The AM Forum

What does the AM forum hive mind say about how to bring up such fils after a long inactive period?

The H&K's have them and I am nervous about just throwing full juice on them. They are 5 volt @ 7.5 amps per tube.

Any recommendation to heating them slowly when first lit? small variac in the primary n slowly let them burn for a while at half scrote?

What say you poeple that know more than me?   ???

The question is prompted by hearing many stories about how the larger tubes like 100th's with such fils have failures. The fils in these tube are much shorter in length.

a variac is your friend

Here's what I do with 4-250's and 4-400's in my filament burn-in jig.   These tubes are 5.0 V @ 15 A filaments.  I run cooling air over them and monitor filament voltage at the socket using a DVM.  I run them for an hour where you can just start to see incandescence, this is somewhere around a volt.  Then up to 2.5 volts for another hour, next hour at 3.5 volts, next at 4.5 V.  Then the remaining 6 or so hours at approx. 4.8 volts.  With 10 filament hours on the tube, I'll run it in the tx for 10 or so hours.  To me this seems like a good yearly procedure to keep spares in good shape.

While these are some of the 'little ones' in the transmitter biz, filament management is very important to tube life.  As far as start-up is concerned, refer back to the way they are lit in the gear that uses them; when the 'on' command is given, they get the whole voltage from the get go.  This may or may not be a good way to wake up some tubes that have been out of service for some time, but I really don't think it makes a great difference over the long run.  If they are going to go, they will do so.

I'd let them heat for a while, and then apply plate E to them gradually to see if there is any gas.  If there is a little, heat them a bit more and it might just go away.  If it persists, the old trick of using the tube as a 'gas discharge' device by passing high voltage (10kV AC) with no filament lit through it for a few minutes can really clean up the problem and get that tube back into service.  Careful here, you can just get really rattled by that amount of juice....   

What makes the most difference is how they are run in the socket.  Pay attention to the ratings, and start the fils at that voltage.  Measure it at the socket at first just to make sure it is right.  When the tube is thoroughly heated, put it to work and get it perking good.  Then drop the fil voltage a bit at a time to see where the power out falls off.  Push it back up a bit and that is where it would like to be for best life.

Bear in mind that overvoltage on the fil will crap out the coatings pretty fast, nothing to be gained there.  Undervoltage can cause them to go south prematurely as well, as the coating won't be up to proper heat and some electrons will be coming from the wrong parts of the filament.

All in all, that series of tubes is some of the toughest glass tubes made.  Run 'em within ratings and they will last a long time.  Run 'em hot, and you will be surprised at just how much power you get out of them.  But in any case, RUN 'EM!

dg

I found the original H&K datasheet finally. gonna upload it here. check out how they say a ma meter is a waste of time, just tune them for minimum plate color and put a current meter in the feeders. Tune for max blowsmoke. Thats a scroteful tube. 8)

Did you see the careful instructions about sequencing the SG vs the Plate voltage?  That's a bit scary.  Guess if you tie the SG in with the plate supply it won't be a problem.

Al

My H&K 354C's, and all Eimacs get the mini variac or a 120/240 transformer step start treatment. Those tubes arent getting easier to find or cheaper.

Eimac triodes went thru several design changes to improve filament reliability just in shipment. A few of you may have seen the slings that 304TL's were shipped in for instance.

Carl
KM1H

yes Carl .... looked like a huge microphone shock mount or sumphin from Frankenstein's lab ...73...John