Film-Tech Forum ARCHIVE: Plates overheating on a 4-400A Tetrode

This is topic Plates overheating on a 4-400A Tetrode in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Paul G. Thompson (Member # 655) on 01-16-2003, 11:10 PM:

First, the operating parameters (according to my handy-dandy high voltage probe)

1. Plate voltage = 2,770 Volts DC.
2. Screen Voltage = 664 Volts DC.
3. Bias Voltage = -169 Volts DC. (almost at cut-off Class "B" Bias)
4. High level Plate Modulation.
5. Tubes are operating Push-Pull.
6. Monitor meters are of tube characterists are calibrated in percent, and not actually calibrated in milliamps.
7. Meters indicate I am driving a little hard to achieve 100% carrier modulation (AM)
8. Screen supply is "fixed" (non-adjustable)
9. Plates cool down to just a small tinge of red glow at the quescent state.
10. Audio feedback to the input of the audio amplifiers from the modulation transformer is supposed to be 16db.
11. Modulators are not oscillating, and the feedback ladder seems to be working properly. (thank god)
12. Cathode currents are balanced during the quescent state.

The RFPA's are working a little too hard, also. Since I have a meter that likes to stick, I cannot get a good reading of the Plate current dip when I adjust the tank tuning. The loading coil seems to be adjusted correctly, because at the plate current dip (400ma) on the RFPA's (two tubes operating in parallel) are giving the rated power output of 1KW into a 50-ohm load.

I wonder if I have a modulation transformer going south on me with a couple of shorted turns.....or maybe the isolation/coupling capacitor from the secondary of the modulation transformer to the the plate inductor of the RFPA's is starting lose its value.

I'll cook up a schematic for you guys, and if the resolution is good enough to post, I'll post it on an edit. Here is a picture of the hot tubes:

Look at those plates glow. Pretty, aren't they?

OK - here is the schematic of the circuit.

Anyone have any comments that will help solve the problem? Thank you in advance.

Posted by Mark Gulbrandsen (Member # 72) on 01-16-2003, 11:32 PM:

Paul,
I don't know what the budget is there at the station, but its alot cheaper to run a solid state xmitter these days that the ole firebottles. A solid state xmitter can pay for itself in electricity savings alone in as little as 5 years...so I've been told by a transmitter engineer that works for KSL on top of Farnsworth Peak. Also, unless your solid state xmitter is hit by a meteor it will keep going even if you loose a few modules....just at a bit reduced power.
Check out this page of Jims site to see a 10kw Harris solid state transmitter. They're made all the way up to 50kw now!
http://hawkins.pair.com/wabcnow.shtml#digmodsect
I can tell you a guy to contact that might be able to help you with your problem....contact jim@jphawkins.com If he can't help you himself, he knows alot of engineers that probably can.....
Also, some transmitters finals plates do glow red. A particuluar RCA model that uses 4-400A's literally glow red hot all the time!
Mark

Posted by Scott Norwood (Member # 30) on 01-17-2003, 12:12 AM:

But, Mark, the tube transmitter just looks so much cooler! [Smile]

For what it's worth, WBZ in Boston also uses a DX-50 (same as the one at the station listed in Mark's link); they also have a DX-10 on site for use with their backup antenna tower.

Posted by Mark Gulbrandsen (Member # 72) on 01-17-2003, 12:17 AM:

Scott,
Check out the WLW transmitter site(museum!)on his site. Thats a really cool place. Pretty cool that the large 20kw final tubes have 11 volt filaments that draw almost 300 amps.

Its mind boggling that the DX50 is 90% efficient!! For 50kw out it only takes about 55kw input.....only through the graces of its class D amplifiers I suppose. Older tube type units required on average of 78kw input power for 50kw at the final. Also, all the problems acociated with the modulation stage and large transformers are totally eliminated. Gee, I should get a job with Harris!
Mark

Posted by Kenneth Wuepper (Member # 1174) on 01-17-2003, 07:38 PM:

Paul,
What is the actual bias voltage measured from the filament (Cathode) to the grid of each tube. I believe that you connot assume that the bias supplied to the grid balancing network is actually biasing the tubes. Typical problems in this RC coupling system are caused by the leaking of DC through the coupling capacitors or loss of Cathode reference.

Are the little rings on the grid leads those Ferrite Beads?

KEN

Posted by Sam Hunter (Member # 1119) on 01-17-2003, 09:15 PM:

Pull the tubes and recheck your voltages on your grids and cathodes. For the tubes to glow red they have to be dissapating to much current and to do that the bias has to be off and for that to happen you have got two choices, cathode (heater) or grids. For both tubes to be glowing seems to indicate a common problem and according to the schematic you provided that common point is the bias supply.
Go get em boy!

Posted by Mark Gulbrandsen (Member # 72) on 01-17-2003, 09:34 PM:

Guys, I might suspect a bit of dc leakage but Paul says the bias is almost at cut off......right about where it should be.
After thinking this over I'd be looking for something on the heliax line, or transmitting arrays....too much mismatch, or RF leakage to ground might also cause this sort of problem. Bad or slightly leaky tower base insulator????? With there being two different makes of tubes and the plates not glowing the same that is not really a good clue. Does the bias on each tube adjust normally????? And are the pots about where they normally reside for normal bias?

BTW" you're lucky to be able to see the plates at all!! Most later tube finals use the ceramic vacuum tubes with no visual abilities, and you might not have a clue to something amuck going on.
Mark

Posted by Paul G. Thompson (Member # 655) on 01-17-2003, 09:42 PM:

Ken, it is. Inthe filament (cathode) circuit, the filament center tap is connected with a 2-ohm resistor to ground. This 2-ohm resistor serves as a meter shunt for the cathode current meter. Nominal value of the cathode current in the tube's quescent state is, according to my meters, 50ma each.

I also agree that one would presume a goofy bias supply. I wrung that out volt by volt, and resistor by resistor. Another area could have been the screen supply being too high, but that seems to be OK. The screen supply also is the power supply for plates of the audio amps (through a voltage divider, naturally) but those plate voltages are normal.

However, during modulation, peak currents of each cathode is a whopping 250ma. That reading is out of the tube operating parameters. And yet, my modulation is only hitting 90%. If I increased the audio drive, the modulators would probably self distruct.

So, I would presume my bias as measured from the control grid to ground is about the value I stated. A 2-ohm resistor at 50ma will drop about .1 volts.

My thoughts are leaning more and more to a bad capacitor on the mod transformer output or the modulation transformer is heading south. I have to find a 2uf 3,000 volt capacitor to substitute.

Mark, this transmitter is our night transmitter. In the daytime, we use the 10KW old Collins. The loading of both transmitters is correct. So that eliminates transmission line problems.

And, yes...you do see different tubes. One went horse shit, and I had to rummage to find two tubes that would balance up on cathode current.

Normally, on ocassional 125% peak modulation, we see about 30 to 35% meter reading on the mod cathodes, which would be about 150ma. But when we hit the mods the way we are, as you can see, those plates don't like it.

Posted by Kenneth Wuepper (Member # 1174) on 01-17-2003, 10:20 PM:

Paul,

Measure the bias from the filament to the grid not to ground. Your cathode metering current may be off as well as the fact that the tubes are glowing red. A gassy tube or tubes would draw grid current and could make the grid to ground reading inaccurate.

What is the condition of the two HF roll off capacitors which are across the modulation transformer primary? I can't read the values of those but they could, if shorted, cause the transformer to appear shorted to AC and the static DC system voltages could read OK.

That modulation transformer must look like a 10KVA line transformer!

KEN

Posted by Paul G. Thompson (Member # 655) on 01-18-2003, 12:13 AM:

Ken, I'll do what you recommend, as you bring up a good valid point. I never thought of doing that to each tube individualy, since the cathodes have a separate shunt resistor.

Believe it or not, I thought of those capacitors you spoke of in the feedback ladder. I did check them once, abd they were not shorted. However, I'll check them again to see if they have not changed value. Those capacitors are .004 uf.

Edit: Yeah, Ken. That transformer is one big sucker....I'll bet it weighs close to 150 lbs.

[ 01-18-2003, 01:31 AM: Message edited by: Paul G. Thompson ]

Posted by Jack Ondracek (Member # 1466) on 01-18-2003, 01:43 AM:

Paul, is that a Bauer 707?

Is there a reason why you're running "A"s in there instead of "C"s?

What kind of processor are you running? Have you changed it recently? Are you clipping the crap out of your audio? If so, a Bauer will look just like that, especially if you're trying to overcompensate for NRSC by jamming the highs.

But... you already knew all that.

Get rid of the Bauer. Convince your boss that a 10kW Harris or Nautel will run both day & night powers, and you'll pay for the cost in electrical and tube savings! [thumbsup]

Oh... that WLW transmitter is a real classic! Did you see the story about how the engineer brought the 50kW "exciter" transmitter online for the 2000 new years?

Posted by Paul G. Thompson (Member # 655) on 01-18-2003, 02:02 AM:

Jack, it is an ITA. I think they were in bed with Collins and Continental....

The ITA is our night transmitter, and the 10KW Collins is our day transmitter. All the audio equipment is common to both thransmitters. We run non-directional power both night and day, our RF line currents are all normal and the transmitters are dumping into a 50 ohm load. The same antennas are used for both day and night.

In essence, at power down time, we just simply switch transmitters. The old Collins won't run at 1KW. The lowest it will go is 5KW and that's why we have the ITA. It will run at 500 up to 1000 watts. Our night time power is licensed at 1KW.

Posted by Kenneth Wuepper (Member # 1174) on 01-18-2003, 08:08 AM:

Paul,

A couple of more thoughts about the load side of that modulator:

The modulation choke in the final supply line could be shorted

The capacitor which sets the power supply end of the audio choke at AC ground could be open or changed value so that the power supply is the only filter for the audio at that end of the choke and could lead to lowered modulation.

The obvious one you have checked is the coupling capacitor from the modulation transformer to the modulation choke this could open or change value and reduce the transfer of audio AC to the finals.

This kind of modulation scheme was used in one of my friend's HAM rigs. It keeps the high voltage DC from the finals out of the secondary of the modulation transformer and so it can be less costly to build and gets rid of those multi-KV potentials on transients.

KEN

Posted by Jack Ondracek (Member # 1466) on 01-18-2003, 12:39 PM:

Paul...
Is this a new problem? Your right mod is pretty bright, but it's also a different brand, so it's hard to tell if they're not balanced. I suppose you've got single metering for both of them, eh?

I've had a couple of ITAs, but it's been a while. The more recent one was a 5kW that used 4-1000 bottles in the mods & a 4CX2500F3 in the final... Sure glad I don't have to by those tubes anymore!

You didn't mention your processing. If you're running a CRL or Optimod, you're going to be pushing the average modulation past what that transmitter was designed for in the first place, so it might not be that unusual... especially if the jugs are starting to get a little flat.

When you get the chance, check the operating specs on the "C" version. If you don't find any component problem, maybe that version will give you a little more headroom. I pulled A's every chance I got because of that, especially when using an aggressive processor.

We usually pushed "night-mode" transmitters pretty hard, considering lower power & the garbage out there that the the signal had to overcome.

Posted by Paul G. Thompson (Member # 655) on 01-18-2003, 01:44 PM:

Ken, I sent you a schematic of the transmitter to you via email.

Jack, I'll forward it to you, too.

This problem has been an on-going and was getting a little worse as time went on. The audio processing gear is the same we use with the daytime transmitter. I have request the boss to order some new bottles, and I asked for 4-400C, they have a little more balls than the A's. Anyway, the processor is a Prism - whoever built it, I don't know. But it has been running flawlessly since the day it was installed. It is not a CRL or Optimod.

I might ass that the 10KW Collins has 4-400 bottles as modulators in the RF cabnet. They run at 2150 plate voltage, and -120 on the control grids. Have not checked the screens, though. But they just run happily along, scratching their anodes, and doing what they do best - "Modulate"... [Big Grin]

I'm also check a few more suggestions Ken made. Somewhere, I have to find a subsititute capacitor for the mod transformer output. Some of that high voltage stuff is getting hard to find.

There is a separate meter for each cathode.

Posted by Mark Gulbrandsen (Member # 72) on 01-18-2003, 03:31 PM:

Paul,
Need a High Voltage capacitor???? Call Salt Lake Instrument on 17th south. Ask for Bill. He has thousands of them!!
Mark @ CLACO

Posted by Paul G. Thompson (Member # 655) on 01-18-2003, 04:32 PM:

Thanks, Mark. By the way, I'll send a complete schematic (zipped)

Posted by Jack Ondracek (Member # 1466) on 01-18-2003, 05:56 PM:

Prism's good stuff... not as common as the others... probably because it's pricy. Your boss made a good move there.

The 4-400s in your rf cabinet are probably intermediate drivers for the finals. As such, they'll run at least 2 or 3 times as long as the modulators in your 1kW. That modulator design was really hard on the 4-400 series. The industry should have stayed with the 833 tubes that Gates used. Those were perfect for the job.

Posted by Kenneth Wuepper (Member # 1174) on 01-18-2003, 06:03 PM:

Paul,

Got the two diagrams and think you are on the right track. Happy to know that this was an on-going and worsening problem and not a sudden result of the tube change.

I used to teach the theory classes for the local ham club. I was servicing 2ways since 1952. Lots of hours on the old commercial ticket. Funny thing though, I never did get a ham ticket, too busy with my film work.

KEN

Posted by Paul G. Thompson (Member # 655) on 01-21-2003, 02:43 PM:

Mark, I called those guys. They had one, and it is being shipped and hopefully, I'll have it tomorrow.

However, the transmitter died this morning. The plate contactor will not close. So, time to start chasing wires again. [Frown]

I did have a half-assed capacitor checker, and it showed that the output capacitor of the modulation transformer has, basically, gone south.

Edit: But the trouble this time was an interlock in the coax switch and a relay failed in a controller. Jeeze, what next?

But at least I got the damned thing on the air manually... [Smile]

[ 01-21-2003, 09:35 PM: Message edited by: Paul G. Thompson ]

Posted by Kenneth Wuepper (Member # 1174) on 01-21-2003, 10:35 PM:

Paul,

The factory engineer was testing a UHF TV transmitter and when the SWR relay dropped out he simply bypassed it and continued to operate the transmitter. BAD NEWS! The SWR was for real and they had to replace 1,200 ft of Helium filled CoAx. The problem was the expansion fittings holding the center conductor. The fingers were of Nylon and the expansion balls were Teflon. Bad dielectric shorting in these places. It's not nice to defeat failsafes!

Please don't get electrocuted, we need you on here!

KEN

Posted by Paul G. Thompson (Member # 655) on 01-22-2003, 02:05 AM:

Ken, I know.....but my predecessor did a real hatchet job on the wiring, including using 120 volts on the shields of audio cables which were used for annunciation. But I was doing what I did on a safe bet with myself. The power ladder part of the schematic (left side) shows a connection from TB8-4 to TB8-6 with an attached note. This connection was the final "GO" for plate contactor potential after the coax switch connected the 1kw transmitter to the antenna feed line. There is a microswitch in the coax contactor, but whether it is connected to TB8-4 and TB8-6 directly or a "loop-DE-loop" to a homemade manual change box of which I have no documentation is uncertain.

But when I climbed up the ladder with my 120 volts to the plate switch, I knew it was going to run. With the antenna connected, throwing S-6 induces a one second delay before the plate contactor is closed. The screen supply powers up first at a low voltage until K-18 closes which bypasses R-140. (A little rat race there.)

As far as the interlocks are concerned, There are only three. They are the airflow (which has been defeated years ago) (which I am going to have to undefeat) and two door interlocks. I just had to jumper TB3-10 to TB3-11 to clear that part of the ladder. (I was running out of time, and only had 45 minutes before that transmitter had to go on the air.) It could be that my predecessor may have used a horses' assed interlock through the Gentner or his little home made box that sits in the middle of a pile of rat shit.

As you can determine, I got my hands full! But unlike the Engineer you mentioned, I am very careful about powering up a circuit for test purposes until I know for sure it is not going to fry something else. [Smile]

When working on these transmitters, I take plenty of coffee breaks and study the schematics carefully and well and explore all possibilities of what could happen "if".... [Smile]

Oh, by the way...K-4 pins 2 and 3 are normally closed. My schematic shows they are not. K-4 dumps the power chain in event of various overloads sensed by K-9 through K-11.

So far, that was the only error I found in my schematic reproduction from a 40-year old rag that was obliterated and in a million pieces. I'm kinda proud of that reproduction..... [Smile]

Posted by Kenneth Wuepper (Member # 1174) on 01-22-2003, 07:36 AM:

Sounds like the station is very lucky to have you there. My old high school buddy, Paul, is a real engineer also. We can fix anything! We make a great team on these old theatres as we were both in the high school media (Movie) department. We both had our commercial FCC licenses out of high school and worked for the same 2way outfit. Paul went into communications for the railroad and I was media director for a school system where I operated and maintained the television studio. We even had our own microwave to the local UHF station for a 30min live show once a week.

Your reproduction of the schematic is nothing short of incredible. Also the scanning has preserved all of the detail so it can be magnified many times and still is extremely clear. Great Job, Paul.

[ 01-22-2003, 05:59 PM: Message edited by: Kenneth Wuepper ]

Posted by Paul G. Thompson (Member # 655) on 01-22-2003, 08:13 PM:

Jeeze....someone must have snuck in on me and took this picture of my equipment racks.

This is about what I inherited from my predecessor. [eyes]

Posted by Sam Hunter (Member # 1119) on 01-22-2003, 11:18 PM:

Jeez, you must be freaking kidding?
Thats a freaking mess!

Posted by Paul G. Thompson (Member # 655) on 01-22-2003, 11:26 PM:

Sam, I have seen worse. All you have to do is look at mine. True, there may not be as many wires, but mine are tied in a zillion knots. Sorta like 100 each 100-foot microphone cables scrunched and inter-tangled and knotted into one cable 1 foot long with 200 ends. [Big Grin]

Posted by Kenneth Wuepper (Member # 1174) on 01-23-2003, 06:28 AM:

Zotts!

The person who did that went to the school of "Listen to my great station~" university. Last week he couldn't spell enguneer now he be one!

I bet that the control room looks "pretty" and that this whole mess is somehow interfaced to a COMPUTER? And we all know that the computer makes it PURRfect.

What brand of headache medication do you use? [beer]

Posted by Jack Ondracek (Member # 1466) on 01-23-2003, 08:11 PM:

Is that a drop light hanging in the middle of that? You'd need more than that to find anything in there! [Eek!]

Posted by Paul G. Thompson (Member # 655) on 01-23-2003, 09:14 PM:

Ken, My medication is 16 packs of cigarettes and 14 gallons of coffee.

Jack, I was amused by the light hanging there, too. Maybe the engineer used it to shine up his butt to see where he came from. [Smile]

Anyway, an update. So far, I got the interlock problem restored, and will test it in the morning. I think it'll run. Also got the autostart of filaments restored, that problem was caused by some wires falling off some terminals in the rat's nest.

I ordered the capacitors I needed, but that floozy still has not sent them. [Frown]

But at least, I think the transmitter will power up and power down just fine by itself during tomorrow's test.

Then, finally, I'll be able to focus my attention to the nice romantic glow of the 4-400's, high cathode currents, and high screen voltages.

According to some data I received on the 4-400A's, the typical operating parameters with the tubes biased class "B" are as follows: Plate Voltage = 3500, Screen Voltage = 500, and Bias = -82 volts. In my application, Plate voltage is low, screen is high, and so is the bias. I might insert a dogbone in line with the modulator screen grids and bring it down a little to see how well it performs. This was also suggested by a guy who went to school with Marconi, and has been rebuilding old vacuum tube transmitters for years.

[ 01-24-2003, 03:03 AM: Message edited by: Paul G. Thompson ]

Posted by William Hooper (Member # 71) on 01-24-2003, 02:27 AM:

"Hey, I'm about to go out to our AM site to look at something. You want to go?"

Hell no! I ain't that big anymore.

quote:
Sam, I have seen worse.

Paul is happy because:
1. It all seems to be from the last half-decade or so, as opposed to one of those with intermixed layers from th 1950's to now with ratty brittle cables, going through dirty normals on telco patch panels, then to Rat Shack relays, etc.

2. The environment is not one of those where there's 4" of silt in the bottom of the racks & coating everything as the result of every time it's flooded, & it seems to be not a rat- or wasp-filled nature preserve.

The odd thing is that about 1 in 10 installations you'd see like that were the work of blindingly brilliant, intuitive engineers who'd understand complex problems & arrive at brilliant solutions immediately, but the actual work to them was just something to lash up. It wasn't important. They're like the folks with shops & offices that look like earthquake zones, but they know where everything is located in x,y,& z dimensions, plus remember *everything*. They creep me out.

Posted by Paul G. Thompson (Member # 655) on 01-24-2003, 02:51 AM:

William, that last paragraph describes my predecessor exactly to a "T"! He was a very brilliant person, and a very kind and passionate human being. Unfortunately, he has passed away as a young man in his late '40's of an illness.

I really admire that person, he would give you the shirt off his back if you asked for it. As a young lad, he had Polio, and he was very crippled, but yet, he forced himself to walk. If he would lose his balance and fall down, he would just pick himself back up and keep going like nothing ever happened. He had a golden sense of humor, and always had a smile on his face, and had a kind word for everyone. I have known him for 15 years, and I have NEVER heard him say so much as an off-colored word. Swear words were not in his dictionary.

When he passed away, it was a sad day and a great loss to all of us that knew him.

I would like to think that the reason why his workmanship was not the greatest in the world was because he was always in severe pain, but he did not let on to it. If you saw him work and worked with him, you would automatically know what he was going through. Sometimes I seem to forget that, and in one post I did. After feeling like a fool, I edited that post.

Posted by Paul G. Thompson (Member # 655) on 01-25-2003, 03:50 AM:

Update: Power-up by itself worked as advertized. Power down time, well, that's a different story. Seems like the plate contactor does not want to release when the power is removed from it's coil.

First, the damn thing does not want to power up! Now when I fix that, the damned thing does not want to power down! Jeeze, that's worse than a woman!!!!!!! It can't make up its friggin' mind!!!! [Mad]

Jeeze, does it want an egg in its beer, too?

Now I have to find a contactor that will last another 40 years (but the way this crap is built these days, that will probably never happen).....Maybe I can dress the clappers to buy some time....The Saga continues....

'Ya know...This is really fun and challenging to work on shit like this... [Smile]

But it is stressful...one slip and I am off the air. It is all a calculated risk. So far, I have been lucky.

Posted by Kenneth Wuepper (Member # 1174) on 01-25-2003, 06:58 AM:

Paul,

You are doing this work in the right spirit. You love the pressure and it feeds you. We restoration people in the theatre world have enjoyed a rush from time to time as we get the old organ to play as it was originally intended. Sometimes it is a piece of lighting or sound equipment that we restore. The theatre restoration is likely to be more costly in terms of ingenuity as there is less of the green stuff or it has to be sought through donations.

Broadcast electronics is a very nerve intensive business and you are right to look at all of the alternatives before making a move.

By the way, do you play chess? It is a good game to sharpen your mind for these decisions you are making. It helps to anticipate what could happen a few moves further up the road.

I can't tell from your photos but am I seeing a few new grey hairs from this one?

KEN

Posted by Paul G. Thompson (Member # 655) on 01-29-2003, 01:17 AM:

Well, I finally got my coupling capacitor. I'll change it tomorrow, and see if it helps my modulator drive. I didn't get the feedback ladder capacitors yet, but I'll report on the results of the coupling capacitor change.

Posted by Paul G. Thompson (Member # 655) on 01-30-2003, 06:50 PM:

I changed the capacitor today, and it did help the modulation a little. However, I am still driving a little too hard to get the desired modulation.

Posted by Kenneth Wuepper (Member # 1174) on 01-31-2003, 07:31 AM:

Paul,

How are you coming on the overcurrent in the quiet state problem? I thought you had made some progress there also. I suspect that you have several problems which are in that modulation power output stage.

Did you say that the screen voltage was too high? I believe that the tubes will act more like triodes as your screens approach 50% of the plate supply. The bias should be adequate to keep those tubes near cutoff before they are driven with audio.

Did you measure the actual bias from each grid to the high side of the cathode metering resistor? That is the actual grid bias on those tubes. If they are still glowing then the tubes must be bad. (Gas inside the envelope)

Still interested in this one!

KEN

Posted by Bernard Tonks (Member # 809) on 02-10-2003, 05:30 PM:

I thought this might be of some interest to Paul and others on this thread. It is an aerial system, with transmitters and generators on an unmanned platform six miles out at sea.

www.longwaveradio.com
View Engineering.

Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 08:31 PM:

Ken, I temporarily "bagged" this one. The transmitter sounds really great on the air, has a nice warm "punch" at the low end, and a very nice clean high end. Since I replaced the capacitor on the output of the modulation transformer, the transmitter does sound much better. The transmitter has a better "splatter" specifications than the old Collins, and both transmitters meet FCC requirements.

With no audio applied, the plates of the 4-400's cool down, to almost no glow at all. The quescient cathode currents of the modulators and RFPA's under those conditions are very normal according to the manual.

However, the harder I drive them, the hotter they get. But as long as the transmitter sounds as good as it does, and it meets all FCC specifications, I'm gonna let 'er run. So I fry more 4-400's - I suppose for high lever plate modulation, I can expect that. The 2 used bottles I matched up seem to be holding their own very nicely, even though they do get hotter than hell. At least, according to the meter readings of today, they have not lost any ground.

Posted by Paul G. Thompson (Member # 655) on 10-08-2003, 01:26 PM:

Bumped for Luciano.

Last night, the 4-400 on the right finally went Pbpthbpthbthp! Flatter than a pancake. No cathode current whatsoever. I had a used one and was able to balance. (quick fix)

I have two sets of new 4-400C tubes, but I have to check the ratings on them. I am concerned they might not have enough balls. My RFPA'a may be getting a little flat also...Although my RF line current is normal, the negative modulation is about 90% but the positive modulation - that's a different story. If I can get the positive modulation to peak 60%, I am lucky.

Posted by Jack Ondracek (Member # 1466) on 10-08-2003, 06:24 PM:

Paul, those "C's" ought to work fine. We usually made a point of replacing "A's" with them, because they seemed more rugged (I don't recall the specs, either).

What kind of final readings are you running? Those mod figures are really whacky.

Posted by Paul G. Thompson (Member # 655) on 10-08-2003, 06:53 PM:

Yes, Jack. The readings are really screwy. I just had a shorted 4-400A in the Collins that practically blew the transmitter out the back door this morning. (damned used tubes) I did replace the 4-400A's with a new set of 4-400C's in the modulator holes and they seem to be performing OK. I checked the specs on the "C" version, they should do just fine. The Collins is happy again and making 10,000 watts. [Smile]

For how long remains to be seen.....that's a good question. The next thing to go is the 3CX-10000 - that costs about 1800 bucks now...

The remaining new set of 4-400C's I have will be stuffed in the RFPA holes in the ITA. I suspect that will bring my modulation back up to where it belongs. The modulator drive seems to be ok and balanced, so I suspect one of the RFPA's is going goofy. It has all it can do to make 1000 watts.

Power change coming up at 1830...I am going to stuff the tubes in there and come back and have my screwdriver handy for screwdrivergetwitzen adjustments to adjust the RFPA Ig current and dip the plate before the 500 bucks worth of new tubes burn up.

As an edit, I did stuff the new 4-400C's in the RFPA Holes of the 1KW transmitter. With screwdriver in hand, I got the adjustments made very quickly at power change time. Only one thing. The transmitter output was too "Hot". If I detuned the loading, I could have brought it down to 1KW, but the RFPA Plate current would have been too high. Don't wanna burn up 500 bucks worth of tubes. To add insult to injury, the power ramping motor does not work. It is jammed and stuck. Result was I had to reduce power by using the hi-low toggle switch instead of the ramp motor. I had two options: 1250 watts or 800 watts. I elected the 800 watts until tomorrow when I can fix the ramp motor and the plate current meter. The meter sticks so badly I have to constantly beat on it with the back of my screwdriver to make sure I was getting the correct reading.

Gee...Isn't old equipment fun? [Razz]

[ 10-08-2003, 10:19 PM: Message edited by: Paul G. Thompson ]

Posted by Luciano Brigite (Member # 1104) on 10-08-2003, 09:27 PM:

Thanks for bumping it up Paul [Smile]

and good luck in having the transmitter back at it's top performance [Smile]

Posted by Paul G. Thompson (Member # 655) on 10-08-2003, 10:23 PM:

Yeah...nothing like having a dead 10,000 watt transmitter and a 1kw transmitter that had all it can do to hit 60% modulation. Makes for "nice coverage" of the area...and makes the owner quite nervous. [Eek!]

Anyway, both the 10,000 watt transmitter and the 1000 watt transmitter seem to be very happy at the moment. Now it is "MILLER TIME" [beer]

Miller Low-Life...The Champaign of Bottled Pee....now you know why I drink Busch or Budweiser...

Same Shit...Different Shovel... [Big Grin]

Posted by Jack Ondracek (Member # 1466) on 10-08-2003, 10:43 PM:

quote:
I did replace the 4-400A's with a new set of 4-400C's in the modulator holes and they seem to be performing OK

Glad to hear it all came back up for you, Paul.
Is that correct? You're using 4-400s for modulators at 10kW?

My favorite old combo was a pair of transmitters I had in Yakima.
This one was the 1kW standby. We had the 5kW version of this design (wish I could find a picture of IT) sitting next to the standby (892R mods & final), which made for a transmitter chain about the length of that Ampliphase you were talking about.. I really liked those transmitters. Lots of metering... and all tuning adjustments were motorized. Parts, like that oscillator box, were interchangeable between transmitters.

Though we could run the standby on our psa power (a whopping 86 watts), they usually just dropped the 5. We dumped the remainder of the 1kW output into a resistor load, which was mounted back in the "cage", and kept the whole transmitter room toasty!

Posted by Paul G. Thompson (Member # 655) on 10-09-2003, 12:24 AM:

Jack, there are 4 each 4-400's in the Collins. Two of them are drivers to the 3CX3000 modulators, and the other two serve as IPA's for the 3CX10000 final RFPA. The Master Oscillator drives a 6SJ7 buffer which in turn drives an 807. The 807 drives the parallel configuration of the 4-400's that serve as an IPA. The transmitter employs plate modulation.

The outputs of the 3CX3000's feed the modulation transformer which in turn injects the modulated signal into the plate circuit of the final. The two "400"'s that drive the 3CX3000 push-pull configuration receive the audio signal from the two push-pull 6SJ7 audio amplifiers.

Posted by Edward Jurich (Member # 1787) on 10-11-2003, 11:16 PM:

If the plates don't glow, current normal with no modulation, but are red with high current under modulation and unable to make 100% modulation I'd first make sure it wasn't a bad match. How does the transmitter perform into a good 50 ohm dummy load. Modulate with tone and check modulation. Make sure the output network is tuned properly into the dummy load. If that's not the problem, look for a modulator problem. That bias supply could be suspect if you can't adjust the glowing down by adjusting the bias. I'd do all my trouble shooting into a dummy load.
I used to run my 4-400's biased to glow red some under modulation, they'd sound much better that way.
Hey, anyone still using mercury vapor rectifiers [evil]

Posted by Paul G. Thompson (Member # 655) on 10-12-2003, 12:18 AM:

Edward, mine do. With zero modulation, they cool down. I thought I had a parasetic oscillation somewhere, but that was not the problem.

Anyway, I just "accepted the fact" that they are going to glow. The transmitter still sounds OK. But I am not getting the mileage out of them like I do in the Collins.

Posted by William Hooper (Member # 71) on 10-13-2003, 01:19 AM:

I think I am. They're the ones that emit that colorful gas, right? They're between the booth & the toilet, so you can't be sure which deadly gas is going on at the time.

Posted by Edward Jurich (Member # 1787) on 10-14-2003, 09:34 PM:

Hey Bill, you better be careful with that gas, don't forget you're running carbon arc's [evil]