Seeking Valves

[AlanBeckett]

I've seen the 'Resistor/Spring' method used on Mains Transformers.

Alan

[Refugee]

Yes indeed that one in the photo is on the mains transformer.
Resistors are either soldered to the under side of two tags so that they fall off if they get hot.
Or the other type that are ceramic with one lead bent bask round the body and soldered so that if it gets hot the long bit that goes back pings off clear of the connection..

[pwdrive]

They used spring off resistors in quite a lot of TV's, BRC/Thorn are the ones I remember, easy to get solder in yer eyes with them, similar to these:

http://www.ebay.co.uk/itm/PACK-OF-2-PCS-...1589399533

Some had an oval cross section profile.

Lawrence.

[Terry]

The drop-off type were used by Philips - you removed the back to repair a set and a resistor fell out onto the floor!

[Nowhere-Man]

I haven't totally enveloped the 7 watt resistor. The heat shrink was taken from the welding workshop and is a big diameter. It does hug the resistor body but doesn't seal it. However, safety must be given total priority. What bothers me a bit is I soldered the rectifier to the valve pins. I made as sure as I could the soldering was good but, even so, the radio will be turned the other way up when in use. I was going to use Veroboard at first but I hated the stuff. Way too fiddly. So, in the end, I soldered the diodes to the valve pins so each diode would get the secondary transformer output. Then I joined the cathodes. I'm not sure what other people do in such a case with silicon rectifiers.

I suppose what makes the most sense is to get the rectifier valve now I know it works. Then just tidy up any wiring (the secondary cable shows gradual signs of fray so ought to be sleeved around that area).

Also as the set is I think 80 watts, I ought to be able to put a 1 amp fuse in line.

The way I did the capacitors: This was something of an experiment. I used the Wima red high voltage caps I know to perform very well. However, these are no good for point soldering. The method I used was to take copper wire and cut three inches or so. At each end of the copper wire I formed a loop. This was done by wrapping the wire around another copper wire and squeezing with pliers. These loops were then tinned and the copper strand sleeved in insulation. The capacitor tinned lead would then be inserted in the loop of copper wire and more solder fused into it. This would fix the capacitor so it can be used as a radial Only argument against is there are now 4 soldered joints instead of two if you'd used just a radial capacitor. So, not sure if I'll still be using this method and it's a bit time-consuming too.  I'd never use flimsy wire so it has to be a rigid copper strand. I should note the Wima capacitors seems quite tough. I've used quite a bit of heat on them and they were heat tolerant so far.

[Nowhere-Man]

Amazingly it turns out I had a few of the valves I'd been seeking. Maybe I'd not looked too hard as I didn't ever fancy removing valves from radios to fill gaps in other radios. Nevertheless, I do have a DH77 in a HMV set. It also seems I have the much-sought X78 Mixer. That was priced up at £30.00 last time I asked but for the GEC it turned out to be the X79 which was much cheaper.

I must admit I'm tempted to try and put some power to the HMV and see how the land lies. Overall, it seems nobody has tampered with it so I could do a few basic checks. It is a bit of a wreck and really shows its age and dust but, unlike the GEC, it has all its valves. It wouldn't surprise me if the rectifier valve isn't the same as the GEC either.

What a pity someone wiped all the letters off one of the dial bands. The only way that might be fixed is if I could enlarge a photo of the same dial online and make it into a transfer, big enough to stick into place. The dial chord also needs attention or restringing.

[Crackle]

Then I joined the cathodes. I'm not sure what other people do in such a case with silicon rectifiers.

I suppose what makes the most sense is to get the rectifier valve now I know it works. Then just tidy up any wiring (the secondary cable shows gradual signs of fray so ought to be sleeved around that area).

I seem to remember that in another very long thread on the same radio you were advised to mount the diodes and resistor into an old valve base. Then when you do get the proper valve all you have to do is swap the silicon rectifier for it.

Mike

[Nowhere-Man]

Then I joined the cathodes. I'm not sure what other people do in such a case with silicon rectifiers.

I suppose what makes the most sense is to get the rectifier valve now I know it works. Then just tidy up any wiring (the secondary cable shows gradual signs of fray so ought to be sleeved around that area).

I seem to remember that in another very long thread on the same radio you were advised to mount the diodes and resistor into an old valve base. Then when you do get the proper valve all you have to do is swap the silicon rectifier for it.

Mike

I probably will just get the valve and remove the silicon diodes. One way I could have done it is the same system used with the HMV I'm now working on. They used a perforated ladder so it's not a modern technique as would be the case with Veroboard. Another thing they did on the HMV is place two 3 amp fuses in neutral and live by the rectifier. I found the fuse holders were so rusted there was no connection at all in an Ohms reading but after cleaning it up, all was well.

I think the silicon rectifier was a step to see if the radio would work and I wanted to try it out to see how it performed. However, now would be a good idea to just get the valve. I've probably even got one somewhere in store if I dig about.

[Murphyv310]

I've been reading through the thread and an a little worried. You say about safety but I feel that some of the precautions you seem to be taking could actually be a hazard. In one instance you say about using heatshrink over a resistor. This is really silly, the resistor is rated for natural cooling in air, wrapping in heatshrink will cause the resistor to overheat and could result in a fire. Like I've said before try to get the right valve or an equivalent. You'll pull more current off silicon diodes too, a fault downstream could up the current to blow your mains transformer well before you blow a fuse.

[ppppenguin]

Like Tevor, I too am worried. Heatshrink over a high power reisstor is a very bad idea. If the resistor gets really hot as they are designed to do the heatshrink won't survive. At best there will just be a nasty smell, at worst it may not be as flameproof as you hoped.

Fuses look simple but are a complicated subject. A 1 amp fuse doesn't blow at 1.1 amp. The more current you take, the quicker it blows. This page will tell you a lot more than you need to know: http://wiki.diyfaq.org.uk/index.php?title=Fuse The important point is that fuses will not protect against small, long term, overcurrent.

In many cases there's no problem if you replace a valve rectifier with silicon diode(s) and a resistor. Yes it can stress the capacitors in the set more than a valve because full HT is available immediately power is switched on and before the valves start to conduct and drag the HT down a bit. Yes you can get higher curernts through silicon under fault conditions. Thes are often acceptable risks. We're here to help you make sensible judgements.

You've clearly been struggling a bit with soldering. We all had to start somewhere and I'm sure most of us made a hash of it at first. Presented with a crowded chassis, wires wound round tagstrips and old solder it can be tricky to do a good job. It can be hard to melt the solder at all at first. One trick is to run a small amount of new solder between the iron and the old joint. Once the old joint is fully molten you can use a solder sucker to get rid of a lot of the old solder. To save disturbing a possibly delicate tag such as on a valveholder you can remove a component by cutting its leads near the body of the component. Since modern repalcements are usually smaller it's then quite easy to join thenew component to the old leads. The purists can make some very neat mechanical joints between new and old leads. I'm not a purist

As I said earlier, we all had to start somewhere and we've all made mistakes. Those of us who are still here obviously didn't make any fatal ones We like to help those who are less experienced but it can be hard to do it from verbal descriptions and even from photos. There's nothing like learning directly from somebody with experience sitting next to you but this isn't always possible.