Firing up the KW Transmitter.. Need help with operating guidlines.

[ke7trp]

I have another post asking for information about my Transmitter labeled KW62. This transmitter has two 4-400s modulated by Two 810s.

I am about to fire it up for the first time.  I am going to use the ranger to excite the 4-400s and to modulate the 810s.  The ranger is up and running and ready to go.

IN all the documentation, I cant find any information on operating guidlines for loading the transmitter. My main question is on the two variacs, I must set.

One is for the 4-400 Screen volts. The other is for the plate voltage on the 4-400s.  IN the hand written notes it states 600 volts screen and 4000 volts Plate.  These are maximums I understand..

These are both set to ZERO now.  Can someone give me starting voltages to do the initial tuning on the 4-400s? I was thinking 2000 volts plate and 300 Screen.  I understand the importance of having both voltages up at the same time.  Should I start lower?

My plan:

1. load the ranger up to 10 watts AM carrier on 75 meters in to the dummy through the Transmitter.
2. Plug Transmitter in, Flip switch and wait the 50 seconds for the HV to come up.. (scary moment)
3. If all is well let the filiments cook for 20 minutes or so.
4. KEY transmitter with NO drive and increase Plate and screen voltages(both have meters)
5. Key Ranger and tune the plate cap for resonance.
6. Tune load for max power and retune plate cap for resonance.
7. Let run at 200 watts or so for 10 minutes.
8.  Unkey and make sure everything is ok.

I need starting Plate and screen voltages you suggest.

I need Plate current that you suggest for the two 4-400s in low power.

I need MAX plate current allowed at high power.

I need Typical Screen current and max screen current.

I need to know max Modulator Current for Two 810s running at 2700 volts.

The overall idea here is to get it up and running at around 300 to 400 watts so I can use it for a day or two to locate and repair any issues I might find. 

All it states in the manual, I have are test notes..  "4000 volts plate, 600 volts screen,  1200 watts AM output-100% mod for 4 hours" ect...  No notes about currents.  I wont run this at 1200 watts ever.. It will likely destroy my KW matchbox and 18Gauge antenna.. LOL  I intend to use this at 300 to 400 watts.

Thanks for your time!  Wish me luck!   The 50 sec time delay scares me.. I think I will have my hand on the plug at the wall ready to yank it out if something goes BANG!

Clark

[w3jn]

Just make sure you don't apply screen voltage to the 4-400s with no plate voltage.  That's a recipe for a couple of melted toobs.

Also ensure that there's fixed bias on the 4-400s or a clamper circuit w/grid leak, if you key it with no drive and no bias it'll draw LOTS of current in a hurry!

Believe you're looking at a max mod current for the 810s of about 300 mA.

FYI, my GPT-750 runs about 3KV to a pair of 4-400s.  Loaded to the book values, Plate = 300 mA, screen =60 mA, Po= 750watts.  Grid drive is about 10 mA if I recall correctly; this will affect screen current.

Best way to tune up is look for a peak in screen current; the plate dip isn't as pronounced as the peak in screen current.

You might look at the manual for the Johnson Desk KW which uses pretty much the same toob set.

You should have that turret any day now.

[WQ9E]

Clark,

I would follow JN's advice and the tube lineup is the same as the Desk KW.  I usually run my Desk in the low power position loaded to around 300 watts carrier output.  In the low power position, voltages are as follows:

HV (both final and modulator) 1,500 volts
Screen  250 volts
Fixed bias (final) -60
Fixed bias (modulator) -33

If there is not already some interlock to prevent screen voltage without plate voltage I would suggest using a relay setup with the coil controlled by the plate voltage variac set up so that the screen supply does not receive power until the plate control is advanced to provide high voltage.  Although the 4-400 screens are pretty tough compared to a lot of more modern tubes applying screen voltage without plate voltage will quickly turn your 4-400's into low mu triodes.  A fast acting low current fuse (in the screen lead) may also provide additional protection.  The supplies should also be interlocked so that neither plate nor screen voltage can be applied unless the grid bias supply is also active.

I know your feeling about waiting for the "big bang".  My Johnson Desk KW was in pieces when I bought it and it took about a month to get everything hooked up again.  I did the first testing with the screen and HV supplies disabled and all went well until I switched the mode switch.  The works were outside of the pedestal and when the VERY loud mod transformer shorting relay changed over I jumped high enough to nearly hit the barn roof.  All was well except for my nerves.  For the Desk, I created a temporary power cord with additional fuse holders and I used fairly low values fuses for the initial very low power testing to make sure that nothing major was wrong.

73, Rodger WQ9E

[n2bc]

Your #3 should be first.   Cook the filaments at least 30 minutes.  Does it have mercury vapor rectifiers (866s or 872s, etc.) ?  Cook them at least 30 minutes (60 is better) with NO anode voltage.

I would disconnect the all HV and LV from the PA and modulator and bring the HV supplies up by themselves, slowly as long as you have variacs.  Another trick is to wire a 100W bulb in series with the AC supply so even if there is a short on the DC side, the AC side current will be limited to 100W or so.

Be careful!

73, Bill  N2BC

[ke7trp]

Thanks for all the replies!!

I got the Turrett!! Its the exact one I need!  Thank you so much for sending that!  If you need money for postage or anything.. Just let me know.. What a guy!!!

I am going to download the DESK KW manual if I can find it.  This is pretty close to that except the power supplys are way larger.

Ok... I think I have what I need for ratings on the Plate and screen.. 1500 and 250 to start.  This transmitter has interlocks and time delays built in.  I understand about the screens burning up..  I will apply some Drive when I first key it. 

Once its up and running and I know the iron is all good.. We will start the upgrade process.. On the list are new diodes to get rid of the old Metal ones.  Different bleeders in the power supply as there are 330 watts being disipated now.  I want to install the tuned input back with the new turrett that was sent to me. I am also going to drive the modulator with hi fi outboard gear and audio amp. I am also going to install a third variac for the huge blowers. At the low cycle of ham use, I just dont need the 100% output of the fans going in this room.

Thanks for all the help... Now, I just need to get better so I can move this thing in the room and get to work!

[Jeff W9GY]

I use a small fuse in series with the screens, say 200 mA or so.  That should protect the screens in case the plate HV goes away,

[W2XR]

Hi Clark,

Here is some data based upon my fairly long experience with both a single and a pair of class C plate modulated 4-400As:

My first homebrew plate modulated rig back in 1978 ran a single 4-400A, plate modulated by a pair of 833As. I ran the 4-400A at 2750 VDC at around 275 Ma Ip, for about 750 watts plate input on 75M. I got close to 600 watts output at this plate voltage, which represents an efficiency of around 78%; quite good in my opinion, considering the plate voltage I was running. 4-400As like to see a minimum of 3000 VDC for really good efficiency.

In Class C plate modulated service, when you run the 4-400A at around 2700, 2500 VDC or less Ep, the Esg must be reduced from the published Eimac specification at the normal level of output loading, or else the Isg will run too high. With 2750 VDC on the plates of the pair 4-400As in my present rig, and at the level of Ip I normally run, I pull about 80 ma of Isg, and the Esg must be held down to 400 VDC at this set of operating conditions. If you run the Eimac spec of 500 VDC Esg, the Isg will come close to exceeding the dissipation rating of the screen.

This is true not only with my rig, but similar type transmitters including the Collins KW-1 and the Johnson Desktop Kilowatt when run at their normal plate input level and an Ep of 2500 VDC. Both rigs ran the 4-250As or 4-400As at an Esg of 400 VDC for the reason I have described above.

When I run my rig in the low-power mode; I reduce the Ep to 1800 VDC, and the Ip to 350 Ma. The Esg is reduced to 300 VDC, and the Isg drops accordingly to 60 Ma. The grid drive remains the same at 25 Ma.

John/W3JN: 10 Ma of grid drive for a pair of plate-modulated class C 4-400As is way too low; the Eimac spec is 12 Ma per tube. I also agree with your comment about watching the 4-400As Isg when tuning the HPA; always tune the final for a peak in screen current.

As others have pointed out in this thread, you really want to watch that screen disippation, both during the initial fire-up/testing phase as well as during normal operation. As a minimum, use a 125 Ma fast-blow fuse in the screen circuit, or some kind of screen overload relay protection. You don't want to roast a pair of expensive 4-400As! I also use a series of interlocked time-delay relays to control the proper turn-on and turn-off sequencing of the various power supplies in my transmitter when going from transmit to receive and vice-versa. Very important!

My experience with BC rigs employing 4-400As is that the Ep is usually around 3100 VDC; this permits maximum efficiency, and the Esg can be run at the normal Eimac spec of 500 VDC for plate modulated class C operation, without exceeding the screen dissipation rating of the tube.

Good luck with what will probably be a fun and rewarding transmitter project!

73,

Bruce

[w3jn]

Clark, that turret does a LOT more good in your xmitter than it does in my garage.  All I ask is that you pass the radio karma around to someone who needs it someday.

Bruce, that figure was from memory... I rarely look at the grid current in the GPT because if you have the right screen current it follows you have the right grid current.

[ke7trp]

Ok.. Good advice.. I understand..  The Rig has meters for Plate volts, screen volts, Grid, plate and screen current.

So no more then 80MA of Screen current?   

I can see this relationship as I have loaded my Globe 500 many times while watching the Screen current. They suggest less then 40 MA of screen current and that rig has one tube.

Thanks again for the turrett. I will try to help someone down the road!

CLark

[WQ9E]

Clark,

Sounds like you are on the way.  By the way, the corresponding voltages for the high power position of the Desk KW are:

HV (both final and modulator) 2,800 volts (the manual spec is a bit lower but that was for lower line voltage)
Screen  400 volts
Fixed bias (final) -90
Fixed bias (modulator) -65

As built, the Desk KW doesn't monitor screen current so I added a small external meter in its own little case to monitor the screen.  The Desk is set up to meter total cathode current so the known amounts were total cathode and control grid current leaving you to guess at how the remainder divided between the screen and plate.  The assumption was if you followed the tuning instructions in the manual it would be correct-adding a meter proves that was a bad assumption and with it tuned as Johnson indicated the screen current was usually high (although not dangerously so for 4-400's).  The desired level for the Desk is 75 mils of screen current loaded for 1KW input on AM.

I would definitely get used to your transmitter (and find any "bugs") running low power for awhile before you crank up the variacs.  Do be careful to match the audio amount so you don't create excessive voltages and blow a mod transformer.  Have fun!

Rodger WQ9E

[Opcom]

Under some loading conditions you can get reverse screen current. The screen supply should have a low enough resistance bleeder to absorb it. If screen current is too high, it could be the loading is too light for the applied drive. It's more of a concern in smaller tubes but worth watching out for.

[ke7trp]

This transmitter also has GRID Voltage knob and Grid tune knob.  The Grid Voltage has a Meter.

Going to light off the Filiments in a few moments with NO HV and NO drive.  Just in Standby. 


Clark

[ke7trp]

Ok.... The good and the bad..

We plugged it in to the 240 vac line. No problems.

Turned the mains on.. Power light lit up.. Great.

Turned the Filiment switch.  810s and 4-400s lit up nicely. 

Waited about 20 minutes.. No smells no noises.. 

Hooked up external Keying relay 110v to a Foot pedal.  Stepped on it. Got volts!

1000V Plate.  100 Screen. Bump in plate current back to zero. 

Turned up variacs to 1500 plate and 250 screen.  No problems. Unkeyed it.

no smells no smoke..  Looking good!

Keyed it again and watched the volt meters move up to 1500 and 250 again.  Perfect.

Keyed it up again and plate Current meter PEGGED OUT MAX.... Unkeyed..

Slight smell of burning. Hmmm.. powered down and inspected. Found nothing. 

Turned back on and warmed up.. Keyed Relay for half a sec... Saw voltages come up and plate current PEG.. Unkeyed..


Thats where I am at..  So for a moment.. The thing looked normal..

Plate volts: 1500
Screen volts 250
Mod current. 40 to 50 mills.
Mod volts 2600.
Plate current bumped up to 100 then fell to zero. Later Pegged the meter.

Short in RF deck somewhere... Should I pull 4-400s and try again with no tubes in line to see if Short is gone?
Check 4-400s for a short? Which pins?

NO RF DRIVE the entire time. My wiring for the plug on the ranger is not correct.  I hooked up the 600 ohm out of the ranger to the mod deck as the manual states. Now the ranger has no RF out.  I need to look into that acc plug further. Hence.. No drive tonight.  Just checking the voltages and the plate meter pegged.

We ended up stopping for the night... To much stress and not enough know how to go further. I guess good and bad news.. The volts and iron are good. The mod deck fired right up.  Just need to figure out why the plate is shorted.

Clark

[Gito]

Hallo Clarck


Reading your problems ,may be some of my comment/replay can help you,looking at your grid voltage of the 4-400 it's to low the spec of grid 1 of the 4-400  is -220 volt in class C continuous service,but most of transmitter used a compromise voltage,just to cutoff the plate current(zero current) ,so we need not to drive it with high  R.F voltage,that you must exceed the -220 v off the bias to make grid current flow.So Rodger said the final bias is -90 v,usually there's a Resistance that connect between this - bias to the grid 1 of  the 4-400.so when we drive this tube there's a -220v dc on the grid (-90 + (R X grid current))

It is normal when you keyed the transmitter the Plate current bump up to 100 ma,and back to zero,it's because the - bias is not settled  ,so when the - bias gets to the operating conditions , The plate current is back to zero."with no RF drive".


After sometimes when you keyed the transmitter ,the plate current pegged out,Yes it can be that the 4-400 is shorted ,just try to put in one tube at a time,if the problem is still there .
I suspect the there is something wrong with the - bias circuit,maybe the resistor that is connected to the Grids of the 4-400 is burnout, so there's no bias in the grid.Or the - bias supply is  defect .(seeing that the - bias is so low/-60 volt?)

When I tuned a transmitter  ,I put the screen at low  voltage , as the screen voltage can control the plate current of the tube,than I drive the final gradually, first with small drive ,looking at the Grid Current,tuning the grid input to resonance,peaking the grid current, and looking of the plate current ,loading and tuning the output of the tube ,get a dip of the plate current/ so it's resonance.
The plate current will be low because there "no screen voltage", then slowly  we raise the screen voltage,the plate current will get higher ,when the screen voltage get higher.
when you get the right screen voltage voltage ,and the plate current is lower than we expect ,increase the drive,by doing this we can get the right operating condition. 
used low drive as possible ,so the screen current is low,the data sheet tells that the screen voltage is 500 v maximal,so you can adjust  the screen voltage to make the plate current lower or higher


Regards


Gito

[ke7trp]

Thanks for taking the time to help me Gito!

I think that one of the 4-400 is shorted.  I am going to pull one out at a time and try again.  It worked perfectly until something caused the short. If its still shorted, I will remove RF deck and check the Grid like you suggest.

I wonder if its possible for the 4-400 to Oscilate out of control with no RF drive?  The plate cape and load cap are probably not set anywhere near where they should be to start. Is this possible?

The good news is that the plate, filiment, screen and modulator HV are all there.  This means the iron and related components are good. 

I hope its just a tube! I have spares!  I have searched the RF deck over and over with my meter tonight and as you can see I am up at 1am.   I have not yanked the tubes out because I am alone and do not want to work on this without someone with me in case of accident or fire.  I will wait until a friend is here to assist me so I can be safe and power it up with one tube at a time.

P.S  I asked about the ranger hook up again.. I downloaded the Desk KW manual and realized my problem. I did not put a jumper on the plug for the Final HV on the ranger. I will fix that tomorrow and will have RF drive to the unit for testing. If its a tube or a simple resistor/cap failure, The unit will be up and running on the dummy load for testing.

Going to bed. Thanks alot for all the help! I feel I am close on this project!

Clark

[N3DRB The Derb]

youre a hero to me Clark, you rescued old homebrew and that means to me you love radio more than money. there's some groovy AM gangstas out that way..... you should get in touch and see if any of em can lend a hand in person. real radio people will jump at the chance to help. 

[Gito]

Clark


of course it's possible to oscilate,but if it's a factory build Tx ,I believed there are neutralizing circuit  in the transmitter that prevents it from oscilating,or the neutralizing cap is un tuned,

what did not make sense to me is the bias voltage,usually the -bias voltage is close to the tube spec,the - bias decide the operating class of the tube,since the - bias data s spec is -220vdc ,and your transmitter has -60 vdc,it again did not make sense to me .
Clarck i'm building small MW transmitter ,600 to 1500 watt transmitter used as gospel radio,so to my experiences ,I always used the - bias close to the data spec of the transmitting tubes,so when there's no drive the plate still get no current(zero current)

Are you not mistaken  the bias voltage of the 810 as the bias of the 4-400?

So I think there is something wrong with the bias supply ,maybe shorted turns in the - bias transformer.

Don't use the data sheet as a must, the goal is to tuned the transmitter for the power output that You want,but of course don't exceed the max rating of the tube.

So to begin just put enough drive,so there's a small grid current showing in the Grid meter,tuned the grid so there's a peak grid current showing,tuned the output of the transmitter for max power,with a low screen voltage,slowly raising the screen voltage,repeat the tuning procedure so there's more power output,get a more /higher drive ,repeat the tuning procedure,and if all is going well,the power output will be the power that you want.
You don't have to drive the grid with 10 watt driver (for example),you don't have to get a 12 ma grid current as the data sheet says,if you can drive it with a 6 watt driver and get only 8 ma grid current but can still load the amplifier to the power output that you want it's okay.
lower drive means lower grid current and lower screen current

Regards

Gito.

ps.Sorry for my English

[K4PDM]

I've seen many a tube-type AM broadcast transmitter oscillate with no drive.

And plate current always goes up as drive drops, in my experience.

I would NEVER intentionally turn on an AM transmitter with no drive.


I'm new here so don't flame me too bad, but I've worked a lot in broadcasting.

And a question--I thought the legal limit for AM on the ham bands was 375W carrier?

[KD6VXI]

I've seen many a tube-type AM broadcast transmitter oscillate with no drive.

And plate current always goes up as drive drops, in my experience.

I would NEVER intentionally turn on an AM transmitter with no drive.


I'm new here so don't flame me too bad, but I've worked a lot in broadcasting.

And a question--I thought the legal limit for AM on the ham bands was 375W carrier?

The legal limit for amateur operation is 1500 watts.  PEP or continuous (as in, RTTY, etc).

If you modulate your carrier 100 percent, 375 watts would be the theoretical maximum carrier.  HOWEVER, lots of people here are running broadcast equipment, and run in excess of 100 percent modulation.

--Shane

[K4PDM]

Thanks for the non-flame response...

I have given away two broadcast transmitters here at work and I hope they are enjoying a good life on 160 meters.

We have a couple more old RCA 1 kW transmitters not currently being used. I have considered taking one home, but if I do, I intend to run it legally.

[ke7trp]

Gito.. Bias is fixed. I studied the schematic.  Its just like the Johnson Desk KW.

I pulled the tubes one at a time. The short is still there.  If I key for half a sec, I see the plate meter peg.. I let off it.  Its not oscilating.

Now, With no 4-400s, The short is still there.

Screen: 200 (adjustable variac)
Plate: 1500 (adjustable Variac)

I pulled the HV off the RF deck.  NO short from connector to Ground on RF deck. Even Manualy closed relay and checked.  No short from Plate CAPS to Ground..  Screen and filiment bypass caps are perfect.

Mod deck seems fine.. Mod volts and current are at normal.

HV power supply seems fine as does LV power supply.

The only part that looks like its ever been hot is the Small Choke from Output to Ground at the RF output conector.  It is a bit brown and has some coloration on the wall where its mounted. I cant seem to tell if its damaged or not. I need to pull it out of line and test it. Its a 2.5 choke as per the schematic. It looks like its been this way for years.  I can smell a Rotten and sweet smell of what ever is shorted.. Just cant locate it.

I tried 10,15,20,40 and 80 meters on the BW 805 Tank.  All short when keyed. I dont want to keep keying it like this so I stopped. I have gone through everything I can think of with my meter trying to find this dang short!

I am also studiing the INTERLOCK system.  I do NOT see the interlock actuator anywhere. I think its bypassed. I can see that it is showing the interlock from Screen to plate voltage. If you dont get plate volts, the screen drops to protect the tubes. This seems to be working as I have both plate and screen volts.

Thanks for all the help!  This is like a soap opera!

I am on a committee against automotive companys so it takes alot of my time. We are trying to get the max speed of any car sold in the US to be no higher then the speed limit.  THis way, NOBODY can have the power to break the law.

Clark

[WQ9E]

Clark,

Given your description it sounds like the coupling/DC blocking cap is suspect.

Rodger WQ9E

[ke7trp]

Thanks for the tip.. I was going to pull the RF deck and check the 4 door knobs.  I have two at the top of the choke, One at the bottom and one at the HV input.

Clark

[w1vtp]

Well, I've read through this thread and wonder if a schematic is available.  As far as RF drive is concerned: if the control grid has cutoff bias applied there should be no problem with no RF drive.  In fact, with my 2 X 813 rig. it was SOP to start with no RF drive and advance it slowly while dipping the plate current.  This would be done until the proper grid drive (ma) was achieved.  At this point, the plate current might be a little low so the coupling would now be advanced until proper plate current, control grid and screen current is achieved.

Do you know if there is a combination of fixed bias and grid leak bias in in place here?  That would be important in my mind.  So, after you check the 4 x 4's for shorts and get a tube in the final that you have some sort of confidence it's back to square 1. 

I really think you need to know what you have by way of design.  Again, a schematic would REALLY be nice.  Fixed bias should be such that the 4 x 4's are just in cutoff with grid leak providing the rest of the bias for class C operation.

After reading WQ9E's post.  I am more convinced that a tracing of the RF deck and control circuitry is in order so you know where things are design-wise.  If this a home brew rig, it's a mistake to assume it was designed after the Johnson desk KW or for that matter, the Collins KW1.

[w1vtp]

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