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The New 4D32 plate modulated project - "SummerBreeze"




 
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October 30, 2020, 03:05:52 PM *
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Author Topic: The New 4D32 plate modulated project - "SummerBreeze"  (Read 4210 times)
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K1JJ
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« Reply #25 on: June 29, 2020, 11:57:35 PM »

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... Grin


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
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« Reply #26 on: June 30, 2020, 08:26:12 AM »

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
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« Reply #27 on: June 30, 2020, 09:11:56 AM »

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

Ed
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« Reply #28 on: June 30, 2020, 11:41:36 AM »

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
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K1JJ
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« Reply #29 on: June 30, 2020, 11:48:38 AM »

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
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« Reply #30 on: June 30, 2020, 11:56:16 AM »

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


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K1JJ
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« Reply #31 on: June 30, 2020, 12:14:35 PM »

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
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« Reply #32 on: June 30, 2020, 11:22:02 PM »

Compliments on the use of metal enclosed plug in relays!
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« Reply #33 on: July 02, 2020, 01:11:19 AM »

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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« Reply #34 on: July 02, 2020, 01:13:03 AM »

More:

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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« Reply #35 on: July 02, 2020, 01:14:18 AM »

More:


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« Reply #36 on: July 02, 2020, 01:15:22 AM »

More:

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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« Reply #37 on: July 02, 2020, 01:16:47 AM »

More:

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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« Reply #38 on: July 02, 2020, 01:17:57 AM »

More:

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


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« Reply #39 on: July 02, 2020, 01:18:54 AM »

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.  


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« Reply #40 on: July 02, 2020, 12:01:25 PM »

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
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« Reply #41 on: July 02, 2020, 09:53:42 PM »

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
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« Reply #42 on: July 02, 2020, 10:51:32 PM »

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
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« Reply #43 on: July 03, 2020, 03:14:43 AM »

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


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« Reply #44 on: July 03, 2020, 03:15:39 AM »

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


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« Reply #45 on: July 03, 2020, 03:16:09 AM »

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.


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« Reply #46 on: July 03, 2020, 10:21:56 AM »

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

But your new build is gorgeous and looks like itíll fill your medium power niche nicely.
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« Reply #47 on: July 03, 2020, 10:24:32 AM »

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
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« Reply #48 on: July 03, 2020, 03:16:19 PM »

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

John
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« Reply #49 on: July 03, 2020, 05:33:22 PM »

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...   Grin

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. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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