Seeking Valves

[Nowhere-Man]

A few points that might help:

Firstly, you'd be much better off using the 8-page GEC data sheet, which is far more descriptive than the sparse two-page Trader Sheet #977.

For example,  the GEC data  has diagrams of all of the valves bases (whichever valves are used in the line-up), and tells you which tag on the valve-holder does what. (Heaters, Anode, Screen Grid, Control Grid etc). It also gives you a table of voltages and current values in a working set, and a chart which gives the colour codes of the eight colours used for the wiring. Obviously that's a great help in identifying which wire is which on the valve bases. There is also a clear diagram of the wave-change switch/coils.

As far as I can tell from the maker's sheet, whatever mix of valves are used, the only valve which has a B7G valveholder is V1 - all of the other valveholders are International Octal (not Mazda Octal).

Personally, before I switched the set on - whether or not through a lamp limiter - the very least that I'd do is to replace the AF coupling capacitor or you risk damaging the output valve and output transformer.  This roles of this capacitor is to pass the AC signal from the IF stage while blocking and DC. However, this capacitor is a known problem whatever the make or model of valve radio, which has earned it the reputation 'that' capacitor. Over time, it becomes leaky in the electrical sense, and can pass DC to the grid of the output valve causing it to draw excessive current, overdriving the valve and stressing the output transformer. On the two-page Trader Sheet that cap is 'C19' and on the 8-page GEC data sheet, it's denoted as C12.  (.02 uF originally, for which a modern polypropylene or whatever cap will be .022 F).

How far you go beyond that point depends on whether you just want to get the set working, 'do it up a bit' or to 'restore it', bearing in mind that it's 65 years old and several components and maybe some of the wiring will be well past their 'best before' date.

I don't know what test gear you have, but presumably a multi-meter for checking voltage, resistance and current. When checking voltages I always clip the negative test lead to the chassis, then you only have to poke around with one test prod - not two. A lot of tests can be done with the set switched off and not connected to the mains - switch continuity, coil and transformer continuity and resistances, valve and dial bulb  filaments, resistor values to see if they're within say 20% of the original values or have gone too high in resistance, and so forth.

Hope that's helpful.

Good luck with the set.

Yes, some helpful comments. I don't know if I mentioned already I replaced the capacitors you refer to. This had to be done when making a new silicon rectifier because it's assumed the D.C. voltage will be higher and closer to peak A.C. I used Wima capacitors at very high voltage. So, there ought not to be any danger of positive voltage getting onto the output valve grid.

Here is a summary of the possible valves. I had the idea to double check and measure the screening cans. This is a crude approach but effective in cases where the choice is between large and small tubes.

The detector valve may be either the DH63 or the DH77.

The DH63 has a diameter of 27 mm. The give-away is it's also 80 mm tall.

The DH77 is 17 mm diameter but only 46 mm tall.

The screening can on my set is pretty much bang on 46 mm. That seems a bit tight but the alternative of an 80 mm can suggests to me DH77 is the likely tube.

The IF amplifier options are as follows:

KTW61 at 36 mm diameter and 98 mm tall. My screening can couldn't contain it.

W61 is at 29 mm diameter and still 96 mm tall. Still too big for the can.

W77 is a smaller tube with a diameter of only 17 mm and stands at 46 mm tall. My screening can seems to fit that one - the others would never go in it.

The mixer offered a surprise. I had assumed it was the X61M. However, this has a 30 mm diameter and is 96 mm tall. I checked again with a measure and it didn't add up. I feared, therefore, it was the dreaded.....

X78 - a rare and expensive tube that would be out of my budget. That would mean fitting a new base maybe to fit a substitute alternative. The X78 is 17 mm diameter and 45 mm tall.

However, there is hope: The last possible valve is the X79. This is 21 mm diameter and 48 mm tall and it's closest to my can. Also I managed to get a lens inside the can and there is definitely 9 pin holes. Some people say even that isn't a guarantee but I hope it is the X79.

Fortunately the company would allow me to send a valve back if it doesn't fit.

Screening cans can be deceptive as the actual valve inside the can isn't going to be identical but I suppose if you have a very small can and a huge tube the tube isn't going to go into the can.

Of course, much of this is new to me. As I said, the radio has been messed with and the cans were all neatly clamped back on to hide the obvious. Mains leads had been cut off. I wouldn't recommend taking on a set that has been messed with unless you have more experience than I do but I'll do what I "can" (pardon the pun)