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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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11th Jul 2020, 9:39 pm | #1 |
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Heater-cathode leakage when hot.
There've been a few threads on this subject in the past.
Recently I been testing the valves in my Trio 9R-59DS, some of which must be approaching 50 years old and have seen a lot of use. I also tested pulls from the set and spares for it. 6BE6's and 6BA6's seemed to be particularly bad in this respect and I obtained the following readings on an AVO CT160. 6BE6 200k and 300k 6BA6 230k, 230k and 600k. By comparison NOS examples typically showed readings in excess of 20M although one example measured 10M.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
11th Jul 2020, 9:45 pm | #2 |
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Re: Heater-cathode leakage when hot.
I have never seen it as a datasheet value, does it matter in the real world?
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12th Jul 2020, 3:25 am | #3 |
Heptode
Join Date: May 2005
Location: Adelaide, South Australia
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Re: Heater-cathode leakage when hot.
After finishing a Fender amp copy build for my son I grabbed a handfull of used untested 12AX7's plugged them in and it all seemed good - the amp worked fine, however as the amp warmed up and he continued to play and experiment with the tone controls etc a hum developed which got louder by the minute, the first pre-amp valve had heater to cathode leakage when hot, according to my Taylor valve tester - over 2 meg when cold and less than 10k when hot.
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12th Jul 2020, 10:54 am | #4 | ||
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Re: Heater-cathode leakage when hot.
Quote:
Quote:
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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14th Jul 2020, 6:08 am | #5 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
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Re: Heater-cathode leakage when hot.
Some valves have a minimum limit test applied during production, especially the more highly specified valves like CV4024, 5687WA and rectifier 6X4WA. Sometimes it is implicitly measured by way of a hum/noise compliance test as per a 7025.
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14th Jul 2020, 10:24 am | #6 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
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Re: Heater-cathode leakage when hot.
Graham, from the 1960's, I've stuck to the CT160's Manual's "10Mohm". Often one sees a rise from 1Meg to 5Meg, then creep to 10Meg, or a similar reverse drop. Once this fannying about starts - just bin it. Unless, of course, folk want to keep a valve for collection/display purposes.
Regards, David |
14th Jul 2020, 10:27 am | #7 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
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Re: Heater-cathode leakage when hot.
ISTR that pretty much every one of the small number of metal 6J5s here that I checked showed H-K leakage between hundreds of kilohms to many megohms when cold- my thinking was that as they were only likely to be used as HFO or BFO in the AR88 here where cathode is connected directly to chassis in both cases, then it was pretty much inconsequential.
Cathode-follower use, especially with a high H-K DC potential difference and high value cathode resistor, would be a different matter. Also applications like PSU regulators and DC-coupled amps. |