The AM Forum

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....

Bob,
Just think how good you will feel when you get it right. You can do it, hang in there.

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

I hate it when something or someone... does Not wana Cooperate...

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..

Patrick said:
I think in one of the old handbooks they show a 5/6 meter transmiter using a single 813 so there is some hope! ;)

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.

Hey,if it was easy NORMAL people would be able to do it!Best of luck...Lee

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?......

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.

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

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

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

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

Pete

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

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
 

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.

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

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.

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.

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?

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

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  ;D

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.

Yes, all four meters now appear to have the correct polarity on the new drawing.
Great drawing, easy to read and follow. 8)

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

Steve said:
Funny, I've a pair running at 32 ma. and have been for sometime. I think they should see roughly 16ma a piece. Now I am using a brute force capacitance coupling from the Ranger but it seems to work FBOM.

From my own tests, I'd agree to run them at spec or harder.  I run 20-22ma of grid current with a SINGLE 813. 

This is the test to make:  Load up your rig to it's sweet spot at max power and best efficiency using recommended grid current. (16ma/tube). Put an audio tone through and carefully study the modulated sine wave on the RF scope.  Increase the audio tone until you see flat topping or some other kind of limitation.  Be sure to have a negative peak limiter in place to prevent mod xfmr damage.

Now, while at this max audio point, slowly increase your RF drive to the final so that the grid current rises and watch the audio peaks as you increase audio further. You may find that the audio peaks get higher and cleaner when driving the final grid a little harder than specs.  At least my 813's showed this. I also run a little more screen current to eek out the highest audio peaks I can. I then load the final more heavily (LESS C2 giving less power output, less efficient, more plate current) and everything really comes to life.

I've found the same thing with 4X1's and especially using triodes as finals.

My 813's are original tubes and show no signs of pissbeat wear after years of driving them hard.  813's are amongst the most strappingly built tubes on the planet.

T

Tom said:
I'll drink to that Mr. Vu. The only ones I've ever seen go bad were those that 'fell' victim to gravity.  :'(

I would feel good with some resistance across the input tank to put a stable load on the input. This will also lower the input Q a bit. I would think making it more stable.

Yep, that's a good idea, Frank.   Though, since I've been going to the trouble to neutralize my finals, I've never needed a swamping resistor across the grid. But w/o neut, yes, it is always required.   But on the bands above 20M, a swamp will help. You think a non-inductive 3K at 20W in series with a .001 to ground would do it?


Rick: Yes, love those 813's.  BTW, I'm glad you pointed out the grid meter polarity thing. For all these years it never dawned on me that the polarity  of the grid current meter was the same as the screen. I usually wire all my meters without bothering with polarity and then simply swap the leads once I fire them up to get them correct... :-) It's a 50-50 thang.

I guess the rule could be to start at the cathode (or gnd) and simply call that (-) and follow the path to each element and call that end the (+). The meter follows the same polarity direction. This works also for cathode current meters using gnd as a reference.  Another point is that the grid is drawing positive current in class C, just like the screen.

It's funny to me cuz for years, whenever I bothered to wire a rig up paying attention to the meter polarities, the grid meter was always wrong - I just figured I wired it backwards and never went back to check or think about it... duh  :-)

Tnx for the info, OM's.

Later -

T

Sounds good to me. Someday I want to build one of these 813 rigs.

Rig is on hold for a while until I get a few more parts to tinker with.
I'll do some measurements in the meantime.

Thanks for the help and suggestions everyone.

Tom JJ, et. al. - just got around to reading this again.

Tom per your request for last minute changes, I think a 25 to 50 ohm, say 20 watt resistor in the B+ line before the mod reactor would be in order. Since this is a bare bones, 1950's style rig with no auto cut-outs etc. this resistor helps to protect the tubes, power supply (allowing time for a breaker or fuse to blow) etc. in case of a plate short, etc.  The 20 watt rating sure won't sustain a short very long but it is beefy enough to keep up the good fight until a breaker trips.

Regarding the grid loading with a resistor my single 813 shoebox had swamping but took the same amount of power 8- 10 watts to drive it as my twin 813 rig w/o a swamping resistor.  Yes, the twin 813 is neutralized and I find no instability.  But a light swamp shouldn't hurt anything,  I'd go for more like 5k to 8k rather than 8k if people feel they must have swamping as long as they use neutralization.  With 3k one probably wouldn't need the complexity of a neutralizing network on 160, 80 and 40 anyway.

BTW, my twin 813 uses the classic, low capacity, HV neut. capacitor of a few pf from plate to bottom of grid circuit with a fixed 800 pf from there to ground.  I assume you used the higher capacity variable grounding cap because HV neut. caps are hard to find.

Your circuit's beauty lies in the cathode circuit relay switching, receiver noise muting, etc. borrowed from later 'linear' technology I'd guess.  And yes men, you have to test your power supply transformers for approximate voltages under load. None of our transformers out here match perfection, even JJ"s were probably resistored up, etc. to get him what he wanted.

For example my VR-75 dropping resistor turned out to be 1.5k instead of JJ's 1.0k.  Don't be afraid to deviate. Use what you have but measure, measure, measure, particularly the screen voltage under load.  350 to 400's fine.

Too many 'etc.s' but you understand  ;D

OK Rick -

Thanks for the suggestions.

I like adding the power resistor in the B+ as an additional safety factor - will do.  I actually use them in my own rigs too.

The 5K RF grid swamp is a reasonable value.  I've noticed that about 2K-3K works well with un-neutralized rigs, whereas, the higher value is better when neutralized to minimize drive requirements.



John, I've updated the list of schematic changes for you... The rig worked FB as is, but this should bring it into the 21st century... :-)

Thanks again.

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 wire wound resistor in series with this B+ line so that one end of the resistor connects to the old B+ point 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 CONTROL grid pin to ground. These are additional components. (add note: "Swamping resistor usually not required unless final becomes unstable, especially on 20M and above.")

T

I've had some time today to sit and go over this thread and previous emails from those who have been helping me with this project. I just want to say your insight and experience is not going unnoticed despite me not saying so or responding to those who have offered their knowledge and suggestions.   Thanks everyone for the advice and encouragement.

I'm still plugging along, not progressing as fast as I like but still progressing nonetheless. Slow is a good thing anyway.  I know this isn't kids stuff and can be lethal if a mistake is made and that 's my problem, sometimes I lose sight of that fact but I know I have to walk away from it and do to avoid short cutting and having an accident. Many years ago I got popped and suffered a pretty severe burn on my hand from not paying attention. Suffice it to say I got lucky.

Additionally, I've taken a little time today also to look at the rig and rehash some of the assembly. 

Here's some info based on the thread discussion and questions posed.

Screen power supply choke, DC resistance: 102 ohms   Not sure if this is low enough.  Will determine voltage drop across it and provide info on next update.

Screen secondary DC resistance: ~70 ohms

Screen bypass caps: .001@6kV  I think I'm OK there Rick.

Installed a fixed padding cap at grid input: 500pF recommended. I found double that value was necessary when I was having difficulty with the grid tuning circuit last year.  The question on grid peaking.  The peak is sharp.  Will add recommended resistor input tuning tank to broaden peak.

Don't have the resistors needed for the screen bleeders yet so rig is on hold for now.
bw

Tom,
In step 7 you say to connect a 5K in series with one of the .001uF caps.  Those caps are the screen grid caps.  Is this were you want the swamping resistor? Or on the control grid?

Hi Bob,

The 5K in series with a .001 cap is a NEW set of components. They go from one control grid tube pin to ground. (The tubes have their grids in parallel with short leads, so one set is enuff. I will add "control" grid to step 7.


Almost anything series/paralleled can be clip leaded into the screen supply to provide heavier bleeding. No need to stop on that account for exact parts.

BTW, consider trying the screen resistor from plate I mentioned above. It will completely eliminate the screen supply and mod choke to tell you quickly if that really is the problem.

T

OK.  Understood on the 5K and .001's. 

Unfortunately, my resistor and clip lead supply is limited in QTY and size.  8)

OK Tom

Check 'em out

John

John,
You have the swamping on the wrong grid. it should be on the control grid.

OK John -

The power supply looks fine.

The transmitter looks good except for the 5K and .001.   Those go from the CONTROL grid to ground, instead of the screen to gnd. The control grid is the first element above the filament as shown on the schematic. You can probably connect the parts to the control grid connection going between the two tubes, and then to gnd.

If anyone else sees something to correct, speak now or forever hold your log.


Other than that, looks good, John - TNX, OM!

T

How many Turns is the tank Coil and Diameter, data could be placed right under 1/4" copper...or above and spacing...

Hi Jack,

There's so many ways to make or buy a coil that I left that up to the builder. Some want to optimize for one or two bands, while others use plug in coils or some want 160-10M in a compact package. I recommend someone check out the handbooks for suggested tank designs.

However, I'll tell ya what I do here.  I usually wind a 4" diameter coil  about  9" long. I make sure it has enuff inductance to cover 160M.  Dimensions are not really critical at all. Then I estimate where the taps shud be for each band and connect them to the band switch. Once the rig is running, I fine tune the taps by trying all the different parameters looking for maximum power out, maximum effciency and best audio peaks. Tap, tap and tap. There usually is a particular tap for each band  where you find the sweet spot for any set of tube voltages.

I find this to be the best way to get maximum performance in the real world. So, as you can see, what coil I have to work with I can usually optimize the parameters.  However, try to make the coil length between 1 to 2 times its diameter for best Q.  This ratio will vary for each band, of course.  For the higher bands, (10-20M) making a separate coil will bring that L-D ratio back in and also permit you to use a larger diameter coil stock.

Then again, many simply use the handbook charts and calculate the coil sizes before building. That works FB, of course. You must know your expected plate impedance.   It doesn't allow for real world problems that crop up, like stray capacitance or other random things, but is close enuff.

T

Sorry, I figured out which was the control grid (the one with the signal and not the screen voltage). Then as I got focused on the trees, I forgot about the forest.

I was going to do this in MSPaint like I used to fix the meter polarity.  Turns out, they disabled the ability to add text in the Vista version of MSPaint. Bast***ds.

I found another program loaded on the machine called "Microsoft Digital Image Starter Edition 2006 Editor". Kind of Photoshop-like, but it seems faster and easier.  Still, it was a learning curve figuring out how the tools work and the idiosyncrasies of the program.  NEXT time, it will take me a LOT less time. :)

One thing I hate about Photoshop is its native format (to preserve the layers) makes the file like 100 times bigger than the original.  MS Editor uses something called PNG Plus.  Files are bigger, but not by a ridiculous amount. (850K vs 250K, for example)

If I wanted to do this thing RIGHT, I would re-do the whole thing in PCAD (schematic capture).  Only thing is, I don't have any tube symbols in my library and would have to create them.  Project for another time perhaps.

John

Hi John,

Yes, it looks good now.

Interesting how you were able to get into that schematic file using generic software. 

Well, let's sleep on these changes for a few days to make sure nothing else pops up before sending it to Gary to repost.

Thanks again for the efforts, John!  I'll bet there will be a few more of these rigs built as a result of the dialog here. It's a strapper. That pair of 813's will run 1KW out carrier at 120% using 3KV. I've done it - into a dummy load, of course... :-)


T

Once all the changes are made, I'll redraw the schematic (correctlY) and put it with the original article on The AM Window Web Site.

Thanks for all the input and corrections.

Hi Steve ... are you thinking a single 813 at 30ma? I run my single 813 with 22ma grid drive, necessary with the -180v fixed bias. The screen supply is from a dropping resistor off modulated HV, so the grid drive of 22ma is also necessary to keep the screen at 400v and 40ma, when loaded up at 400 watts input and 300 output.

OK, Steve -

Oh yeah, that's right - this project is on the original AM Window Website. I had forgotten.. :-)

I'd say the schematics are all set to go now.

Why do you want to redraw them?  They look OK to me in their present form below.... 

T

http://amfone.net/Amforum/index.php?action=dlattach;topic=18117.0;attach=9575
http://amfone.net/Amforum/index.php?action=dlattach;topic=18117.0;attach=9571

With all the editing and saving, they are getting a little fuzzy. I'm picky. I like nice clean schematic, if I can help it. And I have so much free time! I will put up the current versions until I get the new ones done. Many thanks John.

Glad to help.

The original image was only 96 dpi.  It's amazing its not even more fuzzy.  I had to create new text, which when printed, stands out as less fuzzy.  With the components, I was able to create object blocks by copying the existing fuzzy parts so they blended in better,  Unfortunately, I had to double the size of the pic to add the text, because the original scaled down text is about 2 or 3 points and the program could not make text that small, then I reduced the image back to the original size.  I was actually expecting more distortion than I ended up with.  It would be nicer redrawn, but of course, it's more important to convey the correct information than be pretty.

What do you use to draw schematics Steve?

Unless you made some changes, the standard display DPI for Windows XP is 96 DPI. So a higher DPI will not look any better on your screen. I agree the DPI needs to be higher for the drawing though, but for printing purposes. Here, printers can print with DPIs in the 1000 plus range, if needed. For simple line drawings like these schematics, 200-300 DPI is sufficient.

I've used several different programs to make the schematics. I'll might try EasyDraw for these schematics.

Got a new transformer for the 813 screen supply.  It arrived damaged though repairable and useable from Fair Radio.  This is the first time I received a part from Fair Radio with poor packing.  Anyone else have that experience?

once 20 years ago. a 100uF 4KV cap arrived with a bent insulator. They replaced it after I returned it.

The Magnet-O-Sphere is pretty nice!!!!

I guess Steve has to re-draw because of that "other operating system" he uses, called a MAC..................hi :>)

fred

Thank you.. Tried to keep it tidy and avoid platform specific entanglements.

yeah those macs.. The user has the right to choose which o/s they wish to be annoyed by.

since fair radio was mentioned, there's a whole bunch of fall 2007 tourist pictures from it here:
http://www.bunkerofdoom.com/travel/steelsoldiers2/20071002a/index.html

It's sad so many people equate operating system with annoyance. I find it rare amongst Mac users.

Off topic, but I got my first Mac (G3) last spring to play with.  Nothing I could download would run on 10.1.  I found a copy of Office that would run.  Then I upgraded to 10.4 and Office would not run.  I had to load a newer (much larger) version of Office that ran slower.

My impression was that OSX is very fussy about versions.  I have DOS programs and Windows 3.1 programs that still run on Vista.

Never had that problem. Then again, I don't use any Office stuff. It's MS junk and very likely the source of your problems.

Hey guys,
Finally had some time to tinker with the 813 rig today.  Stiffened up the bleed resistors on the screen supply, big improvement and made a small mod to the RF deck.   Envelope is far more stable under modulation now but still I think it can go a little further. Plus, I'm seeing positive peaking!  Frequency response has improved for whatever reason.  It's not the best just yet but the clipping or squashing of the waveform has been significantly reduced.  Back to the grindstone.

OK,
Need to know how to tune this thing.  Should it be tuned up for max output, about 700w carrier and back off the drive to about 400x If I do that I seem to have the best results. Is this proper? Or should I tune it up at the intended output level, 375w carrier to be legal?  I don't seem to get the best result by tuning it up for the intended power level.  The transmitter doesn't like it from the arcing sounds I'm getting.

I'm assuming it's a Class C amp. Hit it with the proper grid drive and load up to the desired output or plate current. That's it.

Bob,
Sounds like your tank Q is too high for reduced power level. When you reduce power the plate z goes up if the plate voltage is about the same. The tank inductor is fixed so as you decrease power the Q goes up. Sounds like you want a bit more L at reduced power.  I'm not sure if you have a variac on the plate supply but you could tune up for 700 watts and reduce the plate voltage to get you down to the power you want to run.
I see the Sunoco opened up again.

Hey guys,
Finally had some time to tinker with the 813 rig today.  Stiffened up the bleed resistors on the screen supply, big improvement and made a small mod to the RF deck.   Envelope is far more stable under modulation now but still I think it can go a little further. Plus, I'm seeing positive peaking!  Frequency response has improved for whatever reason.  It's not the best just yet but the clipping or squashing of the waveform has been significantly reduced.  Back to the grindstone.
Glad the screen loading worked.

& Bob, don't forget to try opposite polarity from the modulator too.  Just switch the mod. plate cap wires if you don't have a phasing switch anywhere else.  I see mushy peaks go to emongus ranges on the scope with this simple fix. 

Frank, Rick, Steve,
Got a variac for the plate supply.  I can lower the plate voltage. May revisit the tank coil design.  OK on the swapping modulator plate leads.  Will mess around with the settings some more to understand how it all works and works best.  Getting closer.

Thanks.

BW

Bob,
Nice 3 way gas thing going on in Colchester. Just drove through. I only saw 2 other stations between here and the coast with gas that cheap. I'll stop buy sometime when I don't have the family in toe. I'll be going that way more often in about 4 weeks. You might also check your efficiency at high and low power to find the best plate voltage for the tank circuit. This will give you a clue to where you need to go but it sounds like you need a bit more L at lower power/higher Q

Hi Frank,
Actually the less expensive gas is in Portland.  It's about a nickel less but seeing the old Sunoco station towing the line with the other 2 stations is surprising.  It's been that way since they reopened.  I think the new owner only sells gas.  There's no auto service. I go to the Phillips joint.  I like the people that run it.

I've experimented with various settings and I'm finding that if I tune the rig up to what essentially Tom/JJ has stated (Is and Ig) in his Maul schematic I can get decent results on the scope and no intermittent arcs with good audio response.  I spent most of the day today securing things and doing some final touches and minor mods to each chassis.  Overall, it's getting close to it's maiden voyage on the air.

I need to work on the audio chain and some more permanent wiring including how this thing will be powered. Got to work around the grounding issue.  Right now the whole thing is a total JS and it makes me nervous.  With my memory being as short as it is these days, trying to remember what I did previously can be a potentially bad situation especially when dealing with HV.

Yes, stop by the next time you come through town.  Just give me a heads-up so I can be here. 

Updated the 813 page with your skizmatics. TNX.

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

Thanks Steverino,
'you are a gentleman and a scholar.'
uh, urk....
Frank made me say it. ;D

Tnx, Steve!

Makes me wanna build a few more -  a quick-on-the-air  RF deck for each band, using a common modulator and powers supplies. 

But the question everyone is thinking but afraid to ask:  "Does it work on 11M?"   (caw mawn)

T

Already done,
1963 handbook, pp 202-205
One Band Kilowatt Amplifiers

complete with switchable circuitry for either linear or class C operation.
and all the coil winding, capacitor data for 80 through 10 meters.

The 19" rack panel had the meters in the top panel, amp decks in the middle, and screen, bias, switching stuff in bottom panel.

"In the class C position of S2, +400 volts is applied to the screens and -150 connected to the grids. In class AB1 the screen is increased to 700 and the grid bias is dropped to a value detemined by the setting of R2.  This later setting should be one that gives best linearity without exceeding a no signal plate input of 150 watts for the two 813's; it depends on the plate voltage available."
 -Cool single band rigs. 

The 813's were horizontal assembled on a 13 x 17 bottom plate with two 5 x 13 x 3 inch chassis used as the side enclosures with grid L/C's on one side, plate L/C's on the other.

Hi Rick,

Yes, I remember those single band 813 amps very well from my first days as a Novice.  The 1964 Handbook was my first one, so I drooled over that one. But it seemed so complex and far away from my first days as a "radio moron" (as the Huz calls it) I just looked and looked.

It really is a feasible idea and if I were to start over, I wud definately do it. There's not much to the RF deck of a pair of 813's. The infrastructure of modulation and powers supplies take up all the effort - but has to be built just once.  Band to band would be a breeze.


BTW, I DID  try to build that 811A linear in the same 1964 hamdbook.  I started with my red chemistry set metal "suitcase" as a chassis. I didn't know anything about drills, so used a hammer and nail to try to punch a hole for the tube socket. (Might as well get started until I get the parts.) The chassis collapsed and that was that. Glad I didn't have the money to buy the parts -  I would have surely electrocuted myself.... :-)

T

boy that brings back memories.
My first chassis was a cardboard box where I rebuilt the power supply portion of the ocean  hopper. Missing the 12at6 and 50c5 caused excessive filament voltage.  good thing the dropping resistor was still in the string.  Yeah, later on I used a cake pan to build a 50c5 oscillator, using guess what the ocean hopper coil, tickler used for feedback and the very same 35w4 rectifier and hot connection to the line.

I did get quite a few shots of 115 in those days.

I'll never forget the day when all of a sudden schematics made sense.  Very similar to when I learned to read in the first grade.   Getting up in front of class to sound out the words, all of a sudden I realized I could read.  Spooky. 

I wonder what ever happened to my red box; it was Gilbert's erector set #5 or 7, I think. Anyway it had a picture of a motor driven ferris wheel on the cover but I never built anything that elaborate.  I think it was good for a small crane.  I can still see those wheel halves, brass axle stops and millions of screws and square nuts in my mind's eye.

Sounds like you actually made some good progress with the cardboard box circuit... :-)

My first RX was a single tube regen. It used a 67 1/2 V battery for the plate power. I never got it to work at all.  I didn't have the nerve to stick my tongue on the battery to test it. When I finally did, after a month of testing, it turned out to be dead after all!


Yep, I had the same Erector set with the Ferris wheel on the front.  My best project was to build a little model race car using the 1.5 volt motor. A friend in class saw it and built one too. The teacher let us race them one day in front of the class. His beat mine.

So I went home and discovered that I could put THREE 1.5 volt batteries in series and make that motor go like a bitch!  I came in the next day and asked for a rematch. My model took off so fast it flipped upside down. I restarted it and still beat him.... HA!

I see those Erector sets on eBay from time to time.

T

Tom,
My first linear was also that 811A 200 watter. Then I added a second tube.
A pair of TV plate transformers in series secondary. (I just found the bleeder resistor from that rig a few nights ago.) 400 watts from my bedroom with a 250 foot end fed. L network in the closet with wire coming through a small hole to the vent above my closet. I got some real RF burns off that set up.
I drove the family crazy so soon moved to the basement and put up a rhombic after investing $176 at Hatry for a KW match box. Ward and Corkey were proud of me for getting the KW Matchbox. 4-1000A came about a year later.
Once and a while I ran SSB on 20 Meters and got some real lip burns.

Pretty fun stuff, Frank.

Yes, that 200 watt 811A linear probably enticed a whole generation of baby boomer Novices.

We probably unknowingly crossed paths many times in Hatrys back in the 60's.

In the early days I built both the swr bridge and the tube keyer from the 1964 handbook. I even tried building that 2M transistorized transceiver - remember that one? Nothing ever worked right. All failures. I had really poor luck getting ANYTHING homebrew to work back then.   I was really a radio moron.... :-)  So I stuck to trading in over and over used rigs at Hatrys for the first 2 years or so.  I went thru quite a few boat anchors looking for the Holy Grail. No one could afford Collins gear - we all knew that was the best at the time.

Corky used to gloat about owning the S line. He would sometimes axe the customers, "when ya gonna get rid of all that shit and get some Collins?"  One day he celebrated and said, "Today I made my last payment to Hatry - the Collins is ALL MINE!"  He passed away about two years later.  Corky gave me my Novice test - great ham and full of ham spirit.

I spent most of my time, paper route money and energy on antenna masts, and antenna supplies. You can go thru a lot of $ when experimenting. At least those worked pretty well for me.

T

Corky was on the rag and had Ward give me my test. He made me take the code test twice. I used to drive my mother crazy for a ride to Hartford when my Dad got home at 4:00. She was into electronics so usually gave in. She busted my butt for years when we drove in traffic to get a mic when my General came and I was into CW soon after that.
I used to buy a few 4 inch spreaders every time we went. They were 55 cents each. I dreamed of the 6 inchers but cost was too high.
Imagine how crappy Hatry pay was back then. Corky must have been paying for years.
I bought my first BC610 plate transformer from them around 1974 after getting my first 4-1000A at HRO near S.F. Ca. while visiting friends around New Years 1974.

My first attempt at radio was when I was in elementary school. A friend of mine had this neat crystal radio set he and his father built.  His father was an engineer and radio man for the Coast Guard. Back then I was fascinated with my friends crystal set.

My father collected all sorts of parts for fixing the old b&w TV. I had no idea what most of his parts were but had a rough idea.  I wanted to duplicate my friends crystal radio set because I thought it was so neat to hear radio without power or amplification.

Granted I wasn't dealing with any type of voltage. I breadboarded the parts I had to duplicate the crystal set using a piece of a split log (the closest I could get to a flat piece of wood), used 2 inductors as the coil and used the sheet metal screws to connect the wires.  Suffice it to say it did not work.

It was quite the abortion.

I wish I still had the galena holder my Dad made me from a hunk of 1/2 inch copper tubing. I think it came out of an old Cub Scouts manual.

Yep, "Hatry pay" and buying a Collins S-line.... HA!  Corky was a REEAAL ham.

Hey Bob -  Back to your rig... You fixed at least one problem. What's the bottom line now - after stablizing the screens, what is left to do? Are you happy with the performance at this point?

T

Hi Tom,
Yes, Overall I'm happy with the rig.

The bottom line is I think I'm close to a maiden voyage.  Screens are stabilized for the most part but do waver no more than 5% under modulation.  The scope looks good when tuned up and a 1KC and up audio signal is used to modulate it.   

One thing I'm not happy with however is the frequency response.  Low end response does roll off plus the envelopes shape becomes more rounded.  I don't believe it is a result of the modulator but more the audio amp and transformer. So I have some investigating to do.

The arcing issue is solved. That had more to do with tuning of the rig. Sort of a learning curve I had to overcome.

Also, I would like to investigate what Frank suggested, that being tank Q being too high. So I'd like to try a different tank coil arrangement if I can shoe horn one in.  I haven't used your dimensions yet.  Just haven't had the time to construct a tank coil yet. 

Aside from the possible Q_tank issue and getting a nice audio response the rig maybe ready for an on air test.

Throw it on so we can tell you how nice it sounds.

Yes, during the new-rig-honeymoon period, everyone has to give out nice reports, so don't worry.

After a few weeks, you'll start hearing the truth. By then, you've had plenty of time to clean it up anyway... :-)

T

OK guys.  I'll be there this weekend. Will put the fire to the wire.

Be there or be square  ;)

The acid test will be getting it by HenryLR.