Document in Post 1 has been updated.

Just curious, I noticed in the diagram that the grid resistor on the 6BD7 is 1M ohm, usually it's 10M - is it really 1M ?

In the absence of any other circuit at the time. The radio museum one linked is back biased

When reverse engineering, I normally go for a layout diagram first I use an AutoCAD from about 1995. Most of the valves types are in it to import, if not I drew & saved them to import into the drawing as needed.

Kevin Chant has a couple that I reverse engineered. STC Chassis 59 was one, with explanations of source & mods.

Many valves are not at all fussy as to what they amplify. Amplifying RF can cause issues if its in the audio, as it can be radiated. In some cases there is a pF cap on the output tube plate for RF decoupling. I have at times used an RF choke, Autodyne style, to keep the RF out of the output tube.

If 6BD7 had a cathode resistor (pin3) that would set the bias & 1Meg grid leak would not be unusual, albeit 6AV6 is often10Meg.

450V is about 10times over kill for 6M5 Cathode.

Robert,
the grid resistor on 6BD7 is odd. It measures 1.1M, but it has a black body with brown/green/brown bands. I can't make sense of it. I think I'll replace it. Should I replace it with a 10M? If it helps, there's -0.4VDC at the grid (pin 2).

Marc,
I don't know why ordered a 450V cap for the 6M5 cathode. The original was 40V and quite large. I think I assumed it was the same rating as the other electros. I guess it can't hurt to have it over-rated for voltage.

Hmm, if the audio is fine then probably leave it as is. You can replace it with a 10M if you want.

I tend to use capacitors that are somewhat in the range of the expected voltage. So you could use a tiny 10 or 20 volt electro on the cathode if you wanted. Those old 25 or 40 volt giant ones are always open circuit by now.

Also you didn't need to use a 630 volt capacitor on the volume control, a 25 volt one would have been fine (and cheaper). Capacitors on the AGC line are much the same, you don't need anything higher than 25 volts. Still, what's done is done, may as well leave it as is now.

As I fix there is a thing called inventory and that costs. So, one tends to stock 630V NP Caps for tube radios as that keeps the inventory down.

63V covers pretty much all cathodes and if you buy some in RB, they will mount on the socket but keep the leads long & buy the higher temp ones for all.

With Plate resistors on detector first audio, they have an attrition rate. Believe it or not grid resistors, be they the grid leak, or the grid stopper also have an attrition rate, especially on 6V6. The grid resistors on horrid little 6AV6's are rarely good. Where there is a cathode resistor, it sets the grid bias.

Best practice, from a commercial point of view says check resistors as best as, to save problems & rework later. If a ceramic, or Mica cap has one end out of circuit: Leakage test it.

One does not replace just the filter electrolytics, you replace all of them.

If the resistor is original and not cooked; If the resistance does not match the colour code, check that you read it right, the circuit if you have it and if its non compliant: Replace it.

"With Plate resistors on detector first audio, they have an attrition rate. Believe it or not grid resistors, be they the grid leak, or the grid stopper also have an attrition rate, especially on 6V6. The grid resistors on horrid little 6AV6's are rarely good. "

Would there be any value in replacing these high attrition rate components with two components of appropriate value in series, so doubling the voltage rating of whatever the function is? Of course a higher voltage component could be used, but doubling up approach would also keep inventory cost down.

Jaycar rate their 1W carbon resistors at 500V. At 65 cents for two, they are as cheap as chips. Are they suitable for use in valve radios? Assuming dissipation is below 1W of course.

I have finished my 455kHz AM modulator and used it to adjust the four IF transformer trimmer screws for maximum volume through the speaker. The IF section was already pretty well aligned. Only one trimmer gave an improvement. Reception is improved.

There is some hum through the speaker when the radio is tuned to a dead spot. Connecting the scope to the speaker terminals shows a complex waveform with a fundamental of 50Hz. The hum is audible with the volume fully down, suggesting it's from the power supply. The hum isn't very loud. I suspect that's because the repaired speaker cone is now too stiff to reproduce 50Hz very well.

I don't know if it's worth chasing down the hum, as it's not audible when the radio is tuned to a station. But I'm a little concerned the hum is over-working the 6M5 output tube.

It's been suggested I add a choke to the power supply in series with the 3K3 resistor between the two B+ filter caps. Where would I source such a choke in Oz?

Hum sounds like normal to me. Don't worry about it "overworking" the 6M5, it will have no effect at all.

You could simply double the capacitance of the 2nd filter cap.

Thanks Ian. It may well be normal. My ears have been spolied by decades of HiFi.

I have finished the restoration.

I was unable to source a replacement for the missing volume knob, so I made one. I made a brass centre with a ¼" bore. I 3D printed a mould and cast the brass centre in epoxy. I drilled and tapped an M4 thread for the grub screw then used an indexing wheel on my lathe to cut the grooves in the knob. I spray painted the original and the new knob with off-white enamel to get a perfect colour match.

I made a new dial window from 1mm thick PETG. A vacuum forming "buck" was modelled in CAD to match the original window, then 3D printed. I made a rudimentary vacuum forming table and formed the PETG over the buck. The result exceeded my expectations.

I made a new back cover from 9mm MDF. It's modelled on the style of a different model AWA Radiola I found on the internet.

The bakelite got a cut and polish. The speaker cloth was removed and given a few light coats of matt white undercoat, resulting in an overall off-white colour.

Thanks to all who contributed to this thread.

Thats the sort of work I like to hear about.
Getting stuff done by applying engineering!
Looking forward to seeing pix.

Fred.

Photos uploaded to Post 87.

Fred,
thanks for your kind words.