New home-brew 40 meter RF deck

<< < (15/16) > >>

W8ACR:
OK Bob, Thanks a bunch.

Chris P.:
I had a pair of 3-500Z that I KNEW had good filaments, but I couldn't get any continuity through the pins even after resoldering them several times. What I had to do was completely clean all of the old solder out and start over with a good amount of heat and flux. They've been fine ever since. It might be worth trying.

W2PFY:
That's too bad Ron! Sounds like Criss has found  a cure for that filament problem and it may be worth another try? It depends how much an effort you want to put into it. The tubes are getting rare and generally when found on eBay, they are as usual big bucks. In amature service assuming you never have a plate current runaway, they should last 100 years. If you were to put a hour meter on the filaments and the plate transformer you would find that your yearly usage would be very low.

I am going to take my one dead 100TH and saw the base off with a dremel blade and see what is really going on? I'll start that project next week so Ron, don't throw them out just yet. I have over the years, given dead large tubes to audiophiles. They love to have them for display! I had another 100TH that became gassy and a friend made a regenerative radio out of it. He commented that it was a bit ridiculous to be burning that much filament current for such a project but despite the negatives, he said it works great!

W8ACR:
OK Chris,

Thanks for that info. I'll give it another try. They sure look like good tubes, and I cannot see any evidence of filament fragments in any of the tubes. In my experience, a broken filament usually leads to at least a few small fragments on filament material rolling around inside the glass envelope. I have some good silver solder and flux that I'll use.

Thanks again, Ron

K1JJ:
Quote from: W2PFY on February 26, 2018, 01:58:13 PM

You should not use a filament transformer that is for example 20 amps where only tem amps is needed. It could result in a thermal shock to the filaments.


Hi Terry,

I've probably  heard that suggestion before, but it never dawned on me why... but it makes sense. The initial low resistance surge would indeed be greater with a filament transformer using larger diameter wire and a larger core.  I've always used Variacs on my fil xfmrs without exception. In addition, the Variac allows precise fil voltage settings which will also extend tube life and performance.

Yes, tubes that are supposed to have forced air or at least adequate ventilation are prone to overheated filaments and solder drip. I had two 3-500Z amplifiers  (both commercial without chimneys) that used fans. They both dripped solder and the fils became intermittent.  Once I added real chimneys and blowers, the repaired filaments never failed again.

Try touching the filament pins of a poorly cooled "forced air" tube and you will burn your hand. My acid test is that once I turn the rig completely off after a transmission, filaments off  - and wait one minute with the blower on - then ALL parts of the tubes - glass, seals and pins should be only warm to the touch. Then you know the tube will have maximum life and will probably never fail due to seals and other common cooling/heat related problems.


BTW, a dummy something I learned the hard way:  Never suck air from the top down thru the tube bottom and out. I once had a blower on the outside of the house that pulled air out. It was a big (and expensive) external anus 3CX tube. It was quiet and efficient. But the tube failed quickly because the hot air thru the plate pushed hot air onto the filament seals and the fil seals failed. Always push air in from the bottom thru the top as most smart people do.. :-)  The filaments deserve the cool air first since they are smaller and have less effect on the plate cooling - compared to the opposite direction of air flow.

T

Navigation

[0] Message Index

[#] Next page

[*] Previous page