KT 66 valve base connections. - UK Vintage Radio Repair and Restoration Discussion Forum

Andy - G8MNM · 26th Jan 2011, 12:17 am

Hi,

Amongst some valves I have recently obtained is an Osram KT66 it is, however, separate from it's base. Apart from the heater connections is there any way of accurately determining what electrode the wires exiting the valve envelope connect to? Or is it destined for the bin?

Any help appreciated

Andy

GrimJosef · 26th Jan 2011, 1:26 am

No

! Please don't bin it ! If you follow the wires carefully as they pass through the glass 'pinch' you can see which electrode support rod each one is attached to.

It's clearer if you open the valve up. To save you doing this with your working (?) valve here's a pic showing one I dissected earlier. You can see the support rods which run between the pinch and the lower mica, some of which have wires connected in the glass. Unfortunately I've photographed this slightly from above, so I can't be quite certain which of the two thick bent outer rods goes to the anode and which to the cathode. But I think the assignments are, working across from the left:

Anode (thick bent rod)
Grid 2
Grid 1 (no wire on this side)
Heater
Heater
Grid 1
Grid 2 (no wire on this side)
Cathode and beam plates (thick bent rod)

Further support for this assignment comes from the appearance of the black 'fur' surrounding some of the wires in the pinch. I believe this is caused by electrophoretic ion migration in the glass and it occurs around the low voltage electrodes only (heater, cathode and grid 1).

The identification job is quite easy if your valve has an uncoated glass envelope. But a lot of the Osram ones had a grey coating on the inside which may well make things more difficult. A bright light can help, but you may have to fall back on an electrical approach. If you just power the heater the resulting space charge around the cathode will influence the potential of grid 1 which can be measured using a DMM. I don't know what to expect in an isolated valve but when plugged into a Quad II amp the KT66 grids go a volt or so negative after the heater has warmed the cathode up. This is a crude but handy test for emission from an unknown valve.

Hope this helps, I'm sure others will be able to suggest ways forward too.

Cheers,

GJ

Andy - G8MNM · 26th Jan 2011, 6:09 pm

GJ,

Thanks.
I was really sad to see the state of the valve when it arrived, and would really like to reacquaint it with its base and hopefully then check it on my valve tester.
I shall give it a go based on your info.

Thanks

Andy

julie_m · 26th Jan 2011, 6:41 pm

If you've got a valve tester, then you should be able to work out which lead is which easily enough in a variation on the old "a transistor looks like two diodes when you test it with an AVO" theme.

First, find the heater connections; they will be obvious, as they will show a low resistance between them. Once the heater is warmed up, apply some low HT (via a suitable resistance; keep the current under 1mA) to pairs of pins in turn. You will be able to detect emission from the cathode to any grid or the anode. Once you have emission going, you will be able to cut it off by applying negative volts to any grid between the cathode and whatever is acting as an anode. The electrode where emission cannot be cut off by applying negative volts anywhere else must be the first grid.

-ve volts on G1 will cut off K-G2 or K-A.
-ve volts on G2 will cut off K-A.

Grids obviously don't like being used as anodes, so don't leave the valve in this state for long.

Lucien Nunes · 26th Jan 2011, 7:04 pm

I happened to have a line of them on the bench here so I've just checked a dozen both clear and coated; they are all as per GJ's notes above. It always seems to be possible to view the pinch even inside a coated envelope, because these only have a getter on one side and you can look up from the clear area near the base.

Lucien

boxdoctor · 26th Jan 2011, 10:08 pm

Be very careful with the lead-out wires, as these can be very brittle, and if they snap off at the pinch, then you can throw it in the bin!,
I hope you do succeed in repairing it - definately worth the effort of a try. Tony.

G4XWDJim · 26th Jan 2011, 10:27 pm

I wonder why the base is off?

These valves have a habit of developing an internal bad connection that stops them working. Initially it could be thought that it could be a bad soldered joint between a wire and the valve base pins.
Perhaps someone took the base off expecting a bad connection and when he didn't find one he realised there could be an internal fault and gave up.

AJSs idea should give you a good idea of whether it will work before you go to the trouble of fitting the base back on.

I spent quite some time removing a base and then refitting it to no avail about a year ago and posted here to that effect.

Good luck,

Jim

26th Jan 2011, 12:17 am	#1
Andy - G8MNM Hexode Join Date: Apr 2008 Location: Perigord Noir, France Posts: 352	KT 66 valve base connections. Hi, Amongst some valves I have recently obtained is an Osram KT66 it is, however, separate from it's base. Apart from the heater connections is there any way of accurately determining what electrode the wires exiting the valve envelope connect to? Or is it destined for the bin? Any help appreciated Andy __________________ Member BVWS https://www.lushanda.com/

26th Jan 2011, 1:26 am	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: KT 66 valve base connections. No ! Please don't bin it ! If you follow the wires carefully as they pass through the glass 'pinch' you can see which electrode support rod each one is attached to. It's clearer if you open the valve up. To save you doing this with your working (?) valve here's a pic showing one I dissected earlier. You can see the support rods which run between the pinch and the lower mica, some of which have wires connected in the glass. Unfortunately I've photographed this slightly from above, so I can't be quite certain which of the two thick bent outer rods goes to the anode and which to the cathode. But I think the assignments are, working across from the left: Anode (thick bent rod) Grid 2 Grid 1 (no wire on this side) Heater Heater Grid 1 Grid 2 (no wire on this side) Cathode and beam plates (thick bent rod) Further support for this assignment comes from the appearance of the black 'fur' surrounding some of the wires in the pinch. I believe this is caused by electrophoretic ion migration in the glass and it occurs around the low voltage electrodes only (heater, cathode and grid 1). The identification job is quite easy if your valve has an uncoated glass envelope. But a lot of the Osram ones had a grey coating on the inside which may well make things more difficult. A bright light can help, but you may have to fall back on an electrical approach. If you just power the heater the resulting space charge around the cathode will influence the potential of grid 1 which can be measured using a DMM. I don't know what to expect in an isolated valve but when plugged into a Quad II amp the KT66 grids go a volt or so negative after the heater has warmed the cathode up. This is a crude but handy test for emission from an unknown valve. Hope this helps, I'm sure others will be able to suggest ways forward too. Cheers, GJ Attached Thumbnails

26th Jan 2011, 6:09 pm	#3
Andy - G8MNM Hexode Join Date: Apr 2008 Location: Perigord Noir, France Posts: 352	Re: KT 66 valve base connections. GJ, Thanks. I was really sad to see the state of the valve when it arrived, and would really like to reacquaint it with its base and hopefully then check it on my valve tester. I shall give it a go based on your info. Thanks Andy __________________ Member BVWS https://www.lushanda.com/

26th Jan 2011, 6:41 pm	#4
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: KT 66 valve base connections. If you've got a valve tester, then you should be able to work out which lead is which easily enough in a variation on the old "a transistor looks like two diodes when you test it with an AVO" theme. First, find the heater connections; they will be obvious, as they will show a low resistance between them. Once the heater is warmed up, apply some low HT (via a suitable resistance; keep the current under 1mA) to pairs of pins in turn. You will be able to detect emission from the cathode to any grid or the anode. Once you have emission going, you will be able to cut it off by applying negative volts to any grid between the cathode and whatever is acting as an anode. The electrode where emission cannot be cut off by applying negative volts anywhere else must be the first grid. -ve volts on G1 will cut off K-G2 or K-A. -ve volts on G2 will cut off K-A. Grids obviously don't like being used as anodes, so don't leave the valve in this state for long. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

26th Jan 2011, 7:04 pm	#5
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: KT 66 valve base connections. I happened to have a line of them on the bench here so I've just checked a dozen both clear and coated; they are all as per GJ's notes above. It always seems to be possible to view the pinch even inside a coated envelope, because these only have a getter on one side and you can look up from the clear area near the base. Lucien

26th Jan 2011, 10:08 pm	#6
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: KT 66 valve base connections. Be very careful with the lead-out wires, as these can be very brittle, and if they snap off at the pinch, then you can throw it in the bin!, I hope you do succeed in repairing it - definately worth the effort of a try. Tony.

26th Jan 2011, 10:27 pm	#7
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: KT 66 valve base connections. I wonder why the base is off? These valves have a habit of developing an internal bad connection that stops them working. Initially it could be thought that it could be a bad soldered joint between a wire and the valve base pins. Perhaps someone took the base off expecting a bad connection and when he didn't find one he realised there could be an internal fault and gave up. AJSs idea should give you a good idea of whether it will work before you go to the trouble of fitting the base back on. I spent quite some time removing a base and then refitting it to no avail about a year ago and posted here to that effect. Good luck, Jim