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Author Topic: WA1GFZ audio driver question  (Read 4462 times)
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KK7UV
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« on: September 07, 2020, 07:38:03 AM »

Still plugging away on my transmitter build and playing around with some layout and lash-up options for the audio.

Regarding the use of the WA1GFZ driver, I'd like to build this as an external unit with it's power supply, and it appears I need three leads to the modulator - two grids and one negative feedback line.   Can this be done with standard audio cable and XLR hardware?  do I need a fourth pin for a shield?

Or is it far better to mount the driver board inside the modulator chassis and have short grid leads?
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WBear2GCR
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« Reply #1 on: September 07, 2020, 10:27:43 AM »

You need ground too.

The issue with any long cable, even inside a chassis driving grids, is the potential for
parasitic oscillations. If you don't have any, then no problem, of course.

I'd probably want to keep the negative feedback shielded from the drive.

I'd put some low value resistors in series at the grid leads as "grid stoppers".

Keep in mind that the grid is typically very high Z, your drive is low Z.
When the grid is driven positive, it switches to a lower Z...

They make 4 pin "XLR" style plugs and jacks...

                   _-_-bear
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w9jsw
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« Reply #2 on: September 07, 2020, 11:20:07 AM »

I have mine up and running. Just checking it out.

Here is how I did it -

- Shielded mic cable from V01/V02/GND on board to each grid. Separate cable for each with a shield each.

- 22 ohm 1/2 watt in series. Value is not critical. 22 is what I had on hand. 33/47/etc work also.

- 300pf from grid to gnd. I had some mica 330pf ones.

- Added grounds on 5 of the 6 bolts. See pic.

- Added a mute relay on the input. I am planning to keep the board up and just mute the input on RX.

- Added 10uF 450V caps in the C5/C6 locations. Maybe overkill depending on how far your power supply is. I am running +/- 285V and the leads are around 1ft away.

I am doing the testing without NFB from V02 plate, per gfz suggestion. I am using a wire from V02 on the board to the nfb with a 50k in series. That will work for the checkout. Later I will build a ladder of around a meg of resistors to get the NFB from the tube.

I am running +8V of DC on each tube. Get around 20ma of idle current on the 813s. YMMV based on tube.

John

PS - K1JJ had some hum issues. He moved the bias pots from off board to just above the board. Also, pay close attention to the pot wiring. The board is laid out in a confusing manner (my bad). Wipers to P16/P18.


* IMG_0543.JPG (164.04 KB, 946x710 - viewed 288 times.)
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KK7UV
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« Reply #3 on: September 07, 2020, 03:06:22 PM »

Great info - - thanks John and Bear.

I think I may opt for mounting the driver board inside the modul. chassis.  Seems like simpler cabling with only 3 external PS leads and a front panel XLR input for line level audio input.

Your 22 ohm stopper resistors are right at the 813 grids?  Likewise with the 330pF bypass caps?

You are muting your input how?  Does your relay simply switch your line input audio to a load resistor during RX?  Is this just to avoid having any audio potentially driving the amp and appearing at the modul during RX?   This seems like a better solution than having my muting relay cutting off the 120v supply for the driver board.

Steve
 
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w9jsw
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« Reply #4 on: September 07, 2020, 05:11:15 PM »

Simple spdt relay. I short the input to the board and leave the audio input open. Board is cap input so no problem with shorting it.

Resistors and Caps right at grids.
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KK7UV
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« Reply #5 on: September 11, 2020, 07:55:41 PM »

I found your schematic John, which generated couple more questions...

 - What bias pot values did you arrive at?    With 8v at the grids, seems like 10 or 20k would would fine here?

 - C12 was 0.47uF in the original GFZ print, but yours is 39pF.   Was this change specific to your own NFB setup?

Thanks,
Steve
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K1JJ
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« Reply #6 on: September 11, 2020, 08:09:09 PM »

Hi Steve,

You seem to be asking all the right questions about the SS board -  sounds like you're about to put the new homebrew rig into a dummy load for testing.... Wink

What did you end up with for the input RF matching circuit you were working on a few weeks back?

Do you have any pics and progress of this 813s X 813s rig?  With the GFZ SS driver it oughta perform quite good.

IIRC, the 0.47 was used to stabilize a  NFB problem. But once I fixed stuff, 39 pF is all that was needed for bypass. That's what I have in there now -  in the NFB input circuit to the board. You will need to experiment and find what value keeps everything calm with whatever NFB level you choose to crank in. I run about -3 to -4 dB NFB these days on the 813 rig.   The 4X1 rig runs about -10 dB.  That's just what worked best for me.

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.  Easily done in DSP.

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« Reply #7 on: September 12, 2020, 07:29:43 AM »

I am using 100K pots. If I were to do it again I might consider multi-turn. The setting of the bias is a bit touchy with a single turn pot.

On edit - Frank says 10K to 20K will be fine for 813's.
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w9jsw
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« Reply #8 on: September 12, 2020, 07:03:10 PM »

I have now blown both fuses. I used 100ma. Not enough for 813's. Tom and Frank just wired direct with no fuses. I will probably drop 200ma ones in since I already have it installed with clips. Too much of a pain to remove it at this point.
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w8khk
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« Reply #9 on: September 12, 2020, 07:08:06 PM »

If you run out of those little fuses, just take a chunk of 1/4 inch brass rod stock, cut it to length, and snap it into the fuse clips.  Let that grid current flow freely.  Pennies will not fit in that type of fuse holder!
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Rick / W8KHK  ex WB2HKX, WB4GNR
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K1JJ
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« Reply #10 on: September 12, 2020, 08:12:07 PM »

BTW, anyone using the GFZ MOSFET audio board should be sure to use only non-inductive resistors for the NFB ladder.  This is the string of 300K - 1 Meg resistors that tap off one cap of the 813 modulators and feeds back a sample to the board input.

For my own 813 rig I had used spiral wire-wound 100 watters and even though they're no problem at lower audio freqs, at ultra sonic freqs above 20KHz and higher the parasitics were dancing all over the scope. I could see them as tiny audio parasitics dancing on my waveform peaks. They had the classic triangular burst look you see in textbooks. After finally figgering it out and switching to a string of metal film resistors (and testing them to make sure they were non-inductive) the rig ran FB and was stable.  

It was one of the more difficult troubleshooting problems I've ever had. I was too stubborn to change out the WW resistors thinking they were OK for the job.  It was a good thing in hindsight because it made me go thru many changes related to grounding, shorter leads, bypassing, groundplanes, shielding, etc. before replacing the ladder. The product was better in the end.

I wonder if that 0.47 bandaid cap found its way in there because of this kind of thing back in 2012 when I was optimizing the 4X1 rig and the GFZ board.

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.  Easily done in DSP.

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KK7UV
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« Reply #11 on: September 13, 2020, 10:19:11 AM »

Tom - - I don't think it was me that asked about RF input tuning recently.  Maybe it was longer ago?   Anyhow, I have not done much with that yet.

Thanks for the added info on the GFZ setup.   As an aside, in your original design you were using the backward audio transformer idea.  I was   wondering what the high-Z side of that transformer should be to feed 813 grids?  I may do this just to get the modulator working and then work on the GFZ setup.

My 813 rig doesn't look like much just yet (see pic).  I have the HV power supply installed and the primary side tested (SS contactors, step-start, etc) and will test the HV secondary soon - just waiting on a couple small items.  I expect between 2000 and 2200v.  I've got an outboard variac if needed.

I got the Thordarson mod tranny and Gates reactor from Dennis W7TFO last year on a spring road trip to AZ  (Thanks again, Dennis!)

The next panel above the PS panel is the PA/Modul. which I am working on now.  It will have the four 813s lined up and visible at the top through a 3u window.   Above that will be the output tuning/bandswitch panel, and then a meter panel in the top space.

Super fun project but wish I had more time to work on it.

Steve





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K1JJ
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« Reply #12 on: September 13, 2020, 11:16:15 AM »

So let's say I had three different input networks, one each for 160, 75, and 40m, mounted on a board with relays.

Rather than a broadbanded toroid transformer with taps, could each network have a toroid transformer with optimized windings and have it be narrow-tuned just like the air-wound coil input network?


 Re: Grid input network for K1JJ 813 rig   on: February 24, 2019, 01:00:30 PM
Has anyone tried this input network with a wound toroid?  Which mix?  Ferrite or iron powder?   I'm stunned by the price of B&W coil stock.
 
Thanks for those responses - very helpful.
What is the grid input Z of the 813 pair?   I may play around with some matching network software.

Grid input network for K1JJ 813 rig   on: February 18, 2019, 04:37:08 PM
I trimmed a piece of B&W miniductor to 40 turns as specified in the schematic.
It measures 12.5uH.  With the specified 150pF variable, it should resonate down to 3675.

If I wanted to take it as far down as 1870 khz, is it as simple as switching in another 430pF?  or should the inductance be increased to maintain the same Q?    How about operation up to the top of40m?

Should the grid coil be tapped and switched for each band (160 - 80 - 40) just as the output tank is?



Hi Steve,

I was referring to some old questions you had (posted above) about the input tuning some time back.  You must have solved it by now...

The backwards audio transformer taps used can be what is usually found; 8 ohms to  2K, 3K, 4K, 5K, etc.  Not critical since it will be a temporary thing and the GFZ board will fix any problems later. Just getting it going is the goal, as you said.  To optimize the taps, just run some tones thru the working rig into a dummyload and see which config gives you the highest and cleanest audio peaks on the scope. The minimum audio power producing the best audio is the optimum setting.  

That is a nice looking 813 build!  Solid construction. The gray looks FB. I should try gray on my next series mod rig.  

Yes, Dennis W7TFO has helped me out several times for parts. In fact he is sending me some meters for the next project I'm working on. Very generous guy.

Keep us informed.  Another 813 rig builder, John / W9JSW has made some big progress on his 813 rig over the weekend. He now has it modulating 100% with tones at near full power. That is a big step indeed.  He may ressurrect his thread and show some pics of the rig and waveforms so others can learn.  The shakedown /  testing part is the hardest and requires a deeper understanding of the rig and test procedures.

* BTW, is that a multi-match 250 watt Thordarson mod xfmr?  Be very careful in testing with tones cuz a pair of 813s with 2200 V can really put out the soup. I use a 250 watt broadcast RCA (with mod reactor) and worry sometimes. IE, don't make any serious mistakes at full power with it.  Hopefully that spark gap will keep it safe.

One time I was running 100%++ audio   tones thru my previous 813 rig - and the primary mod xfmr wire melted at the terminal strip connection. Luckily I just pulled the bell/cover off the xfmr and fixed it, but shows it was near its limits rated at 250 watts. I wud much rather have a 500W or 1KW  BC mod xfmr in there, but hard to find.

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.  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.
KK7UV
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« Reply #13 on: September 13, 2020, 05:44:19 PM »

Yep - it's the T11M77.   Rated 300w I thought?   I plan to proceed carefully in all aspects of the modulator setup, and will surely be hitting up the group with questions.   I suppose I can dial back the juice to whatever keeps the "weakest" iron safe.
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w9jsw
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« Reply #14 on: September 15, 2020, 06:27:25 AM »

In my opinion, I think you should wire in your variac permanently. I am amazed at the scrote of 2 813's in modulator duty. I have a Gates BC1T mod transformer and it is super easy to drive it hard. I am running at 1500V testing and easily get 400W carrier and 100% modulation with tones. I think you will find that it will be difficult to keep the modulator throttled back to 300 watts at 2000V or above. I think the sweet spot for my transmitter will be around 1800V.

I have more testing to do so I will take some notes on the mod current at 100% modulation at various audio frequencies to give you an idea of what your modulator can do.  I am down now installing my relay sequencer and buttoning it up so the fans can do their work. Hope to be back testing later this weekend. Also, start looking for a BIG dummy load if you don't have one. I started making much better progress when I acquired one (MFJ oil can). This transmitter could fry fish if I swapped to peanut oil....just kidding - I don't let it get that hot.

I also think you should go directly to the mosfet board. Mine came up with zero errors and runs perfectly. If you get a solid, quiet +/- 280V supply, and an adequate heat sink you will be FB. It just works!

Here is the latest copy of my schematic so you can see how I installed the variac on one leg of the 240V mains.

Any questions send me an email or pm (or post here)

John

* 813 Transmitter 08292020.pdf (531.18 KB - downloaded 159 times.)
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