UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

radiozero · 20th Oct 2012, 7:24 pm

Let me first talk about the AVO MKI & II test that tends to come first (after insulation tests) - The Anode Current Test.

That test requires all electrodes to be set to a given voltage. The indicator examined, and which is not set on the Tester is - the anode current. The anode current "happens".

If the anode current is lower than what a table says it should be, then something is amiss. Perhaps usually a problem with the cathode supplying sufficient electrons. But, the current could be higher, due to a soft valve.

Depending on the percentage the current is, higher or lower, with regard to what a table says, then the valve can be accepted as suitable or not. But, what is that percentage? Unfortunately, that can vary depending on use of valve.

Now, I have in possession three tables: The manufacturers table for the specific valve, a table in a radio valve data booklet (which is probably just a collection of manufacturers tables for their valves) and AVO has produced a table for use with it's valve Testers.

Take the KT66. GEC in it's manufacturers table (just for the KT66) tells us that if the anode current reaches 85mA, then the valve is considered at the end of it's life. I'm not sure under what electrode conditions or other conditions that refers to. It would seem to me that perhaps it's 85mA when it's going soft. But, that is just a guess. Is that value of 85mA an in-circuit reading, or is it a reading associated with a valve Tester. It is a value that is NOT associated with a reading to be expected on a Valve Tester. I think.

Then we have the table from my Radio Valve Data book. It gives three anode current values: (BT) 62.5mA; (BT) 80.0mA; (T) 62.5mA.

Then we have AVO's tables. Now, the anode current is stated as 65mA. That is under the heading for a Type 160 Tester. There is no anode current mentioned under the heading for AVO Valve Tester (Presumably MK1 & MKII). But, surely I am suppossed to be looking for a current of 65mA. Interestingly, the electrode voltages are different for a MKI & II, compared to Type 160.

AVO made that table so that you can test valves, with their models of valve Testers.

The two tables then of real potential use are GEC's one for the KT66 and AVO's tables.

Whatever can be said of the usefulness of a particular anode current reading, certainly, GEC laid down something of a marker by saying the valve has reached the end of it's life when Ia equals or is above 85mA. In some conditions or other. And AVO give a value of 65mA for a new valve under a set of electrode conditions in a Valve Tester.

I have a couple of KT88's. Two questions: What value of Ia would be acceptable for audio use (AVO table says 57mA). Why does it say "No data Available" under the heading "Data For AVO Valve Tester"? Thanks.

20th Oct 2012, 7:24 pm	#11
radiozero Hexode Join Date: Mar 2008 Location: Wakefield, West Yorkshire, UK. Posts: 444	Re: Valve Testing Let me first talk about the AVO MKI & II test that tends to come first (after insulation tests) - The Anode Current Test. That test requires all electrodes to be set to a given voltage. The indicator examined, and which is not set on the Tester is - the anode current. The anode current "happens". If the anode current is lower than what a table says it should be, then something is amiss. Perhaps usually a problem with the cathode supplying sufficient electrons. But, the current could be higher, due to a soft valve. Depending on the percentage the current is, higher or lower, with regard to what a table says, then the valve can be accepted as suitable or not. But, what is that percentage? Unfortunately, that can vary depending on use of valve. Now, I have in possession three tables: The manufacturers table for the specific valve, a table in a radio valve data booklet (which is probably just a collection of manufacturers tables for their valves) and AVO has produced a table for use with it's valve Testers. Take the KT66. GEC in it's manufacturers table (just for the KT66) tells us that if the anode current reaches 85mA, then the valve is considered at the end of it's life. I'm not sure under what electrode conditions or other conditions that refers to. It would seem to me that perhaps it's 85mA when it's going soft. But, that is just a guess. Is that value of 85mA an in-circuit reading, or is it a reading associated with a valve Tester. It is a value that is NOT associated with a reading to be expected on a Valve Tester. I think. Then we have the table from my Radio Valve Data book. It gives three anode current values: (BT) 62.5mA; (BT) 80.0mA; (T) 62.5mA. Then we have AVO's tables. Now, the anode current is stated as 65mA. That is under the heading for a Type 160 Tester. There is no anode current mentioned under the heading for AVO Valve Tester (Presumably MK1 & MKII). But, surely I am suppossed to be looking for a current of 65mA. Interestingly, the electrode voltages are different for a MKI & II, compared to Type 160. AVO made that table so that you can test valves, with their models of valve Testers. The two tables then of real potential use are GEC's one for the KT66 and AVO's tables. Whatever can be said of the usefulness of a particular anode current reading, certainly, GEC laid down something of a marker by saying the valve has reached the end of it's life when Ia equals or is above 85mA. In some conditions or other. And AVO give a value of 65mA for a new valve under a set of electrode conditions in a Valve Tester. I have a couple of KT88's. Two questions: What value of Ia would be acceptable for audio use (AVO table says 57mA). Why does it say "No data Available" under the heading "Data For AVO Valve Tester"? Thanks.