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Another 813 Build




 
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Author Topic: Another 813 Build  (Read 31996 times)
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w9jsw
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« Reply #175 on: December 01, 2019, 07:16:53 PM »

I got tired of waiting for Lehman's to get more stock of the lantern globes so went searching. These popped up. I mounted the sockets under the chassis with a 8-32 nut to make a 1/8 in space between the socket and the bottom of the chassis. I cut the hole 2.25in which gives a 1/8 in space around the base of the tube. I mounted a small 12v fan that I will run at 9V so keep it quiet. I will fully cover the base of the chassis so the air only has one place to go.

While these lanterns are only 1/2 high I figured it will still properly cool the base of the tube. I will be running the tube at 2000-2500 volts. So I am hoping this give it adequate cooling. These tubes are advertised as not needing cooling under normal conditions. I am near/at the max ICAS rating so a little bit of cooling should be enough, I hope.

I got the lantern glass at a company called American Mantle. They were $7.50 plus shipping. I ordered 4.

John
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K1JJ
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« Reply #176 on: December 01, 2019, 08:29:26 PM »

Very nice, John.  Professional look.

I would suggest some improvements: Remount the 813s  with 1/4" socket sub-mounting -  and drilling 3/16" holes around the rim (between the mounting screws) to add air flow. (Drill 16 holes total, 4 per section between screws)  This will make a huge difference.  One small 12V fan on the back of the RF or Mod chassis will hardly do the trick the way it is.

If the 12 V small fan doesn't do it, try a small squirrel cage blower that can handle some back pressure using a Variac to slow it down. You should feel a moderate breeze coming thru the chimneys.

In addition, usually the chimney goes all the way to the top of the tube and bends inward to push air past the plate seal.  You will find no air hitting the plate cap with a shorty chimney like that.  I wonder how much difference it would make if you placed another chimney on top of the existing one?  You could always order the correct chimneys later on from Lehman when they are available

But all in all, it's true that an 813 needs no pressurized air when run within limits, though there still needs to be some ventilation and heat radiation flow.  IE, just think of how hot an 813 would get inside a sealed metal box with no air flow at all.

T

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« Reply #177 on: December 01, 2019, 11:43:22 PM »

A friend got a  2 x 4x1 HB  old school contest amp that has cut down pickle jars for chimneys..regular glass...how cool is that?
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w9jsw
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« Reply #178 on: December 02, 2019, 07:04:16 AM »

Thanks Tom. All good suggestions to implement, as always.
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KB2WIG
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« Reply #179 on: December 02, 2019, 11:18:41 AM »



Coleman lantern chimbleys have been mentioned in the past.

Just a thought,

 KLC
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« Reply #180 on: December 03, 2019, 09:15:22 PM »

At 1250 VDC plate u need -160 bias in AM Plate modulation, class "C" mode. I'd use a fixed screen supply with a small 10 hy series choke for complete modulation. I like that better than no fixed bias and using a clamper tube. If you use a series resistor for screen voltage and no fixed bias, tube wastes a lot of heat and never really cuts off if keying in CW.

The bias can be derived from a VR105 plus a grid leak resistor of about 4.5 K @ 1 Watt to make -160 volts. The fixed bias is more than 2 X cutoff and resting current with no excitation should be zero. Grid current runs around 12 - 13 ma. with a drive of about 3 watts.


Seth KC2WE

Note: Been doing this since 1959 L o L
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Seth Taylor
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« Reply #181 on: December 03, 2019, 09:50:19 PM »

I'd use a fixed screen supply with a small 10 hy series choke for complete modulation. I like that better than no fixed bias and using a clamper tube. If you use a series resistor for screen voltage and no fixed bias, tube wastes a lot of heat and never really cuts off if keying in CW.

I've seen the choke in the screen supply in a couple schematics. Can anyone explain how this works, what the pros and cons are?

Ed
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w9jsw
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« Reply #182 on: December 04, 2019, 06:31:08 AM »

Looks like Seth's comment got duplicated across 2 threads...
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K8DI
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« Reply #183 on: December 04, 2019, 06:37:44 AM »

Looks like Seth's comment got duplicated across 2 threads...
It did, and he may have only meant the other, but it may apply here as well. The 813 data sheets say that the screen should be supplied from a separate modulated source (eg an additional winding on the mod transformer) or a resistor from the plate, for best linearity. Your design has it from a separate unmodulated source.  I see a choke in your schematics. Is the choke a good add on? What will it do in terms of the screen being modulated? And so on?

Ed
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w9jsw
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« Reply #184 on: December 04, 2019, 06:29:35 PM »

I will let Tom opine. I am mainly a kit builder here. My schematic doesn’t show that i am now planning a regulated FET 350v supply with a clamp circuit for that. I do have a winding available on the mod transformer that we could use if I get help on how to implement it.
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K1JJ
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« Reply #185 on: December 04, 2019, 08:36:37 PM »

  The 813 data sheets say that the screen should be supplied from a separate modulated source (eg an additional winding on the mod transformer) or a resistor from the plate, for best linearity. Your design has it from a separate unmodulated source.  I see a choke in your schematics. Is the choke a good add on? What will it do in terms of the screen being modulated? And so on?
Ed

Hi Ed,

You can use a 10H choke in series with the screen, a resistor in series with the screen or a resistor from the plate to screen of appropriate values.  It won't make much difference in modulation level or linearity which technique you use.  The amount of screen modulation compared to plate modulation is small.

Hook up a scope and SDR spectrum analyzer and run some tones thru. You can try each technique and adjust for best performance, (IMD and audio peaks) though just using a choke works out fine. The choke's impedance will vary based on audio frequency while the resistor remains constant thru the whole range.

The advantage of the choke is its DC resistance is low and the wasted power is small. When running a big tube, this can generate a lot of heat when using a resistor, especially from plate to screen.  A small tube makes it easier heat-wise.

Dean, WA1KNX wrote an article in the archives that shows how to optimize the resistor/capacitor screen network for a DX-100. But again, just throw in a 10H choke and you will be done with it for down to 30 HZ or so.

When homebrewing, I never liked a separate screen mod transformer winding to modulate the screen because you are stuck with whatever the winding ratio is and is more of a specific design for a specific tube line up, like a pair of 4-400As modulating a pair. If you use a different tube line up, then it may produce too much or too little screen modulation.

I have always used a self-modulated choke in the screen for my plate modulated 813s, 4-1000As and others rigs. I notice most of the big guns do the same thing over the years.

Tom, K1JJ
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« Reply #186 on: December 07, 2019, 11:57:24 AM »

Peek A Boo

I see two!

4-1000 that is!

--Shane
KD6VXI


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w9jsw
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« Reply #187 on: December 07, 2019, 03:50:51 PM »

Show me the back side when you get it presentable.
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w9jsw
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« Reply #188 on: December 14, 2019, 04:39:31 PM »

Peek-a-boo!  That large punch that Pat N4LTA loaned me worked FB on the large meter.

Back picture is the back of the modulator. Decided to use the shielded potted filament trans on this deck. Hope that it is shielded well enough to not feed over to the driver board.

Frank has designed a new Balanced microphone preamp circuit to drive the mosfet driver. It uses the unused 6.3V windings on the mosfet driver PS to make +/- 6Vdc to run a series of op-amps to amplify and then shape the audio. Of course I did a board of it. We are awaiting sample copies to test.

John


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* Schematic_Balanced Microphone Preamp-1-2.pdf (21.16 KB - downloaded 51 times.)
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KB2WIG
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« Reply #189 on: December 14, 2019, 04:45:25 PM »



She is starting to look nice..... I had a !Duh? moment; why is that arrow pointing at that meter?

klc
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w9jsw
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« Reply #190 on: December 14, 2019, 04:53:21 PM »

I need to find a face plate for the honking variac. That spacer between the PS and Mod will house the sequencer. I bought a nice bezel for it. Should look sharp. I had planned to have a PS control there but decided it was not needed.

Here is more on the preamp -

Frank sez - "I added a couple op amp stages of active low pass filtering. The green trace below is without the filter and the blue is with two stages. 3dB point is about 7kHz. The roll off is easily shifted with RC values. I tried to use common values for the simulation. So now we have balanced or unbalanced input with the option of gain (2dB now), a negative peak limiter and active filtering. This addresses my concerns with the wideband driver passing extreme highs. I think the layout would be cleaner with 4 discrete op amps. I only picked the OP07 because it is common and in the LT Spice library. FET op amps would be better for RF susceptibility and there are many lower noise amps out there. Sockets would allow for additional testing. The user has the option of populating the functions desired. "

Board pic... 2" by 3" - $30 for 3 of them at Oshpark.

John


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w9jsw
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« Reply #191 on: December 28, 2019, 06:18:10 PM »

Still working a lot of metal.

This punch is a beast! Makes perfect 2.75in holes.

Hooking up the variable caps to the turns counters going well. Will take a pic when farther along.

Going to make the peek holes smaller on the RF deck. Not as much space and I don't like the way the large ones worked out on the modulator. As you look down on them you see a lot of tube base. I guess they will look nice from the operating position, though.

Do these large ammeters have a shunt inside? Some of them track face current properly, some do not. I see 2 bolt heads showing under the main terminals. Will open one up to see, I guess.

John


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K1JJ
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« Reply #192 on: December 28, 2019, 10:49:36 PM »

Going to make the peek holes smaller on the RF deck. Not as much space and I don't like the way the large ones worked out on the modulator. As you look down on them you see a lot of tube base. I guess they will look nice from the operating position, though.
John

Hi John,

That rig is going to be nice inside AND out.

I think almost every homebrewer of a big rig initially makes the mistake of not realizing that the viewing hole will not line up at certain viewing angles, like when sitting in a chair.   I have always used a single 3"  hole but after seeing yours, I think I would have done better stacking two holes for an oval shape. The 4X1s are rather large and tall, so I may get motivated to cut more to better expose the RF and modulators. I hate having to crane my neck to get the perfect alignment.

Viewing the tube plates during operation is a critical and useful requirement for both pleasure and tube longevity.  Notice the broad-cash industry did it in spades when tubes were in vogue.

T  
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« Reply #193 on: December 30, 2019, 03:14:21 PM »

Looking Good! -

Got me thinking about a big rig. I found a modulation transformer.   I have 811A and 813s  I think I have some PS chokes, a plate transformer and 6 - 4uF oil caps. I have a 48 " rack

Not sure what I might use for the RF  tube. I have a used 4-1000 and Eimac air system cast socket. Not sure how good the tube is. I have a new Eimac  3CX1200A7 but I have a multi band linear planned for it. More time is what I need!



Pat
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« Reply #194 on: December 31, 2019, 12:58:07 AM »

More pics

Wanted to show how I did the turns counters as I could find no other examples on the site. They turn real smooth and are calibrated to increase capacitance as the turns count increases.

On the top side, got the components packed up tight with minimum distances between the tank and the plate cap. Problem is that the tank bandswitch and inductor is huge. Has solid detents and should work well. Tried to wash it in the dishwasher but it still came out dingy. Time for a different approach on that. I have some copper sheet that I will cut into strips for the tank interconnect.

Waiting for a SPDT vacuum relay to deliver and have to wind a safety choke. Have 23uh on the 160M setting so thinking a 5 in long piece of 1 in pvc with around 200 wraps of 20ga magnet wire should make a pretty stout choke. Frank suggested a 10X or greater value than the 160M inductor value.


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w9jsw
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« Reply #195 on: January 05, 2020, 09:03:56 AM »

Here is the safety choke. 210uH


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K1JJ
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« Reply #196 on: January 05, 2020, 12:08:36 PM »

Here is the safety choke. 210uH

Yo John -

Now THAT'S a safety choke!   Built to take it and hold up during a direct HV short.  Good going.


I'm seen my share of "commercial ham" rig chokes that are fried and have opened up. They use those wimpy 2.5 mH jobs that are good at smelling and melting.  Dangerous.

You want a big wire choke in the 50 ohm load position that will take a 3KV hit to ground and  blow the 240VAC 30A breaker - and live to tell another tale.  We do not want that choke to blow during an event, like when the plate coupling cap shorts and we have HV on the antenna.

BTW, I'm building up the 3-diode high level limiter circuit for my 4-1000A plate modulated rig as a saftey net when occassionally going over 100% negative. You may want to consider it for your new 813 rig.  Use some low level limiting in the audio chain and use the 3-diode circuit to handle overshoot and mistakes that come after the the low level circuitry. Protect the mod transformer as well as keeping occassional splatter at bay.

T

A thread with the pros and cons:
http://amfone.net/Amforum/index.php?topic=38237.0



* 3-diode limiter circuit WA1QIX.jpg (77.87 KB, 592x417 - viewed 92 times.)
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« Reply #197 on: January 05, 2020, 07:53:28 PM »

Not seeing how this translates to a Heising circuit.



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« Reply #198 on: January 05, 2020, 11:10:08 PM »

It is not true Heising. Often called modified Heising.

The large inductor passes DC to the tube plate. Normally this DC passes through the modulation transformer and saturates the core, interfering with the transformer action. The core of a modulation transformer is wound on a core with an air gap which alleviates this saturation but the transformer will perform better with no DC flow. The two capacitors block DC from passing through the modulation but the AC audio current can pass.

You can use a conventionally wound audio transformer with this arrangement if the turns ratios are correct. This can be done with a large audio tube output transformer in reverse feeding a solid state amp into the speaker winding and using the high impedance winding to modulate the plate.

In a tube audio output transformer, there is no average DC current flow in the winding because the current is fed at the center tap and magnetizing current in one tube is cancelled by the magnetizing current in the other push pull tube. That happens in a push pull modulator in the primary winding also, but the secondary winding has DC only flowing in one direction.

The modulating reactor has the same thing happen, but it is wound with an air gap to help alleviate the saturation.


Pat
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« Reply #199 on: January 06, 2020, 12:40:22 AM »

Just when we think it's safe to go back in the water, I get an email from Jeff/ NBC...

This is a link to W8JI's tests (a FB engr he is) using the 3-diode high level limiter (or variation). He feels it actually generates MORE IMD trash UNLESS it has a low pass filter after it.  This is why I like QIX's low level diode and R/C low pass filter. Without a directly coupled (DC) modulator there is a risk of overshoot from tilt by not having it high level, but I think it may be worth a try - and I will pass on the high level 3-diode circuit for now.  A high level low pass filter would be a kludgy thang too.

https://www.w8ji.com/Johnson%20audio%20mods.htm

T
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There's nothing like an old dog... or a puppy, or a dog in his prime!  Help a good dog - give him a "Forever Home!"
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