833 Mystery

[KA3EKH]

So I have been working on repairing the blower motor on my RCA for a while now and noticed something weird, laying on the floor messing with the blower you can look up and see the 833 modulators and PA tube and I saw something. Apparently the 833 has two sets of filaments in the same envelope run in parallel and the tube in the PA had one set of filaments open on one side. Looking at it the filaments on one side are bright and dark on the other, you can see this better when under the tube looking up at the tube because I never noticed it looking from the front or top. I pulled a couple spare tubes I had laying around and the next tube was just like this except the filament was open on the opposite side. Have one stupid looking Chinese 833 but the filaments were both good in that one. This is where things get weird, running 9.5 VAC on the filament around 1.5 kV @0.38 on the plate I get the same output regardless of if it’s a tube with both working filaments or just one working filament. I like the tube with just half a filament because it’s a Amprex and has a nice logo on the front where the RCA tubes just don’t look as impressive and the Chinese tube is hideous being that it looks like the getter flash had discolored almost one entire side of the tube but it is the only one I have with two working filaments. I have a couple brand new RCA 833 tubes that are still sealed in the boxes from 1978 but for reasons that I cannot explain am reluctant to open them and use them, some weird phobia about opening new tubes when you have tubes on the shelf. Has anyone else noticed this about the 833? Assuming this was because the 833 tubes came with this transmitter and from looking at where it was in service am cretin they ran the filaments too hot. I have played around with lots of old broadcast transmitters in the past with 833 tubes but never have seen this but until now never knew to look.
Ray F

[KL7OF]

I have never noticed an 833 with only 1/2 filament working....Doesn't seem like it would give full output....As far as having new in  the box tubes, Those would be the ones I would have in my transmitter..Save the used ones for spares..Life is too short.....Run the best ones you have..

[k4kyv]

I have heard the same story.  Don't recall if that has occurred with any of mine. Probably enough reserve emission to get by with just half a filament.  Have you checked modulation linearity?  It may make full carrier output but not be capable of making full positive modulation peak output.  The best check would be the trapezoid pattern on a scope.  Also, check for downward carrier shift; the most obvious symptom would be a noticeable decrease in plate current under heavy modulation.

I'd keep the half-filament tube as a spare and use one of the good ones in the transmitter.  It's not good for those unused tubes to sit idle for decades on the shelf; they may gas up.  They should be run occasionally.  Heating the filament and plate tends to activate the getter and suck up any residual gas.  Electrons from the gas molecules floating round inside the envelope tend to sand-blast the thin coating of thorium off the filaments as the tube draws plate current and the emission electrons bombard the gas molecules and cause random scattering.  For  that  reason it is recommended to first run the tubes filament-only for a while, then with grid current, then at reduced plate voltage and finally, with full plate voltage and plate current. RCA recommends something like 15 minutes per step, but if tubes have been inactive for a long time, I tend to run each step for an hour or more, and maybe run the filament and grid current-only step overnight.

Also, I wouldn't advise running the tubes at 9.5 volts.  Low filament voltage can shorten tube life just as much as high filament voltage.  9.5 volts is right at the ragged edge of recommended filament voltage tolerance, and if line voltage happens  to drop unexpectedly, the tube could be inadvertently run with the filament in emission-starved condition which may damage  the tube. It has been suggested that running the filament slightly low may extend tube life, but only if the the starved-filament condition is avoided.  This requires some method of making sure the tube is maintaining full emission all the way up to maximum peak output - something difficult to judge without appropriate test instruments (again, the trapezoid scope pattern showing straight sides all the way up to maximum positive modulation peaks).

[KA3EKH]

Well imagine I will bite the bullet and open the two new RCA 833 tubes and stuff them up the modulator. Will take the best of the modulator tubes and put that in the PA. Have one modulator tube that’s got both filaments working because now that I know what to look for can spot if the bottle has one or two good filaments. The tubes with two good filaments are just a little bit duller but uniform across the top of the tube where the ones with just one filament  are a little bit brighter, assuming that’s because the reduced current on the individual filament transformers with one filament gets a little more voltage and the increased brightness. The problem now will be that I no longer have two new tubes on the shelf, will stick a couple of the spares back in the boxes but just not the same as having a brand new tube sitting there. I see that there are Russian GU-48/PL833A listed on EBay, anyone know anything about them? I have enough spare tubes to last a lifetime so not planning on buying any, just curious.

[KL7OF]

You will have new tubes sitting in your transmitter!!!

[k4kyv]

I guess I spoke too soon.  I have been having an intermittent problem with the audio in my Gates BC1-T.  Sometimes it's OK, and at other times, the positive peaks won't make above about 90%. It seems that about 98% of my radio failures are flaky-intermittent.

Last night the problem appeared again, so to-day I checked out the driver board.  I found a couple of weak 807s and replaced them with good ones, and re-adjusted the balance pot.  I was getting the same audio voltage to the grids of the 833As, but at times the modulator plate current, even with a sinewave tone, was unbalanced by about 2:1, and the modulation headroom had disappeared.  Then I noticed what looked like only half the filament in one of the modulator tubes was  lighting up. I tapped the tube with a sharp blow, and the whole filament came back on.  I re-tested the transmitter, and the modulator plate current was close to balanced, and I had plenty of positive peak headroom.

Looks like that tube doesn't have enough reserve emission to make full audio peak output from the modulator when that half-filament  is out.  I'll have to start searching through my spares for a good replacement.

[W2PFY]

I had the same thing with an 833 in my Westinghouse MW-2. It was driving me crazy as I would get good audio reports and then suddenly while in the same QSO, they would say your sounding mushy again. One day I just happen to be standing in front of the modulator and the tube filament starting turning off and on like a tower beacon. Mushy audio reports can mean weak 833's as well.

End of story