The AM Forum

Except for some more road testing and shakedowns, the 4X1 PDM rig is pretty much finished.

Here's a few nighttime shots of the tubes lit up.  Everything is under Plexiglas for your viewing pleasure....  ;D

T

Here's some shots of the shack.  Notice the 4X1 X 833C's rig - and the 813's under glass on the floor. It's a cozy fit, but completely safe.

Here's the PDM filter coils that were arcing, burning up and giving me Tesla coil problems. They total 2,000 turns wound on 5 gallon plastic pails. Also notice the small coil used as a series resonant trap to suck out the last remnants of the 135kc pulse switching frequency. All the coils are now enclosed in Plexiglas.

Also here's the latest shots of the 4X1 X 833C rig and the Blown 813's. Notice the lantern chimneys used for the 813's. It really helps keep them cooler.

Ah yes,  transmitter under glass! (Or was that pheasant under glass??)

Nice work there, Tom. I look forward to working you with those beauties in the near-term. Numerous radio projects nearing completion here at W2XR.

Best 73,

Bruce

While we're at it, notice the 833C's mounting. I've found that pushing a little air from bottom to top with a muffin fan is an excellent thermal path and great help.  The dissipation of the tubes go from a 300W rating to 450W according to Eimac. Also, the carbon plates of the 833C's are the only way to go.

That's it for now!

Later -

T

Beautiful work, Tom. Congrats!

Way cool Tom!  Your new rig is a "Plexi-Babe."

The 833's look like they're levitating.  I had to look twice.  
OTH, maybe they are....   ;D

Bet your PDM rig cab's will sound every bit as good on the air as they look.

Tom Vu and his triple trash can integrator.
very cool!

This has to be the most unique approach to enclosing a HB TX I've ever seen!  I would have never thought of this approach.  My last HV TX was enclosed in cane metal which is just about impossible to find these days.  I wonder if there would be an easy way to sandwich copper screening with the plastic panels with a real see-through but shielded enclosure.  HMMM.  Perhaps one could use thinner plastic sheets with copper screening as the "fill" in the sandwich.  Gotta think some more about this.  Nice work!

Where did you get the chimneys for the 813s?  That's a nice approach.

73, Al

There's something missing... I don't see a single alligator clip lead anywhere. Sup with dat???

It all looks really nice Tom!

73,

Rob

Thanks for the comments, guys!

Al:  Here's a link to a thread about the Lentz lantern chimneys. They are about $5 each a few years back.  The one in this thread fits perfectly over the 813. I also drill 3/8" holes around the tube socket in the aluminum to increase air flow.
http://amfone.net/Amforum/index.php?topic=6279.0

Yes, copper screening between two thin pieces of Plexiglas is an even better idea. Looking at the tubes is fun, but I also like to look at the parts and construction, cuz it's so easy to forget when they get locked away behind aluminum or steel plate.  I look the rigs over every day - can't do that easily with my external anus linears which are in 19", 6' racks.


Bruce: Are you going to post some shots of your new HB rigs?  What are they?


Frank: Yes, a triple trash can integrator. Trash cans wuda worked, but actually they're 5 gallon plastic pails, often used in construction.  Found in hardware stores for about $7 each. I had to keep them compact to fit into the limited space.

T

Here's a shot of the PDM generator. It inputs 1 volt audio and converts it into a 135kc pulse. The audio modulated pulse drives a MOSFET in the 4X1 switch tube's cathode and gets amplified by this 4X1 switch tube. (for 90% efficiency)  The big pdm filter coils then convert the pulses (integrate them) back to analog audio to modulate the 4X1 class C final.

This pdm generator BOARD is a KIT from the WA1QIX product line and could be used to drive a MOSFET pdm rig as well. It has a built-in audio limiter as well as required pre-pdm filter anti-aliasing low level filtering.  Nice little unit and clean layout.  Years ago it was a major undertaking to build a discreet component pdm generator. But today's chips make it a breeze.

Notice the components on the terminal strips. The RF choke and related parts create a low level pdm "sloping" filter to round out the pulse corner a bit. This permits negative 100% modulation without the usual avalanche effect after 95% or so. Some pdm rigs cover this anomoly using a negative peak limiter band-aid. It CAN be cured with this method. Credit to Frank/GFZ for this idea. Also, the other terminal strip is a pre-driver MOSFET power chip for the 900 volt MOSFET in the 4X1 cathode.   An effective mix of solid state and tube technology, don't ya think? (caw mawn)



T

I know but triple trash sounds better....
Shielding Home Depot sells thin metal with different shaped holes, you  could cover the plastic and still see the guts. It will reduce RF in the shack.

See the operating position with the rigs on either side?  I'd just need two pieces of shielding - one on each side to protect my crotch from the finals.  8)

T

I saw the picture.....BTW do you have any pictures of the 1991 rig?

damn Mr. Vu, looking good. U teh Plexi-King Shaolin-Master. Well versed in fighting stances of plexi-materials, especially crouching tigers and flying old buzzards.

You guys ain't seen nuttin. The 1991 rig made this look PW but when you burn nitro things break often. I was bummed when Mr. Vu gave up on it. I had to mess with him a bit on this one when he almost threw in the towel. The least I could do after Sgt. Anderson.

Tom,

Nice stuff....so if I show at home this weekend do I get to work 'it' on the air?

Peekaboo everything, I like it.

Tom,

The rig looks great!  I love the Plexiglas technology.  I find large sheets of heavy gauge plexi to be costly though.  Have you found a bargain source? I think I'm going to steal the idea.  ;D

Yes -  Frank was instrumental in keeping my motivation up during testing and troubleshooting this difficult rig. We exchanged maybe 80 emails over a coupla month's time. His guidance in PDM filter layout and design was critical as well as suggesting ways to make the switch tubes more stable and achieve negative 100% modulation without severe distortion.  We all help each other here, that's for sure.

No, Frank, I never took a pic of the old PDM rig, sorry.


Derb: Yep, my Plexi Kung Fu is strong.


Dave:  Yes, I expect to get on again this weekend. Put in about 8 hours on the air on Sat and Sunday, so we is back. It was a blast with the  huge group down on 3725.  Most were in rare form again.  Hope to catch ya then!

BTW, it appears many guys don't listen down there. I tuned up to 3885 later on Sun eve and heard some talking about how quiet and dead it's been all evening. Gawd, it was a nut house down on 3725 for awhile.


Eric: I don't know what Plexiglas costs these days. I got mine from a friend who used to use 4' X 5' sheets of 3/16" Plexi to mount his commodity charts on. He used mylar chart paper and Scotch taped them on.  He uses the computer now so gave me about 20 sheets for free. I'm down to my last 4 sheets, so you can see I use it for everything here.

What's neat about plexi is  using a hot torch flame to bend it into various right angle forms for brackets, mounting parts, etc. This stock is about 25 years old and shows no signs of yellowing. 

Oh, one point I didn't mention.... notice on the two rigs on the floor... they are on casters and the plexi covers can be pulled off in a second. The cabling is long enuff to pull each unit out into the middle of the floor for easy access and servicing. That was one of the requirments. If you build a plexi unit, be sure to make the cover a "U shaped" unit that simply fits snugly over the rig like this.  I made sure that there is no way on these rigs to come in contact with HV... even if you tried to stick your finger into a crack on purpose.  (Break break - ah excuse me, this is Chris Hanson from Dateline NBC - we're doing a show on ham radio preditors)


I can't get over how much I enjoy just looking at the insides of these rigs every day. My memory is so short that it's like a new thang every time I look inside...  ;D


Johhny:  hi-hi FB OM.  Your GPT 750 was very whirl wide last weekend, OM. 

T

It is amazing! 95% efficient modulator!

It's almost as though the tables have turned. It used to be that 3725 was wide-open real estate with all the QRM and idiots on 3885. Now 3725 is starting to sound like the gawdawfulness we thought we were leaving behind on 3885, with 3885 being wide open many nights.

I think it's safe to say a lot of the jammers who are strictly in it to jam AM activity are reading this board for gratification, and are well aware of our trends. Hence, 3700-3750 has been packed right in like sardines, while things are nice and quiet up in the ghetto.

Great to hear you're getting back on the air, Tom! That's one sexy looking rig ya got there! Hopefully I can fix the wiring I wrecked last weekend in time for this weekend's festivities. I feel like a bad parent leaving the Junkyard Dawg off the air for so long.

--Thom
Kilimanjaro Africa One Zulu Goat Cheese

NICE!  Hope to hear it on the air.
BW

Tom,
You modulator efficiency is a bit lower due to the slower turn on time of the tubes to get analog compensation. I think you are measuring at the saturation voltage of the switch which is quite good.

Yep, agreed.   I believe Patrick was quoting a theoretical number in the post above.

I actually measured 90.1% efficiency BEFORE I added the analog compensation.  I did it by shorting the switch tube to ground and then letting it run full on and comparing the two.

Of course, this does not include the losses of screen current, grid current, filaments and the pulse slope loss from the analog comp - which will bring the overall efficiency lower.  But all tube modulators have to deal with this anyway - even the slope loss, in the form of class B operation.   

T

That's a good way to check to see if the final is stable.