Fixing a small French radio set.

[Geordie McBoyne]

The 25Z5 HT rectifier valve is difficult to find this side of the Atlantic.  However, according to the Brimar valve and Teletubes book the 1D6 will in most radio receivers replace the 25Z5 except in circuits where the latter is used in voltage doubler circuits.
I've won a 1D6 on ebay and when it arrives I'll continue the restoration of the set.

From the Radiomuseum:   http://www.radiomuseum.org/tubes/tube_1d6.html
                                     http://www.radiomuseum.org/tubes/tube_25z5.html

Geordie McBoyne.

[Geordie McBoyne]

The Brimar 1D6 arrived last week and has been fitted in the set, no modifications required.   This set is designed to work on 100 to 120 volt AC supplies, that's the way it was in France in the old days, 230V now.   The 0.3amp heater chain adds up to 75 volts including the dial lamp so 40 volts has to be got rid of by some means.  A capacitive dropper is out of the question because it will have a value of something like 20microfarads if the set is to be used on 115 volts.
However, it's good to hear about other ideas from other radio engineers and Trevor has come up with a good one.  Convert the set to 230 volts. The heater dropper will then become the more manageable value of just over 4microfarads. Better still, a separate capacitive dropper for the HT supply, this will determined after the HT current has been measured.
At the present time for the heater chain ballast I've fitted a RadioSpares "polomint" type of resistive dropper, that's definitely not nice because of the heat dissipated from this component.
Check out the dropper resistor in the attachment.

Geordie McBoyne.

[AlanBeckett]

20 Mic is no big deal if you get a Motor Run Capacitor http://www.ebay.co.uk/itm/Start-and-Run-...530289?var=&hash=item35fbaaa131:m:mfwrpLrRIRh9xkdPtza_S5Q
Space might be a problem.

Alan

[ppppenguin]

For 115V I make the dropper cap 11.5uF. Within plausible range of a motor run cap.

[Murphyv310]

Hi.
David states the use of a separate HT dropper cap. This will only work if a second diode is connected to the HT rectifiers anode, cathode of the new diode and the anode to chassis.
Cap value of around 0.5uf dependent on HT current.

[Refugee]

Be aware of the HT.
You may well find that your capacitor dropper just charges up from the rectifier valve and then sits there passing nothing to the set.
The heater one should be fine.

[Murphyv310]

Correct Ref.
The addition of a diode that clips the negative half cycles cures this problem. The only snag is if you use two silicon diodes the HT could rise to 450+V before its loaded by the rest of the valves when cold, if the set has an indirectly heated rectifier then there is no problem.
I have done many sets this way and the advantages are many.
1, A cap for the heaters on its own is not a compromise, it only supplies the heaters and the value can be accurately worked out.
2, A separate HT cap can be tailored accurately for the HT you need with of course the exception of class B push pull stage outputs when the HT could sag on loud volume as there is less regulation.
3, Big reduction of heat
4, More efficient and less costly on electricity.

David's set will need a 4.19uf cap for heaters, I always dispense with a surge limiter as it's unnecessary and a 0.65uf cap for HT assuming 45ma HT current.

Run capacitors can be used and there is plenty of choice out there with obscure values too. Under no circumstances use electrolytic caps. I find that "X" rated caps are fine for the HT dropper, any faults will blow the fuse and that is one other mod I suggest for safety and reliability, I normally rate the fuse at 50% above the normal HT current, fit it in series with the Cap dropper.
Have Fun

[Murphyv310]

Hi.
In addition to my last couple of posts, id like to say that using a cap dropper for either heaters or HT should not be a worry. Modern run caps are really reliable especially in this use where they are lightly loaded and not subject to inductive or heavy loads from stationary motors or big pulses from motors being switched off and other switching that is on motors with separate start and run caps.
The addition of a discharge resistor in parallel with the capacitor should always be done, a 100k across the heater cap is fine or a 470k across the HT cap will also be be fine.
I have two Murphy TV's with cap droppers, a V330 & V530, the original design uses a 441 ohm dropper resistor for the heater chain when VHF FM Radio is selected, these 441 ohm resistors get mighty hot and in the V530 it causes the EHT rectifier valve base to degrade.
So in these sets I have a main heater chan dropper cap and a secondary one for the radio chain when it is switched over.
In this case I use a 27k resistor across each cap to reduce sparking on the Radio/TV slide switch. Both sets have been 100% reliable with the caps and the V530 is used for hours on end on Radio with a massive reduction of heat.

[Geordie McBoyne]

Trevor has explained clearly that an additional diode is required in order the capacitive HT dropper will work.   The extra diode is essentially a DC restorer and is wired so that the cathode is connected to the low potential end of the capacitor and the anode goes to chassis potential.
So it follows the AC waveform "sits up" above ground potential.   Without the diode the anode of the rectifier will be hit with only DC pulses.
Actually if a 25Y5 or 25Z5 rectifier valve can be found then the classic all thermionic solution could be realised.  These valves were designed for voltage doubler power supplies.
www.radiomuseum.org/tubes/tube_25z5.html   or    www.radiomuseum.org/tubes/tube_25y5.html

Picture of the reassembled set.  The metal escutcheon can come into electrical contact with the chassis metalwork so before the job can be signed off something will have to be done about that.

Geordie McBoyne.

[Murphyv310]

Hi.
That is one beautiful set David. What a little sexy radio.
Bet you Bridgette Bardot had one