KB 214 "Transportable" radio

[Crackle]

Here is an updated schematic, now with working voltages.
   
Mike

[Murphyv310]

Hi Mike.
I'd say your 175v off the rectifier is low. What voltage do you get from the ht winding of the mains transformer with the rectifier in circuit and removed. Interested to see if it sags significantly with the valve in place.

[Crackle]

Hi Trev
With the rectifier out, the AC voltage is 275v AC, there is a bit of an imbalance between each phase, the other one is 265v AC.
The field coil is drawing about 12mA and the current through the choke L12 is about 16mA.
I suspect even this rectifier is a little low on emissions.

Mike

[Refugee]

Have you checked the resistances of the HT winding sections?

[Crackle]

Hi Ref
Yes I measured them, the values are on the schematic, but I expected them to differ, they always seem to I believe it is because the outer windings are longer because of the different length of the turns as they pile onto the transformer.
There is no sign of heating on the transformer, and the lamp limiter shows no glow. I have had some other mains transformers where the losses in the transformer seem to make the lamp glow faintly even with all secondaries disconnected.
Mike

[Crackle]

I have found a spare U10 rectifier valve I had spare and now the HT is up to 247v, I think its rather too high now. So that proves the rectifier is not very good, but I will probably keep the 506K in, which was slightly better than the original DW2, so as to not stress other components too much.

I have also found a data sheet on the AC064 http://g3ynh.info/valves/old/Data-Books/...G-p046.pdf

Mike

[Murphyv310]

Hi.
I'd say that 245 volts is what I'd be looking for, I would check the total current now. I would think that it would be no more than 45ma (or a bit less) which would be about spot on for a set of this type.

Regarding the resistances of the HT secondary they will be different dependent how they are wound. They would only be the same if the windings were interleaved but this make the transformer more expensive to to wind. I'm sure Ed Dinning would explain far better than myself.

What you have said Mike is what I thought, there is a little drop on the HT winding when the valve is in use, that is normal as you know due to the resistance of the winding but at 175v DC output the valve is either low emission or your pulling too much current, which of course the load isn't, the OE valve is rated at 70ma so as far as I can tell its well down in emission.

[Crackle]

With 240 volts on the anode of the AC064 it is passing 20mA so a little over its spec. I feel. As it is working OK at the lower 175v I am going to leave it at that rather than adding a dropping resistor in series with the choke.
Tomorrow I will be back working on the cabinet. I have done the top, control panel and front, finished with a very thin coat of lacquer to seal it, and then rubbed off the excess to leave a nice smooth satin looking finish.
Just got the sides and base/legs to do now and fit a new grille cloth.
Thanks for all your advice and comments.

Mike

[Murphyv310]

Hi Mike.
If the O/P valve is drawing 20ma @ 200v than I'd suspect either the biassing is wrong or the valve is leaky a bit like the UL41's.
You did say it was low emission earlier, low emission valves should not be able to dissipate the full current they can.

[Crackle]

No Trev, it was 20mA at 240v anode voltage, the cathode voltage was 22v at that current so the automatic bias seems to be working ok.
I dont want to stress the OP valve, I dont think I would never find a replacement, and would probably have to use a PX4 which would probably have less gain.
Mike