Heater-cathode leakage when hot. - UK Vintage Radio Repair and Restoration Discussion Forum

Station X · 11th Jul 2020, 9:39 pm

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.

11th Jul 2020, 9:45 pm

I have never seen it as a datasheet value, does it matter in the real world?

retailer · 12th Jul 2020, 3:25 am

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.

Station X · 12th Jul 2020, 10:54 am

Quote:

Originally Posted by merlinmaxwell

I have never seen it as a datasheet value, does it matter in the real world?

I've never seen a figure quoted either. The advice in the CT160 manual is.

Quote:

It is not possible to state a rejection figure in Mohm for a valve under test, for such a fault will be of considerable importance in some circuits, whilst in a few cases its presence has no consequence at all. The instrument is capable of giving the insulation between cathode and heater, and the acceptance or rejection of the valve can only be determined when the operator has details of the circuit in which the valve is to be used. In those cases where details are not known, it is always better to reject a valve having an insulation resistance of less than 10Mohm. It will be appreciated that there are many circuits in which an appreciable potential exists between heater and cathode, dc amplifiers etc., and the presence of heater to cathode insulation breakdown, even of the order of many Mohm, can often give rise to quite serious trouble. Heater to cathode insulation breakdown either permanent or intermittent can also give rise to noise in valve amplifier circuits.

trobbins · 14th Jul 2020, 6:08 am

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.

David Simpson · 14th Jul 2020, 10:24 am

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

turretslug · 14th Jul 2020, 10:27 am

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.

11th Jul 2020, 9:39 pm	#1
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	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. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

11th Jul 2020, 9:45 pm	#2
Guest Guest Posts: n/a	Re: Heater-cathode leakage when hot. I have never seen it as a datasheet value, does it matter in the real world?

12th Jul 2020, 3:25 am	#3
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	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.

14th Jul 2020, 6:08 am	#5
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	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.

14th Jul 2020, 10:24 am	#6
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	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
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,396	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.