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Another 813 Build




 
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N4LTA
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« Reply #200 on: January 06, 2020, 02:06:13 PM »

I agree that he is a FB engineer.  I always go there first. I was going to add that to my transmitter, but I think I'll pass also.

BTW - You were loud and clear last night into SC. I had my 4SQRP Nouveau am rig on and  listening with it  - but a 5 watt reply was not likely to be heard.

Pat
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w9jsw
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« Reply #201 on: March 22, 2020, 06:08:12 PM »

Thundering along like a herd of turtles...  Roll Eyes

I don't know why it has all of those scratches on the inside. Came that way from Hammond. Annoying.


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w8khk
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« Reply #202 on: March 22, 2020, 08:05:32 PM »

Very nice work, John....

Except for a hint of green printed circuit board, and a couple new toroid transformers, it looks retro enough to be a photograph from one of the 50's handbooks.

i like the use of gear and universal drives to make a very functional layout without detracting from optimal control locations on the front panel.  I am especially enamored of using the barrier strips for wires with lugs on the back.  This makes experimentation, customization, and measurements very easy.

Don't worry about the scratches on the underside, it is masked by the fine workmanship and planning you have invested so much time with.  It will be great to see the entire rig in a photo essay real soon!
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K1JJ
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« Reply #203 on: March 22, 2020, 09:51:47 PM »

John,

I see you made the front panel layout symmetrical, ala K1KW-style.   That took some planning.

Now keep all the RF connection short and fat - and you've arrived!

T
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« Reply #204 on: March 23, 2020, 07:08:49 AM »

Let's talk about hookup wire.

For the tank, I plan on copper strips from the blocking cap through to the safety choke, then RG213 from there down to the antenna relay. The big tank inductor is already tapped about right with copper strips.

For the input side, RG213 from the connector to the cap that feeds C16. Between c16-L11-c17-c29 use strap, stranded hookup, solid? GA?

For the filaments, using #12 copper. That is the gauge of the wires coming out of the fil trans.

General connections I thought auto hookup wire (I have a bunch) that is an mix of 16 and 14 ga stranded in multiple colors. Any RF reason I should be using solid wire?

Up close on the sockets, most connections will be asap (as short as possible). For ground lugs, will put it directly to chassis.
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n4joy
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« Reply #205 on: March 23, 2020, 10:10:19 AM »

Excellent progress!  I am also building an 813 amp and will hopefully have it up and running this week.  Thanks for sharing the details of your project!

P.S.  The last Hammond chassis I ordered also arrived with deep interior scratches.  No big deal but my OCD side would have preferred their abscence. 
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w9jsw
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« Reply #206 on: April 05, 2020, 06:31:57 PM »

Today I did a little testing. Wired up the grid 1 and grid 2 supplies. Was worried that the input voltage was too high. Loaded it down with the expected current load and the voltages came right in line. I am using PCBs that I made from circuits from AC0OB. Thanks Phil! 385V with a 80ma current limit, and a -75V regulated supply.

Most of the wiring underneath is complete. Wired a simple power supply with the 5V and 6.3V extra windings in series on the above supplies. Going to use a large 24V rotron fan running on 15V. Not loud at all. These big fans were a steal. Cost me $5 each. Plenty of CFM to keep the tubes cool. 

Now moved up top to start the wiring there. Measured all the meters. 2 are properly setup with internal shunts. 2 are not. Got online and now have calculated shunt values for them. W8JI site is really good!

https://www.w8ji.com/metering_amplifier.htm

Have plate meter wired. Will have Grid 1 and 2 current, and a voltage meter switchable to measure each grid voltage.

Next will start the copper strips to connect the plates to the tank. Ordered some AL so I can properly mount the ANT vacuum switch.

Drilled the hole and mounted the band switch knob. Need to clean up the windows. They are close. I hate hand filing. May try some door guard covering to mask the uneven cut.

Oh, well. It is home-made...

John


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K1JJ
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« Reply #207 on: April 05, 2020, 09:04:11 PM »

Is the RF choke in the backround with multiple windings insulated from ground at least 1"?    5KV under modulation can be pretty nasty.

The choke in the foreground looks OK since it is a safety choke from 50 ohms to ground.

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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« Reply #208 on: April 05, 2020, 09:07:24 PM »

Looks Great!
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« Reply #209 on: April 06, 2020, 08:38:18 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

The single 4-1000 here uses an 8H choke in series with a 2000 Ohm resistor running on a stiff 600V supply. The screen is very nicely modulated. For fun I subbed in a 50H choke and there was little difference over the stock 8H value.
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K1JJ
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« Reply #210 on: April 07, 2020, 01:33:30 AM »

Hmmm...  I wonder how well regulated the RF final screen voltage source needs to be with a plate modulated rig?  Under normal operation the DC screen current meter stays still, but I never looked at it on a scope to see if there are any fast audio fluctuations under modulation.   My modulators have regulated screen and grid voltages, but not the RF final.


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. 

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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« Reply #211 on: April 08, 2020, 02:41:00 AM »

Mine's definitely not well-regulated, the 2K resistor in series with the 8H choke sees to that. I have bypassed the 2K resistor during testing (it is bypassed for CW mode) and there was not much difference except the 4-1000 needed less drive due to higher screen voltage, and the screen self-modulated a bit less but it was not an annoyance and the mod was still easily 100%. I did put a scope on the screen and found something like 950V peak to peak volts there, and the screen current meter was not moving much if any from the carrier level position.

The 2K res. was not the main thing, only a voltage dropper.  
The screen mod choke is where the modulation happened 8H = 25K Ohms at 500Hz.

By stiff supply, I meant just a typical choke input supply having better regulation over wide current range that a capacitor type.

I think there's tons of leeway in the self-modulating scheme but this opinion is based on only the one transmitter.
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« Reply #212 on: April 08, 2020, 07:35:50 PM »

When you make the interconnects to the bandswitch, make sure you take the additional L into account.  Meaning the L from the bandswitch (minimal) and the in and output strap to and from.

This can kill efficiency.

--Shane
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w9jsw
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« Reply #213 on: April 09, 2020, 06:18:26 AM »

I have not adjusted/changed the straps on the bandswitch yet. Where it is now seems close based on my plate load calculations. My plan is to wire it up fully, then feed it backwards with a nanovna at each nominal AM frequency and move the taps around for best effect. I will have the tubes installed and and some R to simulate the plate load. Is that what you are referring to with "take into account", Shane?

To add to my previous post, got the shunts worked out for the 2 grid ammeters. After calculating, I hooked up a 9V battery, an appropriate R and a digital ammeter in line with the analog one. Used an R to yield approx 3/4 deflection. Dialed it right in on the reading.

McMaster-Carr has a really good selection of rubber metal edging. And only around a $1 a foot for a 10ft quantity. I think that will clean up the window issue.

Not that far away from the final mount of all of the chassis in the rack and the final interconnect.

John
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« Reply #214 on: April 09, 2020, 03:55:59 PM »

Hi John. Nice build. I think Shane is pointing to the stray L which would upset the PI match. That is something the vna test may catch as you set that network up. I think the issue may be tank L, Q degradation; adding series R. Ideal, the unloaded Q of the tank elements are very high compared to the PI match loaded Q. Curious what is your tube total plate load Z? Are you taking into account the Cout of the tube into the match design as well plate block and plate choke parasitic. Ideal, they are minor.

I am running a much smaller set of tubes. Build schematic and the vna test shown attached. I would shoot for 20 dB return loss over the band of interest. It would be interesting to see how bad the stray L attached to the band switch throws that proper tube load Z off. Not sure the vna would pick that up in measurement.

Alan  


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« Reply #215 on: April 09, 2020, 04:28:44 PM »

Just noticed the flexible couplers between the gearboxes and the vacuum caps. Very nice.
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w9jsw
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« Reply #216 on: April 10, 2020, 02:49:38 PM »

I have one on that cap as I needed to transition from 1/2 diam to 1/4 diam. But it makes the rotation so smooth I am going to add one on the other side. These are only $8 at amazon. They make a real difference in releasing side tension.

Also, looks like K1JJ has found another nice upgrade to add to the RF deck. A diode limiter. Will mount it right at the plate choke base and place the indicator on the front panel.

John
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w9jsw
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« Reply #217 on: May 03, 2020, 10:46:20 AM »

Spark Gaps finalized.


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w9jsw
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« Reply #218 on: May 10, 2020, 06:35:48 PM »

Spent the day running experiments on the input pi network and the output tank.

On the output, used a MFJ-269 (a 259 with VHF/UHF) analyzer to look at the tank from the antenna back to a simulated plate load. Was able to get it to a 1.0/1.1 SWR for all bands except 160M. In talking with GFZ (Frank) he suggested that I move to a 1000pf plate coupling cap and a 1000pf padder. That did the trick and 160M came into view. I can get resonance on all bands thru 20M. I think I have too much plate capacitance to get 15M. My goal was 40 and a stretch to 20. Happy. May play a game or two to get 15M. Frank mentioned something to try.

On the input pi network, it would just not work. So I checked the roller and found that it was grounded. Apparently I hooked up the 300pf cap backwards. Swapped it and it came into resonance. Went all the way to 15M.

So, good to go for now. Awaiting some Ruskie caps that can handle the current, then will be ready for power up testing.

Now will mount and solidify the sequencer and start testing that subsystem.

My day job is in IT Healthcare, so been real busy there. Only get a few hours on the weekend to work on this. Thankfully it is a nice diversion to the real whirl.

John

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