Two-tone testing SSB Tx

The two 6146 output tubes (V12 and V13) are in parallel. My guess(tm) is they might be mismatched. Try the two tone test with just one tube.

Intermodulation is another name for a not very linear amplifier. Look at the PA input and output waveforms and check for visible distortion. Also, check if the 6146 output stages are operating in the linear region.

The "loads of grid drive" might be a potential problem. Even a very linear output stage might produce distortion when overdriven.

The ALC (automagic level control) seems rather odd. I don't see any capacitor filtering that could remove audio modulation from the rectified RF after the diodes V16 or around V4. If there was audio modulation superimposed on the ALC voltage, you will see considerable distortion.

Check if the ALC actually works. Adjust the 50K potentiometer near V16 and see if the RF output goes up and down.

Thanks for that suggestion, there is a big difference between the two valves when they are inserted separately. Some of this might be accounted for by the need to re-tune everything because of the changes in valve capacitance, matching impdance and neutralisation but it looks pretty clear that one of the valves is almost dead.. It is possible that even the 'good' one is not as good as it should be, although it gets pretty hot on full power. I haven't got a valve tester, so I can't measure the peak emission.

It is quite easy to alter the grid bias and the drive level independently so as to explore various operating conditions, the circuit appears to be quite tolerant over a reasonable range of adjustment and the output waveform looks good until it hits the flat top.

I can adjust it to give optimum results but the power is still low.

It is there: there is a capacitor from the juntion of the two anodes of V16 to earth.

It sort-of does, although it doesn't have much effect at the power level I can achieve. I have tried unplugging the EB91 and the flat-topping doesn't improve.

It all seems to point to low emission in the output valves, so it looks as though an expensive purchase is inevitable.

Maybe try these to see if there's a difference before deciding whether to get more expensive ones.

Have you tried the various tuning instructions and test points in the manual?

The idle anode current should be about 50 ma. See Section 3-2(4). I think it means 50 ma with both 6146 tubes functioning. Is the idle anode current equally divided between the two tubes?

The manual is for the MkIII, this is a MkII, a lot of ot applies to the MkII but there are some significant differences so I am having to fall back on my own general knowledge of these things, The drive stages have lined up all right (at least, on the 7 Mc/s band) but I have found all sorts of faults in both the hardware and the original design (e.g. cathode followers that didn't, turns removed from the VFO coil, valveholders with pins broken).

There is a note in the few sheets of paper tha pass for instructions (apparently typed by one of the previous users to assist others who couldn't get it to work) which suggest that the standing current with no drive should be lowered from 50mA indicated on the meter scale to 40mA. As I had to replace the burnt-out meter and make a new scale, I put the mark on my scale at 40mA.

Whether that is shared equally between the two valves is not easy to determine as it would either mean disconnecting a lot of the cathode wiring around the valveholder, or putting a meter in series with the top cap anode feed (at +700 volts)- neither of which I would be particularly keen to do.

I suspect that, with the present pair of valves, the currents would be a long way out of balance.