5U4G - to fix broken base index and testing...

[dougietamson]

Along with a used GEC KT66 I also picked up a couple of 5U4G rectifiers, one a Brimar the other unknown/no markings

The unknown one has no markings but looks the sameas the other 5U4G.

Both filaments measure 0.6Ω

It also has the index broken off at the base, is it worth the risk to attempt a fix or just leave it and maybe block-off the pins at the socket?

For a proper test I'm thinking run it as hard as I can and check the voltage loss and current. I have a 4 x 6L6GC(1020W) or 4 x EL34 (100W) guitar amp I could try and stress it with.

Any ideas?

Doug

[GrimJosef]

If you do want to fit another base you'll need to find one which is very similar to the original, or the glass envelope may not fit comfortably inside it.

Otherwise these work pretty well and are almost risk-free

https://www.tubesandmore.com/product...ce-octal-tubes

I seem to recall there is a UK supplier, but off the top of my head I can't remember who it is. And my wife's calling me through for dinner !

Cheers,

GJ

[dougietamson]

Originally Posted by GrimJosef

If you do want to fit another base you'll need to find one which is very similar to the original, or the glass envelope may not fit comfortably inside it.

Otherwise these work pretty well and are almost risk-free

https://www.tubesandmore.com/product...ce-octal-tubes

I seem to recall there is a UK supplier, but off the top of my head I can't remember who it is. And my wife's calling me through for dinner !

Cheers,

GJ

I had the idea to use an old base but all the ones I have are smaller in diameter plus the hassle of desoldering etc.

The part you linked to gave me an idea, I could dremel off the bottom flat part and index, remove the pins glue it on to the current base with epoxy.

Thanks for the inspiration.

Doug

[KeithsTV]

I've had several Octal valves with the central spigot broken off. Occasionally the missing bit is stuck in the valveholder but I've had to improvise a new spigot with most of them.

I found an old bic pen has the right diameter to match the central hole in the base. Cut off a piece equal to the length of the spigot then using a hacksaw make a cut down the length of the piece. Find a piece of old PCB and cut out a piece about 5mm wide and as long as the new spigot. Glue this piece of PCB in the slot using an epoxy glue ensuring about 2 mm is sticking out to form the index. Then glue this onto the valve base ensuring the index piece lies between pins 1 and 8. To ensure it's aligned correctly I then plug the valve into an Octal socket and leave it until the glue has cured.

Keith

[Herald1360]

Just insert the valve carefully and it'll be fine. A couple of dots one on the side of the valve base and one on the chassis in line with the missing key/way will give a bit more confidence if you like. They hardly need frequent removal/replacement anyway,

[trobbins]

Using valves without a history involves some risk that they were discarded or put aside for a reason. If you tube roll them in to an amp, and the valve arcs badly then you could take out the power transformer, or stress it whilst looking at the light show.

The simplest pre-test is to use a valve tester, or a 'megger' type device to apply 1kV and display the leakage current or insulation resistance, or make your own reverse voltage tester (eg. perhaps at least 100V, or use the 400-500Vdc from your power amp) through a current sense resistor that also acts as a current limiter.

[snowman_al]

I'm with Chris here, just mark the base with a dot to signify the key.

IMO pointless trying to replace the whole base unless it was a very expensive valve, then only if they were identical for a proper fit. Gluing a repair piece or the stump seldom works either, they eventually detach again if you are not extremely delicate...

Best is search ''keyway replacement for octal tubes'' or ''octal tube socket saver'' as described by GJ.

[dougietamson]

I thought I'd give the DIY base/key a try, I had a suitable donor of a similar age, same colour of bakelite (no valves thrown out chez moi, even if they have cracked glass/white getter)

I used a junior hacksaw to remove the base end plate/key and then held it in a pillar drill by the key and "lathed" the saw cut flush with the square edge part of a craft knife blade. I drilled the pin holes a tad wider to clear the bulge of the pins, not all the way through though.

As mentioned by others, the glue will probably give way at some point so I don't think I'll go down that route.

This is where I get carried away and think of other cat-skinning scenarios, it might be possible to make some M3 bakelite screws and tap a couple of M3 threads in the base where there are no pins, I'll give this a trial run on more donor parts.

Photos attached, a roll of insulation tape makes a great holder for the glass of a 5U4G!

Last photo has a socket attached and the pins make good contact.

Doug.

[Hartley118]

As has been pointed out, simply regluing a broken spigot is fraught with problems and is very likely to fracture again. What's needed is some form of internal 'splint' to reinforce the glued joint.

I've found that the aluminium ferrule of a cheap artists' paintbrush makes a handy internal reinforcement if combined with a good strong epoxy adhesive. I've used it for reattaching an existing spigot and also for grafting on a spigot from an otherwise scrap valve. Suitable paintbrushes are typically sold in sets by 'poundshop'-type retailers. I paid £1-50 for a set that yielded 3 usable ferrules.

I've restored both a 5U4 and a GZ34 this way, both valves which are worth a little time and trouble.

The pictures show the stages of a quick spigot graft on to the GZ34.

[dougietamson]

I've tried out a possible no glue solution involving 2 x 3M grub screws in 2 of the unpopulated pin positions in the 5U4G base. Suitable glue could be added in addition to the screws.

There is a good 8mm clearance to cut the 3M thread, didn't bother with a tapered tap and went with a bottoming tap only. I didn't go more than 1mm through to the other side of the base plate part. The screws were made with high temp plastic but I'll use bakelite for the final versions.

The extra base plate/key feels very secure even without glue. I've tried it in a few different sockets and stays very solid after each insertion/removal.

For the bakelite scews, I'll cut some square cross sections and file to an octagon then use needle files and a drill to make them 3mm OD round(ish) a lathe would come in handy.

Photos attached

[dougietamson]

Seems I can't cut a thread on a 3mm OD bakelite rod using a die, it just shears a few mm into the thread cutting, maybe could be done on a lathe (beyond my means and 40 years since I last cut a thread on one)

I used the high temp plastic screws I made as a try out, holding really well, I'll have to rig up a test to get it up to full temp.

Doug.