The AM Forum

Construction pix as of today.
Large parts placement complete and temporarily installed on plastic chassis overlay.  Most large holes and cutouts done, just the little stuff to go.  That'll be at least 50% of the work. Plus the wiring.  Still designing the input circuit. Probably will be separate coil/cap. for 160, 75 and 40; not like my single 813 rig, The  "Shoebox" which had one tapped coil and dual 365pf for tuning.  I need to get a decent plate choke; note the homebrew one wound on an oak broomstick handle.  Ho.ho. Well it worked ok on a single 813.

I'm leaving room in front left of deck behind the meter shield for a separate xtal osc. and buffer amp/driver  which may be installed after I play with the amp for awhile lashed to the Drake TR7 for carrier driver/receiver.  Mod drive is provided by a 25 watt PA amp into currently 811's soon to be triode connected 813's.

Iron from left to right is screen audio choke, bias fil./ Ry voltages(OA2 in front) and Raytheon fil. xfor for 813's.  The "Ray" is too big so will be buffered down with a 50 ohm, 50 watt r. in primary.  The bias will be OA2 for neg. 75 into grid resistor for next -75, a la K1JJ.  The 7kv tuning cap is under the quad loading cap (from some ancient TRF radio?)  Boy they sure don't make radios like they used to.

This deck will go into the top of the 32" rack that I picked up at Howard Co. fest a few years ago.  Power supply and modulator mostly done.

Can't wait to get this on 160 particularly in such a lousy 80 meter season. Shoebox is not working now since I "borrowed" a few components.

Rick

Nice work, great looking layout there! Gud Luck with her, I think you got a decent project there.

Very nice but a suggestion. Swap plate tune cap with band switch to make distance between the tubes and plate cap short. Stability issue.

Thanks John, it's been fun so far. 
- And 'GFZ', good point. Your wish is my command.

Meters are now on right side; panel simply flipped around. Nothing like symmetrical layout and Ctr. line drilling to make this sort of thing easier. Also the Screen AF choke is simply placed under chassis, same holes looking up with sufficient clearance for plate tune cap (now on posts, rotor will be grounded of course.)

Hey, none of this homebrew talk is allowed on here. We need special section for it.

Steve,

Please, Pleeeeze, don't move me to Gary's boneyard.



And as an extra added attraction I now have room on the upper left front panel for a VIEWING window.
How cool.

 I'm now debating between rectangular or round with screen. Didn't they used to make screened hole fillers?  HEh, heh,

The round holes will give your audio a fuller sound, with a much more transparent sound stage.

And be sure to use gold plated fuses in your modulator power supply !!

Very Good! You want the connection from the plate to the tuning cap as short as possible, you are there. I learned it the hard way when I tried to build a Quad 572B linear......Then I visited my high school buddy and his wife in S.F. I bought my first 4-1000A at Ham radio Outlet for $22.50 with socket and plate cap included. New tube.

$22.50; those were the days... sigh..  but then I wasn't making $, x20 "new" then either.

Yes, inflation.  ur generic silver dollar, one ounce's worth what now?   -Say, $14.50 now since they withdrew the priviledge of trading in paper for one in '63? 64?   And I don't want to even talk about gold. 'Fraid my fuseees will be plain ol' vanella.

Anyway the links from plates to C,neut. will be Cu strap. Links from 850pf coupling cap to C,tune will be strap too.   Then I can 'STRAP."

you gotta know your crap if you really wanna strap and it don't kum easy....

Yuck! Jeesh! Have you no mercy?

Panel Progress:

Almost all the chassis work, drilling, and parts placement done now.  Panel location of controls and HV/Fil/Main switches done.  Even thought I have a variable timer module on hand, I'm electing to build this Class C amp. with three simple switches, a la 1950's.  First switch, Main 240VAC to fils. and bias/control xfor; Second for HV contactor and Screen voltage; Third for Amp bypass.  Plate HV/screen contactor in a lower panel  will have step start, however.

Unique feature is the optical peep hole shown in front in the pix (panel upper left).  It's a negative lens that shows a lot of the compartment when viewed a few inches away. Yeah, couldn't bring myself to chop a large cutout, add screening, but the first 2/3 of an old Cook lens looked like a nifty way to go.

Rick

RF will cook your eyes so don't look into it too much

What is the finish on the front panel?  It really catches the eye. 

I spy, the RF in my eye...Not gud...

i have another opinion to add to this,.  that by far is one of the biggest failings of this department..RF Safety..or lack of concern for...That picture should be pulled...This Forum is read by millions of people, and the chance of someone seeing something like that and sitting back and thinking Hey I could do that too, and Key the damn thing and put his or her eye against it...Lord forbid something like that happening...

I know we know the RF guidelines, "Us Here"...a board should be put forth on this forum to watch for such things...I can't stand by and watch this...I have to speak up...


jack KA3ZLR.

All too easy to jump on the sacrosanct safety Gods.  Not too worried; middle element of a Cook triplet is leaded flint glass.  Anyway, as I mentioned before, you don't put your eye right up to the lens; it won't focus.  Viewing distance is sort of like a rifle scope, several inches, shows a wide angle view of cage behind.   And the 'hole' is about 3/4" ; Don't think too much RF at 160 meters is going to radiate any more than some 4 x 5 viewing widows on 4x1 rigs I've seen, even those screened with 1/4" hardware cloth.  I guess the operative, scary word must have been "peep hole."   And how about all the 211 to 100T breadboard rigs so proudly displayed in QST, this page, ER, etc?   Just because a 'hole' appears at a panel doesn't mean that suddenly the whole 1kw is magically focused to and re-radiated from that point.  Jeeze, think about it even if you don't re-derive Maxwell's equations.

Reminds me of every PCB and Asbestos scare I've worked with over the years.  The original PCB regs. were written for workers in the friable PCB industry and exposed to dust all day; an act that's since blossomed into an entire Lawyers relief act for every sniffle an errant piece of siding around.
     PCB's have never been proven to cause cancer; some of the original, pre-political literature has posited that the Zylene carrier was actually the culpert... and at  doses that would kill the average mouse from the liquid quantity alone. . Don't get me started on alarmist safety regs.... some of which for the gas and oil industry I'm ashamed to say I helped originate. 

BTW if you all want calc's for a 1" aperture, essentially a hole in an infinite screen with an at most 1kw RF environment behind it, shouldn't be too hard.  We're not talking original point source here, not even inside the RF cage itself. ( Optics 101/  E&M 201, you name it.) BTW I have run the radiative equations for the antennae at 'this here' shack using the ARRL's very cursory graphs and formula but good enough for the log. Wonder how many of us have done that?

RF density of 10 mw per cm squared is considered a problem 1 KW - 50 dB are you feeling lucky
I remember when the motorola training movie when they cooked a rabbit's eyes with low power......Hopefully they cooked the whole animal so he didn't suffer blindness

1 kW radiating from an "antenna" with -50 dB gain doesn't generate much density. And it still must make it through the tiny (relative to the wavelength) hole in the front panel. Add in the leaded glass, the field at the front panel is about zilch. Much ado about nothing.

Hmmm, I must do a little more research.
Interestingly RF at 160 meters has a very low energy fraction compared to microwaves.  Yes, I know quite a few research grants have focused (pun intended) on radiation in the last several decades, so I must ask, how long was the rabbit exposed, at what wavelength, intensity, etc.? I am interested in research of much lower frequencie's biological damage.

... But & oth, light waves at a much, much higher energy fraction than microwaves do damage (e.g., cataracts) only over many decades, given at much lower intensity.  Yes, different waves for different effects; cooking protein at microwaves vs. enjoying a lifetime of sight with visible light waves including enjoying the sight of two filaments lit with an occasional glance..... (you won't see any plate glow in a graphite 813.  I suppose the casual SSB'er passing by the rig operating at a hamfest would be curious.)

   Ah yes, microwaves, quite a peak in the ol' biological damage vs. freq. chart, eh? Yes, dangers abound in this present world. HV I quite respect.  Immediate effects, of course.   Oh, I almost forgot, the lens cell is 1/8 in. thick, leaving a glass aperture of 3/4".  Now I know that some texts and manuals say that apertures should be no more than 1/4" for reasonable practice, usually rigs expected to be operated on 10 meters? How much 160 meter radiation escapes from a 10 x 14 x 17 enclosure through a 3/4 hole anyway? How many rigs have you seen with no meter shields?  How many bald headed hams can attribute their 'loss' to this?

Well, methinks me doth protest too much...   ya' must have hit me nerve. ;)

Almost forgot to respond; Mack, your right. The panel material is exactly that, from a roll of gridded, plastic masking for laying out punch holes.  In the picture angle taken with back reflection of the camera flash it looks pretty good. Kinda like those wonderous shots on Epay.  But looked at head on, in real life with natural light, it's pretty crappy.  Looks just like cheap plastic. Not exactly square, at that.  Too bad; it really looks good in the pix.

Thinking a little more; most of the RF is going out the wire; E&M field from the coil is inside the case. Since it's pretty much a diameter from most metal and sidewalls the circulating currents are pretty much 'unmolested.'  Be interesting to see what the real RF field is inside the cage, given that most power is directed up the line, with remaining 30% dissapated as infrared (heat.)   

"Up and Out" -Yeah, we're all sitting in the near field on 160.  Might as well sit inside the cage ;)

Getting ready for Heavy Metal Night.  Very close to being finished.

Here's the mod. deck and power supply getting ready for RF deck to be placed above.
Will try the 811a's first and if not enough strap will go with triode connected 813's. 

The 811a's are on a little sub chassis on posts that can be changed out with 813 sub-chassis extending into blank space between mod & pwr. supply section if required.

- As seen from the back of the 36" rack pix;
and In front of 811's are left to right:
 20w backwards AF xfor. from Fair Radio (driven by a PA amp w/3 section tone controls); 811's fil. xfor.:  BC610 mod iron behind the 811's.  Middle of pix shows screen xfor, screen filter choke below it; P. Dahl hypersill plate xfor in right rear. with filter choke(tan) in front of it.  HV contactor; step start relay and delay circuit in right front of pix.  All this stuff already proven on the single 813 'shoebox' rig shown some years ago and still on HUZ's website.  Pretty sure it will work fine on twin 813 rig up to a certain power point.

 Well I'll definitely have over 250 watts (already had that with the single 813 come to think of it,) and am working mightily, component by component to achieve the 250 lb. goal too :)

Looking forward to great Christmas and New Year's operating season.
Uh, ok, would you believe Dec 29?

Dual 813 rig's about finished. Here's a view of panel.

Finishing up input coil circuit. Found that a 3 turn link best matches 40, 4 turns on 80/75 and 5 to 6 turns on 160.  Simple method used in finding best match; temporarily wired in about 4k load representing grid of 813's, fed 5 watts or so from the TR7 and tapped/tuned for lowest SWR. Wrote down the 'drive' settings.

Will use same method to pre-set the PI-net output circuit; Temp. add 2k load non-inductive resistors from tube side of plate coupling caps to ground (no tubes in socket) and back feed a few watts from TR7 for lowest SWR on TR7.  Guess there's some advantages to having a very picky 50 ohm SWR foldback solid state final in the TR7.
"Let the 'foldback' do the walkin' "   

 Well, that will get the pi net preliminary settings so I can start neutralization.
Will isolate the output on the other side of the plate coupling cap, add the two k to ground at that point to represent the pi-net load, then feed a very low signal into the tubed circuit, (fil's. and bias on, no hV or screen voltage) and set neut. cap. for lowest feedthrough to a receiver (attenuated as necessary.)

Then I have to go back into supplies in base of rack and add breakers and finish wiring, and build a rack support with differernt clearances.

Very nice Rick! Can't wait to hear it on the air. You'll likely break my S-meter.

there's a complete input cap/coil assembly from a SB200 on ebay. go git it.

Thanks men.

Mack, yeah the gridded stuff was meant for layout; I just used it to protect the finish while drilling, especially making the meter holes - jig saw scuffs, etc.

It looked cool in earlier pix but in real life, view was pretty trashy, scuffed up and just enough out of square to annoy.  Not to mention the peeling edges.  I guess if put on very carefully, after holes are drilled and with some sort of framing to hide the edges, it might look ok.  I even thought about using the u shaped binding that you make notebooks out of but couldn't find any 19" long.

Ya done good. I love good homebrew and de-hamboning stuff. It shows a tiny part of ham radio is still alive.

I'll be building a 160 meter monoband amp later this year health allowing. I hope I can find a good example during the course of hamfest season with all the hard metal work done....thinking about tubes.....

It's super bad how some of the personality of the builder shows through in the final results. I've always marveled at that fact.

Nice work Rick!!

OK boys, done !
(except for a few niggling details..)
like :
have to finish mounting in 32 in.rack, pwr. supply mods., installing breakers properly, etc.

Need to find a couple of coil clips for #12 B&W stock, 6 t.p.i. clearance.  (Temporarily using alleg. clip to find best match. May just solder it up using 5% Ag alloy.)  Anyone have a couple?

And after all the safety issues outlined on site;probably need to get proper Millen HV connectors.  I'll have interlock switch on rear cab. door but you know how it is.....

The front panel's been shown previously so here's other pix's.

Oh and BTW, here's a view through the peep hole.  The negative image is slightly out of focus unless you move your head back 5 or 6 inches. - -  Don't burn your eyes out.  The rear aperture (inside case) of the lens is 11/16 in.  The apertures in the fan's hardware cloth are 3/8 " for that matter.

Thanks for bearing with me on this project and all your thoughtful comments and help.  Probably you can spot some of your improvements.

Rick,
Excellent craftsmanship!  Can't wait to work you on 160 with it!
Joe W3GMS

Yep, thanks Joe; It'll be fun to work 160 again.  I miss the ol' single 813 "shoebox."

And here we are; ready for first light.
-gotta do the coil tap stuff after trying out the first formula guesses though, and drill the cab. rail to accept the 813 deck's non-standard height front panel.  -the holes didn't line up, heh, heh.  Well every rug's gotta have a flaw.

I see several 813 rigs in construction lately.  Might be a run on the bottles. ;)

  Rick, I've been  looking at you 813 transmitter and the progress and got to say Nice Job.I just hope the one I'm building turns out OK....Tim..WD4TC

Tim, you'll be done before you know it.
Here's some interior views.

Paralleled a couple of 5mh rf chokes for the screen, 150ma each didn't look too hefty; just one 5mh in the grid bias circuit.

Series wired two audio chokes in the screen, one 8H from Hammond with a 5H on hand. I don't really need to have low f. cutoff at 30hz, -50 or 60 ought to be plenty.
Liberal use of .01/3kv bypasses just about everwhere on power leads, etc. Screen bypasses are .001  - Don't want to0 much or it'll suck down audio highs.
Anything else you need a pix of?

Rick,

Very nice construction practices and technique! Looks like she'll be a real beauty.

I'll bet she'll sound every bit as good as she looks.

Congratulations and good luck with the new rig!

73,

Bruce

So, very nice work indeed! Now, with this peep-hole thing going, you must add a small interior lamp for accent lighting, perhaps a 12V automotive lamp (or LEDs) running off the 813 fil. xfmr, so that you can see the innards better. A spectrally narrow deep blue light source like an LED (array) or an incandescent lamp with a dichroic filter would not interfere with the glow of the tubes.

Rick,
I think the metal around the base of the 813 likes to be grounded. Somebody correct me if I am wrong.  Nice tight layout to the plate tuning cap.
BTW. Keep an eye on the TR7A Mine the carrier drifts up as it warms up. maybe 5 watts won't be an issue but at 60 watts it is when I drive my class e final.

Francie,?)
I used a brass clip and grounded the 813 in the ol' shoebox xmitter.  None used so far in this twin 813 rig.
Found no difference in stability. Recall that an 813 has an internal bottom shield disk made of nickle.  Interesting that in some bottles the external shield is wired to a pin. Not so in the 813.

Also I notice in the RCA literature 813-4-55, 7 pages (1955) that all interelectrode capacitances are referenced with, "with no external shield and with base shell floating." Cgp 0.25pf, Cg1 to fil&g2&g3 16.3pf, and Cout 14pf.

It certainly wouldn't hurt to ground the base shell but would call for re-neutralization if I were to do it. Glad I wasn't shooting for 10 meters, Cout might be close to unobtanium using the traditional pi. net and base shielding.

I'd be interested in any literature recommending or definitely requiring grounding of the base shield in an 813.

Also I see in many designs that the base shell is sunk in the chassis which is a design requirement for some tubes, (Eimac's "Care & Feeding..." )but not the 813. If Screen caps., grid caps, etc. are grounded to common point, socket above, at or below chassis are fine. In my design, sockets are above a large hole allowing for a % of air flow to come up around the pins, fan blowing out the rear of the cab. In the 1962 ARRL h'book for example, two 813 rigs are photo'd, neither with base shell grounded.  One design shows raised socket, other at deck level.

Regarding shielding, many forget that a large ceramic socket might as well be a large hole anyway. - as well as the ceramic base of the 813 itself.  That's why it has an internal shield.

The maiden voyage was Feb. 24 on 3733kcs. Also loads up fine on 40 and 160. Po's range from 350 to 370 output, no instabilities.

I like the illum. peep hole idea; also thought about a neon pulsing in time with the audio. ...   But then might be too CBisih?  ;D