Eddystone EA12.

[cicero123]

Hello, Can anyone tell me what the rating is for the mains transformer in the Eddystone EA12 receiver. I know that it is 250v 0 250v output. And would any of the other Eddystone sets have a suitable transformer which would do for the EA12. Any other ideas would be welcomed. Thank you.

[G6Tanuki]

Power consumption is specified as 85 Watts. You can add-up the heater currents for all the valves (and scale-lamp bulbs) to determine how much power is going to get things hot, and then anything left-over must be for the HT.

If your mains transformer is dead, there are helpful people on this forum who can rewind it for you. But beforehand, try and work out _why_ the original died, so as to avoid any replacement having a short and unhappy life.

[turretslug]

Eddystone, as a relatively low-volume manufacturer in the scheme of things, used the same model mains transformer type 3937P in several models including the EA12- the ratings are 250-0-250V at 110mA DC output, 6.3V at 4.2A, 6.3V at 0.3A and 5V at 2A.

[cicero123]

Hello Everybody.

Thank you for all the info regarding the Eddystone EA12 transformer. I notice that turretslug says that the output for the 250V 0 250V is 110 mA DC Did you mean 110 mA AC as the output from the transformer would be Ac and not DC. If anyone has a suitable transformer that they would like to sell do get in touch. I live in Scotland.

Thank you all once again.

[Ed_Dinning]

Hi Cicero, a picture of the transformer may help as well as its dimensions. You should also indicate if a larger transformer could be made to fit.
Transformer ratings for this type are often given as the rect DC current; ratings for each "side " of the HT winding are typically 70% of the DC rating, but will vary due to many factors.
These ratings are a bit above that for the average domestic set and it also employs an additional heater winding.

You could also consider rewinding it

Ed

[turretslug]

As Ed says, it's an accepted way of specifying the HT winding capability to simplify the end-user's choice, the transformer supplier does the correction-factor sums to suit. I believe that the EA12, as a set that came towards the end of the thermionic valve era, actually used silicon diodes for HT rectification and noise-limiting so that although the 5V 2A rectifier and 6.3V 0.3A noise limiter windings were still present on the transformer, they weren't actually used. Dispensing with them would make for an easier rewind, or make finding a substitute easier. I sometimes wonder if Eddystone had ordered such a large number of this type transfomer to keep costs down that they still had a store-room full to use up well after the secondary winding requirements had simplified!

[Restoration73]

https://vvttransformers.co.uk/vth12611-1300.htm

[cicero123]

Hello.

Thank you all once again for the information and turretslug is correct in saying that diodes were used for rectification. My next question to you all is the following. The EA12 uses two diodes plus a resistor of 140 ohms in each of the legs from the 250V-0-250V transformer but would it be possible to use a transformer with a winding of 0-250V and four diodes in a bridge configuration. I have found out that the Eddystone 940 uses the same transformer as the EA12 and there may be more models that do so but some of the earlier sets did not use a transformer.

Thank you all once again.

[Radio Wrangler]

Yes, a 0-250v transformer with a bridge rectifier will be fine. Just go by the same DC current rating. Use a single 140 Ohm resistor in the bridge's output or one of the AC inputs (not both) 140 isn't a standard value, so 68 Ohms in each AC input to the bridge would be good, and also helps a little with RF noise.

eddystone made a lot of sets without mains transformers. Maybe for cost saving, maybe for AC/DC power input. They also sold receivers into the marine market where DC supplies were not unusual. These sets have inner chassis connected to neutral (if you're lucky) or mains live (unlucky) You have to be careful with them especially if working inside them. I just avoid all live-chassis sets, I simply don't like them and don't need them.

David

[ms660]

If using a bridge rectifier and a single surge limiter make sure the surge limiter's adequately rated as a single resistor will be dissipating more power than each individual one that was fitted for bi-phase full wave.

Lawrence.

[cicero123]

Hello to everybody, again I thank you all for the help and information. I have one I think last question. If I had a transformer which was configured as 250v 0 250v at 100ma and the ends of the two winding which terminate at the 0v point if these could be separated and the windings connected in parallel I would then get 250v output but what would the current output be. I assume 200ma but would that then overheat the transformer. Thank you all for the replies.

[cicero123]

Hello, As I have not had any replies to my last question I assume that it was not a good question and for that I do apologize. I am well over 80 years old and was trying to enhance my knowledge / I am most grateful to those of you who have answered my earlier questions as I now know more than I did just a few days ago. Good health an happiness to you and yours.

[Radio Wrangler]

It was a very good question. I apologise for not having noticed it.

In theory, you could pick apart the wires at the common point of a 250-0-250 volt winding and put the two parts in parallel.

In reality, you don't do this for two reasons:

Unless the numbers of turns of the two windings are exactly the same, you set up a sort of shorted turns effect with the difference in voltage driving a circulating current around the pair of windings. This wastes power by increasing losses and makes the transformer hotter.

Secondly, when the transformer was designed, the rating was done for the net DC output and allowed for each winding getting a rest over the half cycle its rectifier diode did not conduct. This is allowed for in the choice of wire gauge. Try running both at once and you don't get double the DC current rating, you get pretty much the same.

It's an idea people often get, and builders of high power valve RF amplifiers sometimes use a 350-0-350 transformer with a bridge rectifier across the whole secondary to make 800v or so. They have to work out a suitable current rating and it isn't as much as they usually hope.

David

[snowman_al]

Hi cicero123,
Just wondering what the real question is here?
Do you have a failed transformer in the EA12 and if it failed have you got to the cause?
The rewind route (Ed is your man) will be least expensive and will keep the radio original.
Or is the radio missing the mains transformer all together?