Acceptable grid current ? - UK Vintage Radio Repair and Restoration Discussion Forum

retailer · 9th Sep 2021, 8:43 am

I have a bunch of older pre-amp triodes and a friend was wanting to use them, as they are over 80yrs old I ran them through the valve tester, they all tested good for shorts and gas and spot on for anode current and Gm, a few however have a whitish looking cloud on the inside of the glass, looks like the vacuum is not that great, looking closer there is a band of clear glass with no whitish deposit that is inline with the top of the anode, I tried to show it but it does not show up in the photos - maybe the vacuum is ok ? I rigged up the circuit shown and the grid current jumped between 1 and 2 uA on my Fluke 87, what is acceptable grid current for small signal valves the Avo user manual mentions grid current and how to measure but leaves it up to the user to determine what is acceptable.

kalee20 · 9th Sep 2021, 10:07 am

It does seem a lot of grid current (I'd expect less than a tenth of that), although there is healthy-looking getter at the base of the envelope.

However, critically, you don't show the polarity of the LT supply in your diagram. If the supply is such that positive terminal is to HT- then you will have the other end at -2V, so with your -1.6V bias there's going to be 20% of the filament where the grid is positive to it. And that's going to make grid current flow.

As an aside - isn't it a thing of great beauty?

trobbins · 9th Sep 2021, 11:30 am

Do you have a working item of equipment that uses that valve in a cathode biased cicuit? You can measure the anode voltage with default grid leak resistance, and then short across the grid leak and see if the anode voltage changes. A leaky grid will cause a noticable change in anode voltage.

That's the easier way to check, as I assume you don't have a very high resistance input voltmeter to directly measure grid voltage (and hence grid current).

retailer · 11th Sep 2021, 12:37 am

A gas test on my AVO MKII was negative as was the gas test on a Taylor 45, I wasn't sure if the vacuum had deteriorated there is a good looking silvery deposit around the base of the glass but also a whitish mist around the top, I don't have any type of gear that I can just plug the valve into, I think I might take the time to breadboard a setup so I can test a selection of valves and compare them

retailer · 12th Sep 2021, 9:45 am

I tried a number of older valves 26, 56, 6J5 and 6SN7 with a breadboard type hookup and all showed 0.1uA grid current on my Fluke DMM - with the test probes just lying on the bench it reads 0.1uA and occasionally will briefly flick to 0.2uA, it only reads zero if I short the probes - pretty much what I would expect so I would say that none of the valves I checked had issues with vacuum. Then I thought did I really read the DMM display correctly when I checked the globe type triode, so I hooked that back up and sure enough it was 0.1uA, shorting a 470k resistor in series with the bias supply did nothing to the anode current - from this exercise I've learnt to trust my valve testers and make sure I read the DMM display correctly.

9th Sep 2021, 8:43 am	#1
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Acceptable grid current ? I have a bunch of older pre-amp triodes and a friend was wanting to use them, as they are over 80yrs old I ran them through the valve tester, they all tested good for shorts and gas and spot on for anode current and Gm, a few however have a whitish looking cloud on the inside of the glass, looks like the vacuum is not that great, looking closer there is a band of clear glass with no whitish deposit that is inline with the top of the anode, I tried to show it but it does not show up in the photos - maybe the vacuum is ok ? I rigged up the circuit shown and the grid current jumped between 1 and 2 uA on my Fluke 87, what is acceptable grid current for small signal valves the Avo user manual mentions grid current and how to measure but leaves it up to the user to determine what is acceptable. Attached Thumbnails

9th Sep 2021, 10:07 am	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Acceptable grid current ? It does seem a lot of grid current (I'd expect less than a tenth of that), although there is healthy-looking getter at the base of the envelope. However, critically, you don't show the polarity of the LT supply in your diagram. If the supply is such that positive terminal is to HT- then you will have the other end at -2V, so with your -1.6V bias there's going to be 20% of the filament where the grid is positive to it. And that's going to make grid current flow. As an aside - isn't it a thing of great beauty?

9th Sep 2021, 11:30 am	#3
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Acceptable grid current ? Do you have a working item of equipment that uses that valve in a cathode biased cicuit? You can measure the anode voltage with default grid leak resistance, and then short across the grid leak and see if the anode voltage changes. A leaky grid will cause a noticable change in anode voltage. That's the easier way to check, as I assume you don't have a very high resistance input voltmeter to directly measure grid voltage (and hence grid current).

11th Sep 2021, 12:37 am	#4
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: Acceptable grid current ? A gas test on my AVO MKII was negative as was the gas test on a Taylor 45, I wasn't sure if the vacuum had deteriorated there is a good looking silvery deposit around the base of the glass but also a whitish mist around the top, I don't have any type of gear that I can just plug the valve into, I think I might take the time to breadboard a setup so I can test a selection of valves and compare them

12th Sep 2021, 9:45 am	#5
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: Acceptable grid current ? I tried a number of older valves 26, 56, 6J5 and 6SN7 with a breadboard type hookup and all showed 0.1uA grid current on my Fluke DMM - with the test probes just lying on the bench it reads 0.1uA and occasionally will briefly flick to 0.2uA, it only reads zero if I short the probes - pretty much what I would expect so I would say that none of the valves I checked had issues with vacuum. Then I thought did I really read the DMM display correctly when I checked the globe type triode, so I hooked that back up and sure enough it was 0.1uA, shorting a 470k resistor in series with the bias supply did nothing to the anode current - from this exercise I've learnt to trust my valve testers and make sure I read the DMM display correctly.