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-   -   Philetta BD 244U (https://www.vintage-radio.net/forum/showthread.php?t=161459)

Cathode Ray 7th Dec 2019 10:14 pm

Re: Philetta BD 244U
 
Folks, in operation the dropper produces a fair bit of heat which I'm thrilled about.

I was thinking a 220/110v auto-transformer might be added to eliminate the dropper.
Does anyone have previous experience of this mod?

'LIVEWIRE?' 7th Dec 2019 10:36 pm

Re: Philetta BD 244U
 
Is there any reason why a double wound mains transformer could not be used, provided one with a secondary winding of the right voltage and amperage is available. There is the possibility of course that there wouldn't be room to mount a transformer inside the cabinet, so it would have to be fitted in a suitable insulated case and connected externally to the Philetta.

Cathode Ray 8th Dec 2019 12:38 pm

Re: Philetta BD 244U
 
No reason I guess, whatever works.
I think a small internal trafo is better, I don't fancy an external one.

A trafo with two secondary windings might add unwanted bulk to a tight space. So perhaps a 2:1 step down might be preferable just for physical dimensions.

I'm hoping it wont add any mains hum to the audio chain...

Thanks

David G4EBT 8th Dec 2019 4:42 pm

Re: Philetta BD 244U
 
2 Attachment(s)
Lots of smaller radios use an autotransformer and the heat dissipated is minimal.

The Unitra Figaro for example, has quite a small autotransformer but it's a 'minimalist' circuit with just three valves. Unlike your Philetta, it has parallel 6.3V heaters, and the heater winding is tapped at 1.4V for the DM70 magic eye. The Pye R33 is another dinky little set which uses an autotransformer. The cabinet looks like it's Bakelite but in fact it's injection moulded plastic so would be susceptible to excess heat. (The Pye R33 uses a series heater chain like the Philetta).

Not sure where an autotransformer would fit inside a Philetta though - it's a tribute to the designers that they squeezed so much radio into such a dainty cabinet as it is. Well done on replacing the resistors and caps - it's just a nightmare beneath the chassis! Both the Figaro Special and Pye R33 used a metal rectifier to save space and current. I guess one rather radical option with the Philetta which might yield space internally for an autotransformer would be to remove the rectifier valve and fit a solid stage bridge, but maybe that would be a step too far.

Ed_Dinning 8th Dec 2019 7:34 pm

Re: Philetta BD 244U
 
Hi Ray, a derivative of the autotransformer I designed for the DAC 90 & 90a might work in this set.
If you PM me the details I'll see if it would be possible.

Ed

telstar 9th Dec 2019 3:34 pm

Re: Philetta BD 244U
 
Hi Ray.
Great to hear that you sorted out your Philetta radio, and the outer knob problem. I will have to power up my B2D93A model to hear that work again.
A few years ago there was a car boot sale near my location. One dealer had quite a few of these Philetta radios on offer, some in brown and some in cream cabinets. A shame that the pub car park that the event was held in is now no longer - now a new housing estate!
Cheers
Mike

Cathode Ray 9th Dec 2019 7:23 pm

Re: Philetta BD 244U
 
Thanks David, I think there might just be enough room above the dropper resistor, if I can find a way to mount it. Alternatively I could use an external trafo if it were small enough to be a wall plug type.

And Thanks Ed, I will send the details on PM.

Cheers Mike,
Yes the 3D printed worked out well since they match each other. Nobody here has even noticed. Its a pity about the lost Philetta's, they dont come up too often in UK or Ireland, most are sold in Germany.

Cathode Ray 18th Dec 2019 2:33 pm

Re: Philetta BD 244U
 
Folks,
While researching Dropper replacement ideas , I came across Paul Stenning's guide on resistor / capacitor dropper's.

https://vintage-radio.com/repair-res...per-calcs.html

The Philetta draws a fairly constant 300mA at 230v AC, even with the volume up loud, so it seems like a good candidate for a capacitor dropper

Paul's excel calculator sheet is an excellent tool and by my calculations it would need a 4.7uF to 5uF Run Capacitor to drop 113v and provide 117v to the radio.

The radio can then be configured for 110v running.

Anyone see any issue with this proposal?

I'm aware that the fail mode of capacitor can be short circuit so there is that risk.


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