My latest addition - Radialva Groom

[Geaney]

Thanks guys, I'll go with the resistive dropper for now and keep the supply at 110v. I've got a pair of 270 Ohm 20Watt resistors in parallel to give the 135 Ohms which is the stated value for the mains cord. The challenge with this set is the total lack of available space so I'll tinker around with how best to fit them.

BTW, I have not been able to access the forum for a week so apologies for late response.

Cheers, John

[Robert Darwent]

The two bulbs in series seem to have about the same resistance as the bypass resistor so I've assumed this splits the current about half each via the resistor and the bulbs thus the total voltage drop is less than two bulbs in series without the bypass resistor.

Hi John,

Recently I've been preparing to restore my 'Super Groom' (this thread nudged me in the right direction) and I've been going over the dropper calculations.

The two bulbs in the set are (should be) 7V, 0.1A types. By simple Ohms law that makes their resistance 70 ohms each, giving 140 ohms together in series. As you say they are shunted by a resistance, which apparently is made up of a length of resistive cord with a value of 50 ohms.

Adding 140 and 50 ohms in parallel gives an effective resistance of 36.8 ohms, resulting in a potential difference of just over 11V at the specified current of 0.3A. You have 6.9V at that point in your diagram.

So, the total voltage dropped by the combined heater chain and dial bulbs is 69V + 11V = 80V

The 135 ohms originally provided by the resistive line cord drops another 40V or so at 0.3A making a total of 120V for the mains supply, not 110V as specified in the service sheets.

I thought my calculations were wrong somewhere until a check on the internet confirmed that France indeed used a 120V supply during the period back in the 1940s when this set was in production.

Regards

[Yorkie]

What's your mains supply Voltage in Geneva John - presumably between 220 - 240V? If that's so and you are keeping the set to run from 120V you'll have a drop-down transformer to run it from? From Robert's calculations, if you only need to drop 40V at 0.3A that's 12 Watts, so your two 270R 20 Watt resistors will be well within their 40 Watt capability. However, if you needed to drop another 120V to run off 240V mains, 120V+40V = 160V at 0.3A = 533R. Nearest preferred value is 560R, which - at 0.3A and assuming 240V mains - would drop 168V. 168V x 0.3A = 50.4 Watts.

If you wished to reduce the wattage by using two in parallel (as with the 270R resistors), you’d need one at 1.2k and one at 1K = 545.45R, which would drop 153.5V. The 1K resistor would pass slightly more current than the 1.2K one. (If you were to use metal-clad resistors, they only dissipate their rated wattage if attached to a heat sink).

All of the above is of course irrelevant if you plan to run the set on 120V!

I'd never heard of these sets until John mentioned his - fancy Robert having one as well! Presumably yours too will be designed for 120V mains Robert - what are your plans for dealing with that?

[Robert Darwent]

Hi David,

What's your mains supply Voltage in Geneva John - presumably between 220 - 240V?

From a quick internet search, I believe Switzerland has a 220V, 50Hz mains supply.

So in John's case, the calculations to drop 220 -80 = 140V at 0.3A come out as;

Resistive Dropper: 467 ohms at 42W
Capactive Dropper: 4.76 uF with a 33 ohm, 3W surge limiter (10V drop)

I'd never heard of these sets until John mentioned his - fancy Robert having one as well! Presumably yours too will be designed for 120V mains Robert - what are your plans for dealing with that?

As you say David for a 240V supply, the calculations for a 240 - 80 = 160V drop at 0.3A come out as;

Resistive Dropper: 533 ohms at 48W
Capacitive Dropper: 4.22 uF without a surge limiter, or 4.25 uF with a 17 ohm, 1.5W limiter (5V drop)

Both of those values could be easily made up from a 4 uF motor run capacitor and either a 0.22 uF or 0.25 uF X2-type in parallel.

Of course all of the above just refers to the LT supply. It still leaves the problem of dropping the HT supply to 120V as well. According to the service sheets, the total consumption of the set is 0.43A. So assuming the HT current is in the region of 0.43 - 0.3 = 0.13A, the calculations for a 240 - 120 = 120V drop are;

Resistive Dropper: 923 ohms at 15.6W
Capacitive Dropper: 1.99 uF without a surge limiter, or 2 uF exactly with a 15 ohm, 0.3W limiter (2V drop)

Obviously, 2 uF motor run capacitors are available but the exact HT current of the set will have to be measured and the values 'tweaked' accordingly.

However, I read somewhere that you cannot use a capacitive dropper to drive inductive loads, such as the rectifier anodes directly, without having an additional resistive component. I have little previous experience with capacitive droppers so I am not sure why that is so, but if correct looks like it could rule out using the capacitor dropper method unless the surge limiter itself could serve as the "resistive component"?

Taking everything it account, it may prove easier to fit a small 240V to 120V transformer for the HT and run a 40V dropper off of that for the LT. But that all depends whether a small enough transformer can be obtained to fit in the confined chassis space available.

Also, in view of the small dimensions of this set, I would prefer to fit two seperate capacitive droppers and keep the heat produced to a minimum.

Regards

[Robert Darwent]

A couple of images of my 'Super Groom'.

       

The dropper on the far right is a previous owners work, its not actually connected to anything. The 25Z6 rectifier along side it looks quite 'milky' and no doubt needs replacing but I haven't tested it yet.

On the far left the two 100 uF electrolytics have been replaced at some point and should be enclosed in a aluminium can similar in appearance to the IFT's, unfortunately whoever replaced the capacitors didn't refit it.

Regards

[Geaney]

Thanks Robert, David,

I appreciate all the help with figuring this out. Sets this old seem to have a whole different set of challenges compared to later designs. In my case I have the following two main challenges:
- the valve set is not as per the circuit diagram. Two valves have alternatives fitted, one looks like a reasonable swap but the other looks to be quite a different valve than the one specified. I now have a complete 'correct' set and indeed the metal case 25L6 arrived to day. Assuming all the NOS metal valves test OK I've got a usable set to move forward.

- the dial bulbs appear not to be original and have been replaced with more commonplace 6.3v types rated at higher current - 250mA instead of 100mA.

I've decided to drive it from the variac at either 110v or 120v depending on how things turn out. I will use the 2 x 270R 20Watt resistors to drop from that to the heater chain. Another challenge here is finding space to site these two resistors. I believe I've come up with at least a trial solution so I'll give that a shot.

I hope to get some time on this one today so I will post an update with some photos later.

Thanks again for all the technical and moral support

Cheers, John

[Yorkie]

The dropper on the far right is a previous owners work, its not actually connected to anything. The 25Z6 rectifier along side it looks quite 'milky' and no doubt needs replacing but I haven't tested it yet.

On the far left the two 100 uF electrolytics have been replaced at some point and should be enclosed in a aluminium can similar in appearance to the IFT's, unfortunately whoever replaced the capacitors didn't refit it.

Regards

Yes, the rectifier does look a bit poorly Robert!

There's a good view of the cap can in John's picture. Knowing what a stickler you are for authenticity, doubtless you'll be looking out for a suitably sized IF can to replicate the original cap can.

Did these sets have painted grooves around the cabinet, or is that my eyes deceiving me? Quite a striking looking set. Makes a nice change from the ubiquitous DAC90A!

Is that the original back panel on yours Robert?

[Geaney]

Thanks Robert, this is really interesting. I was puzzling over what was in that big can since my set has the electrolytics 'squashed in' underneath. Now I know I can relocate them and put them inside this large can. There was a lot of greasy gunk around the base of the large can so I now know this most likely means the caps inside have leaked or otherwise given up the ghost. Some of the comments so far have suggested that 100uF is very high for this type of circuit. Are yours definitley 100uF? if yes I'll change mine since I've now got some. I used 47uF based on the general view that that should be ample, etc.

Getting the can off will be a pain so I can see why it was not re-fitted when yours was done.

Funnily enough I am planning to site my 2 x 270R droppers just where you're dropper is situated. I've rigged up a strip of metal to act as a heatsink which I will bolt to the chassis for added heat dissipation.

Correction re my dial bulbs - they're both marked 6.5v 100mA.

When you get your chassis out of the case I'd be very keen to see a photo of the underneath!

Your dial looks in great shape so I may prevail on you to take a photo of it against a black background so I can see about re-creating one for my set.

Cheers, John

[standunmj]

hi robert,great info from you there, absolutely essential that my good friend john needs to know,

im sure you also would not mind supplying a trace of the backboard as johns is awol.

as also is the dial configeration, best regards maitiu.

[Geaney]

Right, the electrolytic can is now removed and cleaned. What a mess. Luckily, because the can is square, I was able to use a parting knife along each side to break the bond with the case and then the rest pulled out fairly easily - no heat gun required.

Unfortunately I had a little mishap along the way - as one does! To get access to the nuts mounting the can from underneath I had to unsolder one end of the antenna coupling cap. when I moved is out of the way (just about half an inch) the wire connected to the other end of the cap broke away from 'wherever it was connected to' - aagh! Hopefully I can find where it was connected and it's on a selector switch, not directly on a coil!

Two steps forward, etc.

Cheers, John