I am trying to get this radio working, but the speaker transformer was oc and the EL3NG was stuffed, I have a new valve and another speaker I can use to test it but it's still no go, I haven't been able to get a circuit as it must be an odd ball set but the valve lineup and circuit seems identical to the crammond 505 which shows the cathode to the EL3NG/ EL33 grounded via a 150 ohm resistor and a 25μF 40v electrolytic capacitor which seems to be right, but could it be possible that the cathode is meant to be grounded, I'm thinking this may have been a mistake by somebody else explaining the oc speaker tranny and burnt out valve? Does this seem logical? I've never known a output valve cathode being directly grounded, it just looks wrong! Your thoughts appreciated!

Hmmn even stranger the cathode on the 6B6G is grounded also and the grid is grounded too and has a resistor going from the ground soldered ground terminal to ground? Doesn't seem like good use of a resistor! The wiring appears authentic, it is so neat, maybe the last tech to work on this wasn't wearing his/ her glasses? or has done this as a puzzle!

I'm no expert but it sounds as if it might be back biased.

I've never known a output valve cathode being directly grounded, it just looks wrong!

It's one of the ways of handling grid bias:

https://robrobinette.com/images/Guitar/Overdrive/Grid_Leak_Bias.png

Ok well I'd better not go and throw 150 ohm resistors in there yet, I wish I had a schematic, its got 6A8G, 6U7G, 6B6G, EL3NG/ EL33 and

5Y3, but the resistor going from ground to ground is downright weird???

Aha the resistor doesn't go to nowhwere, the terminal 6 ehre the resistor is soldered is not connected to the valve internally, it actually has a wire appearing to be grounded but emerges through a blob of solder from the other side of the chassis to the top cap being the grid. I don't understand this back biased thing, I always thought a cathode resistor and capacitor were necessary??? In any case the schematic I have for the crammond despite the same valve lineup must be wrong?

I haven't been able to get a circuit

It might be worth trying the Kiwis, if you let them know the valve line-up:

http://www.nzvrs.pl.net/

Cheers GTC, this is the one I bought with the large dial glass that was smashed in transit, it may well be a kiwi set! Worth a try cheers.

"Back bias" is achieved by placing a resistor in series with the negative side of the B+ supply and ground. The B+ current flows in this resistor, giving you a voltage that is negative with respect to ground.

This voltage can be used to bias several valves and saves components. It also allows the cathodes to be grounded.

Back bias is especially useful if the valve has extra functions like diodes in it that share the same cathode. Example: 6BV7.

By allowing the cathode to be grounded it makes life easier (simpler) in the rest of the circuit.

One can always reverse engineer it: I have had to do that several times & is one reason the XP computer is kept. It runs the ageing AutoCAD.

Its set up principally for drawing circuit diagrams. One does a layout first & plays join the dots & then you transfer it to the AutoCAD...colour is your friend. Tedious but effective.

I love it with the Americans when its not a AA5 & someone shoves the field coil into Neg. Its like giving them an engine that's not "square" (bore & stroke equal) it causes dilemma's; As opposed to the Brits, who ... given an engine, will build a car around it.

This is interesting: They may have done a reverse AWA? Philco I believe was American Philips but I could see valves being sourced from Philips Australia.

http://www.nzvrs.pl.net/aaa/philcosets

I'm getting noises like a buzz in certain spots when moving the tuner in a couple of positions on the waveband switch which has 9 positions?

I'm guessing (and only guessing) that the switch may be the issue, maybe a broken contact as the amplifier gives a good buzz when touching

the grid of the 6B6G and the 6A8. Ah yes thanks Marc, I have come across lots like the B15 Radiola which have the back bias setup! I've

twigged now, where one filter cap goes to ground and the other to the negative voltage from the mains transformer, my Stromberg Carlson

has an extra electrolytic filter cap going from negative at the back biased mains transformer negative with it's positive to ground! A bit of

overkill perhaps, but no doubt it must be necessary for it to be made that way. Actually now you have explained this it's actually the type of

circuit I am most familiar with! I've often pondered with sets which have the mains negative and both filter caps going to ground, how the grid

get's its negative bias? Time to read up on it now I have some good books, Frye's Audio Receiver Servicing and the Valve Radio and Audio

Repair Handbook. It's got me a bit stumped at the moment, just hoping for one of those light bulb moments! Thanks for your help guys!