Mullard MW 31-74 tube - UK Vintage Radio Repair and Restoration Discussion Forum

audiomagpie · 2nd Jan 2011, 10:25 pm

A few months ago I bought a scrap Ferguson 968T. It was cheap (a fiver) and near enough to collect. I already have a good working 968T (actually a 978T, the midlands version) with a cracked plastic screen surround trim (the white bit in my avatar) and a 988T (same cabinet and some trim parts) with a much worm damaged cabinet and no back panel, so using this scrap set will very much improve the two good examples I have. The vendors photographs showed the tube base connector and socket still connected together but dangling out of the bottom of the set. I'd assumed that the tube had been 'necked', so it was a bonus to discover that the tube was still intact and the base connector had been snatched off, leaving the wires protruding from the glass tube end!

So I want to re solder the base plug and try the tube in one of my sets-it might be a good 'un! I've found the heater wires and it glows nicely with an external 6.3v supply.
There are another 4 wires. I've removed the ion trap and focus etc. mechanism so I can see where the wires go inside the neck. One doesn't seem to go anywhere, the other three going to what I think are the cathode, grid and first anode. Are they simply in that order from the bulb pinch as on the schematic?

Also, the wires will need extending as they're not long enough now to reach through the insides of the base pins. I can solder and sleeve them then re fit the base connector plug over, then solder the wires to the pin ends. I would then intend to tape the base connector to the glass first to check things work OK. If successful, I would then want to glue the base back on- what sort of adhesive would be suitable?

The service manual for the set states that either MW31-18 or MW31-16 (ion trap) were used on this set. Am I right in thinking that this tube could be a later replacement?

Obviously I'll keep the tube body wrapped in a thick blanket and wear eye protection when I work on it!

Note from the photo how the graphite coating has cracked like crazy paving- the glass looks OK underneath!

Thanks, Greg

oldticktock · 2nd Jan 2011, 10:33 pm

Quote:

Originally Posted by audiomagpie

what sort of adhesive would be suitable?

I asked the same question on the forum and was advised Araldite the two part slow dry. I did this on my MW22-16 and a 7205 worked a treat.

Chris

swordholder · 2nd Jan 2011, 10:41 pm

I have successfully used EvoStick on valves without a problem, so should be OK for a CRT. Don't use superglue as I've been told it can cause cracking of the glass on valves, and presumably CRTs as well.

Danny · 3rd Jan 2011, 12:21 am

Hi i always use silicone bath sealant for this job as it wont hinder the expansion of the glass. but use a good quality sealant as some of the cheap ones dont set hard enogth to hold the base i dont see any gettering ? but these tubes are quite tuff and well worth saving . I have a firgy in the same state in my to do pile. Danny

Maarten · 3rd Jan 2011, 7:01 pm

I'd go with the silicone, since manufacturers seem to have used something like that in later tubes.

2nd Jan 2011, 10:25 pm	#1
audiomagpie Heptode Join Date: Apr 2003 Location: Rugby, Warwickshire, UK. Posts: 824	Mullard MW 31-74 tube A few months ago I bought a scrap Ferguson 968T. It was cheap (a fiver) and near enough to collect. I already have a good working 968T (actually a 978T, the midlands version) with a cracked plastic screen surround trim (the white bit in my avatar) and a 988T (same cabinet and some trim parts) with a much worm damaged cabinet and no back panel, so using this scrap set will very much improve the two good examples I have. The vendors photographs showed the tube base connector and socket still connected together but dangling out of the bottom of the set. I'd assumed that the tube had been 'necked', so it was a bonus to discover that the tube was still intact and the base connector had been snatched off, leaving the wires protruding from the glass tube end! So I want to re solder the base plug and try the tube in one of my sets-it might be a good 'un! I've found the heater wires and it glows nicely with an external 6.3v supply. There are another 4 wires. I've removed the ion trap and focus etc. mechanism so I can see where the wires go inside the neck. One doesn't seem to go anywhere, the other three going to what I think are the cathode, grid and first anode. Are they simply in that order from the bulb pinch as on the schematic? Also, the wires will need extending as they're not long enough now to reach through the insides of the base pins. I can solder and sleeve them then re fit the base connector plug over, then solder the wires to the pin ends. I would then intend to tape the base connector to the glass first to check things work OK. If successful, I would then want to glue the base back on- what sort of adhesive would be suitable? The service manual for the set states that either MW31-18 or MW31-16 (ion trap) were used on this set. Am I right in thinking that this tube could be a later replacement? Obviously I'll keep the tube body wrapped in a thick blanket and wear eye protection when I work on it! Note from the photo how the graphite coating has cracked like crazy paving- the glass looks OK underneath! Thanks, Greg Attached Thumbnails __________________ Greg BVWS committee chairman

2nd Jan 2011, 10:41 pm	#3
swordholder Rest in Peace Join Date: Aug 2009 Location: East Preston, Sussex Posts: 162	Re: Mullard MW 31-74 tube I have successfully used EvoStick on valves without a problem, so should be OK for a CRT. Don't use superglue as I've been told it can cause cracking of the glass on valves, and presumably CRTs as well.

3rd Jan 2011, 12:21 am	#4
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: Mullard MW 31-74 tube Hi i always use silicone bath sealant for this job as it wont hinder the expansion of the glass. but use a good quality sealant as some of the cheap ones dont set hard enogth to hold the base i dont see any gettering ? but these tubes are quite tuff and well worth saving . I have a firgy in the same state in my to do pile. Danny

3rd Jan 2011, 7:01 pm	#5
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: Mullard MW 31-74 tube I'd go with the silicone, since manufacturers seem to have used something like that in later tubes.