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NEW Project - 6146B Plate modulated by Hammond 1628SEA and 85W SS Amp




 
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Author Topic: NEW Project - 6146B Plate modulated by Hammond 1628SEA and 85W SS Amp  (Read 14941 times)
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KD1SH
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« Reply #150 on: April 07, 2022, 05:12:13 PM »

  Excellent thread - been following it intently. Thanks, Tom, for blazing the trail. I've got some parts here: Hammond 1628 and 1642 (what a beast!), a couple of TDA-7293 modules and also a couple of class D 100 watt jobs - a Sure and a Dayton. For now, I'm holding off doing any drilling and blasting until I see where this project takes you.
  Curious, though: since we're not passing DC through the "primary" - the 4/8/16 winding - does it really matter that these Hammond transformers are designed for "single ended" applications? Hammond makes an output transformer - p/n 1650W - good for 280 watts and probably underrated, 4/8/16 and 1900 ohm windings. Could probably modulate a 500 watt input amp with one of those and easily a KW with a pair. Any reason why we need to use a single-ended transformer?
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« Reply #151 on: April 07, 2022, 09:41:56 PM »

Hi Bill,

Looks like you are about ready to roll with all those FB parts!

Well the main reason I went with the 1642SE transformer last year was building the 4D32 "Summer Breeze" rig.  A friend of mine was using the smaller 1628SEA and reported  tremendous audio results, so I tried it in a bigger form.

The gapped transformer eliminates the need for the modulation choke and coupling cap which can add distortion itself. (so called "modified Heising", better low end, less saturation)   So single-ended was the choice for this little rig. The second big 1642SE (recently given to me by Jeff/W2NBC) has about 55H of inductance which would be hard to replicate physically as a mod reactor using regular chokes. Lots of weight and space needed.   It was intended to be used for class A audiophile enthusiasts, so they did a really great job designing and manufacturing it for hi-fi.

Another thing is there was no requirement for big power on this rig. The original plan was for a Ranger 40-50 watt output power level. To put everything in one package (the mod transformer, reactor and cap) in the form of one single-ended gapped xfmr was appealing. I already have a 4-1000A plate modulated rig with BC iron and two smaller 813 rigs, so the motivation to try the higher power push-pull 280 watt xfmr was not there. This was supposed to be a semi-PW rig. But now it will be about 125W, depending on how the new twin TDA7293 works out.

Let me know how the 280W xfmr works out if you go that route.... or any rig you end up building.

Everything but the 2 watts of drive from the DDS is now self-contained. I just finished putting in the +/- 50VDC supply with  new Variac - and TDA heatsink and other connections for the TDA 140W version. I'm hoping it will work without the audio parasitics this time.  I also replaced the screen bypass caps with 1KV 300 pF knobs and also increased the plate coupling cap's voltage rating  just in case it was arcing that was causing the weird trash below 60 Hz. Maybe the TDA was on the way out, I dunno.  I bought three kit boards from China and six TDA7293 chips from Mouser, so there shud be no excuses if it does not work right this time. The only other unsolved problem remaining is microphonic feedback with the 1642SE.  I may have to surround that pretty xfmr with acoustic foam.


** Once I tie wrap all the wiring under the chassis, I'll take some new pics. There have been a few additions on top too.  There is absolutely no more room for parts on that rig.


Out of curiosity I put Commando on the digital scale;  87.6 pounds!    (out of the cabinet)

T
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« Reply #152 on: April 08, 2022, 10:33:06 AM »

I suppose I'm showing my lack of knowledge - I'm not a hi-fi audio guy - but I always figured that the "single ended" output transformer design was to alleviate core saturation due to passing all the DC plate current for the amplifier tubes through the primary. The DC plate current is going in one direction up through the primary, whereas in a push-pull configuration we're still feeding the plate current through the secondary, but since the B+ is applied to the center tap the currents are opposing and cancel out. In the case of output transformers, the designers weren't concerned about DC currents in the secondary because the secondary is only connected to a speaker coil - no DC current. Now, in our case, there's no DC current flowing in our primary because the amplifier modules supply their own current, which appears across our transformer only as an audio AC voltage. We do however have DC current flowing in our secondary - supplying the plates of our final tubes, so that's where the saturation comes in. It's just the opposite of the design intent of an output transformer; we've got DC current in the secondary but not the primary, where in a single-ended tube amp configuration you'd have DC current in the primary but not in the secondary. Either way I guess you'd still benefit from an air-gap transformer core.
No, I've got no plans of trying to scale this approach up to something really big - like you say, it's probably a concept best applied to exciter-class rigs. Also, the windings on these transformers are tested at only 2KV - if you wanted to modulate KW class rigs you'd probably be looking at higher B+ voltages than that. Anyway, these transformer's aren't cheap; you'd most certainly be able to come up with a "real" mod transformer for a KW class rig for less than a couple of Hammonds. You might not be able to duplicate that frequency response, though.
87.6 pounds - yikes! I toyed with the idea of applying this concept to a B&W 5100B that I have, which has a bad driver transformer, but the 5100B weighs in at 85 pounds already!
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« Reply #153 on: April 08, 2022, 11:25:03 AM »

See my post further up on this, too.

You are right that in a push pull audio amp, DC core saturation isn’t an issue. But the transformer is designed for a particularly boutique style of high end hifi amp, the SET single ended triode. They run a single triode in class A, like a WE300B, or even a transmitter tube (there’s at least one SET amp maker out there using a single 833A) running all the B+ one way through the primary (which we use as the secondary and run the RF tube B+ through the same way).  This, the gapped core is necessary, just like in a conventional modulation transformer.  None of this has to do with the 8 ohm winding, neither in an audio amp nor transmitter..

Ed
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« Reply #154 on: April 08, 2022, 12:03:09 PM »

See my post further up on this, too.

You are right that in a push pull audio amp, DC core saturation isn’t an issue. But the transformer is designed for a particularly boutique style of high end hifi amp, the SET single ended triode. They run a single triode in class A, like a WE300B, or even a transmitter tube (there’s at least one SET amp maker out there using a single 833A) running all the B+ one way through the primary (which we use as the secondary and run the RF tube B+ through the same way).  This, the gapped core is necessary, just like in a conventional modulation transformer.  None of this has to do with the 8 ohm winding, neither in an audio amp nor transmitter..

Ed

Ed, you bring up a very good point.  These high-end single-ended audio amplifiers do make very good sense to the audiophiles.  A triode single-ended amplifier avoids many of the issues of push-pull tetrode and pentode amplifiers, and the difference is truly audible and amazing.  

Not only are they implemented with 211s and 833s, there are now companies producing the SET amplifiers with the beautiful Eimac 304-TL bottle!  
https://wizard-highend.blogspot.com/2016/03/alum-rock-technology-304-tl-stereo.html
Ya gotta admit that is a pretty sight with the golden plates and cool blue mercury vapor rectifier bottles!  Too bad it doesn't do RF!

As we complain about the audiophiles running up the cost and depleting the availability of our beloved classic transmitting tubes, I was thinking that perhaps we might better be thankful to them as they create a demand (and supply) for these gapped single-ended output transformers that fit so well in the home-brew tube final transmitters with solid-state modulators.


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« Reply #155 on: April 08, 2022, 12:47:48 PM »

Is that your HB audio amp, Rick?  Very FB.

As a friend told me: "The current and voltage rating of the high impedance winding in a single ended amp is exactly what you get in your application flipped around. Operating at higher voltages or currents will push the core to saturation and degrade your low end. The only way to extend the power is to heising connect and get the DC current out of the winding."

When you think about it, these TDA and other "easy" solid state amplifiers ARE single ended amplifiers and require a single ended transformer to perform in our service.... so that's another reason to do this single-ended modulator trip.

Though, I wonder if a pair of these TDA amps could be run push-pull using a solid state driver to drive a p-p output transformer without too much trouble?   It gets to the point of then comparing it to a class E solid state rig for an easier build.  

The bottom line is we are mixing tubes and solid state -  hybrids here.

T
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« Reply #156 on: April 08, 2022, 01:03:59 PM »

I had that thought, too. Or, would it be possible to solid-state phase shift the output and create push-pull "artificially"?

Is that your HB audio amp, Rick?  Very FB.


Though, I wonder if a pair of these TDA amps could be run push-pull using a solid state driver to drive a p-p output transformer without too much trouble?   It gets to the point of then comparing it to a class E solid state rig for an easier build.  

The bottom line is we are mixing tubes and solid state -  hybrids here.

T
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« Reply #157 on: April 08, 2022, 01:09:03 PM »

Or, use a two channel module and phase shift one of the channels?
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« Reply #158 on: April 08, 2022, 02:13:28 PM »

UPDATE:  4-8-2022


These pics show the wired bottom tie wrapped "more prettier."  Also added the TDA7293 box and heatsink as well as the Variac  to control the +/- 50V audio VCC.   Still awaiting the twin TDA kit from China.

The rig ran FB (carrier) after being tie wrapped. Sometimes we get surprises, but not this time.  I immediately noticed how stiff and solid the wiring harnesses made the wires.  It adds and overall strength to the individual wires as we would expect using bundled wires.

The chassis is stuffed. As you can see, I had to stack vertically for the TDA box and the Variac to fit.   87.6 pounds is a lot to heft.

Pic 2:  This is the only time we can see all of the front panel gold pin stripping due to the light angle.

Pic 2:  Notice the red freq counter pick-up loop to the right of the tank coil.

** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**

T


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« Reply #159 on: April 08, 2022, 02:18:33 PM »

Yaz 4 is now five years old. English Springer Spaniel.  What a great guy!

** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**


Are you not entertained?  :-)


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« Reply #160 on: April 08, 2022, 02:20:16 PM »

Commando lives.

Pic 3:  The empty slot next to the heatsink -  I plan to add a small fan to blow air thru the fins.  The forced airflow makes a huge difference on the heatsink, assuming the thermal resistance of the chip core is reasonably low enuff.

Pic 1:  That is a 6" high chassis. A lot can fit under there. I needed every square inch. I pulled out the freq counter so you can see that covered area better.


** Blow up the  bottom view pics with your enlarge button to get a better look at the wiring.**


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« Reply #161 on: April 08, 2022, 02:26:33 PM »

Pic 1: A pretty picture of Commando at attention. All self-contained except for the external 2 watt drive from the universal station RF DDS and the 1 volt line audio processing.

Pic 2:  Another pretty Commando. He's getting ready for his 3 mile daily walk/run in the woods.

Except for any surprises with the awaiting twin TDA7293 audio project, we are nearing the end of construction and this thread.  I'll post the results of the overall testing in about a week or so when the parts arrive.  If it fails, then we will continue on until it works.

** And panel labels need to be made up.

Thanks to the many guys who helped this project along!  I will add a credits list near the end.  I see the thread hits are over 8,000, so there must be a bunch of interested builders out there.


T


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« Reply #162 on: April 08, 2022, 02:56:18 PM »

Is that your HB audio amp, Rick?  Very FB.

As a friend told me: "The current and voltage rating of the high impedance winding in a single ended amp is exactly what you get in your application flipped around. Operating at higher voltages or currents will push the core to saturation and degrade your low end. The only way to extend the power is to heising connect and get the DC current out of the winding."

When you think about it, these TDA and other "easy" solid state amplifiers ARE single ended amplifiers and require a single ended transformer to perform in our service.... so that's another reason to do this single-ended modulator trip.

Though, I wonder if a pair of these TDA amps could be run push-pull using a solid state driver to drive a p-p output transformer without too much trouble?   It gets to the point of then comparing it to a class E solid state rig for an easier build.  

The bottom line is we are mixing tubes and solid state -  hybrids here.

T

No, Tom, the amplifier is not mine.  It is a product of Alum Rock Technology in San Jose, CA, the proprietor is Robert Potter.  According to info on the web (which may or may not be current data) he has sold seven units with the pair of Eimac 304-TLs, and one employing 75-TL bottles.  The price for the stereo 304-TL product borders very near 100K ($100,000.00).  Apparently they can afford to acquire the Eimac bottles!

I once considered building an audio amplifier with the 304-TL, the closest I came was the modulator for another pair of 304-TLs, push-pull in the RF final.  I don't have enough spares for those games!

Tom, you mentioned "these TDA and other "easy" solid state amplifiers ARE single ended amplifiers..." and while they have a single-ended output, they are really push-pull amplifiers with single-ended output.  They are actually called single-ended push-pull.  If they were not push-pull, they would be running extremely inefficient class-A, but in actuality they are running class-B, with just enough forward bias to minimize crossover distortion.  Solid state amplifiers have beem using this topology for over 60 years, and some of the "output-transformerless" amplifiers employing tubes (for example, the Futterman with 6AS7s) have been around since the 1940s.

The general idea is the use of a pair of output transistors, either bipolar or FET, in series.  The top transistor collector or drain connects to the positive supply rail, and the emitter or source to the speaker or mod transformer.  The bottom transistor collector or drain is connected to the speaker, while its emitter or source is tied to the negative supply rail.  By alternately forward biasing the top and bottom transistor, the positive and negative currents are produced in the output load device, relative to the ground, which is the return path for the positive and negative power supplies.  Thus you have push-pull output devices into a single-ended load.  Proper biasing and feedback maintains the output point at zero volts DC.

Now you may take a pair of these amplifiers, such as the TDA-7293, and parallel the output, thus increasing the current available, providing higher power output.  

Though I would NOT attempt it with the 7293 device, a pair of these single-ended push-pull amplifiers may be driven 180 degrees out of phase, and the output may be taken from the output terminals of both amplifiers, instead of one amplifier and the ground terminal.  This series output arrangement (referred to as "Bridge Mode") doubles the available output voltage, thus quadrupling the available power into the same load impedance, assuming the device is capable of providing the current demand without excessive dissipation.  The Safe Operating Area (SOA) of the 7293 precludes the use of this configuration, due primarily to the thermal resistance from the die to the heat sink. The thermal energy cannot be dissipated fast enough to allow this sort of abusive operation.

Many, if not most, of the solid state audio amplifiers of today are no longer analog.  They basically operate very similar to the PWM modulators we use for AM, and include an LC filter in the output to remove the switching artifacts.  This is how they can be made so small and inexpensively, requiring very little heat sink and minute power supplies.

So, if it turns out you are not satisfied with the dual TDA-7293 push-pull analog amplifier performance, I would suggest looking at available devices or modules providing high-efficiency audio output through PWM or PDM.  They will play well with your existing power supply and modulation transformer.
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« Reply #163 on: April 08, 2022, 03:14:01 PM »

OK Rick,

Yes, you are correct about the TDA7293 being a p-p analog amplifier that requires a single ended configuration.  I was referring more about these application requirements and stated it wrong.  What impresses me about the chip is it is not a class D switching device for such a small size.  But we see that the thermal resistance is a drawback. The class D stuff never gets to that point heat-wise as the linear chip does.  I plan to be very careful with microphonic feedback this time around. My only TDA-7294 is also dead as a result.

FB on the difficulty of interfacing two TDAs in push pull.

T
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« Reply #164 on: April 08, 2022, 04:40:02 PM »

T,
I haven't seen the underside of Commando in several weeks.  Holy cow! the "miles" of wire.  What an undertaking.  My hats off to you OM.  I really love how you fit stuff in and make it work ie, the little variac with long shaft.  What is in the white tube surrounded by the coil? 
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« Reply #165 on: April 09, 2022, 07:20:25 AM »

Tom,

It speaks //volumes// that your mod xformer is twice the size of your HV transformer!

Great looking rig.  Hope to hear it one day from the mighty blue waters.

--Shane
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« Reply #166 on: April 09, 2022, 01:51:47 PM »

Bob:   Yep, there's a lot of wires underneath.  I always thought a rig looked its best just after the parts are mounted, without wiring.  It's almost like looking at a person's face and then realizing how much intracity there is inside.


Shane:  I sometimes forget that we are using two power supplies. The RF final is powered by that 800VAC CT HV transformer - and the modulator has its own power transformer of +/- 40VAC with lots of amps.  So I suppose we could combine the two cores mentally and say we have a really big [show] -  a big cored transformer equivalent, maybe as big as the Hammond 1642SE.

[BTW]  Bob/W1RKW is working on the panel labels with his label machine.  That shud put on the final touches.  I also want to get a brass engraved plate reading  "Commando."

T
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« Reply #167 on: April 15, 2022, 01:26:09 PM »

UPDATE: 4-15-2022:

Labels and Brass!

Bob/ W1RKW sent me some FB labels.  I need to redo a number of them, cuz the camera flash shows the imperfections of repositioning after I make an error. Some look good. They really should be applied once and scrapped if they come out crooked - not salvaged. Clean the surface with alcohol and try again. TWEEZERS are the key for everything. Clean hands...dirty fingers are NG! Live and learn.   I gotta buy one of these label machines.

But anyway, the "Commando" brass plate arrived and I drilled and tapped the panel with 4-40s to secure it. It's hard to get the proper camera light to show its black lettering on brass.  But it's beautiful...satin brass, like a trophy.

I also decided to do 160, 75 and 40M "presets."   Pretuned marks so that I can change bands and have a full 125 watt carrier in about 15 seconds.  

Next, the Chinese TDA 7293 dual chip 140 watt audio kit board will arrive and I'll get back into the testing mode again.  If that all goes well, the rig is essentially finished, except for inevitable modifications.

T


** Pic 2 low lighting really brings out the brass trim and plate... **   (I need a tripod badly)


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« Reply #168 on: April 15, 2022, 01:29:03 PM »

Commando, get ready for service...


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« Reply #169 on: April 15, 2022, 01:31:16 PM »

My memory is too short not to use labels and preset markings.... Wink    [15 seconds to change band with 125W full power  - no brainer procedure]


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« Reply #170 on: April 16, 2022, 11:37:12 AM »

The brass plate looks excellent. Nice touch!
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« Reply #171 on: April 18, 2022, 03:18:40 AM »

Really classy.
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"Let's go kayaking, Tommy!" - Yaz


« Reply #172 on: April 18, 2022, 12:19:13 PM »

Now that's the two nicest comments I've ever received about a homebrew rig... TNX!

The camera's extremely bright flash brings out insignificant problems. Glare and washout.  Rigs always look better in subdued  regular room light with the naked eye. I just ordered a 72" tripod for steadiness and longer exposures without flash.   Watch the improvement in the future.

Still awaiting the dual [analog] TDA7293 audio power chip kit board.   Here's the rough schematic of two TDA7293s in parallel.... 140 watts.    Rick will be doing a software version soon. He has been doing some research and plans to test a dual chip version out soon. We'll let ya know.


* For those who already have TDA7293 single chip boards, there's no reason why combining [in parallel with mods] two SINGLE chip boards will not work with this schematic. Though, just buying a dual kit board is easier. I have not seen any dual TDA7293 assembled boards yet.

T


* TDA7293 W8KHK dual chip schematic.jpg (39.48 KB, 566x755 - viewed 36 times.)
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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.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
K1JJ
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Posts: 8698


"Let's go kayaking, Tommy!" - Yaz


« Reply #173 on: April 18, 2022, 03:29:34 PM »

Commando's artwork is finished....

Bob/W1RKW sent me the remaining labels to finish up Commando and add some presets to the other rigs.

Check out the new larger Plate Loading, Plate Tuning, fan variac and band presets. Easier to see.  Also the big "Commando"  label lying flat in the tube window.

T


** The camera's extremely bright flash brings out insignificant problems. Glare and washout.  Rigs always look better in subdued  regular room light with the naked eye. I just ordered a 72" tripod for steadiness and longer exposures without flash.   Watch the improvement in the future.


* DSCF0011.JPG (325.01 KB, 1280x960 - viewed 43 times.)

* DSCF0001.JPG (330.15 KB, 1280x960 - viewed 46 times.)

* DSCF0007.JPG (332.1 KB, 1280x960 - viewed 41 times.)
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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.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
W1RKW
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Posts: 4319



« Reply #174 on: April 19, 2022, 08:53:54 AM »

I'm no photography wiz but I wonder if reflection off of the shiny parts messes with exposure to some degree. How one would adjust or compensate for that I do not know other than trial and error.  Maybe a polarizing filter on the flash and/or lens to reduce glare would do the trick. 
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Bob
W1RKW
Home of GORT. A buddy of mine named the 813 rig GORT.
His fear was when I turned it on for the first time life on earth would come to a stand still.
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