starting work on the amp

[W7TFO]

Set your HV gaps a bit bigger than the leg spread on your local spiders.  Trust me...

Dennis

[Mike/W8BAC]

Hi Patrick,

I wanted to put casters under my 20V-3 and was given some friendly advice, yours FOC. If your considering 4 swiveling casters consider this. Most swiveling casters place the load ahead of the axle. The taller the wheel, the higher the possibility of a tip over. Consider 2 swiveling wheels at a narrow end of the rig and the 2 fixed at the other. A tip over is much less likely. Things are looking good. What type of rectifiers did you settle on?

Mike

[Opcom]

Thanks!  It is that marginal spacing. The small spiders in the lab are 1" across.  I am counting on dust bunnies too so it may be some insurance to add the insulators. My concern is not having that huge transformer arc to chassis. May be carnage and scorch marks/holes from something like that.

Today I managed to order casters and picked up some ring terminals for the wiring, hope they fit.

McMaster-Carr had a nice lady (not a machine) answer the phone and she for one thing knew exactly what I was asking about and for the next thing said that if the casters did not fit I could return them, and then she took my order. That is service, rare today.

[Opcom]

Hi Patrick,

I wanted to put casters under my 20V-3 and was given some friendly advice, yours FOC. If your considering 4 swiveling casters consider this. Most swiveling casters place the load ahead of the axle. The taller the wheel, the higher the possibility of a tip over. Consider 2 swiveling wheels at a narrow end of the rig and the 2 fixed at the other. A tip over is much less likely. Things are looking good. What type of rectifiers did you settle on?

Mike

I learned a good lesson on that with the Tucker transmitter when I added a steel plate and casters to the bottom. It originally had none. The probaility of tipping seems to be related to how much weight there is going up. The goal for that on this job is to put as much of the real weight in the center bottom of each rack and avoid heavy transformers and the like being up high.

I have to use 8 casters in this one because it is two racks together. Most of the weight, all of the big power supply stuff, will be right in the bottom and in the center front to back, as best I can arrange that. The exception is the powerstat and it's below waist level.  The original casters have 2.5" diameter wheels. the replacements are 3" diameter and wider having dual wheels. They are a little taller therefore but not by much. Each is supposed to be rated 300 LBs.

When raising the unit on 2x4s to remove the wheels, it was found there is barely room to shove a 2x4 under the rack skirts. Without anything in the racks but the powerstat, It was very hard to tip it enough to slip another 2x4 additionally in, so I think I am good on balance but we shall see when heavy transformers are mounted in the bottom and the RF stuff is put in.

The MV tubes are type 673 and will be a bridge. I still want to use the solid state rectifier stacks from a large BC rig to back up the MV tubes but it's not decided. It will be a matter of room. There should be plenty.

[WBear2GCR]

If you do not need to put racks flush and side-by-side then one way to get casters under them, increase the stability somewhat and not lift the unit very high is to use a set of "outriggers" - usually placed left and right and bolted to the rack on the outside of the side panels (or to the rails). Heavy angle iron is possible, but rectangular box stock is maybe more stable and stronger...  since you can extend these rails front and back by a bit you get a wider base.

                      _-_-

[Opcom]

Today no pictures because it looks the same as a few days ago. It was a big job to get the casters to fit. The holes fit, but when the rack was loaded the caster's chassis that holds the wheel and swings around would interfere with the inverted U-channel under which the casters mount. Some slight mod of the U-channels was done with a rather large set of Channel-Lock pliers. Now it all moves smooth as butter! The whole process though had a few gyrations.. the 400LB transformer was put in, then pulled out, then finally put back in. That's the third time it has been in the rack, so its the charm!

The circuit breaker panel was bolted in place semi permanently. All the terminals can be reached and the breakers can be replaced from above. There is no way to make it a simple rack mount but it's rock solid.

The MV rectifier chassis got some 1" ceramic stand-offs under the sockets. It also got a set of four hash chokes from a BC-5. Each of those perches on its own 3" tall conical stand-off. A couple of the hash chokes needed repair to the mechanical mount, a strip of metal. They have a slot in them to accept the soldering of the end of the choke wire. They are prone to tear at this point if abused, and these were. But they deserve to be used and displayed and are right behind the rectifiers.

The equalizing choke was also mounted beside the powerstat. All this took about 7 hours.

About tipping, there is no way for one person to tip this forward or backward. The weight in the bottom is too much for that.

[Mike/W8BAC]

You have probably seen this before, I'll mention it anyway. The 20V-3 uses 575A VR tubes. They punch a hole thru the chassis and mount the socket below using ceramic standoffs. The metal tube guide and twist-lock of the socket is just flush with the chassis top. Makes a neat chassis and would scratch the separation item off the to do list.

[Opcom]

I've seen that before and it is very attractive. Well the work is already done, insulators installed. I didn't have the special card paper disks that go between ceramic parts so I made substitutions from a piece of panel switch hardware and a rubber grommet. They seem to work great together to maintain screw tension and prevent ceramic to ceramic contact that leads to looseness over time. The screws under the chassis use cut washers to keep the screw from loosening.

[Mike/W8BAC]

I found a Delrin  rod and turned it to the size needed. Like you, I puzzled over the need for them (the cardboard washers) and figured if the engineers saw a need for them, that was enough for me. Delrin isn't the best choice for the job but it is holding up fine.

 Bill Carns, N7OTQ had an interesting problem with his 20V-2. It was a hidden arc within a ceramic standoff. A crack had developed in the middle of the standoff between the ends of the threaded  bolts. The arc, and the arc damage was totally hidden. We think the previous owners gave up and if it wasn"t for Bills determination the transmitter might have been scrape.

What do you plan to use for GTO wire?

Mike

[W3RSW]

I always thought the paper washers were to keep the ceramic from cracking during initial installation, especially antenna cone insulators on the mostly hollow Johnson matchbox and the like.

Sort of intuitively knew it would help keep the screws under tension, but thought that would only be immediate protection from over-tightening during installation. Certainly the paper would soon lose its first period elasticity.  The spring action of metal locknuts are the primary tension actor in the long run.
Paper 'insulates' the brittle ceramic from not-so malleable aluminum or steel panels or chassis during installation torqueing.

[W7TFO]

Early use was thin cork discs.

Later builds used translucent plastic discs.

Sp.  "Delrin"

73DG

[Opcom]

GTO wire would be 15KV outdoor rated 14 GA stranded with PVC blend insulation. There is a spool here. It has to be found.
http://prosites-llcwccd.homestead.com/GTOINFO.html

Any special areas will be wrapped either with 3M self-fusing Ethylene Propylene Rubber tape or covered by some military spec 2-3 mil polyester? dielectric tape. It's been used on a tesla coil of Jacob's to reduce arcing. The 3M '23' tape is super for insulating. the milspec stuff is slick and can also be put on the bottom of an RF deck and on the brackets it sits on, to slide in and out easily. Been tested on the 70 LB Tucker RF deck!

http://solutions.3m.com/wps/portal/3M/en_US/NA_Communication_Technologies/Home/Products/~/Scotch-Rubber-Splicing-Tape-23?N=8694168+3294318588&rt=rud

[Mike/W8BAC]

Thanks Dennis, I corrected the mistake.

[Opcom]

Today some wiring was done on the primary side. The wire is #6 THNN stranded and not very flexible, and all of the crimp on lugs were soldered after crimping as is the practice here so it took a long time. THNN wire has a PCV insulation and a nylon jacket The wire has 19 strands.

The Powerstat sections were wired in parallel with the outputs going into the equalizing coil. From there the red lead goes to the 240V terminals on the plate transformer and the black lead from the "0" tap on the plate transformer goes back to the common terminals on the Powerstat.

The two primaries are wired 'out of phase' according to the terminal boiard however that is the correct way because the secondaries must be out of phase, not in phase as they would have been in the original 3-phase application.

The Line side of the powerstat has only the one side of the line "L1" done. It is going to a triple pole mercury contactor. There is another contactor for the other line "L2" that will control the Common input of the Powerstat.

[Opcom]

more. That is all #6 wire.

The white wire ties hold the wire in place on the rack and are not very tight so there is no chafing. There is ample room in the center section where the racks are bolted together for running all the wires. The space is about 2-3" wide.

[Opcom]

The triple 4-400 deck for the Tesla coil is coming along. The sockets are being wired and the 5VCT 40A filament transformer was mounted to a wood reinforcement on the backside of that shiny panel. The cooling is going to be a trick. Probably a sheetmetal plenum will be made on the blower and some hoses will be run to the 1" PVC pipes coming down from the air system sockets. There may not be enough pressure. It will have to be measured.

[Opcom]

This rack is the power amp section. The other rack shown before is the control, power supply, and modulators.

[Opcom]

No pics of last Sunday's work but didn't get so much done. The other side of the line to the Powerstat was wired to the remaining 3x mercury contactor and the order of the circuit breakers was sort of decided on.

The Mercury contactors had to be moved rearward a little to give more room to the bottom of the MV rectifier chassis. Things will move much faster maybe after all this heavy gauge wiring is done. Its slow because it takes much time to solder the terminal lugs onto the large wire.

[Opcom]

Heavy wiring about done. Power cord made. 50A should be more than enough to run the tube full bore though the power transformer can handle more it won't be called upon for excesses. The smaller wire of #14 can be used for primary on almost everything else because of low current  consumption. The blower may be an exception when starting. The red and green give a festive christmas time look. #14 may seem small but at 20A it is 1V for every 10 FT round trip, and it won't be seeing anywhere near 20A on 240V. The breakers may be as shown from right to left main, blower and controls, filament, plate. The breakers have marked Line and Load sdes, so that is being observed. the Line / inputs are at the top.

 

[Opcom]

The filament transformer is 208V so a 230-208 autotransformer is being looked at. It is designed for stepping up 208 to 230 but is there any reason it can not step 230V down to 208V?

Filer name plates shown, and dug out the manly SS diode banks.

On the choke, it is rated for the inductance at 1.3A. The max current on the amp should be 2A but that is the single tone or CW rating and AM will be lower.

[Opcom]

I think we're well past DO-cased parts at this stage.

next some mechanical work has to be done to mount the transformer. It needs some angle iron to strengthen the planks so the weight is distributed. Same for the other side with the choke and capacitor.

[W3RSW]

What did those 1n3495 diode strips come out of?
Pretty beefy!

And your the wing nut king.

[W7TFO]

Now we are waiting for the "IT'S ALIVE!!!" scream...

73DG

[Opcom]

What did those 1n3495 diode strips come out of?
Pretty beefy!

And your the wing nut king.

Those are from a BC transmitter to my knowledge. Maybe someone can say which one. The wing nuts let a person quickly change out a bank of rectifiers. Almost as easy as replacing a tube.

Now we are waiting for the "IT'S ALIVE!!!" scream...

73DG

I was tempted to plug it in and play with the powerstat, a case of 2500 Ohm 225W resistors, and a clamp on ammeter. Rollicking good time like a German fun house.

[Opcom]

Repaired the h/p M.O. for Jacob's Tesla today, then got to some ampwork.

There was a situation where the floorboards sagged in the center. Being supported only at the outer edges it might have been expected of them. A cold rolled steel angle iron of 0.125" thickness and 2" width was proved out. Just placing the Plate transformer on them straightened up the floor pronto. So, time to buy some lag bolts and do some drilling.

Nearly thought there might be some blasting, but avoided it by observing all safety precautions and taking a healthy spoonful of fear of arc flash. At least I had the decency to plug it in myself instead of cajoling someone else into doing it. I am a decent sort.

The 240 was routed to the main breaker in the rack panel and from the second breaker flying leads were fastened to the Mercury Contactors' energizement coils.

The result was that the main breaker became the on/off switch, the second breaker became the control for the power to the Powerstat, which is also controlled by the Plate breaker. None of this matters right now because all of them are rated the same 70 Amps, but it made me have to turn on three breakers to apply the juice.

The energizing current for the plate transformer turns out to be 5A @ 218V. The secondary made 6430V with no load. There is a noticeable humming buzz from the plate transformer, I guess it is normal on a large unit. With the rack enclosed it should not be noticeable.

The 3 phase blower was played with a bit. I'm leaning towards a VFD over the usual method.  Starting current's a monster.