20's HB set - retain variable grid leak? - UK Vintage Radio Repair and Restoration Discussion Forum

greenstar · 18th May 2016, 4:30 pm

Hi.

I picked up a lovely one valve 20's set at the NVCF, which made my day with that alone. It's cleaning up nicely, and have straightened out the swinging coil mount. I would like this to work if possible, but thereby lies a dilemma. I don't think it's a commercial set (transfers on panel) but I would like to keep it as original a possible. The two capacitors can be replaced with good ones (one seems ok), or I could keep them in place and wire up a modern component if needed.
But there is a 'Bretwood' variable grid leak on the panel, which seems seized up and reads nothing. There seem several possibilities:
Keep the set original and don't worry about it working
Somehow repair the grid leak, which is in fair physical condition but nothing will move.
Fit a modern 5 Meg pot in the panel with an old knob
Get hold of a NOS part (apparently likely to be flakey)
Keep the old part in the panel and wire up either a fixed leak or a variable somewhere else

Others must have been in this position, but found nothing on the net.

Comments appreciated!
Thanks,
Tony

Herald1360 · 18th May 2016, 4:44 pm

I'd try to unseize it and go from there. If disaster strikes, you could still fit the knob on a screw to the panel, but if you can get it open, presumably it could be restuffed with carbon granules or whatever was in there before.

Maybe something could be done with a modern multiturn trimpot? You can get 3/4 RL types up to 2M.

greenstar · 18th May 2016, 5:14 pm

That's one thing - what is in there? If it's carbon, what would do - charcoal/ground up carbon rod/contents of a mic etc - down to experiment? Does it work by compression, or some sort of slider thing? Knowing this would help - opening may disrupt the contents! Are we dealing with the unknown?

Hartley118 · 18th May 2016, 5:34 pm

Variable Grid Leak eh?

I suspect it was a salesman's bright idea to give him something to talk about to prospective customers. The customer could then twiddle it and imagine he could hear the difference.

I'd be inclined to leave it alone and sneak in a little 2 megohm fixed resistor in parallel with it.

Martin

PJL · 18th May 2016, 5:52 pm

Classy bit of home building that. Perhaps it was a kit with the front panel already labelled?

ms660 · 18th May 2016, 6:19 pm

A bit of info on a similar looking grid leak here:

https://www.vintage-radio.net/forum/...ad.php?t=80516

Lawrence.

greenstar · 18th May 2016, 6:20 pm

Martin, the utility of the variable leak has been the subject of some discussion. It seems although knob twiddling was allegedly a fixation for early radio enthusiasts, it was not always futile. Courtesy of Antique radio Forum:
'Here is the reason. If you have a strong signal with a very high resistance grid leak say 5 Meg. In a short time the grid of the detector tube will over load and distort. Using a smaller grid leak say 1 meg and the tube will be less likely to distort since you are able to bleed off the grid. (grid leak) If you were trying to DX and you was looking for those far away stations you would want a high value grid leak so that very little of the signal would be leaked off to ground by the grid leak. Being able to hear that weak station was important. Back then to some, the man with the radio that got the farthest station meant that he had the best one.'
PJL, I suspect it was entirely HB - transfers are used and although good they are slightly irregular - look at 'H T'. The inside's not very neat, but has square wire! Lovely. I wish I could afford those posh sets, but this is good enough for me.

Just seen Lawrence's post - thanks, that's a help.

ms660 · 18th May 2016, 6:34 pm

There's an ad in here (grid leak)...Radio World Oct 16th 1926, magazine page number 24, there's another one on a further page in the same mag (Page number 29):

http://www.americanradiohistory.com/...926-Oct-16.pdf

Lawrence.

greenstar · 26th May 2016, 5:38 pm

Thanks folk, for your advice and comments. Just to say, I have now got this working. I have replaced the leak with a modern pot, for the moment, and shorted across the rheostat, which was duff and an unusual small size, so couldn't find a replacement. The circuit would not have worked as found, as the tuning condenser was connected to the wrong side of the leak. Said condenser was an issue, as it's too short for the panel which the knob is tight up against.
It has options for use with two coils, two circuit, or three. With two it's very unselective and couldn't get much reaction. It's far better with three and needs exact tuning. Daytime I'm just getting the two strong locals although possibly others faintly. Very fiddly tuning. Hard to assess whether it could be better. I use 1.2v LT and 27v HT as it doesn't get a lot better with a 2v valve. I've also put in an EF80 triode connected in the original valve base, which works very well at 3.5v.

18th May 2016, 4:30 pm	#1
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,685	20's HB set - retain variable grid leak? Hi. I picked up a lovely one valve 20's set at the NVCF, which made my day with that alone. It's cleaning up nicely, and have straightened out the swinging coil mount. I would like this to work if possible, but thereby lies a dilemma. I don't think it's a commercial set (transfers on panel) but I would like to keep it as original a possible. The two capacitors can be replaced with good ones (one seems ok), or I could keep them in place and wire up a modern component if needed. But there is a 'Bretwood' variable grid leak on the panel, which seems seized up and reads nothing. There seem several possibilities: Keep the set original and don't worry about it working Somehow repair the grid leak, which is in fair physical condition but nothing will move. Fit a modern 5 Meg pot in the panel with an old knob Get hold of a NOS part (apparently likely to be flakey) Keep the old part in the panel and wire up either a fixed leak or a variable somewhere else Others must have been in this position, but found nothing on the net. Comments appreciated! Thanks, Tony Attached Thumbnails

18th May 2016, 4:44 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 20's HB set - retain variable grid leak? I'd try to unseize it and go from there. If disaster strikes, you could still fit the knob on a screw to the panel, but if you can get it open, presumably it could be restuffed with carbon granules or whatever was in there before. Maybe something could be done with a modern multiturn trimpot? You can get 3/4 RL types up to 2M. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

18th May 2016, 5:14 pm	#3
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,685	Re: 20's HB set - retain variable grid leak? That's one thing - what is in there? If it's carbon, what would do - charcoal/ground up carbon rod/contents of a mic etc - down to experiment? Does it work by compression, or some sort of slider thing? Knowing this would help - opening may disrupt the contents! Are we dealing with the unknown?

18th May 2016, 5:34 pm	#4
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: 20's HB set - retain variable grid leak? Variable Grid Leak eh? I suspect it was a salesman's bright idea to give him something to talk about to prospective customers. The customer could then twiddle it and imagine he could hear the difference. I'd be inclined to leave it alone and sneak in a little 2 megohm fixed resistor in parallel with it. Martin __________________ BVWS Member

18th May 2016, 5:52 pm	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: 20's HB set - retain variable grid leak? Classy bit of home building that. Perhaps it was a kit with the front panel already labelled?

18th May 2016, 6:19 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: 20's HB set - retain variable grid leak? A bit of info on a similar looking grid leak here: https://www.vintage-radio.net/forum/...ad.php?t=80516 Lawrence.

18th May 2016, 6:20 pm	#7
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,685	Re: 20's HB set - retain variable grid leak? Martin, the utility of the variable leak has been the subject of some discussion. It seems although knob twiddling was allegedly a fixation for early radio enthusiasts, it was not always futile. Courtesy of Antique radio Forum: 'Here is the reason. If you have a strong signal with a very high resistance grid leak say 5 Meg. In a short time the grid of the detector tube will over load and distort. Using a smaller grid leak say 1 meg and the tube will be less likely to distort since you are able to bleed off the grid. (grid leak) If you were trying to DX and you was looking for those far away stations you would want a high value grid leak so that very little of the signal would be leaked off to ground by the grid leak. Being able to hear that weak station was important. Back then to some, the man with the radio that got the farthest station meant that he had the best one.' PJL, I suspect it was entirely HB - transfers are used and although good they are slightly irregular - look at 'H T'. The inside's not very neat, but has square wire! Lovely. I wish I could afford those posh sets, but this is good enough for me. Just seen Lawrence's post - thanks, that's a help. Attached Thumbnails Last edited by greenstar; 18th May 2016 at 6:26 pm.

18th May 2016, 6:34 pm	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: 20's HB set - retain variable grid leak? There's an ad in here (grid leak)...Radio World Oct 16th 1926, magazine page number 24, there's another one on a further page in the same mag (Page number 29): http://www.americanradiohistory.com/...926-Oct-16.pdf Lawrence.

26th May 2016, 5:38 pm	#9
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,685	Re: 20's HB set - retain variable grid leak? Thanks folk, for your advice and comments. Just to say, I have now got this working. I have replaced the leak with a modern pot, for the moment, and shorted across the rheostat, which was duff and an unusual small size, so couldn't find a replacement. The circuit would not have worked as found, as the tuning condenser was connected to the wrong side of the leak. Said condenser was an issue, as it's too short for the panel which the knob is tight up against. It has options for use with two coils, two circuit, or three. With two it's very unselective and couldn't get much reaction. It's far better with three and needs exact tuning. Daytime I'm just getting the two strong locals although possibly others faintly. Very fiddly tuning. Hard to assess whether it could be better. I use 1.2v LT and 27v HT as it doesn't get a lot better with a 2v valve. I've also put in an EF80 triode connected in the original valve base, which works very well at 3.5v. Attached Thumbnails