The AM Forum

After a lot of thinking and discussion with Frank/GFZ and Tron/HLR on the air, a new rig is under construction. I considered a lot of tubes and designs, but the mighty 813 won out in the end.  This is the most exciting part... laying out the components and visualizing what will be. It will be built broadcast style for easy servicing and viewing. (like all my rigs)

I realized I have four homebrew linear amps of various power levels, but only ONE plate modulated rig. I'm not going to run the 4X1 rig in the summer due to the heat.  My second favorite tube has always been the 813. I have a 250 watt broadcast mod transformer with a 2:1 step down ratio.   A pair of 813s X a pair is a perfect match!  I will put on 2KV and run the modulators triode-connected, zero bias. I will build another GFZ MOSFET SS driver to provide audio drive and negative feedback. I have a pair of vacuum variable caps but still need two turns counters.  Also need two more 813s as backups.

Here's the rig with the first layout of parts. I need to repaint the front panel and label it as per usual.  This chassis and panel used to be my older 8877 linear, but is salvageable. I'll make it look real purty doing an 813s life.

This should make a great summertime rig. The unique thing about it is there will be no fans or blower noise. 813s can be run as convection and radiation cooled. It will be the first time I will have a perfectly quiet shack. It's the same design as the 813 rig in the AMFone technical archives.  There's quite a few of them on the air.  Everybody has their own implementation.

I'll keep you updated as it comes together..

*** Here is the rig schematic:
http://www.amwindow.org/tech/htm/813/813.htm

813 tube data sheet:
https://frank.pocnet.net/sheets/079/8/813.pdf

T

More pics...  it's like playing checkers as the parts move around.


There is little room left in the shack for another rig.

The new 813 rig will look much like the 4X1 color scheme as pictured.  A nice glass window with black borders will look FB.


* Modulator tubes on the left - RF finals on the right.

Modulation transformer, 2:1 step down, 250 watts broadcash iron, about 50 pounds.   A gift from "The Tron" 25 years ago.     Pictured with mod reactor and 1 uF coupling cap on top.  All parts will get cleaned and painted before installation.

more...


Below:  "Captain of My Heart."

Nice. Those 813s are probably the best tube ever dollar for watts. Rugged, as you know.

Yep, you're right. I've never had an 813 or a 4-1000A  fail in normal operation.  Parts around them, yes - but never the tubes themselves.

I was close to choosing quad 4D32s X quad 4D32s  OR  a single 4-400A  modulated by a pair of 3-500Zs  OR  even a string of 6146s X  811As or whatever.  But the smaller tubes mentioned usually give me problems with arced over screens, gassy or shorts of all kinds when mistakes are made or the voltage is too high.

In contrast, 813s are invulnerable workhouses. Same for 4X1s.    I haven't had an 813 plate modulated rig on the air for at least 10 years, so it was time. I missed the Harley, Spartan/Trojan  feel it has vs: a linear amp.    ;D   Once the NFB is applied with a solid state MOSFET audio driver and BC iron, the audio can be near flawless.

T

Its one of the favorites..

Tom?

What am I missing here??

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

It's ur rig/design!
You never built one??

          _-_-bear

John, KC2FXE's version:

Yep, John/FXE did a nice job on his... I never saw that picture.

This is my third one since 1992. It's been ten years since I had the last one.  I usually run them for a few years and tear them down for the parts to build something else that tickles my fancy.  The vac caps and other parts are too scarce.

But as of late, my rigs have been getting nicer (practice) and I could not bear tearing them down anymore. So I now have four homebrew linears of various power levels and one 4X1 plate modulated rig. The new 813 rig will fill a plate modulated gap.  I'm drilling and mounting parts as we speak.

I plan on doing it up right with the GFZ MOSFET audio board, vac caps and quiet as a church mouse. I am absolutely out of parts now and had to really look hard to build this 813 rig.

I'm still awaiting the latest transmitter creation from Bear Labs!   ;)

T

Yeah, meee tooo...

I've got projects that stretch as far as the eye can see..
YIKES!

                    _-_

Tom,

What happened to the quad four transmitter?

--Shane
KD6VXI

Hi Shane,

Funny you wud axe...  The "Dual Quads" rig, quad 4D32s PDM modulated by four sweep tubes sacrificed itself last week as an organ donor for the new 813s X 813s rig. It was a nice rig, but like most of my tube PDM rigs, they are very dangerous with floating supplies, hot meters, large output filters with HV, etc.   Unless they are used often, it is too easy to get careless.

I have about 50% of the parts mounted for the new 813 rig, so feel it was a good decision so far.  My excitement level is high...  ;D   It will run more power, be just as cool in the summer, safer, and the audio should be close in quality to the Dual Quads..

I know you are always building something over there... what's your latest creation?

T

Hey Tom....

I liked the idea of the Dual Quads.  Something about PDM on tubes is just appealing to me.

I have a GS35b almost completed....  Power supply is, and I keep forgetting to get metric screws to mount the COMET Vac Variables everytime I go to town!  Which isn't often due to the lockdown....  It's being taken pretty seriously here.

I got a 5 watt Hermes Lite a month ago.  Decided I'd go rebuild a 1 2290 > 4 2879 amp.  5 watts pep in would equate to about 200 watts out, and with predistortion, would be pretty clean.

Order up 4 x 2879s.  Install them.  Apply power.  1 pair smoked the 10 ohm base to ground resistors. 

Well, that pretty much ruined the month, if you've looked at the cost of 2879s lately.   Yeah, it happened.

--Shane
KD6VXI

I just saw this recent SWL YouTube recording from England in the 75M DX window.  Conditions were good and I hadn't been on for 3 years...  Amazing they still remember you.

https://www.youtube.com/watch?v=dDeEj8vuwlU



Shane, that's a tough beat to take blowing those chips!

I am making good progress on the 813s rig and will post some more pics soon.

T

Nice recording and a quiet band!

Filament, bias, and screen regulator?
Always pays to be inclusive!

Well, all the parts are mounted and ready to start wiring.

I decided to add two quiet muffin fans to blow a breeze by the 813s.  The fans are controlled by a Variac and can be seen in one of the pictures.  I tested the air flow. By slanting the air flow on an angle towards the front panel and chassis floor, there is a nice reflection and 360 degree air coverage of all tubes. It is dead silent inside the cabinet.  The cabinet has two 6" air ports for good circulation.

The screen and grid circuitry on the chassis floor will be shielded from the output.

The modulator iron will be built on a separate sub-chassis.  My new HV supply will supply up to 2800V at 650 mA.

I still need to do the panel labels in black and install the front glass window which is 12" X 7" for easy 813 viewing.

The bandswitch and pi-network will have short-lead access to the tank coil which is reasonably in the clear.

I have a blue cabinet already built to match the decor and the other rigs in the shack.  

I am ordering parts for the GFZ SS MOSFET audio driver board. The rig will have audio negative feedback.

So far I'm pleased at how it's all progressing and it will be a good addition as a summertime-cool, quiet AM rig.  

T

Official Name:  "HOLLYWOOD"


In the first picture, notice Hollywood's black, white and blue brothers and sisters in the background...

See the fans and no fans.

Find the kitty...


The 4X1 modulators are awaiting a new buddy..

The chassis and front panel are recycled from a homebrew 8877 linear I built 10 years ago.  Everything gets recycled at Tom Vu Industries.

It's amazing what can been seen in a picture but is missed by eye.

Those aluminum supports I painted white to hold the vacuum variable caps stood out like a cheap pimp's sharkskin suit!  

I painted them flat black and now they look like they should... almost invisible. I plan to run copper strap between the two caps to the chassis ground.

Some might call it a compulsive disorder....   but Hollywood is happy!


NOTES:

BTW, Look at pic #3... that little toroidal transformer with the brown Teflon wire is a broadband input with a 3K carbon "Globar" power resistor loading the secondary.  It will do 1:1 SWR input matching without tuning.  Less tuning when rowing around the bands. Class C doesn't need an L/C input tuning circuit like a linear does. That circuit is an organ donation from the "Dual Quads" rig. That area will be shielded.  The "kitty" choke is 150 mA @ 30H, the screen self-modulating choke.   There is an antenna relay and a PTT  CT relay for the RF and mod fil CT.   The two small Variacs are for the screen supply and the two muffin fans.

The white insulators on the sloping support arms is where the two muffin fans mount on rubber fasteners held up by a horizontal cross beam.  

In the back sits the protective fixed grid bias supply and the screen supply, both for the class C RF final.  The modulators are connected as triodes and will just need a few diodes in the fil CT for idle bias.

The Plexiglas board with the string of resistors is the audio NFB HV pickoff. It is already optimized from another rig I once used.

I'm glad all the parts are mounted cuz it was starting to get pretty tight in there.

The big Sangamo plate coupling cap is definitely overkill.  I didn't want talk back. I am trying a common National choke. It may last or may not... it does look nice though. I like to push a new rig to it's absolute maximum in testing then back it off to 1/2 power for best IMD cleanliness and longevity.

T

Looking good! Cool Kitty. Very interesting mounts for the vacuum caps. I think I learned something there. Will the back ends need support as well? Those things look heavy.

The input toroid. Why does it not need to be shielded from the output tank?

He states the area will be shielded, just doesn't have it in place yet it seems.

--Shane
KD6VXI

Thanks, Pat, John and Shane...

The mounting of the vacuum variable caps.... I've seen it both ways; mounted in the front only and mounted at the front and rear.   I have always mounted mine in the front, but try to put no strain on the rear, just like a tube plate cap.  I have at least twelve vac variables in service with no failures so far. If I get worried, there is room in there to add an insulated  vertical support to both cap ends.

I see Don / W2DTC mounted his in the front only as well as many pics on Google show. It does look like a strain when horizontal, I must say. Vertical wud be better here.   In my case, the capacitor housing is pretty strongly built and extends most of the way to the rear, so not worried.  The risk would be the seals, so arcing over would be a clue.  I suppose vertical mounting would put the least strain on a vac cap.  So you're covered, John.

Yes, the small input RF and DC circuitry you see on the bottom will all be shielded from the output stuff with an aluminum plate. I gots to live with it on the air so I have no choice.... :-)

Here is a lot of info from Jennings on vac variable cap use. There are warnings about shaft shear and other stuff that may apply to us hambones:
http://www.amstechnologies.com/fileadmin/amsmedia/downloads/4317_jenningscapacitorshighvoltagevacuumandgasfilled.pdf

T

I better try to figure it out. One thing I have stopped for on my amp build is mounting the big vacuum caps. One is metal/ceramic, maybe pretty rugged, the other is glass but mightly big also. Your advice for vertical mounting is giving me ideas. Space is an issue but so is lead length.

Looking more at the picture of the horizontal cap, I wonder if after many years, the alignment of the plates and shaft will sag slightly. The spacing is so small in the vacuum that it wouldn't take much to cut the voltage rating in half or worse.  The glass caps seem to be of weaker construction.  But I am just going by feel here and have never had any of mine fail over a 40 year period when I started using them in a horizontal plane.  Maybe they will all outlive me... :-)

But the vertical plane definitely appears better mechanically; and for a more compact layout and shorter leads, though a right angle drive is required.

T

W2DTC has converted rf generators.

I have two in the shop now, parts donors.  I've converted probably 20 or more of them. Never seen a problem with them shorting out unless running insane voltages on them.

However, I usually have them close to fully unmeshed.  The voltage capability is sky high there, it's more a current problem.

I think it's more a GShock problem than a time sag problem.

--Shane
KD6VXI

I sugest adding a bit of element 115 if you find any at a local ufo crash site  ;D ;D

yes I used to live in Ca

I lived in Tehachapi, CA for a time.  Amazing the things you would see streak across the sky.  Donuts-on-a-rope, etc.

I've since moved to the central coast....  Weirdest thing here is a large farm tractor lol.

Except when offshore on an oil drill rig:  you see some odd stuff when out there.  Fishing 'trawlers that don't move for weeks at a time, pointed broadside to Vandenberg launch facilities.... The four we service have a decent view of Vandenberg south base.  Not to mention cool sea life... Whales, dolphins, sharks, turtles, etc.

And Vandenberg, that place is really fun.  Was at a Minuteman III site last week, along with being on the flightline.  Couple projects we are working on.

Don't get me started on the "Space Force" buildings.... 

--Shane
KD6VXI

hey its just the natural evolution of the Strategic Air Command!

Tom,

Can you show how you added the 160M ruskie padder and the switch?

Hi John,

On 160M only, I don't have enough C2 loading capacitance available, just 2,000 pF. (vacuum variables)  The pi-net on 1.885 requires  3,000 pF for the 20 uH coil I have.  I need more C2 or L, take your pick. So I decided to keep the L1 at 20 uH and add some more C2.  The higher bands, 75-20M have plenty of range for Q=12.

So, on 160M I usually add in a fixed capacitor of 500-1000 pF in parallel with the loading cap. I use either a relay or just a 20A 120V large toggle switch with the shortest leads possible, right near the loading cap.  160M is very low loss band, so is very forgiving when the Q is somewhat higher as a result.

Do not use doorknobs for this high-current use, but rather, capacitors that are rated for at least 5A or more at 160M.  (6A at  50 ohms is about 2500 watts, so there is overkill)  The common dark brown .001, .0005 mica capacitors about 1.5" X  1"  X  3/8" commonly found in flea market boxes, used for years in military rigs are usually marked for current. I've never blown one up in loading cap service, though I have popped door knobs there.

Use thick wire.  


** I will have some new pictures soon of the whole rig and this cap padder. The wiring is almost done except for the tube sockets and modulator iron. I silver plated the tank and RF final copper straps. Lots of cosmetic improvements to make this new 813 rig look FB and talk whirlwide.  Got the window glass and will make up some panel labels soon. It has gone from an Ugly Duckling (see first pics posted here) to Hollywood, quite a looker, rivaling Fabio, Rico Suave and Dr. Love.  Caw Mawn.


T

Hollywood has come out of the dungeon and is continued in this next thread with finished pictures:


http://amfone.net/Amforum/index.php/topic,45867.msg328910.html#msg328910