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W1RKW
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« on: January 02, 2009, 06:15:53 PM »

Anyone want a homebrew 813x813 rig in the making?  I'm ready to haul it to the dump along with all the other stuff I've collected to build this THING. I doesn't want to cooperate and its pissing me off big time. OK. Back to it....
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Bob
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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.
WA1GFZ
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« Reply #1 on: January 02, 2009, 06:21:01 PM »

Bob,
Just think how good you will feel when you get it right. You can do it, hang in there.
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ka3zlr
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« Reply #2 on: January 02, 2009, 08:28:45 PM »

..Time For Beer....Then... More beer...

I hate it when something or someone... does Not wana Cooperate...
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Opcom
Patrick J. / KD5OEI
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WWW
« Reply #3 on: January 02, 2009, 09:11:19 PM »

hey now 813's got alot of gain and are high strung at high frequencies.. Not their fault, RCA made them like that. They helped with the war too, can't be all bad. Tell us the troubles..
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Radio Candelstein - Flagship Station of the NRK Radio Network.
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« Reply #4 on: January 02, 2009, 09:25:55 PM »

Patrick said:
Quote
hey now 813's got alot of gain and are high strung at high frequencies.. Not their fault, RCA made them like that. They helped with the war too, can't be all bad. Tell us the troubles..

I think in one of the old handbooks they show a 5/6 meter transmiter using a single 813 so there is some hope! Wink
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
W1RKW
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« Reply #5 on: January 03, 2009, 04:24:27 AM »

I'm having a problem with the final screen current and voltage not being stable at any level of modulation and RF power level. Then after a given period of time screen current drops to about 20mA when it should be much higher and stay there.
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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.
N2udf
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« Reply #6 on: January 03, 2009, 06:52:01 AM »

Hey,if it was easy NORMAL people would be able to do it!Best of luck...Lee
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KL7OF
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« Reply #7 on: January 03, 2009, 07:54:55 AM »

do you have a dropping resistor for the screens?   Or do they have their own supply?    What are you using for a grid input circuit?......
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WA1GFZ
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« Reply #8 on: January 03, 2009, 11:28:41 AM »

Bob,
Unstable voltage will cause screen current to drop. Trying to run the rig at reduced power will drive the plate Z very high. Running the rig at reduced voltage will allow you to check things out at the operating Z. 1/2 voltage and 1/2 current you can work at 1/4 power but at the right Z.
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WQ9E
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« Reply #9 on: January 03, 2009, 01:40:23 PM »

Bob,

Is your final tuning changing due to heating of components in the output tank (excessive RF through fixed capacitors in parallel with the tune and/or load variables are commonly at fault)?  Problems with the screen bypass cap?  How is the final grid current holding up when the screen current changes?

Rodger WQ9E
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Rodger WQ9E
W1RKW
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« Reply #10 on: January 03, 2009, 02:28:43 PM »

The rig is patterned off of K1JJ's design located here:
http://www.amwindow.org/tech/htm/813/813.htm
http://www.amwindow.org/tech/htm/813/813ps.htm

Frank,
Let me clarify the screen instability.  Screen voltage will drop when the transmitter is keyed but settles at the final value of 400vdc down from 500vdc. However, screen current continues to drop over a short period of time, say about 60 to 90 seconds of key down.  This occurs at full power, 1/2 power or less and at 0% modulation.  The whole ball of wax gets worse as modulation is applied.

If the screen power supply was being loaded down considerably I would think that significant ripple would be present and increase accordingly but I have very little or no ripple on the screen supply.

Grid current is stable and in spec about 30mA or so.

Rodger,
No fixed capacitors paralleled with the plate and loading caps.

Bald spot is getting bigger.
Thanks.
Bob
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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.
W3RSW
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Rick & "Roosevelt"


« Reply #11 on: January 03, 2009, 03:07:38 PM »

Some other thoughts:
Even if fed with a separate supply the screen needs stiff loading to ground at the tube socket, say 10 to 15k to ground. Screen current can reverse under certain conditions of loading and even the tube operating cycle.  This is local and may not even 'show' on the meter other than that of lesser current requirement. Your supply should be able to supply 35 ma screen per tube plus the loading resistor.  Already mentioned are the screen RF bypass capacitors. Screen bypass caps have given me trouble over the years in several rigs. My 32V2 had similar problems until the cap was replaced on the 4D32.  I think JJ"s 1kv ratings are not high enough.  My 813 rig uses 800pf at 3kv each screen.

I also 'assume' you've tried other 813's in the circuit. 

Also JJ's aware that his screen current meter in the diagram is shown backwards in polarity and some other oddities.  His screen supply shows 60k loading total at the filter caps. Might better be, say 5 to 7.5k across each cap.   JJ's grid meter is also shown backwards.  Positive meter terminal should go to the 75Volt VR reg.tube.

Have faith.  I've just looked over two pages of 'fixes, ommisions, rewiring, errors, betterments, etc.' required to get my 813 rig working properly.  Everything from the backwards meters and an open grid supply RF choke to one cathode wire missing
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RICK  *W3RSW*
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« Reply #12 on: January 03, 2009, 03:20:55 PM »

Can it achieve 100% modulation with no modulation voltage being applied the
the screen? Just curious.

Pete
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WA1GFZ
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« Reply #13 on: January 03, 2009, 03:35:46 PM »

Bob,
Yes you want a stiff load at the tube socket screen pin. 25% regulation is not going to cut it. This is going to have a big effect on the gain of the final. I think this is the first thing you want to address.
First, Why is the screen supply swinging so wildly? A cap input filter with a light bleader could be a problem. A choke input filter with the wrong output cap or light load is also bad. I wonder what the transformer is rated for?
Remember most scope probes are only rated for 300 volts.
Send me an email of the screen schematic if you can. Maybe I can help with some parts. fc
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K1JJ
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« Reply #14 on: January 03, 2009, 04:49:13 PM »

A few comments on the 813 rig....

Yes, the meters have been shown backwards since 2000 now. Wish I knew how to change it on the schematic, but someone else drew it up with software and posted it.

Yes, the screen supply should probably have heavier bleeder values, though, I am running a light bleed as shown in my own 813 X 813's rig and it works fine. I usually like lightly bled supplies for everything to increase efficiency, if I can get away with it. Remember that the screen supply is shown with a choke in the CT already, so regulation shud start off decent. Bob is using a choke there. Though, the tricks screen current can play is a good reason to perhaps bleed it harder.

One thing to try Bob, is to eliminate the screen supply possibility completely and use a big dropping power resistor from the plate to the screen. Look up some sample circuits in the handbooks. I'd say with 2kv,  you'd want to drop 1600 volts at 80 ma = 20K resistor.  Power disp = 128W.  Obviously this is not very efficient method, but is a good test.  Offhand, TWO 813 screens need about 400V at 80ma, if I remember right.

Screen bypass caps: I usually use those 500pf 5kv doorknob caps myself, but the 1KV screen cap rating should be increased when the schematic gets worked on again.  Agreed.

As Frank says, first try the stiffer screen supply bleeder to see if that helps and report back..

BTW, building up a big rig is not easy. There are many times when I've almost threw in the towel myself... ask Frank... :-)  I often wonder how so many guys without a lot of past experience pull it off when guys who have been at it for decades stumble. Everyone has projects that run into brick walls. It's just a matter of sticking with it. It's nature's cleverly disguised way to get you to learn more.


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.
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« Reply #15 on: January 03, 2009, 04:52:12 PM »

Frank,
My screen PS circuit is based on Tom's, K1JJ's screen supply (see link in my previous posting for circuit diagram).

I've horsed around with the capacitance values. I've gone as far as tripling them with only very slight improvement.

It's very possible that the power transformer is not up to the task.  I ordered a beefier xfmr from Fair Radio.  Also, my choke coil does not have a 500mA rating.  It's a 160mA rating so it's possible that the DC resistance is high enough to cause screen sag as well as the power transformer but I'm only guessing at this point.

Will try increasing bleed load as well.  Stand by.
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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.
K1JJ
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« Reply #16 on: January 03, 2009, 04:59:01 PM »


It's very possible that the power transformer is not up to the task.  I ordered a beefier xfmr from Fair Radio.  Also, my choke coil does not have a 500mA rating.  It's a 160mA rating so it's possible that the DC resistance is high enough to cause screen sag as well as the power transformer but I'm only guessing at this point.

Will try increasing bleed load as well.  Stand by.

Bob,

Put a VOM across the choke to see how much voltage is dropping across there.  Keep measuring every point in the circuit to see if something is dropping off as you draw more current. The sag has to be going somewhere.

Your 160ma rated choke shud be enuff for a stiff bleed and the 80ma screen current.  (The schematic should read, "200ma rating" for the screen choke, but no harm using a 500 ma unit.  The 500ma chokes are more needed in the HV supply and are labeled as such.


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.
WA1GFZ
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« Reply #17 on: January 03, 2009, 05:08:32 PM »

Check the DC resistance of the screen choke and screen transformer secondary. This will give you an idea of the voltage drop you would get if you had the right bleeder resistance. It sounds like you have a light bleeder to make up for low voltage. This will make the final stage gain swing wildly. Gain goes down on positive peaks and gain goes up on negative peaks.  
The 813 spec I have says 400 volts 40 ma on the screen is plenty so all you need to do is load the screen. A 160 ma source should work fine.
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WA1GFZ
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« Reply #18 on: January 03, 2009, 05:11:23 PM »

Hey Bob,
Think you are losing hair. Take a look at Tom's PDM rig. I guess I need to put my motivational speaker hat again.
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WB2YGF
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« Reply #19 on: January 03, 2009, 05:29:41 PM »

A few comments on the 813 rig....

Yes, the meters have been shown backwards since 2000 now. Wish I knew how to change it on the schematic, but someone else drew it up and posted it.

I downloaded the pic and swapped the polarity on the 100mA meter at the upper left.  The others look OK unless I am reading it wrong?  Any other corrections I could take a stab at?
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KL7OF
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« Reply #20 on: January 03, 2009, 06:39:50 PM »

30 ma. grid drive for 2 class c 813's???   too much in my estimation...Does your grid tune sharp? ie: very touchy variable cap adjustment? The grid circuit needs to tune very broadly and be close to resonance at the operating freq...if the tuning gets too sharp it will change with varying load and heat conditions ... I have built the Tesla 360 with a screen dropping resistor and it really likes about 15 ma. of grid drive with 400 v on the screens....remember that if you use a screen dropping resistor, the grid current and screen voltage will vary inversely ..... gud luk...Steve
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W3RSW
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Rick & "Roosevelt"


« Reply #21 on: January 03, 2009, 08:11:35 PM »

Quote
I downloaded the pic and swapped the polarity on the 100mA meter at the upper left.  The others look OK unless I am reading it wrong?  Any other corrections I could take a stab at?
Yes,  I mentioned this before.
Reverse the grid meter from that indicated.  Don't forget your measuring current, (conventional, opposite real electron flow) not voltage with that meter.
See any ARRL handbook for verification.
gud luk om  Grin
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RICK  *W3RSW*
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« Reply #22 on: January 03, 2009, 08:47:30 PM »

Quote
I downloaded the pic and swapped the polarity on the 100mA meter at the upper left.  The others look OK unless I am reading it wrong?  Any other corrections I could take a stab at?
Yes,  I mentioned this before.
Reverse the grid meter from that indicated.  Don't forget your measuring current, (conventional, opposite real electron flow) not voltage with that meter.
See any ARRL handbook for verification.
gud luk om  Grin
Sorry, I looked for comments as implied but missed yours.  I don't know tube circuits well enough (heck, at all) to recognize a grid meter by the schematic.  You mention the VR but it's on a different page.  Guess you mean the + should go to -75.  Hope I got it right.


* 813small.gif (50.71 KB, 600x452 - viewed 521 times.)
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W3RSW
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Rick & "Roosevelt"


« Reply #23 on: January 03, 2009, 08:54:22 PM »

Yes, all four meters now appear to have the correct polarity on the new drawing.
Great drawing, easy to read and follow. Cool
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RICK  *W3RSW*
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« Reply #24 on: January 03, 2009, 09:09:36 PM »

I downloaded the pic and swapped the polarity on the 100mA meter at the upper left.  The others look OK unless I am reading it wrong?  Any other corrections I could take a stab at?


Thanks, John!  That's nice of you to take the time to help us out.

Besides the two swapped current meter polarities suggested by Rick, here's a few more updates:

1) At the far right, where the plate current says "= ?"  -    put in, "Load final to 250ma - 500ma, depending on power output desired."
2) See the "300pf neut cap" adj ?   - add a 500pf  1KV fixed capacitor across it in parallel.
3) Increase the two ".001 1KV" screen caps voltage rating to .001  2KV.
4) On the power supply schematic, reduce the screen choke rating from 500ma to 200ma.
5) On the power supply schematic, reduce the 30K bleeders in the screen supply from 30K to 7.5K  (Rick's suggestion)  That's 11 watts dissipation total using two 20 watt resistors. A good safety factor.
6) Transmitter schematic: See the "B+ 2KV" label?  At that point add  a 25 ohm, 20 watt resistor in series with this B+ line so that one end of the resistor connects to the B+ and the other end gets the, "B+ 2KV" label.
7) Add a 5K, 20 watt non-inductive resistor in series with a .001 1KV disc capacitor from one tube grid pin to ground. (add note: usually not required unless final becomes unstable, especially on 20M and above)


Anyone see other errors or changes to be made?

http://www.amwindow.org/tech/htm/813/813.htm
http://www.amwindow.org/tech/htm/813/813ps.htm


Thanks.

Tom, K1JJ
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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.
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