Print Page - Screen or Plate Modulation? Which way should I go?

Hi All, I am requesting input. I started out to build a screen modulated 4-1000 rig for 75 and 40. I planned to put it in a 4 foot Bud rack, but that was not large enough for the power supply components. So I switched to a larger HP computer rack, which uses 19 inch panels, but it is taller and 33 inches deep. So I got to thinking, maybe I should be building a plate modulated rig instead. Here are the components I could use:

4-1000A, final deck almost completed
Temco plate transformer, 225 pounds, primary is 120/240, secondary is 3650 volts each side of center tap, at about 2 amps.
20 amp GR 240vac variac for plate voltage
10 amp GR 120vac variac for filament voltage control on 4-1000
Oil capacitor, 100uf at 4kv
Stancor A-3899 600 watt multi-match modulation transformer
Modulation reactor, 50hy at 1.4amps, 10kv rating, 20kv hipot
2uf, 6kv oil capacitor for modulation
A NOS pair of 4-400As, sockets, filament transformer, and plate cap radiators
A used pair of 4-250A tubes
Relays, switches, and all necessary meters

All this stuff fits nicely in the bottom of the rack, so I am wondering if I should proceed with plate modulation, or go with screen modulation. At first, I assumed it would be a lot less work to build the screen mod rig, but it seems to be very little more incremental work to go with plate modulation. I would still have to modulate the screens to achieve linearity with the 4-1000 tetrode, but that could be done with a high-quality audio output transformer wired in as a choke.

I already have a decent speech amplifier, and a Behringer VX-2000 speech processor, so I just need to press on and get the transmitter assembled and tested. The final is almost completed, just a bit of wiring, and the tank coil and plate choke to be added. I am attaching some pics... I put a couple 807s in the photos just for a size reference. So I am open for suggestions, opinions, etc. What would you do, and why?

Hello Bruce,

Thanks for the suggestion. The BTA-1MX on the left was purchased from another ham over a year ago. I have been doing safety mods to it, and it is almost ready to go on 160. It had no back door (very bad for the cats) no safety interlocks, and no PTT function. I have fixed these problems. It had vac caps for tuning and loading, with the turns counters mounted on top of the cabinet. These did not clear my low ceiling, so I am reworking this problem with some u-joints and right-angle gears. Hope to have it on the air in the fall. The BTA-1M on the right was rescued last May from WABV in Abbeville, SC. I learned on AMfone that this rig was to be scrapped, and made the phone call. The engineer stripped it of tubes, transformers, etc, and the rest was in very bad shape, lots of JS mods over the years. I am collecting the parts to put this rig on 75, but in the meantime I am enjoying homebrewing, which I have not been able to do prior to retirement.

73,
Rick

Hi Brian,

Thanks for the vote for Plate Modulation. There are still quite a few factors I am struggling with, and probably many that I did not even think about. For example, screen mod can provide extremely high quality audio with excellent linearity, just listen to Slab Bacon if you are not yet a believer. I have looked at the overall power consumption, and adding the filament power of a modulator and speech amp offsets the low efficiency of the screen modulated final; there seems to be no winner here. Either way I could run the legal limit without pushing the ratings. Plate modulation requires transformers in the audio path, whereas screen modulation could be done with cathode followers or mosfets, eliminating the iron. Would screen modulation be cleaner and more linear without the transformers???? Will I need to add spark gaps to the mod tranny to protect it, like they do in most BC modulators? What if the Stancor craps out while testing? If I use screen modulation, will it be tricky as to loading in order to have a clean signal? With screen modulation, will I cool the 4-1000 sufficiently without undue jet whine in the shack? Dissipation would be lower running plate modulated class C. So many issues to sort out, that is why I was looking for input from the experienced folks on this forum.......

Maybe I just need to build it to work either way, switchable, and compare the two. But I would rather get on the air sooner, and take the best route....... so opinions and suggestions are welcome!

Hi Rick ... I know how you feel ... to get 400 Wts carrier with a 4-1000 screem mod requires 1400 Wts input and 1000 Wts plate Dissipation ... it will certainly heat the shack, also note that tube life may be less because emission drop will be noticed quickly ... even so, if the fil temp is held nearly constant between tx/rx cycles then fil life is a constant at a particular fil power ... If you can cleanly modulate the screen as well as find a way to diminish the carrier level say 6 db or so between words, then could drop 750 watts off the static plate dissipation and the screen mod could be a clear winner in the efficiency dept ... this has been done using controlled carrier modulation (which seems to use a roughly 9 db or so reduction - Heath DX 60) but can be annoying with some receiver agc circuits ... maybe a lesser ratio would be better, make it adjustable ...just some blue sky.....73...John

Hi Derb, thanks for your input. Yes, screen mod is just as clean as plate mod if set up right, I have never done it so half the fun is trying it out. From all I have read on this forum, it is better to get it set up linear and avoid the detected carrier feedback. If I use screen mod, then I can save the heavy metal for the 304TL restoration. Can't screen mod that guy!

Brian, I was planning on using a couple parallel 1625s in cathode follower for a combined screen voltage regulator and modulator. Still thinking about how to mod the control grid, probably some HF audio xfmr... I am aware the screen load is non-linear, but it should not take much to wiggle the control grid and screen, certainly not anything as big as a 4-400. I planned to use the reactor to keep the plate current off the mod tranny secondary, improving the low frequency response. This should not endanger the mod tranny.....

John, the dissipation to get the legal limit out of the 4-1000 is one of my biggest concerns. I have only one 4-1000, no spares. Cooling is a concern, I wish I read Slab's discussion on cooling before I bought the chimney, his idea is really cool (squirrel cage under chassis for tube base seals, and muffin fan up top, switched on with the PTT, no chimney). For a summer rig, I am thinking that this much dissipation may not be the greenest way to go. Controlled carrier is a kwel idea, if not done to extreme, as it makes for annoyances on listening. I suppose I could integrate this into the screen regulator/modulator. BTW, this is a summer rig; the RCA BC and 304TL rigs will heat the shack in the winter.

John, funny you should mention Allen Bond, WB4GNT. Had an eyeball with him just last week. He provided the chimney for the 4x1, genuine Eimac, for a very good price. Allen is almost a neighbor, lives only 3 miles from my QTH. He comes to all the local hamfests, IIRC I met him first in Orlando around 2006. He sure has lots of good stuff!

Yes, If I go the screen route, I will definitely come up with something new for the modulation and screen regulator. Main thing is to use parts I have. Only things I bought new are the 4x1 toob, chimney, and socket. Speaking of sockets, I first bought a chinese aftermarket job offa eBay. Bad Idea! Good quality, but the design blocks most of the airflow past the base of the toob. So I found a genuine Eimac socket, NOS on ebay, paid the same price for it, much better. Allen did not have any 4x1 toobs when I was looking, so I took a chance on one on eBay. Got it from a ham club in SC, club was selling it for an SK. They packed it very well and it arrived alive!

Unless you drop it, or it is already spent, that 4-1000 should play for a long time for your TX. Dude, you got the mod transformer already, plus reactor. Put a 1 or 2 uF coupling cap in there, take the DC off the Polypedance, and get that baby on the air via plate modulation. Why were most vintage 1 kW broadcast rigs plate modulated? Cause they were easy to build this way, and they sounded great. Sure, if you run square waves into them, you might get trouble. So don't.

I think more fun would be had playing with 4-400A modulators or going to a pair of 833s in class B, than investing in screen modulator, but I hear you, the call to innovate and play with something different is there too.

Put spark gaps across the Stancor, and watch out during testing with tones not to run into negative cutoff (100%) frequently. You can always find vintage BC iron too, if you look on 'ebay and other sources, like location stations getting rid of tube rigs.

Hi Rick,

As we all know, there's many, many ways to get a good sounding AM rig on the air. If each method is set up properly no one will really know what you are running. Be it, grid, screen, plate modulation; class E, class D, solid state or tube; low level ricebox driving a linear, etc.

It becomes a "mind picture" of what you are running to the world. What you say is what the picture becomes.

It's the same analogy as riding into a Harley Biker club with a scooter. All bikes will get you there, but some bikes have more "image" than others.

I believe half the fun in ham radio is hanging with a bunch of like-minded ham buddies and then "riding" together admiring and encouraging each others creations and projects. If it was a matter of simply putting out a signal and sounding good, a simple ricebox and linear is all you need. The overall efficiency it not too much different than other methods when everything is added up.

However, since we are in this for the fun and hobby, we restore old tube stuff, build big tube rigs and love to talk about what we've done ... just like any other hobby where there are builders.

With this in mind, I say build the biggest and baddest rig you can. It will give you much joy and pleasure - and something to talk about every time you get on the air. Your buddies will be inspired to build something as competitive and your interest in the hobby will grow.

I've always thought the 833A and the 4-1000A were the most revered tubes (available too) out there. A 4-1000A plate modulated by a pair of 833A's is outstanding. It's riding a Harley. Since you already have most of the iron and parts, stretch yourself a little more and put one together. 4-400A's are fine, but they are very very common and do not command the respect of 833A modulators.

It's funny how we are usually remembered from a past one-time QSO not by our personality, call or location, but by the rig we were running. How many times have you heard an OT say something like, "yeah, he's the fella running the pair of 750TL's modulated by 450TL's.. now what's his call???"

Add a unique touch to the physical layout of your rig and post some pictures for us later. There is no better way to express your ham personality than to customize and run your own homebrew rig.

Good luck.

Tom, K1JJ (Bought a kayak and been out paddling with Yaz almost every nice day for the summer)
http://www.pxlpilot.com/don/photos/135_01.jpg

Here's my own 4X1 X 833A's. I have a short memory for past work I've done, so I built it under Plexiglas to look at... ;D It's an early shot before the full case was built and final shack positioning.

http://home.comcast.net/~k1jj/wsb/html/view.cgi-photo.html--SiteID-3019502.html

Jack, thanks for the class E suggestion. Maybe when I get burned out doing it the old fashioned way, i will try my hand at some real efficient solid state stuff.

John, thanks for the suggestions in caring for the mod tranny. I am really leaning toward using it. Yes, I have been thinking the 4x1 will last longer plate modulated.

Brian, that 4-400 modded by 833s sounds really kewl. Plenty of strap!

Tom, thanks for sharing the info about your plate modded 4x1. It answered one question I had. Many of the guys use perforated metal cages to shield their class-C finals. Plexiglas lets the entire spectrum thru - RF through visible and beyond. I can probably get away with building a rig without lots of shielding, although not quite as open as the one in Quito.... I have side panels and a back door for my rack. I was thinking of using 4-400A modders instead of 833As because the common plate supply would be in the correct voltage range for both mod and final. If I used 833As I would likely need a separate plate supply. Also I was thinking of running 4-400s AB1, with a very simple driver stage following my Behringer VX-2000.

The kayak looks like lots of fun!

One question you folks might answer.... What method do you use to protect the screen of the final in the event the plate voltage craps out? I thought I might sense the current on the plate volt meter at the end of the meter multiplier resistor string, thus releasing a relay to cut the screen voltage if I lose plate voltage. Any other ideas?

Hi: This the first time I have seen a need to reply to a question, I have and run a plate modulated tnx and have and run 3 screen modulated radios. The screen modulated radios look perfect on the scope the plate rigs looks almost perfect. If I was going to show someone how a AM signal looks (text book) perfect. I would show the screen grid radios. They require a slight differance in tuning the final but the efficency is very high. The carrier idles at a lower power level but when modulation is applied the power gose up and so dose your ac meter on the back porch. Belive it or not there are commercial short wave and broadcast transmitters out there running every day, the price of commerical electricty has driven them there.

Quote from: Knightt150 on July 19, 2008, 04:25:48 PM

The carrier idles at a lower power level but when modulation is applied the power gose up and so dose your ac meter on the back porch.

John, thank you for your interesting info on screen modulation. I wonder if your efficiency is accomplished by controlled carrier rather than pure screen modulation? In any case, one reason I would favor screen modulation rather than plate modulation is the ability to eliminate the transformer entirely, and its inherent phase shift and bandwidth limitations. Perhaps you could share more techical info with us on your implementation - sounds very interesting!

Brian, your textbook examples do not provide for simultaneous control and screen grid modulation. The literature and practical experience indicate that this is required to achieve the optimal linearity, and truly compete with plate modulation. Perhaps we should look at Bacon's post on "Scream Modulation", as he is using this technique.

In summary, I have still not decided which way to go..... So perhaps I will do both. Either method requires stiff regulation of the control grid bias and screen voltage; adding modulation to either or both should be trivial. I am thinking of building the modulator with 4-400As with a very simple resistance-coupled driver stage to interface the VX-2000. In addition, I could use a professional solid state stereo audio power amplifier to drive two audio output transformers in reverse, one for the screen grid and one for the control grid. Add sufficient swamping resistance on each transformer to provide a constant load impedance to the SS amplifier, level controls on the amp would allow continuously variable adjustment of the modulation level on each electrode. A relay could be used to kill the 4-400 filament supply and short the mod tranny secondary and reactor, providing pure DC to the 4x1 plate when testing the screen mod option.

Via this configuration, I could sweep the transmitter using a dummy load, evaluating each option, and determine how much we need to wiggle the control and screen grids to achieve the same, or better linearity, than plate modulation. I thought of using 833As or 304TLs for modders, but this would require lower plate voltage for the mod, and thus a separate HV supply. The single supply has sufficient scrote for both, and a separate supply just takes more space and adds weight.

I think most of my time will be spent on PS, control circuitry, step start, TR sequencing, etc, so I will probably just build to run this as a SKITZO 4-1000. The information gleaned should be well worth the additional effort. PS- I am still open for ideas, suggestions, and advice from all the experts.

Plate modulation is much easier to adjust for good modulation linearity. With screen modulation or linear amplification, you have to adjust the loading and grid drive just right, and supply just the right level of audio. With plate modulation, as long as you are in the ballpark with grid drive and bias, and have enough audio to fully modulate, it will modulate with good linearity regardless of plate loading, up to a certain maximum, and to change frequency all you need to do is peak the grid current, dip the final and maintain the desired plate loading, and the modulation will be close to perfect.

As far as total efficiency goes, ratio of power drawn from the a.c. mains to rf input to the antenna, there isn't a lot of difference between low-level modulation (linear amplification and grid modulation) and plate modulation.

Plate modulation is a lot less critical to set up, but has the down-side of more components (and $$$), and the extra weight and space requirements.

As for screen grid protection for your 4-1000A, the relay in the plate multiplier is a fine circuit to do this. It is about the most protective method, as just looking at the primary plate power to the transformer, and then assuming that there is plate voltage, is risky. For example, if the plate connection is left unhooked, or the rectifiers crap out, or a fuse blows, you might have screen voltage still going, and toast your screen. The trick to putting a simple relay coil in the multiplier string is that you need a high z relay, also called a 'sensitive' relay. a 5000 - 10000 ohm coil resistance that needs only a mA or less to close the relay is the ticket. That way, when the plate voltage gets up to about 1/2 or 2/3 of it's normal value, the screen voltage then switches on too.

Quote from: w5hro on July 20, 2008, 03:44:56 PM

The 3 circuits are right from the 1959 Bill Orr handbook. They don't show the additional circuitry that would be required. They are just simplified examples showing the 3 different types of screen grid modulation configurations.

Thanks for the Orr reference, Brian. I am familiar with that, as I have several of the Orr handbooks, including 1959 (fifteenth edition). I recall reading about simultaneous control and screen grid modulation, and I was surprised that I could not find reference to this in the Orr handbook.

A bit further research in the ARRL HB, 1965 page 265, with reference to screen modulation, I quote:

"With proper adjustment the linearity is good up to about 90 percent modulation. When the screen is driven negative for 100 percent modulation there is a kink in the modulation characteristic at the zero-voltage point. This introduces a small amount of envelope distortion. The kink can be removed and the over-all linearity improved by applying a small amount of modulating voltage to the control grid simultaneously with screen modulation."

This is what Slab Bacon and others do, and it is impossible, at the receiving end, to tell that they are using screen modulation.

With all the study and discussion, I am leaning toward implementing plate modulation, at least for the first pass. With everything in a rack, it is easy to swap out modules and make changes; nothing is in concrete!

Quote from: k4kyv on July 20, 2008, 05:37:01 PM

Don, thanks for offering your point of view. This is exactly the type of information I was looking for. If I had to select one feature of plate modulation to make my decision, it would be ease of operation in order to produce a quality signal. Operating has to be fun! All else considered, initial expense and operating costs are a wash in my situation. And whether I use the mod tranny and reactor or not, the plate transformer weight has pretty much locked me into the basement shack. I will initially implement the 4-400A modulator, but leave options open for experimenting with screen modulation in the future.

BTW, I have gained a lot of insight and understanding reading your posts that I have not found in many of the handbooks. Thanks!

Quote from: John K5PRO on July 20, 2008, 05:49:32 PM

As for screen grid protection for your 4-1000A, the relay in the plate multiplier is a fine circuit to do this. It is about the most protective method.........

John, thanks for confirming my suspicion. I was going to use either a sensitive relay, or use a transistor or darlington pair biased by the plate multiplier to drive a 24 volt relay. Lots of control logic to work out with step-start, safety interlocks, and TR sequencing. I am planning to leave the HV on during standby periods, and cutoff the modulators and final, leaving only the bleeders loading the PS. 100uf oil caps are a bear to charge initially, and I do not think I want to cycle them with the PTT logic.

BTW, I still have to restore my dad's 304TL rig modded by push-pull parallel TZ-40s, and my grandfather's 450TH rig, modulated by 810s. That would be quick to put back in original condition, but I feel that it requires overload protection, step-start, safety interlocks, etc. before putting all the old components and myself at risk. The old rigs used plug-in coils, but at my age I would rather just get them running as individual single-banders and avoid messing with plate coils on a daily basis.

John, my present goal is to eliminate as many transformers in the audio chain as possible. I was looking at an article in QST for March 1948 page 13. Here they run a 500 watt modulator with a pair of 4-250As driven by RC coupled 6SJ7s. Using tetrodes in class AB1, no driving power is required. The author said he left room in the design for inverse feedback, but it was clean enough that feedback was never needed.

I expect to implement something similar, driven by the 600 ohm balanced output of the VX-2000. We are down to one transformer, the modulation tranny. With screen mod, I would do it with zero transformers, but that is a few months out.

Speaking of doing something with the screens, In the 60s I ran a pair of parallel 4-400s on 75 thru 10 using the G2DAF design. No screen supply. Just take the RF input to the linear and pass it thru a voltage doubler rectifier filter, and slap it on the screens. Used it for years, still have it but I want to get on AM, not slop bucket.

The reason I bring this up has to do with my thoughts on cooling the tube. I used a pair of Johnson ceramic sockets for the 4-400s, and a single four-inch muffin fan to cool both. The only air they got was thru the socket and thru the tube base. I did not have any holes in the chassis around the tubes. Being on a budget while in the Air Force, I used two Coleman kerosene lantern chimneys for the tubes. The original 4-400s are still working like new, even after long key-down tune-ups and testing. That is why I have the spare tubes. But when testing the 4-1000A under just filament power, it seemed very hot, even with a large fan and Eimac air system socket. So I got to thinking, with no spares, running it with variable efficiency modulation at high dissipation, might tax the tube, or require lots of air. One of my ham friends once told me how you can tell if you have TOO MUCH air flow thru the tube. It blows the tube out of the socket! You can never have too much air, but you can have too much blower noise. Dissipation - just another factor pointing to high-level plate modulation.

73,
Rick

John, I have met very few folks who knew what the G2DAF amplifier is, but they did know about the G2DAF receivers!

When I was at Davis Monthan in Tucson, I was running SB-101 barefoot, and of course I was collecting parts for a linear. One of my ham friends gave me two 4-400A tubes to get me started, and he handed me a photocopy of a G2DAF schematic, hand drawn. I still have that original paper. He said: "Before you go building screen supplies, have a look at this." Seemed too good to be true.

No neutralization because the input swamping resistor was mounted right at the tube socket. Negative grid bias cuts off the amplifier, and with no drive, no screen voltage. Very simple PTT switching. I remember modifying an old xmit cap for plate tuning, adding spacers to double the spacing. I used 2 6AU4 tubes for the doubler, and it worked fine the first time, never modified the rig since building it in 1968!

I have been thinking of using 4X250s (four of them) to make a small G2DAF linear in an SB-400 cabinet, complete with power supply. I suppose this amp might not even need a doubler. I have not thought of using a 4x1 to build the linear, I just want something on AM soon. I spent months restoring a 32V2, but it still is not right, but that is another story.

I would be interested in seeing more data on your DAF design. That would make an excellent thread. Wonder how well the DAF would do on AM???

I have wanted to do a multiple 7034/4x150/4cx250 anp for years ... I worked for the us navy in the 60's at a site that was starting to rebuild/test TWT's. there was a reg hv supply using a 4cx250 as a series pass reg and to simplify cooling and insulating it it was oil cooled ... it should be possible to do this and might eliminate chief problem using these which is plentiful blower noise ...specific heat MUCH higher for liquids ...... G2DAF should be a natural with this tube as well ... I even made some cooling jackets for a related tube the 8072 ...maybe one of these days... 73 ... John

I have a stash of 7035s, the 26.5v fil version of the 4x150. According to spec sheet, it has a diss rating of 250w and can handle 2kv on the plate. Never thought of oil cooling, that could prove very interesting, and quiet! Would not even need any special sockets. Something to think about.

The temp of my 4x1 with fil only might be subjective, I did not measure it. Also did not measure the delta p, but I will before I put any plate voltage on it. Just had a test jig with socket and fan, all air thru the socket, none around it. I think the filament is ok. I measured 21.5A at 7.5vac. Have a 10v 30a transformer on a variac. I also performed a simple "edison effect" test, just to see if it had emission. Connected a milliameter between electrodes. Fil to plate, 0, fil to control grid, 4ma, fil to control and screen grid, 7 ma, fil to control, screen and plate, 9.5ma. So it is "emitting". I am going to punch some holes in the chassis around the socket amd test delta p before and after, and if it does not meet Eimac minimum spec, a bigger blower goes in. BTW, it is a GE tube. Used, from a SK in SC, sold by ham club via epay. Has very slight discoloration on glass in line with holes at top of plate, so I know it was used, but I doubt it was abused. If it is toast, I will probably go with 4-400s for rf and mod, but I think it will be ok.

Hi Rick ,
Very interesting thread here. Whether to go with plate or screen modulation to me depends of what you have for plate tank components . If you have the right stuff I would plate modulate the 4-1000 . If you have piss weak tank components I would go to screen modulation.It would be a shame to limit the maximum power you could run by the modulation scheme. A 4-1000 will make alot more power being plate modulated than screen modulated. The question is do you want a kilowatt transmitter or a 250 watt transmitter? The so called legal limit can be defined in a number of ways. FACT: 1000 watts am modulated 100% with a sinusoidal tone is 1500 watts raw bulk rf power delivered to the antenna. Some people perceive this to be running over the legal limit. I disagree. The fact is it is FeeCCee law that one must run the minimum power to maintain communication. Having a transmitter that can easily fold back to 250 watts is convinient .The Stancor mod transformer does have rather poor high end audio response . One must make full use of primary and secondary to make sure you are not subjecting the transformer to uduly high audio voltages under operation.Protective Spark gaps on the primary are a MUST. Use of the 50 Hy mod reactor with the Stancor mod transformer may be a waste of time being that the A-3899 was designed to handle unbalanced DC. Being that you have 4-400s or 4-250s I would use them . With 3Kv you should be able to get a comfortable 1Kw out of the equasion. Nothing beats power when the chips are down and the slopbucketeers are quacking up your backside.Screen modulation has it's benefits too. Using cathode follower screen modulation with 3Kv on the plate you should get a clean 250 watts with good modulation headroom. Some years back I designed a cathode follower screen modulator for a 4-400 using a 6DQ5. I almost finished it when the fire of '92 badly damaged the transmitter. I never picked it up after that. A good screen modulated transmitter will have unparalleled fidelity compared to the plate modulated scheme with what you have for components. I would plan to build a plate modulated transmitter with a screen modulator as a back up modulator as well as a modulator to use when you don't need a kilowatt of power. Good luck
Tim WA1HnyLR

Quote from: Tim WA1HnyLR on July 27, 2008, 11:18:21 PM

Use of the 50 Hy mod reactor with the Stancor mod transformer may be a waste of time being that the A-3899 was designed to handle unbalanced DC.

Years ago I ran a UTC VM-5 mod transformer with a UTC LS-103 50 Hy reactor. The final ran 2000 volts @ 500 mills, the maximum rated current for the LS-103. I noticed a slightly better waveform on the scope with the reactor, and reports said the reactor made it sound better, although there was not a tremendous difference. The big advantage of using the reactor was that it practically eliminated the talkback from the VM-5. Running unbalanced DC caused it to sound like a small speaker was inside the transmitter cabinet, with the resultant acoustical feedback squeal. With the reactor, the talkback was almost imperceptible.

Hello Tim,

Thanks for the detailed response and advice. I believe the RF components I have will be sufficient to run a KW plate modulated. Very heavy duty tuning and loading caps, and the coil is 1/4 inch copper tubing, 3-1/2 inches in diameter. The band switch is from a BC-375 tuning unit. B&W 800 plate choke. All similar to my G2DAF linear parts, which never had a problem at full strap.

I am looking at capacitor input, full wave center tapped supply for the final, with 100uf 4kv oil cap. For the modulator, choke input full wave center tapped supply, using the same plate transformer. which is fed via a 240 vac 20 amp GR variac..... I calculate that this will provide 2500 volts or so for the modulator when the final is at 3KV full strap. The modulator plate impedance is around 10,300 ohms, and the final should draw 333 ma at 3KV for 1KW input. This is around 10,000 ohms dc impedance as seen by the modulator, so I should be able to use the 3899 tranny at 1-1 ratio, using entire primary and secondary, with spark gaps.

Since I have the reactor, I will probably leave it in, just to take the DC IR drop off the mod tranny, but I could always short it to see if it makes any difference in the audio quality. I would expect screen mod with no iron in the path would have much better high-end, and I plan to play with that; but with the plate mod I expect to attenuate the high end in the audio chain.

For plate modulation I expect a suitable reactor in series with the screen would be sufficient; I am thinking a high-end audio output transformer here, perhaps a UTC LS-58.

Screen and bias supplies will be electronically regulated and swamped, adjustable so that the biass may be set to work with any reasonable plate voltage, meeting the FeeCee power reduction requirements. The regulators will include audio modulation, so via front panel controls I may adjust any desired level of grid and screen modulation. A relay will short the mod tranny secondary and reactor while removing filament voltage from the 4-400 modulators, when using grid modulation.

This rig will use solid state rectifiers, no xenon or MV tubes. I plan to use a string of 10 amp, 1000 piv diodes. I have been told that it is not necessary to use equalizing resistors and capacitors with the newer quality diodes. Not sure if this is true, I will find out either by experiment or advice... I will of course design conservatively - lots of diodes, MOVs on the input, and step start circuitry.

Sorry to hear the fire killed your screen mod rig, would be interesting if you were able to finish it. One of the things I want to do is determine actual bias levels, and modulation swing needed on control and screen grid to get linear modulation, and to sweep the frequency of the rig in both grid and plate mod modes. I have no idea what swings will produce a linear modulation, but at least I have an idea what biass values to start with, thanks to Slab Bacon's input.

73, Rick

Quote from: k4kyv on July 28, 2008, 12:05:20 AM

The big advantage of using the reactor was that it practically eliminated the talkback from the VM-5.

Hello Don,

I wonder if the Stancor 3899 has a reputation for talk back? If it does, I assume the reactor would help. I am thinking about shock mounting the mod tranny and reactor to minimize this effect.

This brings up one other question I have had for some time. I wonder if I should insulate the mod tranny and reactor from ground, and let them float, to reduce the risk of high-voltage breakdown between windings and core. Of course I will wire the mod tranny secondary to float at PS output DC level, not at ground potential.

There will be no safety issues floating the iron, as the equipment will be fully enclosed and interlocked, and sufficient clearance between iron components. How would you do it?

Thanks for your input....

73,
Rick

Quote from: w8khk on July 28, 2008, 11:11:43 AM

For plate modulation I expect a suitable reactor in series with the screen would be sufficient; I am thinking a high-end audio output transformer here, perhaps a UTC LS-58.

Screen and bias supplies will be electronically regulated and swamped, adjustable so that the biass may be set to work with any reasonable plate voltage, meeting the FeeCee power reduction requirements. 73, Rick

Hi Rick,

Be sure that the LS-58 output transformer (or whatever impedance device you select for a screen audio choke) can handle the unbalanced DC and still maintain the value of inductance you require to provide the desired low-frequency response. The LS-58, like most audio output transformers, is designed for push-pull operation, and cannot handle the 50 to 100 ma. of unbalanced current that the screen of a typical high-power tetrode tube or tubes will pull in class C plate modulated service, without a radical drop in it's inductance.

In my homebrew rig, which consists of a pair of 4-400As plate & screen modulated by a pair of class B 833As, I use a 30 hy choke as my screen reactor. It is mounted above ground on four ceramic stand-offs, to reduce the distributed capacitance losses to ground, thereby improving the audio high-frequency response of the screen circuit. Based upon the screen impedance (ratio of screen voltage to screen current) when the final is loaded to normal plate input, the predicted/calculated -3 dB low-end response with this value of inductance is about 30 hz. This is more than adequate for really good high-fidelity quality.

I see no point in regulating the screen supply when plate modulating tetrodes or pentodes. It is not done in commercial broadcast transmitters, as there is no technical advantage in doing so, and will only serve to drive the cost, complexity, and reduce the reliability of the design. Besides, once you stick the screen reactor at the output of a regulated screen supply, the series impedance and DCR of the reactor will negate any power supply regulation.

Additionally, you do not want to have a stiff or regulated bias supply for the grid bias for the final amplifier, when utilizing plate modulation. A portion of the bias voltage (about one-third of the required operating bias for class-C plate modulated service) must come from the fixed bias supply (for protection of the final amplifier tube or tubes in the absence of grid drive), and the remainder must be derived from a grid-leak resistor that is in series with the bias supply and grid circuit. This arrangement provides for much more linear modulation of the final amplifier, and further details can be found in any the ARRL Handbooks from the golden age of AM.

Good luck with the new rig. There is nothing in the world like running a high power transmitter that you designed and built yourself. Well maybe I'm somewhat overstating this............let's say almost nothing in the world.

73,

Bruce

Quote from: w8khk on July 28, 2008, 11:18:24 AM

Quote from: k4kyv on July 28, 2008, 12:05:20 AM

The big advantage of using the reactor was that it practically eliminated the talkback from the VM-5.

This brings up one other question I have had for some time. I wonder if I should insulate the mod tranny and reactor from ground, and let them float, to reduce the risk of high-voltage breakdown between windings and core. Of course I will wire the mod tranny secondary to float at PS output DC level, not at ground potential.

There will be no safety issues floating the iron, as the equipment will be fully enclosed and interlocked, and sufficient clearance between iron components. How would you do it?

Hi Rick,

I float the modulation transformer and modulation reactor in my rig above ground, primarily to eliminate any potential failure issues of winding-to-core voltage breakdown. Each device is mounted on ceramic stand-offs so they stand way above frame ground. The modulation transformer is also protected on both the primary and secondary sides with spark gaps.

Very cheap and easily implemented insurance, considering the difficulty nowadays of sourcing good quality modulation iron.

73,

Bruce

Bruce, thanks for the suggestions. I will definitely float the iron on porcelain.

I had not thought about the unbalanced DC in the hi-fi audio transformer used as a screen choke, guess I better look further for some appropriate iron here too. I also had not thought about distributed capacitance losses, good point indeed. I planned to provide regulated bias to run as either screen or plate mod, but with all the discussion it seems to be getting a bit complicated, so I should probably stick to either one or the other.

Based upon Tim's point on heavy duty RF components, I went back to the book and looked at tuning capacitor spacing. I have 1/4 inch spacing on the cap, according to Orr this is good for up to 3500 vdc for cw or ssb, with no dc on the cap. 100 percent modulated would double the peaks, and it appears I would need to go with either 1/2 inch spacing, or use a vacuum cap instead. I wonder if this is being too conservative.....

Thanks also for the advice on the combination of fixed and grid leak bias. On the rigs my grandfather ad father had, only grid leak bias was used, but they were built in the 30s and 40s. Certainly safer to have fixed bias to protect the rig in the event of loss of drive.

Still many options to sort out. But all the suggestions here encourage me to do more research and try to get it right the first time. With all the screen issues that have come to light when plate modulating tetrodes, I may still go the route of screen/control grid modulation on this rig, and get on and join the fun. Still have several other boat anchor rigs to restore, and it just may be easier to plate modulate the triodes- either the pair of 450THs or 304TLs. If I use the mod tranny in a rig that could be screen modulated, then I will be short a tranny for the triode rig....

BTW, still the open question on whether I should bypass each diode in the string with a cap and resistor, or whether that is no longer needed with modern diodes. If no advise, I will try it without any equalization components, if I zorch a few, they are cheap.

73, Rick

Hi Rick ... If the diodes are from the same production lot, generally no eq/surge parallel stuff needed ... modern mixed production diodes do well with just eq res ... snubbing caps/mov's should generally go across windings for spike protection, if you can find parts with sufficient ratings ... easiest thing is just to use more diodes in series, if you can tolerate the V drop ...

I'm still considering my refill of a Gates 250gy. I have a Dahl mod xfmr from Gates BC-1 (833's mod by 833's) rated for 750 W audio. With a ratio of about 1.86:1 and a tertiary winding, the plate mod option is very attractive. I am currently considering 2 rf decks (one mf and the other hf) to give 10-160 mtr coverage. Pretty unorthodox, I guess. The strange thing is that the stock panel hole location supports this notion. 73 John

Hello Don,

Seeing your mention of the VM-5, and Rick's mention of TZ-40s, challenged my long term memory back to 1933 when I was first licensed as W8KHK. From then until 1936 I operated 40m cw exclusively, first with a Hartley '10 osc, and later adding a 203-A. In 1936 I began grid modulating the 203-A as my first foray into AM on 160m. But in 1937 I built a new rack-mounted rig with a 59 tri-tet osc, 6L6 amp, followed by a 35-T driving a 250-TH, modulated with four push-pull parallel TZ-40s and a VM-4.

I guess the main reason for writing is that you said your VM-5 gave talk back, while mine gave no talk back at all, even when running full tilt.

I regret it like mad now, that I deep-sixed the entire TZ-40 mod unit with the VM-4 when I retired from RCA in 1980 in NJ and moved to Florida. At that time I didn't think I'd ever use AM again. How wrong I was. However, I saved the four TZ-40s, which Rick now has and plans to use, even though they're more than 70 years old!!!

I'm going to attach a coupla pics of the 250-TH rig I built in 1937.

Don, I want to thank you for the excellent advice you've give Rick. In the earlier days I was Rick's Elmer, as well as his brother Bill's, W2WM, but now it's the other way around, I'm now learning from them. Rick wouldn't tell you, but I will. When he retired from Agilent a coupla years ago he was one on Agilent's top-level computer engineers, coming up with many innovative techniques in LAN and global networking used on their own world-wide network.

Walt, W2DU (Rick and Bill's Dad)

Quote from: wb4bfs on July 30, 2008, 06:57:04 AM

Thanks, John. I have over 100 of the 10 amp 1kv piv diodes, all from the same lot, and plan to use at least ten, maybe twelve, in each leg of the center tapped full wave supply. Voltage drop is insignificant considering the plate voltage, which must be reduced from maximum available via a variac, to stay within ratings of the other components, and the FeeCee limitations.

FB on the Gates BC rig. When I first started working on my BC rigs, I had a notion that I should keep everything as stock as possible, but the more I get into it, the practical modification route wins. Moving to 75 meters requires lots of changes. Interesting idea to have two separate RF sections for different band groups. Lots of room in the old BC cabinets for customization! I had a slightly different approach in mind - since my second RCA is missing most of the iron, I thought I would set the two side-by-side, and use the HV PS and Mod from one, to drive the RF sections in each one, just by removing filament power from the final that is not in use. Switching from one band to another would be very simple.

73,
Rick

Quote from: KX5JT on December 14, 2008, 07:00:01 AM

What oh what has become of this project?

Thanks for the interest, John. The project is not dead, just sidetracked for a while. It will definitely be screen modulated, primarily because the plate tuning cap plate spacing is not sufficient for plate modulation. If it is not considered a sacrilige, I will probably feed unmodulated RF from a Kenwood TS-830.

Several familiy illnesses have kept me out of town for weeks at a time. Another delay is the "ready to operate" Valiant I picked up in Huntsville this summer, turned out it was in need of major repair, so I stripped it and it is almost completely rebuilt with many mods and upgrades. Will finish this project before going back to the 4 by 1 rig. Hope to have it finished soon enough to enjoy the heat output during the winter!

The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: w8khk on July 18, 2008, 04:37:32 PM