter of finding the sweet spot on the grid, if there is one.


Anyway much moving forward has been achieved. I have sorted the cathode follower, a change in the bias set-up means that it no longer has a DC integration of the oscillator, still only using a percentage of the osc output. The cathode follower output is 11/12 Vpp, but low enough impedance that scope leads, or fingers make little difference.

The final valve is now biased differently I had to use some semiconductors!!!! rather than a resistor and capacitor to bias the cathode at 3.6 Volts, I have used a Zener, or rather for the time being I have used two LED's which when used are acting like a Zener. Also there is a small diode in parallel with the 470K grid leak resistor. Without using the Zener and diode, I found that the valve would self bias it self to stop any chance of it drawing grid current The valve would start to draw mure current and bias the cathode voltage higher, it would also dc bias the grid circuit negative again to stop grid current. I did wonder how much stray capacitance I have using strip board?

The Zener in the Cathode now locks the potential to 3.6 Volts and the diode in the grid input limits the negative charge on the grid from going less than 0.6 Volt. In effect I now have drive peaks from the cathode follower going to +6 Volts, 2 Volts over the Cathode bias and would therefore be driving grid current on peaks. If I am reading the spec sheet on the 5840 correctly at 0 grid Volts I would have a plate current of 16 mA, if I guess with +2 Volts on the grid peak current could be around 25 to 28mA Max limit is 16.5 mA but peaks should be OK.

The un-tuned output across the inductor on the anode is 95 Volts peak to peak, which does not look that bad to me.

Anyway the above is all my mind workings, hope some of it is correct. Amended circuit attached, next to try and do is sort the sweet spot for the g2 modulation.

Adrian
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Old 20th Apr 2020, 10:26 am   #25
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Default Re: 1.8MHz A.M. bit of fun build.

A few minor changes to the circuit. One problem I was havng was to drive the audio stage enough to give a reasonable swing in the PA voltage. A 20 x voltage gain on the single triode in the 6021 was not enough. Then I realised my mistake, using the headphone output of the PC it was expecting to drive something in the 15 Ohms area and not around 500K input. Not having a suitable audio transformer I am trying out a small mains transformer, which is in the area of 240 in 12 Volts ac out. So voltage wise should step up by around 20 times impedance to around 6K which is a bit better.

A slight change to the biasing to g2 of the 5840 and for now just running a small length of wire from the 5840 Anode. I am getting reasonable A.M. results, a bit Bassy which could be the transformer or i have too much decoupling, but overall I am happy, wish I could load a video to here.

Adrian
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Old 21st Apr 2020, 3:06 pm   #26
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Default Re: 1.8MHz A.M. bit of fun build.

I have a Heathkit DX40 which uses G2 modulation of a 6146. It seemed to be fairly pathetic, I changed to a DX100 which had anode modulation of a pair of 6146s. I used this for some time to talk to my sister in Canada until SSB users objected to another sideband.

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Old 21st Apr 2020, 4:58 pm   #27
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Default Re: 1.8MHz A.M. bit of fun build.

Hello Trevor;

It is just a bit of fun for me and keeps me busy, sometimes into the early hours.

I spent decades fixing radios on the bench or installing systems in PMR. So learning about circuit design especially valves in the first place was never a requirement. So now no longer working and being in lock down, it keeps me occupied.

I have been playing trying to match the output into my HP 100MHz scope that has switchable 50OHms inputs and managed a magnificent 34mW of power. I also put the recovered audio through a mod meter and distortion meter on my old IFR test set and at 85% mod I get 10.5% distortion, so it will never win any awards.

I do not have the required spares to try things like mains transformers for screen and anode so g2 mod is the best I can do for now. Struggling working out how to best match it into a small whip or anything really. But still fun.

Stuck up a video on the facebook page, hope it is OK.

Adrian
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Old 22nd Apr 2020, 11:32 pm   #28
TrevorG3VLF
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Default Re: 1.8MHz A.M. bit of fun build.

Some years ago I had a play with making a double balanced frequency mixer. I used a pair of EF80s with cathodes commoned and anodes also. I drove the grids in antiphase and the screens also. The idea was to produce a mixer with the two carriers phased out. This worked but I had not got a strong enough source to drive the screens since they take current.

I then put both frequencies into the grids with a cunning circuit and got a massive output.

I wondered about using the suppressor grids as one input but would need a large negative voltage It works by diverting the anode current to the screen so the screens may be overheated. This modulation method is used in a GEC crystal frequency standard.

I suggest you try g1 modulation
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Old 23rd Apr 2020, 8:26 am   #29
AdrianH
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Default Re: 1.8MHz A.M. bit of fun build.

Would that mean the final has to be in class A if driving mod into g1?

I should be getting a couple of small transistor style audio transformers, basically to replace the mains transformer on the input and drive a bit higher impedance. I was wondering about using the primary winding as in the Codar AT5 (Other thread Top Band Transmitter).

The best output I have managed to get is 60mW unmodulated so requirements for driving the mod stage are still quite low.

Adrian.
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Old 12th May 2020, 5:21 pm   #30
AdrianH
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Default Re: 1.8MHz A.M. bit of fun build.

I did a board with Kicad and had it made, a few bad choices with footprints, but it is easier to play with then on strip board, will never be a hi-fi TX but was fun, quite liking Kicad.

Adrian
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