Maximising Tube Life

[jkeny]

I'm replacing the caps & rectifiers in an old Rogers Cadet III (already rebuilt into a Baby Huey) and I was worried about the new power surges damaging the ECL86 tubes.

I came accross this article from Audioexpress http://www.audioxpress.com/magsdirx/...lespie2544.pdf & was going to implement them, specifically:

- CCS & Heaters turn on when unit plugged in i.e always on when plugged in
- Heaters brought up slowly using a LM317 CCS
- front panel switch turns on B+
- 100R resistor on screen pin

I may change the CCS & heater turn on to the front panel using a relay which turns on B+ after a time period. This is complicated slightly by the fact that I have shoehorned a Beam Deflection Tube preamp into this chassis & I want to be able to have the options of preamp on/off, amp on/off.

Any downsides to this or am I missing anything?

One other issue that occurs to me is a reference in Tubecad to running the heaters at 5V (for a 6.3V heater) as a means to prolong the life of the tube. http://www.diyaudio.com/forums/showt...29#post1594229

This can't be accomodated with CCS feed can it?

[leadbelly]

If I was going to leave the heaters always on, I'd use the method that Morgan Jones shows in Valve Amplifiers. That's voltage regulation with 2 different setpoints, 1 for off, 1 for on.

CCS is nice too, but I'd switch the tube off to implement that method. I like to use the guitar amp technique of having a separate on switch and standby switch.

[jkeny]

Yep, I was a bit worried about the heaters being on all the time - I'll just unplug the unit, when not in use, until I implement the delay & relay solutions.

In fact, as an aside I was thinking that houses should have a switch on the wall next to the light switch that would turn off all sockets (which you would turn off at night) - this would go some way towards energy reduction - i.e not having equipment in standby mode using up unnecessary energy

How does the low heater voltage (5V) square with using a CCS? Should I run the CCS at a slightly starved current for the tubes? I guess too little is better than too much current?

[Wavebourn]

Inrush current limiting and slow starting are equivalent in terms of tube life saving. I implemented both: voltage regulator with current limit just above the current drawn by hot tubes, and voltage regulator with big RC time constant in a reference voltage. Results are the same.

[jkeny]

I was really talking about running the heaters slightly current starved using a CCS - I wasn't going to run the tubes with voltage regs or current limiting, although I might consider it later.

What about the 100R on the screen - is this a good idea? I seldom see it implemented!

[leadbelly]

Originally posted by jkeny
What about the 100R on the screen - is this a good idea? I seldom see it implemented!

It's not uncommon. It's certainly common on amps with triode/UL switching.

[jkeny]

OK, then - it's all good, as my daughter would say!

What about lower current in the heater CCS? Good idea or not?

[leadbelly]

Originally posted by jkeny
What about lower current in the heater CCS? Good idea or not?

Sorry, don't know enough. There's a very non linear relationship between voltage supply and the heater draw, which is a good thing so that heaters are sort of self regulating. I don't remember what happens when you're way off the sweet spot on that curve by using a low CCS setting. One of the gurus will have to answer.

[jkeny]

One other question that occurs to me - do I need to put in a heater CCS for each tube or a CCS shared between the 4 ECL86 tubes?

Let me explain my thinking - the ECL86 has a heater current specified as 660mA if I under-supply the total current, say 2000mA instead of 2640mA, will the first 3 tubes in the circuit not take 660mA each leaving only 220mA for the last tube (2000 - 1780)?

Edit: Sorry, I remembered that the max current on LM317 is 1.5A so I would need 2 CCS anyway but the same question applies - is the total current supplied by the CCS split evenly between the tubes?

[kevinkr]

Hate to tell you this, but with a CCS you run the tube filaments in series otherwise you actually have very imprecise control over the current to an individual tube, also if the filament in one goes open the voltage will rise in an attempt to force all of the current through the remaining filament(s) - if the raw supply voltage is high enough you might just burn out that filament.

Frankly you are better off with wavebourn's current limited voltage regulator - no concerns with voltage soaring in the event a filament goes open.

Tubes run with filament voltage and no plate current for extended periods of time (like when you forget to unplug it) are subject to cathode poisoning. Limiting inrush current - again wavebourn is on absolutely the right track will prevent burnout and give you the maximum service life. Instant on was a convenience for people who had to watch their "telly" NOW, you can probably wait a minute or two for the tubes to warm up in your amplifier.. (And those tubes were designed for this operating mode - still tube life was shortened, we had one set that had this feature, my dad shut it off pretty quickly.)

Finally, with the possible exception of a couple of "super nova" Mullards I've yet to see a tube fail for an open filament, old age or other defects generally get them long before the filament burns out.

Ventilation and operating conservatively is the best thing you can do for tube life.