The AM Forum

As promised I am posting the beginning of my new 125 watt single 4D32 class C final -  plate modulated by a 150 watt Dayton class A/B solid state amplifier. The cool thing is I will be using a Hammond 1642SE 75 watt single ended audio output transformer. This is a super hi-fi transformer with 53H of inductance, 300 mA unbalanced current,  and weighs 28 pounds, costs $260.

Check it out: (Look at 1642SE specs)

https://www.hawkusa.com/sites/hawk-dev.ent.c-g.io/files/manufacture_group/HAMND/Series%201627-1642/spec/1627-1642.pdf

The Dayton 150 watt amplifier will drive the Hammond to plate modulate the 4D32 at about Valiant power, 125 watts output.

The new rig will be called "Summer Breeze" because it is a cool summertime rig with a small muffin fan. Probably the most efficient rig in it's class, besides a class D or E type rig.

It will serve a triple purpose.  One as a "little rig" to round out the plate modulated harem. Second, as an RF driver to provide a carrier for the bigger plate modulated 813 and 4-1000A rigs. Third, as a hi-fi exciter for my various homebrew linears.  The FT-1000D is the current driver but I am pushing it too hard. A single 4D32 in class C with the Variac moved low will work FB.

I included a temporary handwritten schematic of the whole rig below.  I was asked to suggest one for another ham on this forum building a 4D32 too.


I've been wanting to build a 100 watt class rig for some time now because there are times when I get on 75M in the late afternoon and it's foolish to run any of the big rigs when conditions are so good locally.

I also included some shots of Hollywood, the new 813 rig in it's resting position. I have it under Plexiglas for now until I modify a metal  enclosure.  But I am liking it under Plexi for viewing and really easy servicing in seconds.  I optimized the tank for 160M and 40M today. It worked FB except for the plate choke arcing on 40M. I had used enamel wire and rewound it using thin Teflon. Works like a champ now and Hollywood is band-agile. It is very stable for both RF and audio.

You will see the 17"X 22" X 4"  chassis for Summer Breeze as well as the front panel already mated to the chassis - in the staging area.  I'm just moving parts around for now and deciding just what to do.  I was up in the air about using 6LF6 sweep tubes as modulators, but decided to go the Hammond route.  My friend Jeff/ W2NBC has already built a rig like this using the 1642SE xfmr and says it's the cleanest and best sounding rig he ever built. This oughta be a fun project.  I will be using a 0-800 volt plate supply, separate screen voltage on a Variac and double tank loading caps to give me plenty of C2 load capacitance down on 160M.  One 4D32 is about 2400 ohms plate impedance, so there is a need for a lot of tank C.  I am using a roller inductor for the coil with turns counter. I shud be able to switch bands in seconds with presets.

More to come as I start drillling and blasting.

Tom, K1JJ

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That's it for now...

Why muck around with 4D32s when one can step up to the latest in vacuum tube technology?  As they say at the dogsled races, "Unless you're the lead dog, the view never changes."

http://www.tubecollectors.org/archives/606.pdf

73 de Norm W1ITT

I had a bad case of those.  Cured by sending them back...

73DG

Check your bias rectifiers...small error there...

Curious on PTT.  Looks like you’re switching the screen and HV into a lower voltage using a step start?



Ed

Slimatron?

Hi Ed,

Thanks for the heads up on the bias supply rectifier error. Yes, the diodes should look like the screen supply, except with the diodes backwards. I will replace the schematic later today.

The PTT step start brings (keys)  the HV up slowly at the same time as the screen supply.   The fixed protective bias is always on with the initial switch.  I am using this system with the 813 rig now and works FB.  I also use a "snubber" across the mod sec winding that shorts a 3K power resistor to kill any unkey magnetic collapse. It uses a series resistor with a vac relay that is normally on when in receive and lifts when in transmit. I did not need a sequencer with the 813 rig except with for the HV step start.. No arcing.

I learned that the screen and grid supplies need a good bleeder equal to the operating current to maintain stability. The grid bias can charge up the filter cap and make the rig impossible to drive when using smaller bleeders.   Regulated supplies wud help, but unnecessary with class C.

Another thing I found yesterday with the 813 rig fine tuning is that the loading and tank tuning is important to get the very best audio waveform above 5KHz or so.  As the modulator struggles to maintain good form at 7, 8, 9, 10+ KHz, the modulator load (the RF final) has to be near perfect or the fragile modulator causes ripples or slanting at the extreme highs. I see the IMD and harmonic distortion increase too.  No problem below 5KHz and tuning can be more general. The intra-turn and general loss of mod iron at the extreme highs makes the tubes need more current to maintain a good waveform too.  Bottom line is I have precise settings for C2, L1 and C1 on the various bands thats gives me the very best extreme high frequency waveform as possible and at the same time good power output and efficiency.  It takes some juggling to find these sweet spots, but when done, the high end looks really FB when limited to 6.5 KHz and there is plenty of clean bandwidth headroom above.

Norm:  That old buzzard tube looks like the ancestor of a 750TL. I ran a pair back in the 70's.  A dead, dumb triode with an amplification factor of 12, but oh what a look! Like a movie star.  I had six at one time, but one by one the filaments went bad due to the manufacturing shortage of tungsten during WWII.

T

Updated schematic for 4D32 rig:

Looking at the new schematic there’s one other thing that caught my attention— the cathode to filament voltage. Specs say they should be connected, or less than 100v apart. Since it’s only the one filament tying them together is simple enough...

Ed

Yep another good catch, Ed.   I will connect the fil xfmr CT to the cathode. That will keep the voltage differential to minimum.

BTW, I just noticed that the 500 pF plate coupling cap needs to be moved to the right to let the DC pass to the tube. Corrected and I will post a new schematic soon.

I don't mind making mistakes on paper at all... :-)

T

"SummerBreeze" (named by Jeff, W2NBC) has been Brillo-padded down and is ready for painting the front panel. The front panel will get a few coats of paint to fill in any scratches.  Except for terminal strip placements, all of the drilling and blasting is finished.  Time to paint and then start mounting parts.

You will see the transformer iron painted in gloss black, ready to mount on the chassis.  The 4D32 tube sits right in the front next to the big, rectangular viewing window.  The 3" hole on the bottom left is for the turns counter controlling the roller inductor.  There is a second loading cap mounted underneath the 4" chassis giving a total of 4,000 pF, good for 160M and the low 2400 ohm 4D32 plate impedance, with a Q=20.

I think it will weigh close to 80 pounds when complete...  just about like a Valiant.    Very strong custom-welded chassis and side supports. The panel is made from a very hard aluminum. All came from the scrap yard for cheap - bought by the pound.   The extra holes in the chassis will be filled in with screws - and were free... :-)

Same power class at 125 watts carrier out.  All power supplies onboard. The Hammond audio transfomer alone weighs 28 pounds.  The 800 volt HV xfmr is about 20 pounds. Six beer-can-sized filter caps. Three Variacs and two very heavy loading caps all start to add up.  She's gonna be a beautiful lower powered standalone rig /carrier driver/ linear exciter capable of a wide power range using the Variac HV supply. The single 4D32 tube will make a great final. I'm happy with my overall decision.   The 4D32 is my third favorite tube for AM work - with the 4-1000A first and the 813 second.

T

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Yaz had a little mishap running into a rusty hay rake at night.  He is healing from a cut on his leg that he kept licking.  One more day wearing the Elizabethan collar.

I thought he was working moon bounce. The coyotes around here try it without the collar.

klc

Heheheh… good one!   And he's looking up at the moon too.

I got SummerBreeze some paint and he's out in the sun baking it on.


I may fill the chassis and panel with parts later tonight if it dries well. It's very dry today.


Nothing like a rig filled with parts but not wired yet. It will never look better than that. Something like meeting a blind date in a dim bar.

T


BTW, I was going to paint the panel Baby Blue, but may save that one for the next Pissweaker rig.

Always love to follow your projects!
Full metering!

Elizabethan collar LOL

Tom,
Nice looking chassis!  Did you do the bending and welding?  Nice looking welds. TIG?

TNX, Bob.   I picked up the chassis and sheet aluminum at the scrapyard for $1 per pound. The chassis was already built. I'll cover up the extra holes soon.


Just like a Heathkit, the new rig went together quickly. Now it's time for wiring which always takes a lot of time....


I'm missing the Hammond audio transformer. I took some pics with the tube fan mounted and without to clear the view. It will be a fun 125 watt rig to run.
There is another C2 2000pF loading cap under the chassis in parallel with the topside one. I'll take some under-skirt shots once it is wired up.

Notice I mounted most of the heavy iron towards the middle of the chassis to make it easier to carry.  The RF stuff HAD to be in the front for access and tube viewing.

T

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Notice the cooling fan above the tube. It is on a Variac to be slow and quiet. This pushes air DOWN around the tube very well. The socket is sub-mounted for some base seal circulation too.

Tom,

Which direction does the fan move the air over the PA tube?

Tom,

I realize it is against the grain cuz heat rises, but I experimented a lot with muffin fan placement when I had the 4D32 dual quads. I wanted to cool four tubes in a square with two fans. Without a doubt, this works best: The air is focused down from directly above and comes down to the base, the air then bounces off the bottom and spreads out evenly as it moves to the edges of the chassis. It requires a ventilated cabinet.  Air hits the plate cap, all glass and even some bottom seal area if the tube has a mounting gap.  I usually test it by leaving the filaments on for a long period and increasing the airflow with the Variac until I can touch the glass without getting burned. You can feel the tube all around its 360 degree circumference and see if there are any blind spots. But one tube is very easy to service with one fan.   There is excellent air control when it is forced directly down vs: from the side.  

Of course, a pressurized chimney airflow from the bottom is best with a fan that can handle back pressure.  But I wanted this to be very simple since a 4D32 needs no forced air anyway.  

T

Better closeups:

I have 3/16" Plexiglas in there now... I usually use 1/8" glass.   I think the glass is clearer.

The L - C  input network is mounted under the chassis, as well as the HV Variac, the fil xfmr, screen and grid circuitry and a few other things to hide the wires.

T

Well, I finished the wiring and fired her up.  A bunch of bad parts and a few errors later - after a few days or so I got SummerBreeze working on 160-40M.  Very stable with the expected 125 watts of carrier with a single 4D32 at about 650 V.  I have the Hammond 75 watt audio transformer on its way thanks to Pat/ N4LTA who gave a me a great deal as a Hammond dealer.

The front panel is almost all labeled and the wires are getting tie-wrapped. It's really turning into a FB transmitter weighing at least 80 pounds self-contained.  I can change its power out from about 25w to 150 watts carrier  output by changing the HV Variac and screen Variac.  This will make a great "little rig" with the clean SS modulator and Hammond xfmr.  A nice addition to the AM harem.

I should have a new series of completed pictures posted in a day, so keep watching...  Unlike my other rigs with the usual JJ "on top" broadcash style of building, this rig has a chassis and the underneath is filled with circuitry. Made a cleaner top area and better isolation for the input circuitry. Still, I like the BC style for easy servicing of big, heavy stuff. But I can still pick up SB and roll her over on her side without too much whining... ;D


A NEW TROUBLESHOOTING MYSTERY:   I always seem to run into a few tough problems that take me a day or so to figger out. No exception here. I was getting very unstable plate, screen and grid meter readings when I keyed up. Power would start at 125 watts out and quickly drop to 60 watts. There was also tuning instability like a feedback loop. I tried loading the grid with a 1K non-inductive resistor and it did not help. I measured all DC supplies and saw they were reasonably stable.  My voltmeter readings got unstable when I traced out the cathode relay PTT circuit. I found I got full output by DC grounding the PTT circuit.

Once I uncovered the gremlin and made a correction it worked very FB and the power is stable and tuning is great on the three bands. (even with NO swamping resistor - 3 watts drive to full power) The RF layout is pretty tight.  What was the problem?  Second hint: It was an incorrect part value. This is not an easy one unless you consider the clues closely.

The lesson to be learned is to to check your circuits in real operation using this simple but clever test before the stamp of approval...

T

Tom,

   This is a tough one!

As a guess, the Bias transformer CT and the plate meter minus (-) are tied together (good), but that circuit also needs to be grounded, and it wasn't.

Jim
Wd5JKO

Meter shunt wrong/heating up/increasing in resistance, causing drop in current due to both resistance and bias shift...

Ed

It sounds like something was misbehaving RF-wise in the PTT relay for the cathode. Maybe you used a wirewound cutoff resistor or RF was coupling to the relay coil?

Jon

Jon, you were the closest, BUT - no cigars yet...

Hint:  This incorrect part value is depended upon by all other tube elements. Without it, DC will flow fine but RF doesn't know where to go, causing instability that even affects the DC readings too.

The test I should have performed could have  shown this culprit quickly if I had thought of it earlier.  

Who IS this masked man?

T

Then I'm going to guess my second choice --
 The .001 cathode bypass wasn't .001 or didn't maintain .001 as it got hot.  I didn't include it in the first guess sine you said an incorrect value, but .001 doesn't seem too incorrect....

Ed

DING - DING!   That's close enuff to ring the bell, Ed!  

The .001 uF cathode bypass cap was too small in value.        0.01uF fixed the problem.    The RF was floating around the rig in the PTT circuit.

I found when I made DC voltmeter readings on the cathode, the meter went crazy and the power changed, like RF was floating around. The RF appeared all the way to the plate meter. The RF shud have been bypassed at the tube pin with the .001 bypass cap. When I grounded the cathode directly to chassis ground with a clip lead, the power came up to normal.  I measured the .001 and it was OK.  I increased it to .01 and the circuit worked perfectly.  I checked the rest of the cathode circuit from the tube pin, relay, meter, etc., and all was fine.

I have seen .01 bypasses used for directly heated filaments in common use.  But I have also seen .001s used in indirectly heated cathodes like the 4D32.  But now I  figger the 4D32 is a low impedance tube, so the .01 was better.

The test I was talking about is to take a clip lead with a .01 to ground and go around connecting to the cold end of any RF circuit, where the RF choke connects to the DC supply.  With the rig operating, the power out and meter readings shud not change when the bigger cap is added. In my case, by touching a .01 to the cathode lead, it wud have showed up as needing more C bypass.  

So, I went around and tested all my RF circuits this way and all were fine (no change) except for the cathode.

Does this mean there is something else wrong by needing .01 rather than .001? I don't know. But with the .01 (X10) the rig is very stable now.  The scope shows no RF on the cathode PTT lead as it did before.


T

Compliments on the use of metal enclosed plug in relays!

Well, here we go.  Lots of pictures coming.  Both top and bottom shots.  SummerBreeze is almost ready for audio testing and then on the air.

I got the Hammond 75 watt, 28 pound audio transformer today.  What a beast!  It will make the rig close to 100 pounds, about a pound per watt.  I think I may make a separate chassis to mount the 150 watt amplifier and the Hammond. SummerBreeze is just too much to man-handle for service or carry around.  There will be a big space left who who knows what?  Maybe a VFO.

Anyway, I put the labels on and started to mark the tuning presets in pencil on the front panel. I will make band labels so that I can change settings in 30 seconds to go from band to band.

I optimized the fan position from the top and the muffin runs quietly hanging from the plastic straps. It was too loud when I bolted it directly to the front panel.  But now as quiet as a summer breeze. I let it run at 125 watts output for 10 minutes and the tube stays too warm to hold your hand on, but not really hot. Air is good, even for a convection cooled tube.

The RF and power supply sections all run cool and  work flawlessly now and I will clip lead in the Hammond tomorrow to do some tone tests.

This turned into a fun project and will probably get a lot of use when conditions are local and 125 watts is the norm. Very conservatively built rig for sure.  Yes, I DO build chassis rigs once in a while... :-)

T

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Notice there are TWO 2000 pF loading caps in parallel = 4,000 pF.  One on top and one underneath. I need about 3500pF on 160M.   The tuning cap almost maxed out at 500 pF.   The roller is all the way in on 160M.

The L/C input matching circuitry works like a charm to give a nice 1:1 swr input match.  The big relay on the bottom is the step start.  There is also a VU circuit and a snubber to quench the unkey arcs from the iron.

There is a lot of mass in this rig with the Variacs, power transformers and heavy duty RF components.


The schematic has the latest corrections and value changes. I shud do a computer drawing once it is tested and finalized.

T

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Notice I used stand-off insulators for the Hammond transformer connections so that I can easily remove it if I need to move the rig. The jury is still out if I will build a separate chassis for it.

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If you look closely near the tube socket underneath are two 50 watt TO-220 1K resistors in series as a non-inductive grid swamping resistor. Works very FB to keep things stable. (2K total load on the grid)   This gives a nice stable load to the l/C input match.

I tried to lay out the topside tank circuit as tight as I could. I used copper strap and direct connections. So far no problems.  Notice there are two plate RF chokes in series to cover 160M. The National buffers things well on 75M and 40M.  The second choke just adds some 160M needed inductance.

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I used a temporary  Plexiglas window until I can have a real glass one cut. The glass is definitely clearer for viewing.

That's all folks!

Look at that monster Hammond in the last shot. It's like a TV transformer on steroids. The TV HV transformer to the right is as big as they come, but the Hammond dwarfs it.  55 Henries for the deep low end sound.  

The step-start and PTT on the schematic may be a lot to digest.  

A brief explanation:  Basically there is always power to the filaments, fan and fixed grid bias. This is controlled by the main 120VAC switch.   When the PTT is keyed with 24V, the screen and HV supplies get AC power thru a series set of relay contacts - and thru a 10 ohm resistor to limit charging surges. The 10 ohm resistor has a second set of contacts across it with a 300 mS delay that then comes in to bring the HV and screen to full voltage. This step start relay uses a steering diode, a electrolytic cap and resistor to produce the proper delay. Values will depend on the relay coil resistance.

When unkeyed, both the screen and HV slowly drop to zero unless the PTT is keyed again.  Thus, when in standby, there is no HV or screen on the rig, just fil, fan and fixed grid bias.

The HV (usually set between 450- 700V) is monitored with a 500K resistor ladder and a 25 ohm resistor across the meter to ground.

The Variacs are all fed by this same circuitry right before their respective loads.  I use a simple 5A fuse right after the main 120AC switch to protect all circuits as well as a breaker on the AC line strip.

You might axe, "How is the screen protected when the HV is off?"   Well, hopefully the screen and HV are always keyed together, but I found that the always constant fixed grid bias of -35V  is enuff to keep the screen current at a safe level when there is no HV and full drive.   But if you wanted to, a relay coil in series with the screen DC circuit could be used to drop the screen voltage out when it gets too high.  But again, the fixed bias protects the plate, screen and the grid current to reasonable levels based on my testing...

T

Tonight I hooked the Hammond 1642SE transformer up with one 75 watt audio channel. I tried an 80 watt carrier and put some tones thru. It immediately worked and the sine waves were clean from about  15 Hz to over 16KHZ!  I ran some triangles thru and a 50 Hz triangle looked flawless.   At this point I could actually hook up the mic and audio chain if I wanted.

I was able to modulate well up to 150% positive. (using tones)  Next I will hook the audio amplifier up in bridge to give me 8 ohms at about 150 watts. (Dayton APA-150 class A/B amp) It shud modulate a 125 watt carrier with no problem.

I let the tones run for 10 minutes and the Hammond did not get warm, still room temp.

Jeff, W2NBC, turned me onto this audio design and said his version was the cleanest and best sounding rig he ever built. I can see why. It has revived my faith in using audio transformers.

I was especially amazed that it would go past 16 KHz without a hint of distortion. Jeff tells me that there is no need for NFB since the amplifier is already internally compensated.

The neat thing is the transformer needs no mod reactor chokes to help pass the lows. I can run it at up to 300 mA unbalanced with no problems.  I'll see what the limitations are next with the 150 watt audio amplifier bridge mode, but it will probably saturate since the Hammond is rated at 75 watts.  No problem. It's a little 100-125 watt rig with lots of headroom and I have plenty of big bros to lean on.

Now we'll see how much she can take, captain!

T

I ramped up to the 150 watt amplifier bridge mode and 700V on the 4D32 final.  I was getting about 125 watts carrier out with easy 150% positive peaks.  I found the high freqs near 18 KHz were slightly cleaner with the 150 watts of  audio drive capability.  The audio peak saturation point was also higher which shows the 150 watt mode is the right choice.  I will probably run the HV at about 600 volts, 110 watts carrier out with huge audio overhead.

The audio sinewave looked fine all the way down to 10 HZ and then fell apart below 8 Hz.  Not bad.

I quickly tried the 4 ohm winding impedance and the amplifier tripped out. It warns about going below 8 ohms, but I just had to try it...  The 16 ohm winding was almost the same as 8 ohms, so I settled on 8 ohms. Maybe 16 ohms is a better choice to make it easier on the amp and xfmr, I dunno.

I want to run some tone IMD tests tomorrow using the SDR spectrum analyzer.  I'm hoping for a clean rig.

I think if I had used two 4D32s, I could have run the carrier output up to about 175 watts out and still had enuff audio to do the job.  But that's infringing on the 813 rig's territory, so one tube is probably enuff with lots of headroom to give the cleanest signal.

This is one of the few 100 watt rigs I have ever built. All in all, this rig build was worthwhile and I think I'm gonna like Summer Breeze.

T

The Hammond xfmr is working so well I decided to mount it inside the rig for shorter leads. Less stray capacitance and less instability that way.

The rig is too heavy to carry by hand now, so will have to use a wheeled cart.

Looking at the pics, the xfmr really filled in some space.

The good thing is if I need to, I can remove it quickly cuz it's sitting on studs and the connections are on standoffs with screws.

T

More Hammond xfmr pics:

The VU meter will work now. It is connected to the 8 ohm winding with a bridge rectifier to show actual power from the audio amplifier.  I plan to calibrate it for 0-150 watts of audio power. There is no modulator plate current to watch so this will do.

I have a snubber circuit across the secondary (5K) to quench the magnetic field on unkey.  Uses a vac relay.

After some SummerBreeze IMD tests tmw I hope to get back into the 813 rig, Hollywood, and get the audio section finalized with the MOSFET driver.

T

What a brute...

All this artillery and infrastructure for one 4D32 tube in a 125 watt rig...  Reminds me of the old buzzard 1940s and 50's when the huge racks housed 100-200 watt rigs.

So look again at the 32v.   Compact, modular and a tough beast. ;)

But your new build is gorgeous and looks like it’ll fill your medium power niche nicely.

Glad that big Hammond worked out. With a low cost SS amp, it makes for a reasonable cost modulator , even with the cost of the transformer.

I think I am going to order me one.

Pat
N4LTA

Where did you hide the snubber and vac relay? Are you using a normally closed one?

John

Hi Rick -

Yes, it does remind me of a 32V transmitter in overkill. A Valiant is another iron-laden example.

John:  Here's a few shots of the underside showing the snubber. Yes, the same NC vac relay from MaxxGain.. cheap cuz no one wants Normally Closed relays. I'm not real happy with the picture lighting. The rig actually looks better in person. Pics can really beat a rig up... :-)  But this rig took me only about 2 weeks from beginning to end. I don't spend a lot of time on the finer details, but it is built strong and all connections rarely fail. Every spade lug is soldered, etc.  I don't have the patience to over-dwell on my work when I could be dreaming and building the next project...  But it will work and perform perfectly when I'm done - I can guarantee that. I never give up on a project unless it is a total failure in design.

I'm already thinking of a 25-30 watt rig. My harem has rigs with a X5 or 7 DB  difference between them.   So there is a gap to fill.

The Hammond 1628SEA  is a similar 30 watt unbalanced 50H xfmr for a little over $100. It would modulate the pants off a 25 watt carrier.  This chip or slightly bigger, could even be the 8 ohm audio driver:

https://www.ti.com/lit/ds/symlink/lm1875.pdf

Modulate a single 807 in class C or something of that class would do an easy 25-30 watts..

John, have you made any more progress with your 813 SS audio board testing?  I just put mine  back on the bench and hooked it up to the 813 rig.  I was hoping yours is working FB so I will have the confidence to proceed...

In the second pic, notice the 50 watt! 1K resistor chips next to the 4D32 tube socket. They are in series giving 100 watts at 2K grid swamping. From Mouser, they are very low inductance rated.  The rig was stable before I added them and even more so now.  Would also be FB for the 813s rig.

BTW, 99% of all parts in this rig and other rigs (except the Hammond transformer) are from years of ham flea market used parts accumulation. I keep thinking I'm out of parts, but then there just might be enuff in the cellar for another one...   ;D

T

Tom,

For your little rig idea, why not a triode? Less PS voltages to manage. Say perhaps an 809?

John

I finally got around to doing some final mods and tests on Summer Breeze, the single 4D32 plate modulated rig.  The 813 rig and GFZ audio driver took me away for a few days.

Anyway, I couldn't be happier about how well the 4D32 rig is performing.  In the design phase I couldn't decide whether to run one or two 4D32s plate modulated by the Hammond xfmr and 150 watt Dayton amp. I was getting about 135 watts out carrier max at 700 volts under load. I wasn't satisfied with the power level so I added in a 24V transformer in phase with the HV transformer primary. (Like a bucking transformer but the opposite)   It brought my HV up to 850V under load. I am able to load up to a max of 200 watts carrier out now.  At 850V X .270 mA = 230 watts input.  200w/230w= 86% efficiency. (I subtracted the screen current thru the cathode meter)    I know that efficiency sounds high, but that's what the Bird says into a dummy load with the swr near 1:1.    It's like a Viking Valiant on steroids.  So bottom line is that ONE, not two,  4D32s is a perfect match for the Hammond xfmr and audio amplifier.  Two tubes would bring the plate impedance down to 1200 ohms which would be a little low for the Hammond and require larger tank capacitor values. Right now the plate impedance is about 3100 ohms with one tube... FB!

The peak modulation voltage on the tube plate is probably approaching 2KV under heavy positive modulation. The Hammond  xfmr is rated at about 3500V. I'm glad I used overrated tank components now.  Even the PS filter caps are  rated 1200VDC total with eight caps in series/parallel. It all adds up fast when pushing the DC HV and modulation levels hard.

At 200 watts out I am able to modulate it very well with tones that go well over 100% positive peaks.  That's a little above the xfmr rating but the amplifier is within spec.  I will  back it off in real use somewhat, but am very pleased with the power a single 4D32 can put out.  The plate is dissipating about 35 watts with no color. Like a finely tuned grenade.... :-) It has a muffin fan so stays reasonably cool.  I always beat the crap out of my rigs in testing to find the weak parts and faulty designs -  and optimize it past the tube data sheets. After making it a super hero, I then back everything off to the tube rating  or less and it rarely breaks after that abuse.

At full power, I do see a slight bit of blue flash activity in the tube when I unkey, like a screen acting up. But this tube was abused for months in the PDM dual quads rig, so it may be already damaged. I have some more good tubes around once the testing is over.

The audio quality is probably the best of all my rigs. About 5 Hz to 20 KHz… unreal.  The audio is very simple and of high quality components.  Summer Breeze will fill a nice niche below the 813 rig.

It runs cool like its namesake suggests. The fil requirements are low, the plate efficiency high and the SS amplifier hardly breaks a sweat.  Not bad for approaching a quarter KW  rig.  

Next, I'm trying to come up with a simple 40-60 watt RF driver for the three AM rigs to take the strain off the FT-1000D.  The 4D32 is such a nice tube - I'm thinking of one in linear service. A single 813 is also a possibility since I could even borrow 850 V from Summer Breeze and run the 813 class C as a driver.

I'm about ready to put SB on the air soon.  I might take a few pics of the completed rig in its operating position.  Very quiet and about 10' away, so background noise is nil.

John, anything is possible for a tube choice at this point.

T

I think I will order a 1642SE for myself. I should have the shop back in 8 weeks or so and  I have  the 4D32s and power supply iron  on site. I am starting the wiring on the 807 60 watt modulator but cutting metal is a bear on the apartment table. Instead of series modulating the dual 4D32, the Hammond transformer looks like the best way to modulate. A small Crown class D amp should work well on this project. I had multiple audio amps destroyed in the fire and I can get a Crown amp as a replacement cost amp.

Pat
N4LTA

Pat -

I'll be curious how your pair of 4D32s work out with the Hammond.  I'll bet if you run 600-700V and load the finals so that you are doing 150- 175 watts out, it will be a clean setup with ample audio with the xfmr limitation.  IE, if you load it right and reduce the plate current, you can bring that plate impedance up towards the 3K area, just like my single tube.  

I played around with the amplifier taps and found the 8 ohm tap > 3100 ohms was the best in the bridged mode.

Right now I'm trying to decide on a small master linear tube to RF carrier drive these AM rigs.   Two 6146s, a single 4D32 or an 813 - decisions, decisions.  I like GG, but if I can put the linear in grid driven service, class C,  it will take only 1 to 2 watts at most to drive any one of these prospective tubes to 60 watts out - and the FT-1000D fan will never come on.

The FT-1000D passes a squarewave at 5 Hz with the modified balanced modulator. So it can also double as the station Pissweaker rig driving the little linear at 25 watts AM carrier or so.

T

I have a Hammond 284X  transformer rated at 1200 volts Ct with a couple of 6.3 volt heater windings and a 5 V 5 amp winding. It also has a 125 volt tap to bring down the voltage a few percent.

A  cap input filter would have about 825 volts peak and loaded should be in the range you are talking about. A couple of 500 volt snap in caps on my existing board should work fine.

Choke input would be about  540 volts and maybe a few more percent with the 115 volt tap. Maybe 570 volts. Much better voltage regulation. I have a 10H 300 mA choke in hand.

Two 220 - 470 uF or so will fit the board. Plenty of filtering.


Pat
N4LTA

Pat, did you ever build up a rig using the pair of 4D32s and the Hammond 1642SE?

T

I lost quite a few parts in the fire but the transformer survived. I have re-aquired most of the other parts. Might be interested in startig it again. I can't remember what the plate and screen voltage are for the 4D32. Might be interested in doing the 6146 version.

Pat