The AM Forum

It was a busy week. Lots of small issues getting this rig up and running. Tonight, we finally installed it and made a 75 meter am contact with the rig. 

The specs:

Class C AM transmitter.  Two 4-400a's with its own HV supply. Three voltage levels.  2300, 3000, 3500.  Big tank with 3 bands, 40, 80 and 160 with vac cap. Tuned input on all three bands. Full metering.

Modulator has its own HV supply running 3000 volts to two brand new 304 TL Triodes in AB2. Adjustable bias and full metering. Modulator takes 8 ohms in from a small mono TOA studio amp.

So far so good. Running the low voltage setting tonight and the rig seems stable.  Since I have 800 watts of audio power here, I can run 150% if I choose.  Tonight, I ran about 80 to 90.  The 304Tls sure look incredible with current on them. I got good audio reports. Running an MXL large diaphram condenser mic with NO EQ or processing right now. Here are a few shots of the Rig!

C

another pic showing the Transmitter stacked on top of the modulator.

I forgot to mention, the rig is excited by a Yaesu FT450 in CW mode.  There is a seperate Sequencer box that is keyed by a switch.  Then, the exciter, the Transitter and antenna is all keyed up at the right times.

C

Hej !

WoW ! That rig sure looks nice! Impressive!

Clark,
Congratulations on getting your big rig running.  It sounds like everything fired up real well.  By your photo's I think your Power Supply is external but I am not sure about that.  The photo's were a bit blurred. 
I am sure you will enjoy its operation!
Regards,
Joe, W3GMS

Beautiful job!

Great Job! I especially like the chicken wire viewing windows ;D ;D ;D

Nice looking rig, Clark.  That pair of 4-400A's with 3500V, modulated by 304TL's will certainly put you into the "ChannelMaster" class in your area .... ;D

Expect a lot more work to come, cuz it can take as long for shake down, adding improvements, as it took to build.  Have the local guys with Flex radios email you some hi-fi recordings as an initial baseline to work from.


Have you added negative feedback around the modulator/ driver yet?

T

Robert built the modulator.  The power supplys are seperate. The Mod plate Xformer is behind that Huge Reactor so you cant see it.  This rig is not power supply limited :) 

I cant turn the power up over the air since I am using a KW matchbox. I need to build a big tuner.  IN the dummy load it will carrier at full legal limit power.

 The heat from the modulators is outragous. I am thinking of ways to put glass in the front and duct the air out the back so its not blowing right on me as I sit there and talk.

The RF deck has a BC edge wound coil with Vac relays to switch band taps from the band switch knob. This way, We have no bandswitch to Zorch.  The tuned input has a roller and cap.

I got called into work so I cant fire it up on 40 meters today. Maybe I will try when I get home later.  This thing took 2 years to complete and lots of money. 

Next up is to make a plate current overload trip.  I have not needed it yet but the T368 has a neat system that kicks a breaker out the instant the plate current gets over a set limit. I would like to have that feature incase I have an accident and key up on the wrong antenna or no antenna.

Clark,
Build them big and run them hot.  Nothing like plenty of fire in the wire.
Have fun after your 2 year build cyle!
Regards,
Joe, W3GMS 

What a beauty!

Very pretty indeed!

Love those big bottles!

NICE! We shouldn't have any problem hearing out here on the East Coast this winter.  Enjoy.

Nice Clark. Looks awesome and just the right size too.

There goes the heat load on your air conditioning lol.

Its has not been that bad so far.  Running on the low tap there is no extra heat on the 4-400 deck that I can tell.  The modulators do blow some hot air out if you get long winded.  Lucky for me, The weather has cooled off here. It was 80 most of the day. 

I think I am going to change the cooling around a bit though.  I want to put Glass on the front of each unit and isntall fans at the rear to blow the air out that way.  I might even run some duct work out the wall using a Dryer vent.

On the air now on 3870..

C

Clark,


Dryer duct can be noisy and less efficient for air flow due to the bumpy inside rib's air turbulance. It can also melt if you push it hard for exit air.

I use flexible fiberglass 4.5" ducting that has a smooth inside. It costs more, but works really FB. Find it on the web. It is used for car ducting sometimes.

I have three rigs that suck air from the outside. This decreases the noise maybe 70% over having a blower inside on the chassis. When you have a long run of ducting, it is important to have a smooth inside ducting wall.

Mounted outside, you can use a big mo-fo blower.  I use a Variac to tone it down - and use a digital temp gauge to monitor exit heat. The temp gauge is on the web too.

T

Good idea.   This morning its been kind of nice. I woke up and it was in the 60s here.  Unreal. I cant remember the last time I felt this type of weather.  Going to fire it up on 40 meters a bit and make some calls.

Yes, I use glass for viewing windows. The days of needing extreme shielding because of TVI are over. Harris used plexiglass windows in some of their 1 KW rigs, which didn't last or stay transparent after a few years.

I assume you do have a fan or blower on the 304s. Put as much wind on them as possible, including the base. That's why the Johnson sockets for the 304s elevates the tubes, so air can circulate at the bottom and through the hole in the base. As old as they are, I've had several go gassy on me after running bright orange for a few months. Don, K4KYV has had problems in the past with their base pins getting so hot that they unsolder themselves.

I use a stack of two muffin fans for cooling, their combined height is just about right to cover the base pins to the plate cap. Not the quiet ones, but the industrial fans that really wail.

304s are made from a glass called "Nonex", which is similar to Pyrex but also is minimally expanding with heat, hence the name. When they're new, you can run them until the glass almost gets red hot, but a questionable practice when they're 70 years old. Pyrex won't begin to soften until it reaches nearly 1,500F.

I have plenty of airflow on them and there is no current when its not keyed up.  I am running them down so low that they do not get to hot.

The big issue now is I feel that I am getting to much radiation out of these viewing ports.  I am sitting in front of the rig. I noticed my right arm has some color to it. Its about 1 to 2 feet from the Tls when I sit and talk. Currently, we are using aluminum expanded metal over the ports.  What can I do to ensure I am protected?  Is this a concern or am I over acting?

I thought going down and getting some house screen and cutting some panels out to lay over the windows.

C

Interesting question Clark. I wonder if it's IR?

Some years back a ham in this area was running the more modern 304TL's with the plates that have a carbon coating on the anodes. He said he could get 30 percent more power out of the tube and he attributed it to the coating. Anyone agree with that finding? This is not a high-jack  ;D ;D

It may be just the heat.  My Arm is right there taking the brunt of the heat output.  It just got me worried.

Never seen the carbon coated 304s.  Mine are original NOS eimac tubes that say VT129/304TL.   They are just wonderfull to look at and make audio power I only dreamed about before. This is a huge step up over an 810.

C

Hi Clark, here is the type I was referring to.


http://cgi.ebay.com/NOS-GENUINE-EIMAC-304TL-POWER-AMPLIFIER-TUBE-L-K-/270634001477?pt=Vintage_Electronics_R2&hash=item3f030b2445 (http://cgi.ebay.com/NOS-GENUINE-EIMAC-304TL-POWER-AMPLIFIER-TUBE-L-K-/270634001477?pt=Vintage_Electronics_R2&hash=item3f030b2445)

There are no plate fins on this type. I like the older type as they have a much nicer glow. I don't think these would like a yellow glowing plate color? Perhaps this type would emit less IR?

 

Its the IR that was getting my arm. I moved back from the rig.  I know a guy that has one of those carbon tubes. Maybe I will try to bid on this one and have a pair. I am told they do not Cherry up so they are not as near as cool looking. But can make more power.

I had my first failure today on 40 meters.  I started hearing a snapping sound in the RF deck.  I loaded up the deck higher and the snapping stopped. The it started again. I took a close look and found there was an arc about half way up the plate choke. It looks like the choke was sitting in a box of parts over the years and got some nicks and dents on it.

I plan on ordering the RF parts plate choke. Its 4kv, 1.5 amp and 225UH.  This one is out of a BC rig.  Robert said he would cover the cost of a new plate choke. They are $25 at RF parts. Anyone had any good luck with this unit?

For now, I brushed on Corona dope over the spot.  THen took a sheet of plastic film and wrapped the choke around and pulled the plastic tight. This forced the dope into the windings of the choke. I applied more until it was level in that spot. Also found some other questionable spots so I decided to dope them up as well.  Lucky for me this case has a hinged top so it was a 10 minute job.  I let it Dry for an hour, The turned the filiments and blowers on. I hope it will be ok for tonight. I doubt it would even do this on 80 meters anyways.  So I might have to stay off 40 until the new choke is in. We will see how my Quick fix works!

Roberts going to make me a tuna.  A big one.  He has the parts and a spare HT32 chassis to mount the tuner in.  I cant have an open tuner here with the animals and curious people around.  The idea is to be able to open the hinged top of the HT32 case and gain access to the taps for the coil to change bands. This should cure my issues with teh KW flashing over.  I know why they call em Flashboxes now.  Even on low power, mine flashed over on audio peaks.

C

Welcome to the wonderful, wild world of the living on the edge of wetness...

T

Ok. Corona dope fixed the plate choke for now.  The method I came up with really worked.  I used the plastic to inject the dope into the windings.  Going to replace it anyways.

Two KW matchboxes flashed over now. I used the T3 for a few years at full tilt and never had an issue. At the same power level this rig takes em right out. Must be huge audio peaks I cant see on the scope or meters going on. Really strange. If you get anywhere near 100% audio, BANG they flash over. 

My theory is that with the T3, You have a hard limit of modulator and cant go past that, If you turn up the gain, You just compress the signal so you end up with dense audio but hard limited at 100%.  With this rig and no limit of the modulator, any word spoken into the mic can shoot that peak way up there.  I guess I need to get a peak limiter going and hard set it to 1500 watts.  I bet the KWs dont flash anymore. I might try this today.

My brother gave me a Bird 500 watt Dummy load. Model 802.  I ran the rig into that dummy and finally had some time to listen to the audio and make a few adjustments. This bird load took the transmitter at 500 watts for 10 minutes before showing any sign of warmth.  I guess 500 watts is a rating for 24/7.  The transmitter sounds incredible. A huge jump up over the king and T3 that is for sure. The biggest thing I noticed is that I can now boost the lows with no huge current swings or distortion. Just smooth clear audio and as much as you want.

Initial testing is that at 500 watts carrier it will do 150% pos mod easy with a perfect wave form.  At 750 watts it is real happy to do about the same.. maybe 140%.   At 1000 watts the transmitter will do at least 120 clean.  At 1500 watts carrier you are down to 100% clean modulation.  I just start to see color on the 4-400s at 1200.  Thats about the limit of the 4-400s anyways.  Even at 1200 to 1300 you see forward on RMS meter.

 ITs nice to have a super power modulator :)  These figures work out perfectly as my modulator is currently set in AB1 and on paper it should produce 750 to 800 watts of audio. The only difference is that my wall voltage is high.  So we have an extra 200 to 300 volts on the mods and finals. So there you go...  Better performance then a BC Rig,  400lbs and a small foot print that fits into the avg ham shack with ease.

So the plan for this wonderfull sunday, is to get the rig up and running with a hard peak limiter if 1500 watts. I will be legal and I wont flash my Tunas over.  Thats all I want out of this thing, I cant run it higher on the air anyways, I dont want to tear up the bands or neighbors. Just want to get out there and enjoy myself with supa audio soundz.

C

Happy to report that the DBX 166xl tamed the beast.  I set the 166xl for very little compression. a couple DB.  Then I rotated the "peak stop" limiter control back until the rig would not go over 1500 watts on the bird in PEP mode. I ran the gain up so my avg is high but there is just a slip bit of action from the peak stop limiter.  I can scream or clap into the mic and it will not go past legal limit. The audio seems decent. The compression gave me some audio punch. The KW matchbox no longer arcs.  Now I can enjoy my new rig without worrying about flashing the Flashbox over or bothering the neighbors!

Hope to see some of you guys on 40 meters today. Will be on 7293.

Just goes to show you that you really need a peak limiter with this much audio power. Otherwise, Certain words will really spike the output.

C

The KW matchbox can easily handle 3KW PEP unless you have a weird load.
Old KW rigs ran about 750 watts carrier.

Not these two.  They ran the T3 full tilt but it wont take the heat of this rig. Not even close.

I have run BIG SSB power through them with out ill effects.  It wont take this BIG MAUL.. HAHA
C

Will it do 20 meters & 10? Maybe you already posted what it will do but i'm toooooooooooooooooooooooo lazy to go back & read. And where are you located and what time do you operate?

l

Nope.. Just 40, 80 and 160.   The only bands an ole AMer needs.

C

My monitor shows the 304's as white, that is asking for trouble if its true as thats a lot more than 600W PD out of the pair.

Its just the camera, They run a nice warm cherry color.  Class AB1,  125 mills idle.  About 750 to 800 watts for the pair at this voltage and current. Normaly, one low power, they only pop up 200 mills.  On High power and full modulated I only see 300 to 350 mils.  THe Transformer is 1 amp.  Read the attached 304Tl spec sheet and you can see I am running then way below specs.

 In class AB2, A pair of these will do 1800 watts. The modulator was built for full power and has the iron and PS to back it up. I just cant imagine what type of carrier needs 1800 watts of modulator though!

Neat tubes!  

I kinda wish I had two 450 THs in the final instead of the 4-400s.  I think it would have more "cool" factor.

C

Here are some more shots of the rig.  I figured out how to hook this EVO phone to my computer.

C

Very nice rig Clark!  I have a pair of 6C21s (sort of like pulse-rated 450TLs)  that I would like to run in the RF and modulate them with a pr of 304TLs.  The cool factor should be quite high.  Those big bottles are really neat to see all lit up.  I have been planning that rig in my head for years.  I obtained some broadcast xmtr iron to do the chores.  Maybe I will finally get it built one of these days.

73,  Jack, W9GT

20 can be fun but I think most of the old timers who used either gave up or are SK's.

what do you use as an audio driver? A cathode follower would be great for class AB-1 so your driver could be transformer-less for clean audio. I heard JJ on his driver transformer set up and his new J-Sweat fangeled driver and he sounded great on both set ups. Unless he AB'ed them, I don't think I would have noticed the difference.

I use the king 500 on 20 meters Phone.  WOrks great and gives me a good 250watts.   This rig is just for the low bands.

Robert W0vmc built the modulator for hifi.  It is Driven by an external Toa Amplifier.  There is no real limit to its audio bandwidth. It runs 20 to nearly 20,000 if uncapped.

The 450TH just looks cool!  I think you have a fine idea.

C

Ok.  I got the RF parts plate choke installed today. part number RFC-3.   1.5 amp 225uh- 4kv.  So far so good. I have not run 40 meters yet.  I am scared to do so.  This plate choke looks light duty to me. I did run full smoke into a dummy on 75 meters. No problem.  I also ran full legal limit tonight on the rig on 75 phone.  I had the tuner arc AGAIN but was told I had the audio up to high.  I put the rig back into the dummy load and reset all the audio rack and peak limiter so I was almost closing the carrier but not all the way.  Now the tuner does not arc. 

I just had a pleasant QSO with W1IA in NH from Arizona. He said I was booming in and sounding great on 3870.  I guess the rig is working now :) 

Right now I am trying to decide if I should put the rig on 40 tomorrow or if I should just let it be on 75 for now.  I hate to blow that plate choke out!  It was pointed out to me that in class C plate modulated phone, That choke is probably seeing ALOT more then 4kv voltage on it.  The HV is at 2300.  I need a less on how much that voltage increases when modulated.  I am going to grab a handbook from the 50s and read tonight to try to learn.

C

I believe at 100 percent mod, you will see twice the dc voltage imposed on the PA place so if you can do 125 percent or even higher, the voltage go's up more.

I read the 1964 hand book last night.  In AM, class C, The voltage is double at the carrier.  ITs doubled again at 100% mod.  So the total is 4 times over the HV supply.  On low power, I am running 2300 volts.  Nearly 10,000 volts!!!


C

Are you sure? I think somewhere it reads that for 100 percent mod you need double the voltage.

According to that handbook, AM in class C doubles it. When you add the audio to 100%, its 4 times HV voltage. They show in the book 13,000 volts calulated from 3250 volts HV.

C

Sounds right.

Got the transmitter on 40 meters now. 95 % positive and 80 negative.  No arcs or sparks!!!!    7293

C

Recorded from Global tuners in Grand Junction, CO.

Cool. Sounds decent for a mic right into the rig!  THe audio has alot of his in it. I wonder how hard I was hitting that RXer? That was 95% mod on the gates mod monitor.

Thanks alot!!! 

40 is full of activity on AM right now. I am about 9 contacts deep and more are signing on.

C

You were doing 20 over sometimes. That was recorded in the 15 kc bandwidth.

The other guys were weak. Lots of noise on the receiver. I'm near Albany, NY.

 Your welcome!!

Cool!   There are a bunch of guys. Can you hear kd7HVL??? Frank is on a PRC174 backpack radio on battery power!  He is S9 over here.

C

No, too weak in that receiver.

I plugged my Sony headset right into the computer and gave those two clips a listen. That transmitter sounds damn good ;D ;D

I got a bunch of 304tls around here and a bunch of 450TH's and just about everything to build a HB. I think it will happen this winter. I have three mod transformers that can handle the power and I would be able to see which one sounds the best.

If your going to be on tonight on 75, let me know the time & freq, I'll try to record you on a local receiver.

I just did the same. I plugged my Beats headphones (studio quality) into the laptop and had a listen. WOW.  Full audio.  If anything there is to much bass.  Maybe I will try to use the onboard mixer EQ to cut some of that bass and boost the mid a touch. 

Your transmitter plans sound great.  I wanted 450THs also but was talked out of it.  The main reason is they are Triodes and you are going to have hell to pay to get them neutralized on multi band/freq operation.  If its a single band use them!

The plan right now is to switch to 75 meters in about a half hour.  The BC stations are going to take over 40 here shortly. 

Try Smeter.net for another RXer. I seem to hit that one loud.

C

Your bass is fine. Maybe you have your EQ set up for more bass on your computer? set if flat and then give a listen.

450TH tubes can work on an all band transmitter. The secret is having a very small vacuum variable cap as a neutralizing cap (2-45uuf) and re-neutralize it from the front panel when making band changes. Once you have the settings noted, its just another step. I don't know how it would work for a pi net but if you had a balanced network in the grid circuit it may  work fine. An MBL 150 network in the grid would handle a 450TH. My Westinghouse is shunt fed to the final and has a balanced network in the PA final. Resetting the neutralizing cap becomes automatic. 

Thats what my elmer did with his 450th.  I guess that would work!

I got Stickam up and running if you want to see the transmitter.

Go to www.stickam.com/ke7trp1tube

C