Heater to cathode over voltage issues.

[catalin gramada]

Hello. I try to understand what h-k overvoltage symptoms are ? I used to use 12at7 /ecc81 as dc coupled stages for years keeping the cathode of one of triode over 110-125 Vdc, whilst the datasheet clearly state no more than 90V as max voltage (meant ac+dc). Is a circuit I often use and used a lot of brand of tubes. Sometimes I have the opportunity to check the tubes I used it and shows no signs of worn, normal conduction, no extra noise, etc...in a word no sign those tubes was affected by exceeding isolation stressful condition. May I understand a failure will broke down the tube completly or what are the effects in time please ? Thanks.

[Helmholtz]

A little info here: http://www.valvewizard.co.uk/accf.html

In his book (page 123), Merlin adds that the problems like crackling noises/popping noises take time to develop and may show after hundreds of hours of operation.

[trobbins]

For starters there is a long term lowering of h-k resistance due to migration of impurities, as well as quite a wide distribution of h-k resistance for new tubes, so the h-k resistance can vary widely initially and over time. That may or may not be noticeable in a particular amplifier stage situation, and may only be noticeable if h-k resistance falls below say 1-100 Meg and start injecting hum in to the stage.

There is also a tungsten filament oxidation process occurring over time when there is significantly high positive Vh-k present at the touching points of the heater assembly to the inside cathode wall, which can lead to shorting between the filament and cathode (see ref below). Similarly, there is a much slower oxidation process between the cathode nickel wall and the heater assembly for high negative Vh-k. These processes are slow and polarity dependant, and although their impact is mainly related to shorting (as that was the test outcome looked for), they may well be influencing the heater-cathode resistance differently with respect to Vh-k polarity, and be very dependant on application history of a particular valve.

If you are keen on some vintage technical reading - although the pictures are great, and it gives some background to the use of 'SQ' for tubes:
https://www.dalmura.com.au/static/Th...alves-1956.pdf

[Mick Bailey]

I built a Hi-Fi amp, each channel with a SRPP (series regulated push-pull) input stage using both halves of a 12AX7 which exceeded the h-k voltage of one section by a good margin. It was a build that i was intending to sell, so didn't want to risk moving it on and the new owner finding there was a design issue if it ever materialised, so I used a simple voltage divider across the HT supply to elevate the heaters. What did happen by doing this is the initial slight background noise that was there before completely disappeared and even with my ear to the speaker I can't tell the amp is on. I never did sell the amp - it worked so well I kept it as my main living room amp.

[loudthud]

There is a version of the 12AT7 that has a higher H-K rating, although it draws more heater current. 12AZ7. Probably hasn't been made in decades but you might be able to find NOS somewhere.

[Jazz P Bass]

"If you are keen on some vintage technical reading - although the pictures are great, and it gives some background to the use of 'SQ' for tubes:
https://www.dalmura.com.au/static/Th...alves-1956.pdf"
The article has some interesting insights on 'cathode poisoning', the subject of which has been brought up previously.