I made changes to the receiver and later to the transmitter.
After a while I had difficulties with the paraset to get on frequency.
Mainly to some drift from the oscillator, and the tuning from the regenerative receiver.
So I decided to try change all that.
The paraset I ended up with is nothing like I started out with, he is completely different.
After I discussed it with my friend Egbert PA0EJH and his advise on the receiver and transmitter I proceeded with the modification.
As Egbert indicated the receiver must be simple otherwise it would t be a para set any-more.
Having said that the receiver must be simple, because it is an Direct Conversion receiver, doesn't mean this receiver is easy or simple to build.
It is not for the beginner or the faint-hearted, I did find this out for myself in the very early years.
Everything from the receiver has to be shielded (compartments, valves) including the antenna.
The antenna doesn't need shielding when the receiver stands on it's own.
And to make life more difficult, I have to work with an powerful oscillator who can overwhelm the received signal .
First the transmitter.
For the oscillator I had to search ( study) the old books for a fitting solution.
It is an Hartley oscillator, but in a different setup and is very stable.
Normally you have the capacitance between grid to cathode and the temperature factor etc from the valve what in turn changes the frequency , now the valve doesn't respond to those factors any more.
The valve doesn't care about voltage drop, varying current, or the load, the valve stays nicely on frequency.
But that change from the oscillator comes with a price to pay.
The valve needs extensive shielding (grid anode)and decoupling, so he has to be boxed in completely.
At 250 volts, the signal was approx 55 volts top top with 6SJ7/6AB7, grid and anode must be shielded from another.The coil is 40mm 22 turns(20uH) closely wound.
I did drop t the 250 volts to 150 volts, after that change, the signal coming from the valve was still huge, perhaps the voltage must be even lower than that.
Normal setup with the coil and tuning capacitor in place the frequency would run from 3 to 8 MHz, and that is not what I was looking for.So I changed that and it's working well.
The oscillator now runs from 3.4MHz to 4.2 MHz, and there is band-spread involved in two ways.
The first is using fixed capacitors in series and parallel with the tuning capacitor.
In addition I fixed a second small variable capacitor from 10 picofarad parallel to the main tuning capacitor to be able to tune properly.
The end stage is 6V6 (Grid and anode must be shielded also)with 50 mm wide coils with 17turns(17uH anode) and 20 turns(23uH antenna). Both coils closely wound.
Upgrading to a bigger diameter tunes a lot better.
Both valves oscillator en end stage must be shielded from another.
So everything has to be put in compartments. You have to check the coupling (excitation)from the oscillator to the end stage, 6V6 has to have the right load!!
A low-pass filter is in place for the antenna output,frequency drops off by 3.9 MHz, above that nothing gets out on the air.
The 6V6 as end stage, the max is 320 volts.
At 250 volts I was able to get 3 watts HF.
The receiver.
This blog PD3WMZ exists now a number of years now, I made this Channel ON2WMZ that is now available on YouTube:
Video's over a lot of project's restorations building and so on.
Click on the link below and you go Straith to my channel ON2WMZ.
