Seeking Valves

[Nowhere-Man]

What resistor are you measuring?
It sounds to me like the cathode resistor (connected to pin 8 of the KT61) and it should be left at the marked value.
The most likely cause of low anode volts is bad biasing on G1 of the KT61 (pin 5).
Measure the voltage from the chassis to Cathode (pin 8) and post it here. It should be about 4.5 volts.
Do the same for G1 (pin 5). It should be less than 0.5 volts.
If pin 5 is above 0.5 volts replace the capacitor that is connected to pin 5 with the other end connected to the other audio valve.
That should reduce the anode currant and allow the anode voltage to rise.

I've been swamped overall with work as I'm trying to fix up 2 seventies canal boats before the weather turns nasty towards Winter. Plus, I seem to have a growing workload of more valve radios all waiting their turn. I tend to work on my radios on the rainy days and, as it was raining yesterday, I packed up my boat work and went to Maplins. Then today, it was sunny weather again but I decided to carry on and do some radio related work. Why not?

The output valve you refer to is, in this case, an N78 although the manual cites the KT61. Funnily enough, I do have a KT61 on a stored HMV and it's bulkier than the N78. Anyway the GB resistor was measuring 160 Ohms and the manual specifies it rates at 100 Ohms. Also, the GB resistor itself seemed to have leaked and bubbled in its past so I'd decided to replace it for a modern 100 Ohm from Maplins. I was expecting this would also lift the voltage a touch on the output anode of the N78. Anyway I kept with the specified 100 Ohms resistor for now as that's what the manual lists although I know when you fit a silicon rectifier, you get into murkier waters.
Soldering that resistor was tricky as the modern one is larger in bulk and I found no way to safely squeeze it in. I winded up using a short run of DC wire to one end of the resistor and the valve pin as the only option and I seemed to get a really strong joint in the end. I then tested for Ohms and all was good.

On testing, it seemed to me output anode voltage was a bit lower if anything. I tested the rectifier ripple voltage and that was fine (around 275). I tested my new 100 Ohms resistor and it was merely 2.4 volts between pin and chassis. However, I ought to point out it's a 7 watt resistor and not a 5 watt!

So, now we can consider the 1 K series resistor between rectifier cathodes and the HT capacitor. That, as I'm aware, tends to be a big bone of contention since a lot of engineers feel strongly you don't need the series resistor, just the diodes and capacitors. Or others may recommend a token 200 Ohm resistor to smooth the current rush a bit on start-up. As I'm not a designer, it would be out of my scope to argue any case for or against. I do have a 500 Ohm resistor from Maplins and I'd be interested to see how that might alter the voltage readings. Surely it will go up a bit as I already measured a 30 volt drop across the 1 K that I have.

Lastly, I find myself wondering if at this point I may be better off getting a rectifier valve now I know it works. It would be a bit safer perhaps than silicon diodes soldered to the valve base pins. Of course, that would add at least another tenner to my costs (I think I spent close to 35 pounds to get to this point).

To Refugee: thanks for the tips and I'll follow up on it tomorrow.