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Author Topic: Plates overheating on a 4-400A Tetrode Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 01-17-2003 12:12 AM                        But, Mark, the tube transmitter just looks so much cooler! 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.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 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  |  IP: Logged Sam Hunter Jedi Master Film Handler Posts: 779 From: West Monroe, LA, USA Registered: Jan 2002 posted 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!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 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  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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 ]  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 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! 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?  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 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  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 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.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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"... 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.  |  IP: Logged

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