The AM Forum

My grandfather, William Maxwell, W8YNG, (Mt Pleasant, Michigan)  procured a P-P 250TH transmitter modulated by a pair of P-P 810s in 1937.  It had four 866A rectifiers in a bridge for the final, and another four in a bridge for the modulator.  He used this rig until he became an SK in 1958, when I was only eleven.  You can see a picture of this rig as it existed in 1958, with the natural aluminum front panels and windows for the 250THs and two banks of 866As.  It had a 100TH buffer stage, driven by an external Meissner Signal Shifter.  Dad always referred to it as a Signal Drifter, as it was not very stable.   But it sounded good.  I remember listening to it in New Jersey, while dad operated with a PP 304TL rig, modulated by four Taylor TZ-40s.  (He built this rig in 1949, This will be the subject of a near-term future restoration.)

Edit:  Brother Bill, W2WM, still has the SX-28 in the photo.  I still have the RME-69, and the Meissner Signal Shifter.  To the left of the aluminum front 250-TH transmitter are two more racks, one with 450-THs, the other with 813 finals.  I still have the 450-TH RF deck and 3 good tubes.  This might someday get restored too.  I do not know what happened to the telephone, or the word processor to the left of the phone.

My dad, Walter Maxwell, W2DU, used this rig as originally designed for AM in the early 60s.  I remember Bill, W3DUQ, suggested that he swap the plate caps on the 810 modulator tubes to attain a higher percentage of positive modulation.  Later on, he disabled the modulator, and reconfigured the final as a linear amplifier for sideband, and drove it with a Heathket HW-100.  He discontinued using it in 1970 when I gave him the dual 4-400A amplifier that I built in my barracks room while I was in the Air Force, stationed at Davis Monthan in Tucson.
When he retired and moved to Florida, most of the rig was saved and made the move.  But other parts apparently were discarded.  The final RF deck, the modulator and driver transformers (Thordarson CHT T11M78) meters and modulator panel, and the front panel for one bank of 866s remains, along with the rack cabinet, remain.  The 100TH deck and power supplies did not make the move.  Other than having two 866A bridges, nothing else is known about the power supplies.  I do remember it had one large pilot light and three "wall switches" for filaments, modulator HV, and final HV. So it will be impossible to accurately restore to its original condition.   

I spent some time hunting for 250TH tubes, and bought three to get two good ones.  The third was gassy and would function well as a VR tube at about 150 volts, with no filament voltage applied.  866As and 810s were easier to obtain.  While trying to maintain much of the vintage look, I felt it needed to be modernized, not only for safety, but to preserve and obtain maximum life from the unobtanium tubes.  So in this thread I will provide some photos, and descriptions of some of the safety and reliability changes I made.  I am not sure I should call this a restoration, rather, it might better be termed a renovation, or maybe even a reincarnation.
 In 2007 I had time to fully restore the Final RF deck, but it sat idle until I had some help to continue the restoration.  I have worked with another local ham friend, Richard Austin, W4BYT on several other product restorations, and he has offered me almost unlimited time and assistance in the restoration of this rig, and I am extremely thankful for his generosity.

A few views of the front of the transmitter appear here....

We proceeded carefully, and restored the transmitter in sections, testing at the completion of each phase, with lots of clip leads and distance between our bodies and the units under test.  The final was tested first, followed by the modulator.  All testing was done with resistive loads; we did not yet test the final with the modulator.  We also restored a UTC MLF (Multiple Loop Feedback) 25 watt High Fidelity amplifier, that UTC sold as a kit in the early 50s. This will be the speech amplifier for the transmitter.  Today we performed the first integrated test, with no clip leads needed.

Kudos! :D

73DG

The rig originally used a 3 wire power cord, 220 volts with neutral, but no ground, no breakers, no fuses.  It now uses a similar 3 wire cord, feeding a two pole 20 amp breaker, followed by a 5KVA high isolation transformer with dual 120 volt output windings.  

Now the rig is properly grounded, with the neutral derived at the output of the transformer.  It can now be powered by 208, 220, 230, or 240 by choosing the correct taps on the input side of the isolation transformer.  We no longer have 220 volts in our neighborhood, so I have the transformer strapped for 240 Volts AC.

The plate transformer is an old pole transformer, or "pole pig", sans the oil and can.  It takes either 120 or 240 in, and provides 2400 out, with a center tap.  Output winding resistance is less than 10 ohms.  Output voltage sags less than two volts at 2 amps load.

Following the isolation transformer, buss fuses accessible from the front panel are provided for the rectifier tube filaments, modulator filaments,  final filaments, and bias supply.  Separate Variacs are provided to adjust the filament voltages, and metering is provided for the filaments, sourced directly from the bases of the tubes.
Also following the isolation transformer is a pair of 10 amp breakers, feeding the HV power supply.  This consists of a 240 volt, 15 amp Variac, followed by a 10 amp breaker on the load side of the Variac.  The plate voltage will be on only during transmit, and a pair of contactors are employed, a two pole to power the input to the Variac, and a single pole contactor to short a series resistor for soft start.  When the voltage at the output of this resistor is sufficient, the contactor closes and provides full variac voltage to the plate transformer.  This takes about 200 milliseconds if the modulator and final loads are present.

Here is an underside view of the control and bias supply, along with some of the wiring to meter filament supplies, variac output, line voltage, etc.  All the interlock circuits connect through the control section as well.

The rectifiers for the final are solid state, 1KV PIV at 10 amps in a bridge of 20 devices.  The final filter consists of 16 tubular electrolytic capacitors, 700 uf each at 250 volts.  Equalizing resistors also function as a slow bleeder.  The modulator power supply uses the negative half of the previously referenced bridge, but uses a quad of 866A rectifiers, two for each leg of the positive side of the bridge, with equalizing resistors.  This feeds a filter of two 8 henry 500 ma smoothing chokes, followed by a series parallel group of four capacitors, yielding 8 mf at 4000 volts, also coupled to equalizing and bleeder resistors.  Using the 866s is optional, but it retains some of the nostalgic appearance of the original transmitter.

I wish to acknowledge the gracious assistance provided by Stu Personick, AB2EZ, with LTSpice simulations to save much time and effort in the development of the power supply design.

While the variacs provide safety for the tubes, allowing filament voltage to be raised and lowered instead of the surge and impact of switching the filament power on or off directly, several safety features have also been added.  An interlock circuit consists of a back door interlock, an HV enable toggle switch, overcurrent trip relays for both modulator and final, and a PTT relay, with a PTT simulate panel switch to enable testing without enabling the RF exciter.  A separatepanel light indicates whether either the Modulator or the RF Final Amplifier overcurrent relay has tripped, and the relays can be reset from the front panel.

In addition to filament voltage metering, HV metering is provided separately for the modulator and final.  Line voltage in and variac output to the HV transformer are also metered.  The original implementation had the plate curent meters for the modulator and final in the high voltage circuit.  The filament transformer for the modulator 810s has separate center-tapped windings for each tube.  This makes it possible to meter the plate current at the cathode of each tube, and it is wired such that the plate current meters do not also register grid current.  Separate isolated bias supplies are provided for the modulator and final, and each modulator 810 grid bias is adjusted from the front panel, allowing accurate balance of idle current in each modulator tube.

During modulator testing, additional swamping resistors were added across each half of the secondary of the audio driver transformer, thus improving the driver linearity under changing load when the 810s draw grid current.

The push-pull final originally used a manually adjusted link for the output.  I added a swinging link with couplers to enable adjustment of the link from outside the cabinet.  At my age, I feel it is important to have door interlocks and be able to perform all adjustments at a safe distance from the high voltage.  I will use my KW rigs as "single-banders" and avoid changing coils, to reduce risk of shock. 

Notice the final plate tuning control is on the side of the cabinet to maintain electrical symmetry and balance.  The control is adjacent to the swinging link control knob.

A few more views of the back and front of the transmitter appear here.....

This restoration has required several months work.  The rig will be on the air on 40 meters soon, but first the DX-100 driver must be restored and tested, and an ugly antenna tuner must be built to handle the AM mode.  Final efficiency has been tested at about 55 percent, and output approaches 400 watts carrier before the overload trips.

The Bird wattmeter shows final output at around 400 watts input.  Testing and operation will be done in a very conservative manner.  High power is not necessary on 40 meters most of the time, and the 250TH tubes are rather hard to find today.

In addition to sharing the history of a ham transmitter restoration, it is hope that some of the changes and features will be of interest to folks on the forum who are presently building their own HB AM transmitters.

The 250TH tubes are both of Eimac manufacture, but the plate structures differ.  One has a cylindrical plate, the other has fins, supposedly to increase IR radiation and cool the tube more efficiently.  One of these might make a nice avatar.......

Thanks for the detailed posting, nice rig and good job.
73 de k7iou

Beautiful and well engineered work, thanks!

Great history and a great story. Thanks for sharing and congratulations on bringing the transmitter back to life. Not too many 3rd generation transmitters out there!

Superb work Rick.  Thanks so much for making the effort to share with us.  Bill

Beautiful! You are right to resurrect it in a way as to prolong its life. This stuff isn't getting any newer!  I think you are in the running for best radio pron of 2015.

There are what look like three pushbutton circuit breakers in a row. I had a box of those and every one was a radium button one. They were not broken or dangerous but it is worth checking if they are the old type.

I'm looking forward to working you on 40 meters Rick.

Thanks everyone for the kind comments!


Steve, I did not even think about a third generation transmitter.  My daughter and grandson have just recently become interested in ham radio.  Makes me wonder just what is in the future for this rig!

Tom, I hope to work you and many others on 40 soon.  So far i have only been on 75 with the restored Viking II and 75A3 or 51J4. 

I still need to get the DX-100 fixed up to use as a 40 meter exciter.  Will not worry about any of the audio section.  And I need to get some copper tubing for the fugly tuner, it will probably look a bit like a backwoods still sitting atop the transmitter rack.

Thanks for the caution, Patrick.  I do not believe there will be a radium problem with these breakers.  The ones in this rig are rated at 10 amps, 250 volts AC.  Two feed 240 volts to the HV variac, and the third is used on the output side of the variac, feeding the plate transformer. 

The breakers were recovered from a Boeing B-47 in the DMAFB boneyard prior to the aircraft submitted to the aluminum slicer for recycling.  MARS members on the base were offered the opportunity to have a field day stripping some of the electronics from the old bird. In addition to the stash of breakers, I scored a Collins ARC-65 sideband rig and an automatic antenna tuner, both with motorized vacuum caps.  Still using parts from the old relics.

Wonderful job Rick.  Both your Grandfather and your Dad would be proud to see the wonderful job you are doing on this project.  I as well as the others, will enjoy working you on 40 meters after you get that DX-100 and tuner up and running!

73,
Joe - W3GMS   

Joe, thanks for the compliments.  

Rick (W4BYT) was with me again today, and we did some testing of the final with the modulator.  Ran all the tests that the west coast handbook recommends for testing neutralization and stability, it is rock solid.  Note: other than a complete disassembly, cleaning, and reassembly with all new teflon insulated wire, the final RF deck is exactly as it was in 1937.  I did bring out terminals to the back panel to enable metering the filament voltage, but that would not affect the performance of the amplifier.

Then we turned on the modulator and started with conservative tests, using the HP audio generator in sine wave mode.  With the cantenna dummy load, the output waveform looks very clean on the Tektronix scope.  

We ran the HV up to 2000 volts (capable of 3000, components rated for over 4000) and ran an input of 350 watts, and the bird wattmeter indicated 250 out.  Verified the dummy load is 50 ohms, and bird says no reflected power, so the output power reading should be relatively accurate.  

After some sine wave testing, we tried a 1 KHz triangle wave, it was sharp and clean at 350 watts input, 100 percent modulation, no apparent distortion.  We initially set the overload relay threshold at 400 MA for the final and modulator.  

We connected the MXL-990 to the Mackie 1202VLZ then fed the UTC MLF into the 810 modulator.  Audio looked clean, but on voice peaks that were over 100 percent we tripped the modulator overload relay and it shut down the entire power supply, just as designed.  Quite a bit of thought was expended in the interlock and overload circuitry.  It is important to be sure that if the final amplifier overload trips, the modulator is not permitted to run unloaded.  Not so simple as using a clamp tube, they don't work very well with a triode final!

Rick has a Tektronix spectrum analyzer that we will put on the rig next week and see what the bandwidth looks like, and later with the dummy load we will let Eric Clapton test the audio system.  

Although the rig is capable of a full 1KW plate input with 500 watts modulation capability, I see no need to run it at that level.  Less than half that is needed for consistent communications on 40 meters, and I do not think I want to hunt for more 250TH tubes or another Thordarson CHT T11M78 modulation transformer.  If I decide I want to play with a rig at higher power levels, it will use broadcast iron and commonly available tubes.

All testing thus far has been without failure, except in one test we could not get the RF to display a trace on the scope.  Had channel one enabled, and the input was on channel 2.  Good thing I was not using the 4 channel Tek scope!

Excellent and well thought out restore Rick, Thanks for sharing!

It has been a year now since we moved our QTH and moth balled all the Heavy Metal Projects. Renovating always has hidden issues and the old QTH at the River House was no exception. We hope to list the River House this month and when it's finally sold, I can get to work on building the shack at the new QTH. Once this is accomplished, I can then start again on the Heavy Metal Projects.

Rick, your project has inspired me!
I can hardly wait.

Rick,

Well your data shows all is as it should be thus far.  My mentor Harry-W3FDY used a pair of 250TH's in push pull in his KW rig.  Had a pair of 450TL's as modulators.  It also used the B&W HDVL push pull coils and the Faraday shielded link.  His did not have the link control so one had to go behind the transmitter and manual push the link in and out to get the correct loading....yikes!!

Keep up the great work and we need to mark the occasion with a special day on 40M when you put an antenna to it and fire it up! 

73,
Joe - GMS

From the looks of the link coupled output the output connector seems to be a BNC connector.  I suggest that be a UHF SO-239.  The modulation peaks depending on the Z of the load can be as high as 377 volts assuming a Z close to 50 ohms.  The BNC might work but a safer bet for legal limit output would be a SO239 / PL-259 combo.

Nice detail Al

Mike, thanks for the kind words of encouragement.  I know it can take a long time to get around to some of these big projects, this and another one have  been waiting for many years.  I am glad I inspired you to press on.  Brett, N2DTS, has inspired me to get back to work on the 4-1000 screen modulated rig, and to try another transmitter with a quad of 4x150s, also screen modulated.  Several other heavy metal projects in the wings, but I must get out and get some antenna work done now that it is warm, before the leaves are on the trees.

Joe, dad used the same technique for adjusting the loading with a wood stick on his 304TL rig.  It did not have the HDVL coils, just homemade coils with banana plugs and jacks on polystyrene strips, and a loop of hookup wire for the link.  Later on he used the faraday shielded links. 

I have been more forgetful as I age, and that is why I put the HV interlock switch on the door, so an external control for the swinging link seemed to be a necessity.  Boy, 450TLs would certainly make fine modulators!  Dad's P-P 304TL rig was modulated by a quad of Taylor TZ-40s in push pull parallel.  I modified it some time ago to use another pair of 304TLs in the modulator.  It needs the power supply reworked with the soft start and overload circuits to protect the vintage components.  Not sure which project will be next, but I will have the 40M 250TH rig on the air soon.

You are correct, Al, that is a GR adaptor from 3/4" spaced banana jacks to a BNC.  The BNC connects to an "N" connector feed-through in the top of the cabinet.  I plan to use all "N" connectors instead of SO-239 and PL-259 connectors.  I will mount an "N" coax relay inside the top of the cabinet with an "el" type "n" connector.  The open terminals at the right of the control deck are reserved for the antenna relay, and the sequencer logic is in the control deck.  The challenge will be mating the leads from the swinging link to the male "n" connector.

I used BNC connectors almost exclusively with my dual 4-400A linear at legal limit, and never had a problem.  But I will go with "N" for the output circuits on the AM rigs.

Great looking project. Maybe it will make it through a few more generations!

It's a family heirloom now!   :D

What I like about the project are the pictures.  Seeing the rig in the 1937 photo with Rick's grandfather, then with Walter, Rick's father.  The xmtr has a great history and it's nice to know, with all of Rick's efforts, it will live on for more generations.

Fred
KA2DZT