Bestone model 20?

[egerton]

I spotted a "loft find" going locally - a Bestone chassis marked as model 20 ac/dc so I took the plunge and bought it mainly because I want a 1930s radio and I believe this may be from that period. It's not too bad condition the wooden case is very dry so I need to read up how best to treat old wooden cases. Internally it's all intact but evidence of some early repairs and original mains lead with 2 pin plug!

I have not had any luck finding any info. on a model 20. The valve lineup is:

6D6 Brimar BVA England (var. mu RF Amp)

6C6 RCA "Cunningham Radiotron" USA

25A6 not legible

1D6 Brimar BVA England (1/2 wv rect.)

L-55-C "Clarostat" octal plug in ballast resistor

So with my beginners knowledge I think it could be a 3 valve + rectifier TRF set.
Any of you good folks come across such a set or know where any info is? I'd love to have a go at restoring it, it's a really neat little set for such an age.

Many thanks

[PJL]

This will be a USA set designed for 120V AC and modified by Bestone to work in the UK. The modification normally involved using a line cord dropper which is a resistive power lead to dissipate the additional 120V. These are a fire hazard and are very likely to include asbestos so need to be disposed of.

Personally, I like these sets BUT they are not really suitable for a beginner. They paid very little attention to safety with live chassis, metalwork easy to touch and single pole mains switches.

To make them a little safer, I convert them back to 120V operation and use an isolation transformer to power them.

[egerton]

Thank you that is very interesting. I thought the power cable was strange so that may explain it. I am a beginner to radio restoration but have many past years experience as an electronics test engineer, so feel confident regard to safety aspects.

I wonder if I could use a large wire wound dropper resistor mounted on the chassis to replace the resistive lead, or better - a capacitive dropper as discussed elsewhere on this forum?

[egerton]

Having refreshed my memory by reading the excellent guide on here by Paul Stenning, I realise the capacitive dropper is usually used for series heater dropping. Whether it could safely be used to drop the mains down to 110V to supply a whole set I can't quite decide, but as the total load of the set is not purely resistive I think it could be dodgy. I will source a 240 / 110 isolation transformer to run this set and mount it inside if possible. #2 here suggested it was an American set designed for 120V. I thought USA supply is 110V or was it 120V in the past? 10v shouldn't matter just wondered.

Capacitive dropper calcs.html

[Boater Sam]

To be pedantic, 117V was the aimed for voltage.
110V tool transformers are cheap but not designed for continuous duty.

Could two capacitor droppers be the way? One for the heaters, t'other for HT.

If the cabinet is not quite large, an internal mains dropper resistor will fry it.

['LIVEWIRE?']

Some time ago I made up a Capacitive dropper, which supplied the heaters and the 110v AC supply, for a Canadian-made Mitchell Table Lamp/Radio. AFAIK, this radio isn't in continuous use, it has worked well ever since. Somewhere I have the circuit diagrams and my notes as to the value(s) of the caps. I used. There are also threads on here relating to this conversion.

[TonyDuell]

Originally Posted by Boater Sam

110V tool transformers are cheap but not designed for continuous duty.

They are not designed for continuous duty at full load, sure...

But in typical use on building sites, etc they are left plugged into the mains all day, then tools are intermittently connected to the output. So they will stand no-load for a considerable period (I would say continuously). And the typical portable ones are rated at 3kVA for 20 minutes or something. A small radio is going to be under 100W, so I would think a power tool transformer would run it for a considerable period without the transformer overheating.

Interestingly the maximum continuous load is not quoted for such transformers (and also not for the 230V output isolating transformers in a similar case that RS sell) and the manufacturers didn't bother to reply when I asked them.

[ms660]

Originally Posted by Boater Sam

Could two capacitor droppers be the way? One for the heaters, t'other for HT.

I don't think the HT one would work.

Lawrence.

['LIVEWIRE?']

AS I noted in post#6, a Capacative dropper I made up to use in a Mitchell Table lamp/radio, and which supplied both the heaters and the 117vAC mains did and does work. The Mitchell is basically an 'All-American Five' AM superhet circuit, so, IMHO, a capacative dropper should work with the Bestone, although I'm not familiar with that make or model.

[ms660]

Are you referring to my Post#8?

If so, rectifier anode only connected to the capacitor and nothing else? If not, my apologies/ignore this post.

Lawrence.

[Herald1360]

Provided that the heater load is >>HT load it will work, but if the heater rms volts are correct, the HT dc will be a bit off, which may or may not matter. You could feed the whole set with positive dc by using a bridge rectifier with a series capacitor on the ac input, but then the whole set will be up at half mains or so.

120v from an isolation TX(ideally) or an auto TX wired so that its common terminal and the set's chassis are neutral connected is "nicest" way to go.

Tool TXs may need a centre tap earth removing from the secondary to give true isolation. Their intermittent full load rating is unlikely to be a problem for one or a few radios fed from them.

[PJL]

The line chods are sometimes tapped for HT and LT. Also the speaker may have a field coil which needs to be added into the equation. Lastly, they often get messed about with.

First, post some pictures and draw out a circuit of what you have.

[egerton]

Thanks for all the replies, I'm finding it an interesting conversation, and useful to gather my thoughts on the options and possibilities for tackling the restoration. I will post some photos shortly. I have run Mr. Stennings cap. dropper calculator to see what it gives for 240 - 110V drop if I were to contemplate powering the complete set this way. It became clear this is not a viable option unless a constant current is drawn by the set. I assume any radio will draw more current at max. volume for example - therefore a cap. dropper is not suitable as it assumes a fixed current. I also noted that rather large capacitor values would be required when I made an assumption the supply current could be 0.5A then a value around 7.6 microfarad type X would be needed with a 20 ohm 5 watt surge limiter resistor.

Paul

[PJL]

Let's have the circuit as some USA midget sets put the field coil across the incoming mains.

[egerton]

It will take me some time to trace out the circuit. I can't find anything online about this set. I will post some photos ASAP.

Paul

[Ed_Dinning]

Hi Paul, even given all the ands and buts associated with a capacitive dropper, it works well enough.
Allow for the valve heater current (assuming 300mA) and say 50ma for the HT; typically 350mA total. Voltage to be dropped 240-117 = 123v.

Apply Paul's formula's and you should find a value about 2-4 uF.
Use class X caps and maybe use a smaller value (lower current) first

Ed

[julie_m]

Capacitive droppers really need a symmetrical load, otherwise you will get a badly distorted waveform.

Probably the best way to get a transformerless radio to work with a capacitive dropper is just to wire a bridge rectifier around the set; so its rectifier, as well as the valve heaters, will be passing current on both half-cycles.

It is a bad idea to use class X rated capacitors as droppers. The self-healing property (which is required to earn an X rating) is undesirable in this application. Instead, use a series resistor (ideally fusible, but definitely not carbon comp) chosen to run close to its power dissipation limit in normal operation.

[egerton]

Originally Posted by PJL

Lets have the circuit as some USA midget sets put the field coil across the incoming mains.

I started tracing out the circuit last night and found that the speaker field coil is connected to the mains as you thought it might be. This is new to me I will find out how that works. I'm away for a few days now but will get around to finishing the circuit off. That is the priority I think. Many thanks. Paul

[TrevorG3VLF]

In time past, I did some LTspice calculations as a mental exercise.
Thread Capacitive droppers 20 Feb 2013

https://www.vintage-radio.net/forum/...ad.php?t=93802

This looked at different circuits and dealt with distorted waveforms and the heat generated due to the HT rectifier. I forget how I did it now and would have to relearn to do more in this area. I do remember what I had for breakfast.

[ms660]

I'm still trying to understand a capacitor, diode and resistor connected in series connected across the mains.

Lawrence.