headphone tube amp concern

Tubes are unlikely to fail non-conducting in a really sudden manner. If the filament burns out or the emission drops, these are not all that sudden.

Inter-element shorts (say, grid to cathode, or plate to grid) and internal arcs are far more likely.

I do not know of any attempts to mitigate the volume in case of such a failure. Hooking the primary of the output transformer straight between B+ and ground will undoubtedly make a loud pop but this isn't too much different than any output driver in any situation failing shorted. Not only have all us had loud pops in our ears, we've also played with mercury when we were kids, and you can see that we're all PERFECTLY normal here!

Tim.

Did you know that old lead house paint tastes really good?

Of course, I know everyone is "normal" here ;)

So it is more likely a short between grids and/or plate that could produce this sudden pop. I was thinking of adding some clipping elements but this can be tricky not to have an audible effect on normal sound peak.

The problem with putting a varistor or transient suppressor directly across the headphones is...?

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That's a problem. These don't come gold plated ...

< duck and run >

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Regards, Joerg

http://www.analogconsultants.com

Tubes do not have that problem; usually the gain slowly decreases due to lower filament emission long before the filament would even think of opening (and i have never in 50+ years heard of a tube filament opening if powered properly).

Do not worry about these extremes.

On a sunny day (Fri, 1 Dec 2006 15:45:39 -0500) it happened "John Cash" wrote in :

If you never experienced that problem then... I go on with my project!

Thanks!

So, the hundreds of bad tubes with open filaments I replaced in the '60s and '70s were imaginary?

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Michael A. Terrell
Central Florida

Are you prepared to certify that the filaments were "properly" powered?

Filament widings on power transformers tended to be after-thoughts, with a couple of extra turns to deal with peak loadings, that didn't do anything good for the life-time of the filament.

--
Bill Sloman, Nijmegen (but in Sydney at the moment).

Robert may be right...

He probably meant that if filaments are softly powered on or off, not the crude switching through a conventional switch. Same problem with incandescent lamps, their filaments (usually) break when you turn on the switch. Hardly ever they go off in the middle of evening or night unless they are stressed by a mechanical shock or severe overvoltage peak from the mains.

On a sunny day (Sun, 3 Dec 2006 10:57:31 -0500) it happened "John Cash" wrote in :

Yes, we had the U series tubes, those had 150mA heaters (mainly radio), and the P series tubes with 300mA heaters (TV). Those tubes were put in series to work on exactly 220V (Europe). Thsere were ceramic decoupling caps from heater to ground, and thse sometimes shorted, or a heater touched a cathode or some flasho-over in the TV HV stage, and the heater would get a lot more then 300mA as no only a few tubes were on the 220V.

T1 T2 T3 T4 T5 T6 0 mains -^---^---^---^---^---^- 220v mains | | | === === === C3 | | | etc /// /// /// ------> + 311V | |

220V mains + | bridge rectifier === - --- 0 mains | | /// ///

You will note high DC / AC voltages between cathode and heater! Think wat a defective C3 would do, or a heater-cathode short for tube T5.

Later European voltages were upped to 230V, and that sure must have cause problems with electrolytic caps and heaters and flash over for those who did hang on to their tube TVs. Before that time I have replaced tubes with defective heaters in TVs too.

Series tubes saved a transformer.