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TX Faffing

Thu, 06/11/2009 - 14:21 — m0njp

Currently trying to make a valve transmitter.View from the bottomView from the bottom

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I bought a partly assembled project from another Amateur with no wiring or plan and I am trying to make use of the nice parts.

I have now made a fairly OK Vackar oscillator and buffer amplifier using a 6AQ8 twin triode and will work on the PA final next. Lab Book with DiagramLab Book with Diagram

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tesla-VackarTesla-Vackar

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Vackar-USVackar-US

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The big mains transformer the kit came with gives out 350V HT.  I have full-wave rectified that and I fitted a VR150 voltage regulator valve via some dropper resistors to create a reasonably constant supply for the oscillator.  There was a series smoothing resistor between two large capacitors which I am contemplating replacing with a mains choke to give a better supply voltage under load.  It will be an interesting experiment anyway!

TX SideTX Side

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I have no experience of PA finals so I am looking forward to it - although I am a little worried about the high voltages involved!  My concern at the moment is the more than doube increase in amplitude of the oscillator signal between 3.5 and 3.8 MHz.

I have to think how I will connect the PA final output to the 50-ohm aerial...

Here are some bench pictures:

Some Bench InstrumentsSome Bench Instruments

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Components Leads IronComponents Leads Iron

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

72 cheers

Nick m0NjP / on4Nic

Comments

Mon, 06/15/2009 - 14:55 — m0njp

TX Progress

My Vackar VFO was not good, low output and taking a long time to start oscillating and so I examined the circuit and changed the capacitor between the anode and ground to a smaller one, 1nf to 500pf and now it is much better.

I built a triode buffer amplifier and then put in a 6L6 valve ino the existing octal socket and built a simple PA using a home wound toroid output transformer.  This worked but the output was very low.  I fiddled with the transformer and increased the power but not by much.  I then fiddles with the grid bias resistor and managed to get more power but not much!  I then decided that the input signal should be greater and so I removed all the VFO and buffer amp. circuitry and replaced the twin triode with the triode-pentode which the hardware came with, a 9U8.  I then remade the VFO and fiddled (I use pots for all resistors until the final "design") until it was working well, incidentally better than before with the twin triode. TX-bench 15.6.9TX-bench 15.6.9

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73 cheers
Nick m0NjP / on4Nic

Mon, 06/15/2009 - 15:13 — m0njp

Some Watts Procured!

My 6L6 final managed to generate a few Watts!  I am now working to increase them.  I have a better drive now but need even more if I am to exceed 10 Watts.Some WattsSome Watts

I am using the famous NorCal QRP Power Meter for the first time properly.  My previous 12V Valve TX didn't generate enough power to register!

 

 

 

 

I will try using a tuned anode load for the final and maybe the driver pentode too.WaveformsWaveforms

I am trying to ensure a clean signal from all the stages, oscillator, buffer/driver and final of course!  The optimum power signal is not the optimum shape unfortunately.  The power meter registers over 6W but the waveform was rather zig-zag so no good at all.

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73 cheers
Nick m0NjP / on4Nic