Band 1 Channel 1 Rod Pentode modulator

[peter scott]

Trevor,

I think it is very commendable that your 405 line modulator will still function after Putin sends us a massive EMP but is it not an oversight that in powering it from your solar system you have a transistor inverter in the way? I really think that you need to redesign the inverter using a rod pentode or two.

Peter

[Murphyv310]

Hi Peter.
Indeed you are totally correct, I hang my head in shame.
Perhaps I should use 9 PP3s and a Ni-Mh instead.
A Rod Pentode inverter...... Now that "is" a challenge!

[BusyBee]

One thing I have often thought about this EMP thing is that there are many other components which work on semiconductor principles. I like your comment Peter in that it's easy to overlook things and the weakest link will enable a system to fail. I personally have many doubts about the sometimes said, deliberate use of vacuum technology to thwart the effects of EMP. I am no nuclear expert though.

It would be an interesting side project to build say a 1п24ь invertor. Slow startup at low voltage with feedback of higher for normal running? Maybe we are back to TV line output stages lol. I have not looked up valve invertors but they used vibrators so maybe it was seldom considered?

Tracy

[peter scott]

Yes, of course! A vibrator is the answer. I don't know why I didn't think of that because it is only last week that I dusted off my old Radiomobile HMV 100. It has the power pack separate from the radio but you can link them together without the umbilical cable. At one point whilst checking voltages in the radio I was horrified to read an HT of over 300 volts! I think that without a proper ground connection between power pack and radio the various filter capacitors were operating in voltage doubler mode.

Peter

[Murphyv310]

Ah yes a Vibrator!
Mind you I could take the easy route and run it off the mains, HT Current is less than 1ma so a voltage multiplier would work nicely from a low voltage secondary mains TX.

[peter scott]

There might not be any mains after Putin sends us the EMP.

Peter

[Murphyv310]

True Peter.
No point in the modulator then as there won't be mains for the 405 TVs

[Murphyv310]

Hi.
It may be quite possible to dispence with using the 1ZH37B and use a 1ZH24B or 1ZH18B. I'd say normal Anode modulation could work with the type of transformer I am using in the existing circuit. The advantage would be lower filament current and possibly lower HT, RF output would also increase.
Perhaps worth a try.
The above two types of valves are also more common and cheaper.

[peter scott]

True Peter.
No point in the modulator then as there won't be mains for the 405 TVs 

I think there will be loads of solar though so let's keep the essentials like 405 line TV running when everything else around us is failing. Standards must be maintained.

Pete

[Mike Watterson]

Also as all the simple manual tuned UHF modulators I have are only LC for the RF, so perhaps a crystal isn't needed. AFAIK the audio uses a 6MHz resonator for the FM carrier which is simply added to the video baseband.

However I think 405 needs a separate RF AM for the audio.