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Still working on this RCA conversion... tube questions




 
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Author Topic: Still working on this RCA conversion... tube questions  (Read 1029 times)
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K8DI
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« on: December 10, 2021, 08:48:17 PM »

So, I am still fussing over getting the BTA-1R1 fully functional before ripping the tank out of it and moving the frequency.

Out of 8 4-400A and 4-400C tubes that came with my two transmitters I still can't get the output I expect.

Three pairs of tubes can make the 1kW of unmodulated carrier I expect. One pair makes under 300; it is just done.  Of those three pair, one looks funny/gets hot spots on the plates and plate current is well above the others and the spec in the manual.

Question one:  If a modulator can modulate well past 100% Negative, is it safe to assume it has sufficient output to modulate to 100% positive?
Right now, the peak to peak RF voltage from the mod-monitor output (that is calibrated to nothing) on my Agilent scope is 85v unmodulated. If I crank audio up until it is just at 100% negative, that peak to peak voltage goes up to 150v -- about 175% of carrier, or 75% positive. If I keep going, well pat 100% negative, it goes up another volt or two.  In other words, I cannot get 100% positive -- not even close.

Question two:  In a Class C final, with a pair of parallel tubes that are NOT matched, if one is good, and the other is weak, will you get power out equal to twice the weak one or the halfway point between weak and good?  In other words, if I put in ONE good tube, will I be able to tell or will it only put out as much as its weaker mate?  I bought a "NOS" Eimac off eBay that came today. I's not new, the glass has the typical faint tobacco color of a tube with plenty of hours on it.  When I tried it, there didn't seem to be either an improvement or a decrease in performance, so I am asking if with only one better tube, would I be able to tell?

Last question:  Any of you have two or more known-good 4-400 type tubes you'd sell me? New prices are astronomically high and eBay is full of charlatans...


Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
WBear2GCR
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« Reply #1 on: December 11, 2021, 04:53:52 PM »

I'm no expert on BC rigs, actually zero personal experience working on them.

Beat tubes are just not going to make the full power or snot that one wants.
Are we to assume the rig is 4-400s modulating a pair?

Being able to go negative fully and not positive?
I'd start by swapping tube locations.
To make negative peaks, the tube on the "negative" side of the iron has to conduct
not turn off.

4-250s may be going for far less expensive prices (until you can get to a hamfest?) and
will not quite make the same max power, but pretty close. Maybe good enough to see what
is going on?

One fresh tube in the finals, and one beat? Yeah, I'd expect full snot from the fresh tube + whatever
less snot from the weak tube. Adds, pretty much, minus plate impedance changes causing less of
a good match to the output network? That's my guess.

                      _-_-bear

PS a number of folks on here have beaucoup BC xmitr experience...
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KD6VXI
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« Reply #2 on: December 11, 2021, 05:38:48 PM »

A mishmash of beat and good tubes in the final means that the one tube with more emissions is getting beat to hell while the one with lower emissions is getting a free ride.

If you have a bunch of lower emission tubes, running 4-250s as finals with 4-400s as modulators can extend the peaks.

If your able to modulate 100 pct neg and not attain 100 pct positive, try swapping audio inout leads and ensuring phasing is proper.  That can be as easy as swapping pins on an XLR connector or swapoi g plate leads in the modulator.

You could also try an equally unobtanium 4-1000 for the final. That's a lot of work, however, especially if you don't have a bunch of 4-1000s on the shelf.

Where it me, I'd swap plate caps on the modulators aif possible and see what happens.

The Chinese are making 4-400s still.  A new pair for modulators and tired pair of Eimac's with similar plate currents at idle may extend the watts per dollar of your tx at ham levels.

--Shane
WP2ASS / ex KD6VXI
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W3SLK
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« Reply #3 on: December 12, 2021, 04:39:28 PM »

Turning the 'Wayback Machine' to about 1991, Duane, then KN0R and myself went to a station's transmitter site where he purchased an RCA 250K that was their standby rig. The station was using a BTA-1R at the time. While we were perusing the basement site for spare parts for the 250K I came across a log book of the 4-400 rig which previously did duty in Hawaii. Paging through the log I noticed they had the same problem of not being able to make even 90% modulation. RCA technicians responded but I can't remember in detail what they did to resolve the issue. Maybe put some audio chokes in the screen circuit so you can modulate the screens with the plates would help. I thought long and hard about this but I'm sorry I don't remember what was the exact resolution to not achieving 100% modulation.
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WD8BIL
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« Reply #4 on: December 13, 2021, 08:17:44 AM »

Try reversing the audio phase to the transmitter and see ifn the 100% positive is achieved.
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KD1SH
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« Reply #5 on: December 13, 2021, 08:53:43 AM »

No experience here with any rigs that huge, but I've seen smaller rigs fail to achieve 100% positive mod due to low RF drive to the final PA. Proof as to whether it's the modulator or not is seeing the modulated B+ rise to twice the unmodulated value. If it is, then a properly operating final PA must follow square law and quadruple the RF out.
Of course, your personal comfort zone and available test equipment will determine whether or not you want to watch your modulated B+ on a scope, but that would show you if it's a modulator problem or not. As always, maintaining an "I can get killed doing this" mentality, and using the proper test equipment, is crucial. I use a X100 scope probe on my lower powered rigs, but your big rig may have several KV of B+.
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K8DI
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« Reply #6 on: December 13, 2021, 11:49:48 AM »

Thanks for the replies, comments, and ideas, all.

To address a couple things,

I have not probed the modulated B+ as of yet. While I have a 100:1 scope probe, it is not rated for the 6kV+ it would see. I am going to give it a go using a HV probe next time I work on it. While the HV probe will not give a calibrated result because it is supposed to be seeing a 10M input DMM, not a 1M input scope, all I need to see is if whatever voltage it does display on the scope doubles.

As far as grid drive for the finals, the 6146 buffer/driver is pushing the finals to about 110% of the grid current specified in the manual, so I'm assuming that I have adequate drive..

W3SLK, any memory trigger for you if I mention that these two transmitters both had the RF finals screen resistor modified? Manuals call for 50K, and a tapped point at 26K on low power, both had 25K, tapped to 19K on low power. This and a couple other mods were reversed before I ever powered it up (partly because the 25K was realized by two resistors in parallel and one was open). Even at stock resistances, screen voltage is 15% higher than specified in manual and tube data sheets.

Shane -- my experiments with one good tube were about finding out if I could tell it was good...that is, really, if the other tubes were bad.

I like the ideas of sourcing 4-250's as they seem way cheaper, and while they will be running flat out, they should still make nearly full power. Longevity in that state in ham use is not a concern like it would be in 24/7 broadcast operation.

Swapping mod plate caps is not simple, as there's feedback ladders and such that would all have to swap.  The source is a sine wave from a signal generator, polarity isn't an issue. I have swapped the mod tubes and their drivers back and forth, no change in positive vs. negative modulation capabilities.

I've swapped around all sorts of combinations and positions of tubes amongst those I have and have determined that I have another bad tube, it and its mate make full carrier and look fine for long periods of operation, but in mod duty, no signal, they bias the same and look alike, and then when you turn up the audio to 50-80%, one sits at the normal current and the other's current slowly climbs and the plate gets brighter in step until it gets way high and trips the overload. It only does this with audio drive, and once it starts removing drive does not stop the slow climb. It's either got a parasitic in only that one tube of the push pull pair or some kind of thermal runaway (grid sag/shorting?? I don't know).

I am really trying to be able to say "all is well" so I can pull out the BC band tank and install something that works for 75m (and others).  I have parts in had and brackets made up but haven't started yet because I don't want to be figuring out this stuff with a bunch of added variables...

Ed





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K8DI
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« Reply #7 on: December 13, 2021, 08:26:48 PM »

I have not probed the modulated B+ as of yet. While I have a 100:1 scope probe, it is not rated for the 6kV+ it would see. I am going to give it a go using a HV probe next time I work on it. While the HV probe will not give a calibrated result because it is supposed to be seeing a 10M input DMM, not a 1M input scope, all I need to see is if whatever voltage it does display on the scope doubles.

Update:  Using HV probe, and scope set to DC coupling, and a 300Hz sine wave to modulate the transmitter (and trigger the scope on the second input), and a pair of ok-ish Eimac 4-400C tubes as modulators (they make 1kW carrier when used as finals):  The finals made as much as 75% positive, way more than 100% negative, as before.  B+ started at 3100v with audio off and went as high as 7100v on the peaks, and all the way to zero volts negative. The bottom negative part of the B+ sinusoid was distorted; I expect this as the tubes cut off. Even at 7100v positive, the top was clean. It seems I have enough audio to go to 130% positive. I stopped there because there's gotta be a limit before something breaks down..

Conclusion, the finals do not have enough suds left to pass any more current after 75%, no matter that the instantaneous plate voltage was well high enough pull it.  Looking for a new (or good) pair of tubes to use as finals...

Ed
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k4pf
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« Reply #8 on: December 14, 2021, 04:34:21 PM »


 While the HV probe will not give a calibrated result because it is supposed to be seeing a 10M input DMM, not a 1M input scope, all I need to see is if whatever voltage it does display on the scope doubles. <snip>


Hi,
If you're using a Fluke divide by 1000 HV probe (model 80K-6),
there is a voltage divider resistor (75K) across the two banana plugs,
in addition to the 75 Meg Ohms in series with the probe tip.
So, only a slight loss in accuracy in using a 1 Meg instrument to meter the HV,
instead of the expected 10 Meg input DMM. (I figure a reading about 7% low).

73,
Ed Knobloch
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K8DI
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« Reply #9 on: December 14, 2021, 06:26:00 PM »

If you're using a Fluke divide by 1000 HV probe (model 80K-6),
there is a voltage divider resistor (75K) across the two banana plugs,
in addition to the 75 Meg Ohms in series with the probe tip.
So, only a slight loss in accuracy in using a 1 Meg instrument to meter the HV,
instead of the expected 10 Meg input DMM. (I figure a reading about 7% low)

Thatís what Iím using..   thanks for the info on the resistor, makes sense, keeps the plug from having a high voltage across it when not plugged in. In my case, the scope voltage reading is a bit off spec, but linearly so, so I usually measure some known voltage and then scale the reading as needed, which covers both the scope and the HV probe being loaded more than designed at the same time ;-)

Thanks,
Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
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