[dominicbeesley]

Originally Posted by Sean Williams

Dom,

Looks to me like you are applying modulation to the B+ (HT - we are English!) of the oscillator valve.

Hi Sean, the Triode of the second valve (the oscillator) is powered straight from the HT (top end of the 22K anode load goes to HT) which at the moment is a regulated bench supply. The wave length stays rock solid and there is no discernable FM...I may have to put some extra decoupling in for this stage when I come to building a real power supply for the thing from scraps.

Sorry if the diagram is a bit unclear with the cross overs. The second pentode is a cathode follower to match the high impedance of the audio amplifier triode to the low impedance required to drive the modulator. Hence its anode and screen are connected to B+ and it's running as a beefy triode and only its cathode and grid should be modulated by the audio.

Originally Posted by Neil Breward

Dom - Must say I've never seen PCL82's being used like that before. Excellent use of resources!

Cheers,
Neil

I must say they're one of my favourite valves. My HiFi Amplifier is the first valve project I designed from scratch (see http://brahms.demon.co.uk/software/e...2_1/index.html). And the sound is wonderfull almost up to audiophool standards, all for a few quid's worth of telly valves!

Originally Posted by Aub

It would be interesting to see what would happen if you increased the HT voltage to, say, 150V, as everything's working in starvation conditions, especially as the RF amp is in series with the modulator valve.

I supspect that there would be more available RF energy to go that bit further round the house. Still, it's an interesting circuit which obviously works!

All the best

Aub

I probably will up the voltage a little but am very wary of upsetting the neighbours! These PCL valves work very well at low voltages though. My first attempt was with ECH42 as a modulator valve but this was rubbish at lower voltages (I guess due to the variable mu nature of the thing?). I plotted all the curves up to 30V and the pentode sections give about 10ma of usefull poke even at these low voltages.

Also I don't want to push my luck too much with the heater-cathode voltage, as I'm running the first pentode as a cathode follower and they're not really meant for this

Originally Posted by Neil Breward

Dom - What's the function of the feedback loop from the aerial - it doesn't seem to do anything?

Cheers,
Neil

It demodulates the final signal as close to the aerial as possible and feeds it back in anti-phase to the input audio signal to provide overall negative feedback. The diagram is wrong in this respect, there should be a 1000pf in parallel with the demodulating resistor - new diagram in a moment. This all, reduces the modulation by about 3dB (countered by turning up the CD player source) and makes a positive difference to the overall sound quality. (Without this feedback a 1kHz square wave source looked pretty ropey at the end, with it present its almost perfect!)

Thanks for the interest

Dom