I'm Finally Building It, Imagine That! WHOO HOO!

[W9ZSL]

   I'VE BEEN WORKING ON GETTING THE PARTS FOR A TRUE LOUDENBOOMER for about 12 years.  I didn't even have a license untill 2 years ago about this time, though I was known as K9ZSL around 1962.  I'm ready to start the build.

I have the bottom half of an Atlas rack.  It's about 4 feet high, black wrinkle and cherry.  Casters came today... the cheapos on EBay.  They'll work.  I'm not going to make a skate board out of this thing hihi.  Sheesh!

I have some tempered .1" aluminum custom cut sheet metal coming in and am going to build a power supply chassis 20" wide by 16" deep and 2" high.  The sides will be made from the same stock and will form the base of the power supply.  I already breadboarded it.  Some of you may have seen those old pix.  The plate iron came from a Gates 250 watt AM rig using a pair of 833s.  It was later upgraded to a KW.  That xfmr has about a year on it at most before it was replaced with the KW iron.  This was from a Gates BC-1T-250.  The transformer is shown operating with 120 volts on the primary.  At 120 VAC primary, 900 VDC with a 100K bleeder.  I'm going to do 240 volts on an internal split circuit so the 120 volt supplies can be separate.  I WILL need help.  

So, maybe I can ask for an Elmer or two? This is going to be my masterpiece.  1500 volts at an amp is no sweat from the power supply.  I decided to use one supply for RF and AF.  I have all the power supply parts right down to the pilot lights...2 chokes including a swinging, 12uf @4KV, an 8 at 3KV and a 4uf at 3KV oil caps.  

This will resemble vintage Collins, Simpson model 27 meters and all.  I'll start with a 4-125A final and allow for a 4-250.  811A modulators into a Hammond 175 watt NOS modulation iron will do the job.  If it's OK, I'll start a thread and maybe we can document something.  Remember the pix I've included is the power supply with only 120 volts.  When I build the chassis, all this will be at the bottom of the Atlas rack.  The excitation is up in the air because this is basically going to be a cheap versioin of the 4-250A amp shown in the '62 West Coast handbook modulated with a 1949 vintage 811A push-pull class B.  I can dial back my Kenwood to a few watts in the CW mode.  Since there will be only 1 power supply there will be just under 1500 volts on the plates of the 811As.  There will be adjustable bias so even 1500VDC won't hurt and there will be a lot of overbuild on the audio...the F factor... some leway on the audio end.  I will keep a log and take photos.  The aim is an all-purpose amp rated at about 300 watts CW/SSB and 225 AM.  Wish me luck!    

73, Mike - W9ZSL

[N0BST]

Congratulations!  Nothing like getting all the parts scraped together after many years.  Will definitely be following your progress.

Scott Todd

[WBear2GCR]

You do or do not have the Gates mod iron??

Suggest you leave physical space for larger mod iron if what you have presently is only the Hammond, or you are planning on the Hammond.

Some nicer bigger iron may show up in the future.

I'd leave space for 4-400 tubes since they are more or less plug-n-play for that series of tubes.

Be sure to plan for cooling the 4-125/250/400 tubes - blown air.

If you are planning on a 300 watt *carrier* rig, it seems to me that the pair of 811a tubes might come in a bit light. Not sure. But they will not do more than about 200 watts of audio, afaik.

Maybe a pair of 813s would do it better... others may have input here.

Looks like a fun project!

                      _-_- bear

PS. on the exciter, I'd plan on using the Kenwoody at higher power than just a handful of watts. Put a divider in the exciter input so that you can run more like 25 watts, that way when the output power drifts a little you won't overdrive RF into the rig!!

[flintstone mop]

The FUN begins.....I have been through that too. It takes so long to GET STARTED. Once the wheels are spinning it's hard to stop.
Good luck

Fred

[kb3ouk]

811A's in class B modulator service will do 340 watts at 1500 volts ICAS ratings.
shelby

[W9ZSL]

Thanks for all the suggestions.  So far so good.  My componants will work.  I'm shooting for at least 250 watts input.  The 4-125A will loaf.  Understand, the parts I've been collecting go with the tube.  I eyeballed a few 813 designs because they can give great results at the lower voltage, but I already have a 4-125A complete with socket and heat-sink type plate cap. Everything is going to be built "roomy" in fact I have the BC-1T service manual including pix that I'm using as a guide for my layout.  That way I can shift things around.  

I have a 4.5 KV at 300pf (2-section Johnson) for plate tune.  My power is going to be limited to 500 watts because the AirDux coil is a #195-2.  It's not silver plated so it's not rated for a KW continuous.  The loading cap jobber, with geared reducer built in, is a multi-section 2,000pf.  Like our old xmtr at WOBT locally (worked there a total of over 16 years), a Gates BC-1T, I'm mounting the pi-net separately just above the 4-125A.  I have a nifty ceramic feed through, the output hardware from said Gates.  RF choke is an NOS National R-110.  The pi-net is the recommended substitute for the vaccap and B&W switched tuning of the unit shown on Page 646 in the 16th. ed. of the Radio Handbook.

As for the modulator, I think I'm going to keep it super simple and feed the 811A's with high level 500 ohm balanced from one of my home-brew limiters fed through my recording studio system.  Talk power!

[W9ZSL]

  Hey Bear, love your logo!  

Got plenty of room for a big mod iron.  That chassis is a 3" x 17 x 13.  All that will be on it would be the mod iron, 2 x 811A's, the filiment iron and maybe a 500 ohm to universal class "B" rated driver. The Gates iron has been long-gone because the KW plate iron that replaced the one I'm using in this rig was shorted!  That's what killed it and I was on the air at the time.  I had a couple days off.  About 10 years ago I was allowed to salvage it.  

I put 6 volts on the primary and blew a 15 amp breaker!!!  The core got very hot.  No wonder the BC-1T died, hihi!  I may have kept the mod iron but with a useless plate iron to go with it, no good reason.  That was the first commercial rig I yakked on starting on my 16th. B-Day.  Guess you could say I was with her when she was being converted from 250 w to a KW...and I was with her when she died.  Guess I'm building her "Adapted" kid, hihi!  73!

Mike
W9ZSL aka Passage Sound Studios

[W9ZSL]

  Shelby, the cool tool is if I want I can build a separate PS for the modulator 'cause I gots a slick Thordarson plate tranny and parts to git me 1250 DC under load at 250 MA.  Rectifiers will be the solid-state replacements from the Gates complete with mounting hardware.  Too much fun!    Mike aka Da Mix.

[N0WEK]

Looking good!

It'll have a cousin here in Minneapolis, since some of the parts of the parent transmitter ended up here. I just got a great deal on a 5 ft Bud rack cabinet to put it all in.

I'm going to finish the Gates BC-1J first though!

Greg

[W9ZSL]

   Hi Greg!

[W9ZSL]

I just scored a NOS Kenyon 500 ohm to universal class "B" grids iron.  The little sukkah has 12 pins.  It's good for 7 watts.  I built a complimiter from pirate schematics in the mid 60's and added a second in the late 60's.  It was used at Full Compass Studios (the original...I was one of the first founders).  It showed up home a few years ago and I rejoiced.  The thing is unreal.  One of the tubes costs over $250 to replace and I have a pair.  The output consists of 4 x 12AU7 tubes running parallel push-pull parallel.  It's the white thing on the bottom of the rack to the right in that pix.  I can feed it directly into the 811 tubes.  I picked out the metering.  I'm using 5 Simpson model 27s in a 4" rack panel on top. There's a 0-25 ma for grid current, a 750 VDC for screen volts (I used a 300 as a stand-in) along with a 100 ma for screen current.  I have a 2KV for PA with a 300 ma for PAI.  By the way, this thing is called George after a pet name for Bugs Bunny given him by Gossimer the monster that belonged to Martin the Martian.  "I will love him, and pet him and squeeze him and call him George."  Whatever.  I got the output net parts pulled.

I'm lucky because the guy who does sound for my church also has a plasma torch and can build panels.

[kb3ouk]

811A's sound like they will be perfect even at 1250 volts, if you go with a seperate power supply, they will do a little over 200 watts audio with the 1250 v 250 ma transformer. plenty of power for only a 250 watt input RF final. 200 watts audio modulating 250 watts input will give around 125% positive peaks. i want to build a rig that uses a pair of 811As modulating a pair of 813s.
Shelby

[The Slab Bacon]

Try using a pair of 572Bs, they are basically a drop-in for 811As and would be poifikt for modding a PAIR of 813s   

a single 4-400 by 811As is basically a Johnson 500.

[WA1GFZ]

4-125 is a better RF tube than an 813.

[W9GT]

I don't understand what you guys have against 811As!  I have been using a pair to modulate the old Federal 167B that has a pair of 813s in the final.  This arrangement has worked very well for over 20 years and I continue to get great reports.  I run legal limit power and the modulation looks great on the scope.  300 watts plus of audio should be sufficient!!!  Overkill may be a good design philosophy, but can get quite expensive.

73,  Jack, W9GT

[WBear2GCR]

I was suggesting 813s in the modulator, not the final.

I like 811a tubes. If you guys are squeezing over 200 watts out of them, then that's pretty darn good... wonder if running them at 1500 B+ puts the plate impedance high enough to make it hard to find mod iron to match? No idea, having not looked at that question any time in the recent past.

Not sure how a 200 watt modulator can modulate a 250watt final over 100%... but I am not sure of anything.

                  _-_-bear

[kb3ouk]

the rig that I was saying I want to build will use 813s in the final. mike is looking to have 250 watts input to the final, not 250 watts output. for 100% modulation, he only needs 125 watts.

using the formula:

P_audio=(m²P_input)/2

where m is the percentage of modulation, solve for m to find modulation percentage. so with 250 watts input and 200 watts audio, you get:

 200=(m²250)/2.

now multiply boths sides of the equation by two:

400=m²250

divide by 250:

1.6=m²

then take the square root of 1.6:

1.264911064067352

Percentage wise, that is roughly 126% modulation. Are there flaws? Certainly. This is giving you the percentage under perfect conditions. Transformer losses are not considered.

Shelby

[kb3ouk]

I was suggesting 813s in the modulator, not the final.

I like 811a tubes. If you guys are squeezing over 200 watts out of them, then that's pretty darn good... wonder if running them at 1500 B+ puts the plate impedance high enough to make it hard to find mod iron to match? No idea, having not looked at that question any time in the recent past.

Not sure how a 200 watt modulator can modulate a 250watt final over 100%... but I am not sure of anything.

                  _-_-bear

Look at this datasheet for the 811A. http://frank.pocnet.net/sheets/049/8/811A.pdf
Running class B modulator service under the 1500v ICAS ratings, they are putting out 340 watts with a small amount of bias. the plate to plate resistance is 12400 ohms. 1250v CCS with no bias gives the same resistance. BUT 1250v ICAS gives 9200 ohms plate to plate, with 310 watts out. for his 250 watts input, he could run them at 1000v ICAS, which gives 248 watts out at 7400 ohms plate to plate. that will give him close to 140% modulation. so going with a lower voltage will lower the impedance, and still have power to modulate a 250 watt input final.

[KE6DF]

Another great option for a modulator is a pair of 805s.

More power out the 811a's, plus prices are getting more reasonable on ebay (although not a low as 813s).

Plus new Chinese tubes are now running under $50 ea.

The biggest advantage of 813s that I see is they are able to run on higher plate voltages therefore working well in TX with a single HV supply.

And don't forget the old 803s. You can run them in triode mode and get as much, or more, out than you can with a 813.

I happen to have a ton of 805s and 803s, so those would be my choices for a QRO modulator.

[K5WLF]

W9ZSL --

Nice looking control room. Is the turntable a QRK or an early Russco?

ldb

[The Slab Bacon]

Proper theory calls for 1/2 the DC inpoot power in audio for 100% mudulation.

Howeva, general "rule of thumb" has always been to have whatever your RF power outpoot is (carrier watts) available also in the mudder. I.E. if you are running 500w of carrier you should have 500w of audio available for a little extra "headroom" to not have to push anything too hard.

or.................

Whatever you are using for finals (yes I said s) use a pair of the same for mudders.  

or....................

What ever the plate dissapation rating of your final iz, you should have the same or nearly the same for the mudders. Examples:

A 4-250 final with a pair of 4-125s for mudderz. or......

A 4-125 final with a pair of 75THs or a pair of 811As mudders. or.....

A 813 final with a pair of 811As for mudders. or.....

4-1000A final and a pair of 4-400s or a pair of 833s for mudders.

These would be somewhat ideal scenarios. The final chouces would be influenced heavily by the iron that you are able to get your hands on.
Keep in mind that the impedances are not as critical as the turns ratios.

Also try to build a transmitter with readily available tubes that wont cost you a kings ransom when the time comes to replace them

[W9ZSL]

Like I said, I have all the parts for a rig with either a 4-125A or 4-250A modulated with the 811As.  I appreciate the suggestions for alternate tubes/designs but then I'd have to start collecting more parts.  I want to build this thing while I'm still alive and relatively healthy, hihi!

According to the info I found with 1500VDC on the plates of the 811A's and -9 volts bias they are capable of 220 watts...more than enough.  The Hammond will handle 175 with 15KCT primary and various secs.  Input to the RF will be about 250 watts...no more than 300.

Yes the turntable is a QRK.  It needs a new idler because there is some rumble.  Those are available on eBay for under $50.  I'll get one eventually.

[The Slab Bacon]

Power ratings on mod transfomas have sometimes left me scratching my head.
It seems that the power rating of the transformer is usually no where near the the power level that it is working at. I would think that it should at least be rated at the modulation audio outpoot, but many are considerably lower and work / last just fine.

I kinda think it is the working voltage more so than the current that becomes the critical factor. At what point does the insulation start to break down. Higher current and lower voltage is probably easier on them than the other way around.

Don (KYV) you might want to chime in on this one.................

[W9ZSL]

  Wellllll nowwwww!  That's known in broadcasting as a 250 watt drawl, hihi.  I'm leaning toward a separate supply for the modulator and may even mount it on the 13 x 17 chassis.  My Thordarson is a classic.  It's a T19P69, 115V @550VA and two secs: 3120 and 2500VCT at 300 madc.  I have a nice Navy choke or I could use a UTC both on hand.  

Because I scored the Kenyon iron, there won't be a speech amp/driver leaving a ton of room.  I will have provision to add them on another chassis and switch between the 500 ohm balanced input from the studio or a driver.  It would be a 6J5, 6SJ7 and 6V6 single-ended into a UTC S-8.  I even found the exact power transformer recommended in the schematic.  I'm basically building a 1949 classic from the West Coast Handbook.  The schematic and componants shown gives the modulator 100 watts into a 15K load  Note in the diagram a single 1250 supply is used for both RF and AF.  If I build around the 3120 windings, BINGO!  I hate to use that Gates 3600VCT for one lonely 4-125A, hihi!!!   Toodles!

[W9ZSL]

Yo Shelby!  UR rite!  I'd be really stupid if I didn't put the modulator and the Thordarson PS on the same chassis.  I'd have 1560VAC into a choke input filter no load.  I have a 300 ma meter and the front panel.  I may modify the meter panel to 4 and use that 2KV Simpson volt meter both on the modulator.  The more meters the better...lots of fun things to watch!

Greg, you out there?  I still have those BC-1T blower motors and some other parts including the door-closer thingie.  If you're ever in town, pick them up.  Free.  Give me a shout at W9ZSL@yahoo.com and meanwhile since we are right next door let's set up a sked.  That goes for the rest of you.  I have a nifty Kenwood TS440AT rebuilt by the infamous K9FuzzyWhiteRabbit...a high school dude friend who got me into hammin'.  Antenna is tuned to 3885 (what else!!!) with a 40 cut for the middle of the CW band hung below it.  My SWR is so close to !:! and it's only 20 feet up.  That WILL change iff I can afford to trim some trees!  73!

Dammix