The 40 minute junkbox wonder! - UK Vintage Radio Repair and Restoration Discussion Forum

HMV 1120 · 22nd Nov 2009, 8:50 pm

Yes, it's yet another glowbug built with no more than 40 minutes between up-ending my box of bits and hooking up antenna, earth etc.

The valve is a single EL91 being used as a diode in an otherwise conventional crystal set circuit. All the parts have been scavenged from dead and dying beasts - the 500pf cap' is from an old phillips set and proudly sports 'Made in Great Britain' on the other side. I remember putting it aside thinking that it might be useful one day...

Operation is reasonable, selectivity is enough to seperate R.5 Live from BBC R.Cambridgshire, and around six stations are heard on a 16' length of hook-up wire. Volume into a crystal earpiece is plenty enough on Five live. The idea came from the following website:

http://www.makearadio.com/crystal/26.php

NB: I've just realised the HT supply is shown with incorrect polarity. imagine it has been reversed!

Robs

Neil Purling · 22nd Nov 2009, 11:25 pm

What is the HT voltage?

Herald1360 · 23rd Nov 2009, 12:43 am

At a guess, since it's just used as a diode, there isn't any.

Is the +ve grid voltage to neutralize the space charge?

HMV 1120 · 23rd Nov 2009, 1:05 pm

For HT I've only used 12vdc, with which it seems to work well enough. Strong stations come in moderate to uncomfortably loud.

I'm not entirely sure what the grid is doing, - I originally tried connecting +ve HT to the anode, but this had no affect (My thoughts being that it needed to be +ve to attract sufficient electrons from the cathode in order for it to work as a diode). Connecting the first grid resulted in life however....

Have just looked up the valve data sheet and it turns out Grid 3 is connected to the anode internally.That makes slightly more sense. The set works better with grid two tied to the anode.

If anyone has suggestions for improvements they'd be most welcome. I might try and convert it into a simple regenerative set instead.

Robs

HMV 1120 · 24th Nov 2009, 8:10 pm

I've replaced the EL91 with an EF91, resulting in a slight increase in volume and sensitivity.

Revised diagram to show the set with an EL91. The connection between cathode and grid three is internal (pin six).

22nd Nov 2009, 8:50 pm	#1
HMV 1120 Hexode Join Date: Jan 2007 Location: Weeting, Norfolk. Posts: 465	The 40 minute junkbox wonder! Yes, it's yet another glowbug built with no more than 40 minutes between up-ending my box of bits and hooking up antenna, earth etc. The valve is a single EL91 being used as a diode in an otherwise conventional crystal set circuit. All the parts have been scavenged from dead and dying beasts - the 500pf cap' is from an old phillips set and proudly sports 'Made in Great Britain' on the other side. I remember putting it aside thinking that it might be useful one day... Operation is reasonable, selectivity is enough to seperate R.5 Live from BBC R.Cambridgshire, and around six stations are heard on a 16' length of hook-up wire. Volume into a crystal earpiece is plenty enough on Five live. The idea came from the following website: http://www.makearadio.com/crystal/26.php NB: I've just realised the HT supply is shown with incorrect polarity. imagine it has been reversed! Robs Attached Thumbnails __________________ Real radios glow in the dark M6GLD Last edited by HMV 1120; 22nd Nov 2009 at 8:55 pm. Reason: Brain dead moment!

22nd Nov 2009, 11:25 pm	#2
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: The 40 minute junkbox wonder! What is the HT voltage?

23rd Nov 2009, 12:43 am	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: The 40 minute junkbox wonder! At a guess, since it's just used as a diode, there isn't any. Is the +ve grid voltage to neutralize the space charge?

23rd Nov 2009, 1:05 pm	#4
HMV 1120 Hexode Join Date: Jan 2007 Location: Weeting, Norfolk. Posts: 465	Re: The 40 minute junkbox wonder! For HT I've only used 12vdc, with which it seems to work well enough. Strong stations come in moderate to uncomfortably loud. I'm not entirely sure what the grid is doing, - I originally tried connecting +ve HT to the anode, but this had no affect (My thoughts being that it needed to be +ve to attract sufficient electrons from the cathode in order for it to work as a diode). Connecting the first grid resulted in life however.... Have just looked up the valve data sheet and it turns out Grid 3 is connected to the anode internally.That makes slightly more sense. The set works better with grid two tied to the anode. If anyone has suggestions for improvements they'd be most welcome. I might try and convert it into a simple regenerative set instead. Robs __________________ Real radios glow in the dark M6GLD

24th Nov 2009, 8:10 pm	#5
HMV 1120 Hexode Join Date: Jan 2007 Location: Weeting, Norfolk. Posts: 465	Re: The 40 minute junkbox wonder! I've replaced the EL91 with an EF91, resulting in a slight increase in volume and sensitivity. Revised diagram to show the set with an EL91. The connection between cathode and grid three is internal (pin six). Attached Thumbnails __________________ Real radios glow in the dark M6GLD