Valve screening can tip - UK Vintage Radio Repair and Restoration Discussion Forum

Testgear John · 8th Oct 2006, 9:29 pm

I needed to change the screening cans on My Ferranti 145 due to rust, and had got off Ebay four cracking good ones, albeit HRO. so they did not fit snugly. I hit upon the idea of using a single plier type paper punch to make 3 dents or impressions around the bottom, so they fitted more neatly on the skirt of the valveholder... being a bit ham-fisted I managed to cut right through! No matter, it left the flap joined at the top due to the leading edge effect of the punch, is adjustable, and it worked a treat!

John, BVWS Member.

Skywave · 10th Oct 2006, 9:19 pm

On the topic of valve screening cans, it is worthy of note that in the mid-sixties some research was carried out which established that if the plain ali screening can - as fitted to B7G & B9A valves - was painted matt black, and a good thermal contact was made with the valve envelope, the valve ran a lot cooler - as much as 20 deg. C !

Obviously, this measure substantially prolongs the valve's life.

Also of note is that fact that the Racal RA-17 receiver has all of its screening cans black anodised. This is a receiver designed for continuous duty in all climates / environments, with reliability a major design feature.

Al / G8DLH

Leon Crampin · 17th Oct 2007, 9:50 pm

During the restoration of a Bush VHF 61, I hit upon the idea of substituting a black screening can for the plain aluminium one fitted to the ECC85. The valve runs appreciably cooler which will prolong its life - ECC85s are not the best in this respect.

An added bonus was less oscillator drift. Although the Bush sets are pretty good in this respect, the improvement in tuning stability at the HF end of the band was worthwhile - achieved with not a lot of effort.

Leon.

paulsherwin · 18th Oct 2007, 12:15 am

Quote:

Originally Posted by Skywave

On the topic of valve screening cans, it is worthy of note that in the mid-sixties some research was carried out which established that if the plain ali screening can - as fitted to B7G & B9A valves - was painted matt black, and a good thermal contact was made with the valve envelope, the valve ran a lot cooler - as much as 20 deg. C !

All the 1960s military valve gear I've come across used black screening cans. I wonder why this wasn't done for domestic equipment

Paul

Skywave · 18th Oct 2007, 12:21 am

Leon -

I have an Eddystone 888A that has a ECC85 in a cascode arrangement for the RF amplifier (this is a modification). It too has a black ali can fitted - and I haven't had to change that valve for a very long time!

Al / Skywave.

newlite4 · 18th Oct 2007, 9:20 am

Back in the mid '70s, I acquired an old telefusion line amplifier. This contained maybe seven B9a valves. Each one was fitted with sprung steel black cans which had six cooling fins radiating from the circumference. Where this equipment was designed to run 24/7 this must have been essential.
Neil

8th Oct 2006, 9:29 pm	#1
Testgear John Tetrode Join Date: Apr 2006 Location: Larne, Northern Ireland, UK. Posts: 64	Valve screening can tip I needed to change the screening cans on My Ferranti 145 due to rust, and had got off Ebay four cracking good ones, albeit HRO. so they did not fit snugly. I hit upon the idea of using a single plier type paper punch to make 3 dents or impressions around the bottom, so they fitted more neatly on the skirt of the valveholder... being a bit ham-fisted I managed to cut right through! No matter, it left the flap joined at the top due to the leading edge effect of the punch, is adjustable, and it worked a treat! John, BVWS Member.

10th Oct 2006, 9:19 pm	#2
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Valve screening can tip On the topic of valve screening cans, it is worthy of note that in the mid-sixties some research was carried out which established that if the plain ali screening can - as fitted to B7G & B9A valves - was painted matt black, and a good thermal contact was made with the valve envelope, the valve ran a lot cooler - as much as 20 deg. C ! Obviously, this measure substantially prolongs the valve's life. Also of note is that fact that the Racal RA-17 receiver has all of its screening cans black anodised. This is a receiver designed for continuous duty in all climates / environments, with reliability a major design feature. Al / G8DLH

17th Oct 2007, 9:50 pm	#3
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Valve screening can tip During the restoration of a Bush VHF 61, I hit upon the idea of substituting a black screening can for the plain aluminium one fitted to the ECC85. The valve runs appreciably cooler which will prolong its life - ECC85s are not the best in this respect. An added bonus was less oscillator drift. Although the Bush sets are pretty good in this respect, the improvement in tuning stability at the HF end of the band was worthwhile - achieved with not a lot of effort. Leon.

18th Oct 2007, 12:21 am	#5
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Valve screening can tip Leon - I have an Eddystone 888A that has a ECC85 in a cascode arrangement for the RF amplifier (this is a modification). It too has a black ali can fitted - and I haven't had to change that valve for a very long time! Al / Skywave.

18th Oct 2007, 9:20 am	#6
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,805	Re: Valve screening can tip Back in the mid '70s, I acquired an old telefusion line amplifier. This contained maybe seven B9a valves. Each one was fitted with sprung steel black cans which had six cooling fins radiating from the circumference. Where this equipment was designed to run 24/7 this must have been essential. Neil __________________ preserving the recent past, for the distant future.