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Zener filament protection

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8/14/1999 4:54 PM

Zener filament protection

Browsing through an old "1001 circuits" book,I came across a recommendation to use zener diodes in parallel across filament connections to protect them from overvoltage. One example for AC supply had two 6.2V zeners anode to anode across the filament, and a single 6.6V zener across the filament on a DC filament supply. This looks good on paper, and seems to make sense, but since I've never seen this recommended here or in any of the many tube amp books I have, I question why it never caught on. Since zeners cost more than standard diodes, is it a matter of economics, or is there another reason I've not seen this idea. Or, have I unveiled a long lost secret of tube amp lore that I can subsequently have patented and then sue the pants off of anyone else who trys to use it, ala Randy Smith

8/14/1999 6:31 PM

Pat:

    So for 6.3vac, the two 6.2v zeners would be wired up in series, anode to anode?

    The only problem I can think of would be the higher current draw from the filament windings if the voltage was too high (the zener would just dump any voltage higher than 6.2 to its cathode- right? Or have I forgotten what I learned in Elec 101 at the College of Hart Gnox?)

    Maybe I'm wrong on this, but I didn't think that the voltage going to the heaters was that critical and that they are not unlike a light bulb. If the voltage is too high they will run a bit hotter and probably burn out sooner (then again how many tubes are replaced because of a bad heater?) and if it is too low they may not get quite hot enough. I've measured voltages as low as 5.9v and as high as 6.5+v and the amp seemed to work okay without blowing out tubes left and right. (On another thread I mentioned that my Pignose G40V came out of the box with a heater voltage of 7.1VAC- I checked my notes and it was actually 7.2vac! I rewired the heater supply for dc per the Torres book, using a 0.22R series resistor to bring the voltage down to 6.3vdc.)

    Other opinions invited!

Steve Ahola

    I believe that when you get into high end tube audio gear the heater voltage is more critical.

8/16/1999 4:43 PM
ADM	I was wondering about using zener diodes to protect the DC circuitry if a tube fails and shorts the plate to the heater. Is this something to worry about? Andrew.

8/16/1999 8:03 PM
Ken	FWIW, I've never seen a tube with it's plate shorted to heater. Or cathode, for that matter. Normally it goes to the grids. KG

8/17/1999 5:52 AM
Bruce	Well sorta! I've fixed a ton of amps with the plate voltage arcing over the tube socket from plate to heater pins! As far as the consumer end goes, it still does the $ damage. Bruce

8/17/1999 2:37 PM
Ken	Ahhh.. Yet another reason to use ceramic and keep them clean! Dust and grime WILL conduct. KG

8/18/1999 1:10 AM

You're right, it doesn't seem likely that the plate could short to the heater, but something like this happened to me. There was some kind of output tube failure that caused both 100 ohm heater resistors to explode (they were A-B carbon composition 'flameproof' resistors. Apparently 'flameproof' does not guarantee 'shatterproof'). I don't know if it was a plate to heater short, maybe it was the screen grid that shorted to the heater.

Also, the plate pin (3) is right next to a heater pin (2), so a probe can slip and induce a plate to heater short. I've done this before and a metal oxide heater resistor burned out. No explosion this time.

As for the Zener, my guess would be that the Zener would just get blown up if the plate shorts to the heater.

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