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Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig




 
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Author Topic: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig  (Read 2941 times)
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w8khk
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« Reply #75 on: September 09, 2020, 10:18:09 PM »


Gonna order some 1/8" aluminum sheet stock off eBay. Good deals there and cheap shipping. I want to make a chassis for this rig so I can hide the fil transformers and any power supply stuff underneath.

Would like to see a link to the stock you are ordering.  I WISH I had a decent sheet metal shear and bending brake.  Then I would make my own chassis too.  What do you use for fabrication?  At home or contract out these steps?  Inquiring minds want to know!

For the plate tune and load, for this size project, I prefer the easy tuning with bread-slicers.  Don't even need a vernier drive.  Just some nice BIG knobs!  I guess I am just old school.

Can't wait to see Frank's circuit, especially the carrier level control he has designed.
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« Reply #76 on: September 09, 2020, 10:53:04 PM »

Hi Rick,

There's lots of 1/8" aluminum sheet on e-Bay:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=aluminum+sheet+.125&_sacat=0

Or other thicknesses. Some large pieces and some decent shipping prices.  There are copper sheets at ridiculously low prices if one were to use copper. (Like a Drake rig)   I just bought a 24" X 48" aluminum piece for $68.  


To cut the aluminum sheet stock (and Plexiglass) I use a 12" round blade table saw with a finer diamond blade.  The coarse blades are too dangerous. It is a scary operation either way. Wish I had a brake and shear.  To make a chassis I use 1"X 1" aluminum angle stock and 1/8" sheet, drill it and use 6-32 hardware to hold it together.  You can build anything like that including a panel / chassis connected by side struts.   I use hole saws on the drill press for old buzzard round 2.5" / 3" meters and various tube socket holes.  

If anyone has a 3000V ~400 pF breadslicer for sale I'd be interested. Hopefully smaller than what I am using now. I could use the vac cap as a backup for the other rigs.

T
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« Reply #77 on: September 10, 2020, 03:25:44 PM »

Looks great so far and I'm just getting my learning.. the trap shut and ears open thing on this. I have only 3 comments.

1.) the 6LF6s, why 4? not for the current..


I'm a 6CD6 man for the high current low voltage sweepers. It has only half the rated dissipation with a 20 Watt anode, but also only 6 bucks. It also handles high currents well.

If the whole series mod+RF circuit is running at 1200V with a 100V minimum drop across the 6LF6s and 550V across the chass C stage at carrier of 70W (all from the tube data), four 40W tubes gives a great margin. 160W dissipation and only 115W dissipated at carrier level. The bare minimum would be three 6LF6 or six 6CD6. I investigated how the plate dissipation changes over the modulation half cycle.

The heater is same voltage as the 6LF6, but uses 2.5A, vs. 2A for the 6LF6. I like to think that it means the cathode is larger, but the 6CD6GA is an older tube from 1954 compared to the 6LF6, which apparently began as the Philips/Amperex tube N30EL in 1966, and was registered as the 6LF6 and revised/improved until 1972. So there's 18 years of evolution between the two.

The datasheets present the class A plate current differently: as average for the 6CD6 and as peak for the 6LF6. The class A data is what seems to be given for sweep tubes to higlight the high current @ low voltage capability during the pulse operation.

design-center values JETEC
tube | Heater | (des ctr) | Cathode | Cathode | class A typical
type | W | Plate W | Avg. mA | Pk. mA | operation
6CD6GA | 15.75 | 20 | 200 | 700 | Eb=60,Eg2=100,Eg1=0,Ib=230
6LF6 | 12.6 | 40 | 176* | 800 | Eb=50,Eg2=175, Eg1=0,Ib=176


*6LF6 Data sheet gives only peak current values. Chart figure is based on datasheet's 22% duty cycle @ lowest class A plate voltage (22% of 800mA). Note that the 6CD6 data specifies the pulse duty as 15% (15% of 700 is 105mA) but does give the 200mA average value. The discrepancy is probably related to the 2x plate dissipation difference between the two tubes. The 6LF6 has a design maximum rating for cathode current of 500mA in an older Amperex data sheet, very impressive.

My inquisitiveness was related to how many 6LF6s are needed and why.

It was stated that four 6LF6 are probably excessive and that number were used for linearity enhancement purposes. I can only presume that linearity will be worked out with negative feedback and a solid state driver for the 6LF6s. A suspicion is that the screen voltage has much to do with sweep tube current, so the supply ought be well regulated for this topic's use as a linear DC amplifier, whereas it is not as important in the pulse operation as used in TV sets.

For series modulators, the required plate dissipation capacity of the modulator tubes can be calculated, assuming the DC load presented by the class C stage is constant (as it's usually assumed).
In the 4D32 datasheet telephony plate and screen modulation section, which should behave the same way whether the modulation is supplied from the positive (plate) side or the negative (cathode) side, the data is given:

Carrier:
Plate voltage 550V
Plate current 175mA
Carrier output 70W

crest of modulation cycle:
Plate voltage 1100V
plate current 350mA
peak output 280W

From this I used a spreadsheet to graph the dissipation of the series modulator. It's not the current rating but the plate dissipation rating where the 6LF6 comes out on top.


2.) Other tubes mentioned:
The 813 is supposedly good for suppressor modulation.
The 6550 appears in an ARRL handbook as making 20W on 20 meters but was said to be poor above that frequency band.

3.) The series modulation via the cathode+grids is "A-OK" by me but I've only ever tried it in series with the plate, that is, as the plate supply.
If it were supplied to the plate as in conventional plate modulation, then an old laboratory type variable regulated power supply could be modded and used to give a plate voltage swing from 0 to 500V or whatever voltage the supply was rated for at full current. Many were something like 0-500V @ 300mA. The advantage is a DC coupled amplifier already with feedback for good regulation. The main mod is to remove various bypass capacitors that would otherwise prevent the supply being modulated by an audio voltage. Removing all those caps sometimes causes instability, so the simpler the power supply the better. The voltage swing of a 500v supply is too small for this project LOL but is enough for a true 25W carrier transmitter. I did this a long time ago using a single 807 as the class C amp making about 5-6W carrier. The PS had two 6L6GCs as series tubes.

Not suggesting any changes, just remembering and thinking about it, figured out some things from the discussion.
I'm watching this as eagerly as everyone else.


* series modulator calculations 04.png (131.97 KB, 1638x879 - viewed 18 times.)
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« Reply #78 on: September 10, 2020, 05:06:13 PM »

Thanks for the analysis effort, Pat!

Let's see if I have this right and it agree roughly with your spreadsheet...   Say we are using 1500VDC as the main HV supply.   And let's say we have about 450 volts on the 4D32 at 120 mA  and 1000V across the 6LF6s in dead carrier state.  So 450V X .120 A = 54 watts input  X .75 efficiency = 40.5 watts RF carrier output.   At dead carrier  (the maximum dissipation point for the class A 6LF6s) there is 1000V X 120 mA = 120 watts dissipation.   Being 40 watt tubes X 4 equals 160 watts rated.  So pulling 120 watts from tubes rated at 160 watts is reasonable.  Even though they may not need it, I definitely plan on some air forced down from a quiet muffin fan.

Notice the rig is only putting out 40.5 watts. This is 34% efficiency for the modulator. (40.5w / 120w)        The 4D32 is running at less than 1/3  power capability in class C.   The bottleneck, if we want to call it that is the 6LF6s.

The current could be increased to 160 mA =  72 watts input to the 4D32 X .75 = 54 watts output. The 6LF6s would be singing for their supper at a full 160 watts of diss, but probably hang in there nicely.

Now as far as modulation %, with the  voltage going from 450V to 900V is 100% modulation.  What would 450V to 1500V produce?  Is this 200%+ ?  

I think four 6LF6s may be a good choice.  They will scratch out a cool 20 to 45 watts of carrier with huge positive peak headroom available for driving a big linear. (or barefoot)  A good working NPL will be needed to keep the bandwidth clean.  There are some rigs on the air running 150-200% audio that are reasonably clean. If the receiving station uses a sync detector we have a lot of possibilities.

I'm thinking that the 6LF6s will show their value when we start creeping up the HV to the 2KV area; with a heavily loaded 4D32 final and 40 watt carrier - seeing how far the peaks can be pushed cleanly.  I ran 2200V on my old Dual Quads 4D32 PDM rig, (series modulated)  so I know the 4D32 can take 2KV peak. Brutal, I know.   Had a lot of fun doing this with my little 5 watt 6AQ5 series modulated rig. Was seeing 200-300% audio when the carrier was backed down to 1 watt.  Again, we may find that in the real world of diode detectors we are limited to 125-150%, but it will be fun experimenting with this series modulated "big rig"...  Grin

T
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« Reply #79 on: September 10, 2020, 06:29:34 PM »

I would suggest planning your output tank values to allow optimum Q for a range of plate voltages on the 4D32.  With the roller and overkill plate tuning and loading caps, you probably already have that covered.  

The reason I suggest this is that you might find a higher voltage and somewhat lower current will play better with the series modulator.  Since it is not switch mode PDM at the modulator, you will not likely run the series tubes up to saturation without a hook in the audio peak waveform.  Instead of 450 volts at 120 mA,  I would suggest designing to allow up to 500 or 550 volts steady state at around 100 mA.  So long as your power supply provides enough voltage, and your sweep tubes can tolerate the highier cathode to plate voltage drop, you will handle the peaks cleaner, with somewhat less dissipation in the modulator.

Since this was intended to be a PW rig as a low end or driver transmitter, it seems appropriate to focus on plate voltages and modulation percentages that provide the cleanest signal before amplification.  I am not sure that positive peaks above 125 percent buy you as much as a good, dense, loud and clean average modulation percentage.  

When you start running audio through the modulator and look at the modulated waveform at on the plate voltage, it will become clear where the sweet spot is, and just how far up you can go on the peaks with cleanliness.  It is not necessary to consume all the excess dissipation available, let it coast and last a long time.  I was thinking of getting some of those 6LF6 bottles until I looked at how the audiophools have escalated the market price!
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« Reply #80 on: September 10, 2020, 08:52:30 PM »

Yep, running  500-550VDC with less current sounds like a good idea. SummerBreeze, a single 4D32 rig, works very FB at 550 volts.  But it can get a mean streak and run 200 watts out fully modulated using 750V.   Of course, if only 20 watts RF is needed to drive a common smaller linear, then the HV can be moved lower.  Lots of flexibility with the roller inductor and lots of C1 and C2 to play with as well as a Variac on the HV.

I looked at the eBay 6LF6 tubes and they are higher than before. Back in 2012 I bought mine new for about $15 each.  And I thought that was high back then.   Notice I have them mounted on a plate that can be easily removed and replaced with something else if I blow them out. But it would be hard to replace such a low voltage, high current tube with one.

While I wait for my aluminum sheet, the choice to make a normal-sized rig like the last three or a smaller compact one with the HV and audio board/PS outboard is still open.  A total effort could be using an internal DDS VFO w/ 5 watt SS driver, HV supply and the GFZ board and PS all in one box.  That would become another 100 pound rig. Then, where do I put it?  A smaller, compact rig could sit on top of Baby Blue and borrow its HV and RF feedthru coax.   If I were really to scrounge, one side of the GFZ 813s  Hollywood MOSFET audio driver could power the series modulator, but that's getting real lazy.

T
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« Reply #81 on: September 10, 2020, 10:00:43 PM »

It's not the audiophools that drove the price up, it's the CBers.  The 6LF6 is the tube of choice for the sweep tube amps.

I remember seeing data sheets that showed 33 watts PDiss for the 6LF6.  That was probably the avg power dissipation.

Like I had said earlier, I've seen them run at 1100 to 1200 volts, 1 x 3 x 6 tubes.  Guys would see 2200 pep or more.  But the tubes sure didn't live long like that!

If you can find them, the 8950 compactron is pretty much the same tube, but 12 volt fil.  There is also the 20LF6 and the 30KD6 that will work, but have different voltage for the filament.  The 6KD6 is also the same tube damn near.  The 6HF5 is a hotter tube than the 6LF6 as well. As I remember, it's interchangeable.

Then their is the holy grail, the M2057.  That is an 8950 on steroids, designed to interchange with it, but made for rf amplification and not sweep tube use.  That tube has 40 some watts PDiss avg.  I once owned an amp with 10 of those as a final.

Hope that helps if you ever need to find replacements!

--Shane
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« Reply #82 on: September 11, 2020, 10:18:40 AM »


The 7984 Compactron tube might be a candidate, and this one is still inexpensive!

A similar Compactron tube with a plate cap is the 8150.

PDF's attached.


Jim
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* 8150-1.pdf (298.51 KB - downloaded 9 times.)
* 7984.pdf (691.36 KB - downloaded 7 times.)
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« Reply #83 on: September 11, 2020, 01:38:46 PM »

Tom.....I am following with great interest..Could you provide details on how you change the power supply leads on the "same power supply, different deck" setups?  I want to use the same supply for multiple decks and I'm wanting a "quick change" pit stop type of connection(s)...Tnx....I like the cast mount on that Vac cap....
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« Reply #84 on: September 11, 2020, 02:44:30 PM »

Tom.....I am following with great interest..Could you provide details on how you change the power supply leads on the "same power supply, different deck" setups?  I want to use the same supply for multiple decks and I'm wanting a "quick change" pit stop type of connection(s)...Tnx....I like the cast mount on that Vac cap....

Hi Steve,

You could simply tie the various rig's HV together and have HV on them all the time.  Or do what I did for my three big rigs... I use a single HV supply that generates 2500, 3500 and 4 KV via primary taps.  I put a big rotary bandswitch in the power supply and simply select the rig I want to get HV power to. Use a bandswitch of about the same size as you would for the tank of the biggest rig. IE, a 4KV supply should use a 4"-5" diameter bandswitch  assuming the contact spacing is adequate.   Also, you can get by with just one step start and one glitch resistor in the power supply as well as just one HV metering and breakers/ switches, etc. Lots of advantages to using just one big HV supply.

Yes, that cast alum vac cap mount is one of a kind. I got it from Dave / K2IJY 30 years ago and it has been in MANY recycled parts rigs since. And it lives again.

Jim, yes, those  7984/8150s looks like they should work. There are lots of choices and price should not stop someone from building this rig. Frank GFZ mentioned that there are similar versions of the 6LF6 tube running higher filaments for cheap prices on e-Bay.  Check out ebay for the 36LW6 ,  26LW6 and 6LW6. Where there's a will there's a way.

Rick, I was thinking about the grid meter for the 4D32.  Should I add a .001 5KV  cap from the bottom of the grid leak to ground?  I could connect the grid leak resistor directly to the tube pin and the other side (.001 to gnd) would be a short path for unwanted RF.   The  500 pF at the cathode to ground would complete the RF path to the cathode.   Watsa?

Shane, yep, those 6LF6s have taken center stage with the CB builders. We could say they are near the peak of tube engineering before money was stopped and then thrown at solid state devices for the next 60 years.  And here we are still appreciating them, just like an old muscle car or Harley.

T
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« Reply #85 on: September 19, 2020, 01:36:53 AM »

You thought I forgot youse guys?  Nope.  The new series modulated 4D32 by four 6LF6s is now at the point of no return. I am definitely going to build it.  

Yesterday I received a 2' X 4' sheet of aluminum for $60 from eBay. Today there's a clean canvas awaiting a new life as a radio transmitter. All made from 1/8" aluminum including the chassis.

This may be the nicest chassis/ panel combination I ever made up. Rick was axing about how I build these, so here is a good example.  No welding, just 1"X 1" angle.  I usually recycle rigs but this is virgin aluminum.  You will notice there is room underneath for subterranean parts.   I decided to make the rig totally self contained with a HV supply, the new GFZ MOSFET audio driver designed for this rig, the +- 300V supply it needs as well as an internal DDS VFO and 5 watt driver for the 4D32.

A little secret... instead of using the table saw and getting wavy-gravy cuts, I traced out the nine cuts with a sharpie pen and brought the 2' X 4' piece down to the local metal shop. For only $20 they sheared the aluminum to perfection. Beautiful cuts! Well worth it. I might even have them cut the panel meter holes, viewing window and turns counter slots with their plasma cutter too. I'm getting too old for this sawing, cutting and filing stuff...  Grin

The four meters will be mounted behind the front panel using Plexiglass so they will be safe for the HV float.   Actually, when the bottom and top are added together, there is enough room for a party inside.

This is a desktop rig and the panel is 22" wide and 13" tall. The bottom chassis is 4" high.  I will build my own custom cabinet. The extra 3" (horizontal panel space) over a standard 19" rack is worth the custom work. There are lots of possibilities.  I have only one big 19" rack here and dislike them cuz they tend to hide the work later. I prefer to look at the insides often for a thrill. Table top is more my style for servicing too. Easy, slide-off cabinets I like.

**  Notice the parts lined up for selection and cleaning. I am running out of work space so will wheel in a table tmw.

** BTW, I decided to name this rig "Yaz" in honor of  Yaz 1, 2 ,3 and 4.  


T


* DSCF0034.JPG (315.96 KB, 1280x960 - viewed 25 times.)

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« Reply #86 on: September 19, 2020, 01:40:20 AM »

BTW, I forgot to say... the chassis / panel combo is very strong!  It is of simple construction but those side struts really give it a boost.  The panel is rigid and there is no movement anywhere.  The alum angle technique works well.


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

We die three times; when our body expires, when we're buried and when our name is uttered for the last time.  So, all my dogs are named Yaz.
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« Reply #87 on: September 19, 2020, 01:45:14 AM »

The second photo, 31.jpg is a rare shot of Fabio,  the 4X1 rig built 10 years ago -  next to Hollywood, the 813 rig, built this year. They are the two big plate modulated AM rigs in the shack.


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« Reply #88 on: September 19, 2020, 01:54:38 AM »

I plan to put big rubber feet on the bottom and leave the bottom open for ventilation.  It will probably be another heavy desktop rig, like 90 pounds or so.

I wondered today where I got all the parts.  For years I would go to flea markets with a camping knapsack on my back and filled it up throughout the day, dumped it off into the car and went back out.  I am getting seriously low on parts these days though.


* DSCF0001.JPG (312.85 KB, 1280x960 - viewed 22 times.)

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« Reply #89 on: September 20, 2020, 12:56:45 PM »

Johnson 154-3   NOS  500pf, 2 kv variable capacitor

https://www.ebay.com/itm/500-pf-2-kv-variable-capacitor-EF-Johnson-154-3/114410331788?hash=item1aa3631e8c:g:qCoAAOSwwc5fXRaH

e-Bay from a ham - Here's the perfect breadslicer for this series modulated project:   a Johnson 500 pF at 2KV.  I plan to use 1200 to 1500V.    500 pF will cover 160M, no problem.  I've been looking for something like this instead of this big vac variable I was gonna use.  $55 and he has seven left in case you need one for your own project.    

This will also save me the cost of a turns counter and I'll be able to build the RF tank with shorter leads. Wat's not to like?

T
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« Reply #90 on: September 21, 2020, 12:30:52 AM »

Yesterday I received a 2' X 4' sheet of aluminum for $60 from eBay. Today there's a clean canvas awaiting a new life as a radio transmitter. All made from 1/8" aluminum including the chassis.

This may be the nicest chassis/ panel combination I ever made up. Rick was axing about how I build these, so here is a good example.  No welding, just 1"X 1" angle.  I usually recycle rigs but this is virgin aluminum.  You will notice there is room underneath for subterranean parts.   I decided to make the rig totally self contained with a HV supply, the new GFZ MOSFET audio driver designed for this rig, the +- 300V supply it needs as well as an internal DDS VFO and 5 watt driver for the 4D32.

A little secret... instead of using the table saw and getting wavy-gravy cuts, I traced out the nine cuts with a sharpie pen and brought the 2' X 4' piece down to the local metal shop. For only $20 they sheared the aluminum to perfection. Beautiful cuts! Well worth it. I might even have them cut the panel meter holes, viewing window and turns counter slots with their plasma cutter too. I'm getting too old for this sawing, cutting and filing stuff...  Grin


I really like the 1/8 inch thick aluminum approach.  The angle aluminum and stout sheet aluminum make for a very sturdy assembly.  I used to use the steel chassis that had "two bottoms".  No top surface, just a lip to be able to attach a top or bottom.  Recycle the chassis just with one flat sheet of aluminum.

I got hooked on making my own chassis when I was in the USAF at Davis Monthan in Tucson.  Befriended a ham that worked in the metal shop, who was responsible for custom-crafting skin and other components for the McDonnell Douglass F4C and D Phantom.  He offered the use of the shears and brakes, all hydraulically operated, as well as the drill presses and lathes.  Lots of scrap aluminum, big sheets, left over from other work, free for the taking.  All aircraft grade aluminum, so home-brewing in my barracks room was easier then than it is now!

I also like the fact that it will be self-contained with power supply and the VFO/driver circuits!

Tom, I like your move to a bread-slicer instead of the vac cap for the plate tuning.  Much easier and smaller, and quicker to find the dip with only 180 degrees rotation required.

Keep the updates and photos flowing!
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« Reply #91 on: September 21, 2020, 07:16:10 PM »

Hi Rick -

FB on the access to the aluminum and shop in the military.  Hams usually take good care of each other once they find out.

I may start the drilling and blasting tmw. I'm just laying out the RF deck now.  Got the table in the room and ready to go.  It will be a slower going as I'm working on a few other things at the same time.

Hope you find the time to complete your 304TL X 3CX3000 series modulated rig this winter.

T

**  EDIT:  Schematic correction on page 3:  The screen meter "+" should go to the "+" side of the plate meter, directly to the B+.  Right now the plate meter reads the plate current and screen current combined.
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"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #92 on: September 22, 2020, 09:58:03 PM »

I need a power transformer for this project that will give me 1000-1200 VAC bridge for a 1500 VDC output under load.  It needs to be 10 pounds or less.   I am using a Variac, so anything will do that is 1000 to 1500V or so, about 200 mA, no CT.  

These microwave oven transformers for $25 are plentiful on e-Bay.  Has anyone used these and will they work for my application?  The common 700V TV transformers may arc if I use them as a bridge, but these microwave oven transformers seem like possible candidates.  I don't see any voltage ratings in these offerings.

I'd like to do it without using a voltage doubler or rewinding turns.  

** I see that one of the secondary legs is connected to the core. Can this be floated like a conventional transformer or is the insulation on this leg  poor and will have a breakdown?

Edit:     "Correct grounding is important with MOTs. The inner end of the secondary, which is near to the core, should be connected to the iron core. In many MOTs, this is already the case. The reason is that the insulation between core and winding is usually insufficient to withstand the full output voltage."   So this must be the downside.

T


Sample microwave transformer:

https://www.ebay.com/itm/Microwave-High-Voltage-Transformer-MD-903AMR-1-120V-60Hz/153948548777?_trkparms=aid%3D1110012%26algo%3DSPLICE.SOIPOST%26ao%3D1%26asc%3D20200420083544%26meid%3D8abfc6c972674086a0d859c0a9a68638%26pid%3D100008%26rk%3D3%26rkt%3D12%26sd%3D154032746913%26itm%3D153948548777%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DPromotedSellersOtherItemsV2&_trksid=p2047675.c100008.m2219
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

We die three times; when our body expires, when we're buried and when our name is uttered for the last time.  So, all my dogs are named Yaz.
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« Reply #93 on: September 23, 2020, 12:16:19 PM »

The other downside is the duty cycle. MOT's are rated (and used) a few minutes at a time. Note in pictures (or get one from a dead microwave) that the core is welded -- the laminations are sheets to reduce eddy current losses, the welds defeat the laminations being, well, laminations vs. a solid core.

Get one and put 120vac on the primary with the secondary open, for an hour, and measure the core temperature (I'm guessing you have an ir thermometer you can use, since they're also great for keeping track of tube seal and envelope temperatures).  My bet is the rise will be excessive...

I've scrapped a few microwaves out; unfortunately I've been to the recyclers between the last one and now or I'd just send you one to play with.

Ed



* s-l500.jpg (53.97 KB, 500x454 - viewed 17 times.)
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
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"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #94 on: September 23, 2020, 04:10:21 PM »

OK, Ed, tnx for the info.

From what I read, the older MOTs back 15-20 years ago had a decent insulation and could be run with the two secondary leads above ground. But since then the manufacturers have cheapened out and engineered every possible savings, making them $5 cost with poor insulation to the core.    So maybe an older one might work.

I suppose a pair could be run with the gnd sides connected together.

I also read that the idle eddy currents are big as you said, so forget it.

All in all, I will look for a real transformer. I have one but it weighs 25 pounds - that would put too much weight into the rig making it past my 100 pound limit for moving around.

If anyone has a <10 pound HV transformer for sale, 200 mA,  1200-1500V AC, no CT, please let me know.   120VAC pri preferred.         (Or 1100-0-1100 CT or whatever)

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

We die three times; when our body expires, when we're buried and when our name is uttered for the last time.  So, all my dogs are named Yaz.
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