A 548

[boater sam]

I think its an A 548.Supposed to have an EL3 output valve but its got an octal socket.

Sam

[boater sam]

It would seem that the equivalent for EL3 is an EL33 which is an octal base, I will try this checking the connections first.
However the rectifier, supposedly an EZ33, has no equivalent that I or AI can find so I will have to rewire the base for a 6X5GT as it needs to be an octal with a 6.3v heater.
Sorry if others are finding this boring but I like a challenge.

Sam.

[boater sam]

It would seem that the equivalent for EL3 is an EL33 which is an octal base, I will try this checking the connections first.
However the rectifier, supposedly an EZ33, has no equivalent that I or AI can find so I will have to rewire the base for a 6X5GT as it needs to be an octal with a 6.3v heater.
Sorry if others are finding this boring but I like a challenge.

Sam.

[Paul_RK]

I'm very suspicious of that EL3, I don't think I've ever met one in a thoroughly British post-WWII radio, and even the set photographed at RM.org sports an octal output valve. So I've just been up to the attic to dig out my own A548 - I repolished its cabinet about 30 years ago, fairly sure it was working at the time - and its current output valve is a KT61, rectifier a U50, for whatever that may be worth.

Paul

[boater sam]

Thanks Paul, that is interesting. I will have to remember that when I get round to restoring it. 
KT61 is very probable but the U50 confuses me as its a 5v heater of course and the EZ33 was 6.3v which is why I thought of using a 6X5G. I will have to check the transformer for a 5v winding.

Incidentally, why were most of the rectifiers of the period 5v?   And why 6.3v and 12.6v, is there some magic reason?

Sam.

[Paul_RK]

Incidentally, why were most of the rectifiers of the period 5v?   And why 6.3v and 12.6v, is there some magic reason?

Sam.

The usual reason offered for 6.3V is that it's the voltage provided by a fully charged 6V lead-acid battery... which only raises the question of why, as 4V and 2V heaters were commonplace in the heyday of lead-acid batteries, they're not quoted as 4.2V and 2.1V respectively.

It's also claimed that 5V was chosen as the minimum useful voltage from a 6V accumulator as it discharges, which seems to make even less sense as (a) there's no rationale for specifying a heater according to the minimum voltage it's going to see, and (b) rectifiers never run from lead-acid batteries anyway: ah well. The U50 like most other rectifiers is directly heated so has to have a transformer winding independent of the supply to the other valves, so pretty much anything could have been chosen and there was no advantage in powering it from the same winding as them.

In the very early days of mains radio, pretty much anything was chosen, hence I'll have to make a few changes if I ever try to get my 1929 Columbia radiogram up and running: its three RF valves are present and probably correct, all 4V heater types, but missing are the output valve, a PX650 power triode with 6V heater - highly coveted by audiophools, hence or otherwise almost unobtainable - and the rectifier, which was an oddity with an 8V heater.

Paul

[boater sam]

I suppose that 12.6v is similarly the charged voltage of a 12v car battery, ignoring the fact that early cars often had 6v batteries!

Your 8v rectifier will be a real challenge, to bad I no longer do any glass work else I could have made you one. Perhaps Glasslinger could be bribed with a few frocks  or ear rings to make you one?

You could go DC heaters and use a buck or boost module or course but that would make it a hybrid. I have used an 807 as a rectifier once.