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Transmitter Cabinet layout?




 
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KI4YAN
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« on: November 12, 2017, 07:28:28 PM »

I have finally gotten my radio room built, and my rack cabinet rolled into the corner where it will sit. I've had some of these parts for almost 10 years now, and things have been put together, taken apart, put together, taken apart a few dozen times...

I am working on trying to get things organized and actually get the framework of this cabinet going, and am looking for advice on how best to manage airflow in and out of the cabinet, how to deal with RF and AC in and out of the cabinet, and ideas on how to mount some of the heavy iron. I haven't built a door for the back of the cabinet yet.



It's a 1950's DuKane intercom rack, the rails are 19" wide but they were originally not drilled or tapped. Equipment was just stuck in the rack and holes drilled willy-nilly. Some of them line up, some don't. The plan is to lay the rack on it's back and set panels in until the front face is full, preferably the panels in their final resting places, and drill & tap the rails for the correct screws in the correct spacing-if I need to weld up and grind back a hole, I can do that easily.



I have accumulated a small pile of blank steel rack panels-these are the ones that are bolted into assembled racks during shipping to help them keep their shape, so some of them are 20g, some are 18g thick. There's also a 1/8" thick aluminum one in there and another chunk of aluminum that's 1/8" thick and 20" wide-I'd have to shave an inch off to make it into a rack panel.

This is the HV transformer, it's 5" wide and 7" tall, and about 4" thick, as far as the core dimensions go. 1KVA at 2300V.



The modulation iron is roughly the same size-it's a Kenyon T-495.

Over the years, the plan has slowly evolved, and I've finally nailed down some specifics. I have my RCA Ham Tips exciter built, the VFO is temperature stablized (after several years of goofing with it) and the drift isn't too bad. I also have the little SS rig I have been working on, and plan to have an auxiliary jack that will have keying and audio outputs so I can use it as an exciter as well.

The final amplifier will be a 4-125A, modulated by 4-65A's. I considered a pair of 4-125's modulated by a pair, and also a pair of 4-65's modulated by a pair, but this configuration seems to take best advantage of the parts I have already, and I can crib directly from the Eimac data pages, where data for the 125 by 65's is layed out and listed, with grounding recommendations and all. I also have a few ARRL handbooks that have similar final amplifiers.

Tonight, I am thinking about how to route airflow through the cabinet, and how to keep the cabinet stable on the casters. Airflow seems like I might put some vents in the bottom panel, and vents in the top of the cabinet or in the top panel. Stability seems to demand putting the heavy iron as low in the rack as I can get it, but then I wonder about putting the modulation transformer down next to the power transformers-worried about hum pickup.

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Patrick J. / KD5OEI
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« Reply #1 on: November 12, 2017, 08:31:12 PM »


Opcom comments
BTW sorry for the text size, it was not able to be corrected, no idea why it happened.

I have finally gotten my radio room built, and my rack cabinet rolled into the corner where it will sit. I've had some of these parts for almost 10 years now, and things have been put together, taken apart, put together, taken apart a few dozen times...

I am working on trying to get things organized and actually get the framework of this cabinet going, and am looking for advice on how best to manage airflow in and out of the cabinet, how to deal with RF and AC in and out of the cabinet, and ideas on how to mount some of the heavy iron. I haven't built a door for the back of the cabinet yet.

If you are absolutely sure everything will fit in the cabinet, that is great, and a fine time to work on the cabinet and the mains input cable/fixtures for it, and cooling.
It is unconventional but if you anticipate a dusty floor then it is not a bad idea to intake the air at the top and exhaust at the bottom. It works well as long there's enough airflow to create gentle turbulence. It also gives an opportunity to push air down onto the final RF section if that's to be at the top. It also presumes a closed cabinet except for the inlet and outlet. That's a real nice looking cabinet!




It's a 1950's DuKane intercom rack, the rails are 19" wide but they were originally not drilled or tapped. Equipment was just stuck in the rack and holes drilled willy-nilly. Some of them line up, some don't. The plan is to lay the rack on it's back and set panels in until the front face is full, preferably the panels in their final resting places, and drill & tap the rails for the correct screws in the correct spacing-if I need to weld up and grind back a hole, I can do that easily.

Perfect! Done than several times and on steel Bud racks so full and heavy that a block and tackle from the ceiling beam was required to stand them upright! Watch out for pinching/severing fingers when putting the final panel into place, that is the only dangerous part of the exercise.



I have accumulated a small pile of blank steel rack panels-these are the ones that are bolted into assembled racks during shipping to help them keep their shape, so some of them are 20g, some are 18g thick. There's also a 1/8" thick aluminum one in there and another chunk of aluminum that's 1/8" thick and 20" wide-I'd have to shave an inch off to make it into a rack panel.

This is the HV transformer, it's 5" wide and 7" tall, and about 4" thick, as far as the core dimensions go. 1KVA at 2300V.



The modulation iron is roughly the same size-it's a Kenyon T-495.

Over the years, the plan has slowly evolved, and I've finally nailed down some specifics. I have my RCA Ham Tips exciter built, the VFO is temperature stablized (after several years of goofing with it) and the drift isn't too bad. I also have the little SS rig I have been working on, and plan to have an auxiliary jack that will have keying and audio outputs so I can use it as an exciter as well.

The final amplifier will be a 4-125A, modulated by 4-65A's. I considered a pair of 4-125's modulated by a pair, and also a pair of 4-65's modulated by a pair, but this configuration seems to take best advantage of the parts I have already, and I can crib directly from the Eimac data pages, where data for the 125 by 65's is layed out and listed, with grounding recommendations and all. I also have a few ARRL handbooks that have similar final amplifiers.

Tonight, I am thinking about how to route airflow through the cabinet, and how to keep the cabinet stable on the casters. Airflow seems like I might put some vents in the bottom panel, and vents in the top of the cabinet or in the top panel. Stability seems to demand putting the heavy iron as low in the rack as I can get it, but then I wonder about putting the modulation transformer down next to the power transformers-worried about hum pickup.

Cooling is twofold then, cooling for each power tube and then general air clearing of the rack.

If you have the room to allow a 1U space under the lowest panel/chassis, then the bottom floor of the rack could be an air outlet/inlet easily. If I may suggest that one of those 1U fan panels will slide right in. If equipped with the usual 9 screaming TA450 fans, some DC fans would be much quieter. You only need 100-200CFM. A couple rows of holes or a screened slot 1-2" tall in the rear door at the top could be the outlet/inlet for the cooling air. I am guessing you will have a small blower with hoses arrangements or two small chassis-mounted blowers to cool the modulator and RF power tubes.



Why I mention it is from experience with internal blowers but no cabinet blowers in the AM transmitter here. The builder put a blower under the 4-1000 and I added one under the 3-500Z modulators when I installed those and removed the previous tubes. There was no forced air to clear the cabinet and the set got extremely warm, having only heavily screened louvered back doors. The convection was nowhere enough. The subchassis of the bias and screen supplies and everything else inside there was being heated by the big tubes' circulating waste heat to where it was all very hot.

The problem was totally solved after adding a 400CFM low speed centrifugal blower from a VAX6000 on top of the rack where there had been a heavy aluminum panel with many very very small perforations. This kind of fan is not just the most common thing, you will probably want something else, although it is about 4" deep, exhausts sideways 360 degrees for 'zero clearance' between fan and wall, would fit perfectly in/on the backside of the door of a small rack.



That blower is 24VDC but runs silently at 14VDC and does a wonderful job. This set is tall, yours is not and I assume you don't want to put anything on top. The point of that is a large low speed blower run at low voltage, or a bunch of low speed high pitch fans in a 1U tray should be so quiet that you will hear the internal blowers more than the cabinet blower.



[/size]
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Opcom
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« Reply #2 on: November 12, 2017, 08:36:20 PM »

sorry about the mess.
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« Reply #3 on: November 12, 2017, 09:16:05 PM »

A few thoughts that are my initial impulses.

4-65s are a bit light.
Use the 4-125s, you can always go up to 4-250s or 4-400s... same sockets.

You said you can weld. It might be easier and simpler to buy pre-drilled strips that are for
racks from one of the rack manufacturers. The biggest ones are Bud and Premier. You may
find others online, since pro audio people use them to go to wood or 'glass road cases.

Have you decided on your B+ voltage? Aka, cap input or choke input PS?

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KI4YAN
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« Reply #4 on: November 12, 2017, 11:03:39 PM »

I thought about 125s by 125s, but then I look at the power limits-375 carrier is the legal limit for me, 250W carrier isn't even 2db down there...not even half an S-unit on receive.

I had planned on capacitor input, with a diode bridge. The power transformer isn't centertapped, so it needs a bridge rectifier. That would give 3250V, so I started look for a choke for choke input...would give me 2070V. I am thinking maybe run the transformer on a variac and cap-input for 2500V output, is the correct answer.



I have five of these for filter capacitors, though. They're rated 115uF at 2300V, and were removed from defibrillator units. It's very tempting to run 2kV and give up some power, but I could also put these in series for 4600V rating:



If I put them in series, they'd be ok for 4600VDC at 57.5uF each. And I'd have one left over. What I don't have is a filter choke yet-have been more focused on other things and minding my bank account than seeking out one right now. I figure everything else has come cheap and easy if I waited long enough, the choke will too.
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« Reply #5 on: November 13, 2017, 07:13:35 AM »

Well, speaking from only my personal point of view, I'd not go through the trouble to build a large transmitter with less capability than the "legal limit".
Certainly I'd want >100% positive peak capability for the modulator.

The 4-65s are a bit small... really.

Otoh, the Collins 30k transmitter is ~250w rated and puts out a nice signal. (4-250s, iirc)

Imo, a variac is a very good idea, especially in the case of output tubes with greater than the "legal limit" potential.

At 2kv the 813 is maybe a better tube choice. You might want to look at the "K1JJ" 813 design that has been built by
quite a number of AMer's...

Suggest that you check into those Aerovox caps, see if you can find the mfrs specs?
The reason I suggest that, is that these caps may not be designed to handle high ripple.
They're rated very high for the size package, which is terrific, but check into the ripple handling thing.
That would make them excellent for a "second position" in a "pi" filter in a power supply.
And, no, don't run them close to the limit... unless ur 100% certain that the PS voltage is incapable of
rising, no load, above the max rating.

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« Reply #6 on: November 13, 2017, 10:48:51 AM »

Bear bear me to it, but beware of the photo flash style caps (which is what those are commonly called).

In the early days of the internet, stories galore about people blowing power supplies up with those style caps.  The CBers bought them in droves.

Turns out they would tolerate about 60 pct of published ratings and any high current demands would make them heat up and kaput city.

They are designed to charge up once, dump their stored energy and then sit until called upon to do so again.  They are NOT designed to be run near limit nor to be called on to continuously filter.

Two, in series, I'd call good for maybe 3kv, thereabouts.

--Shane
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KI4YAN
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« Reply #7 on: November 13, 2017, 02:18:26 PM »

The Collins 30k was a single 4-125a modulated by a pair of 75th, for 250W AM output. RCA's BTF-250 was a 250 carrier FM transmitter, again a single 4-125a, driven by another 4-125A in the doubler.

I am aware of the legal limit thoughts, and plan on making sure the RF deck components can accommodate a 4-250 instead of the 4-125a. I just plan to run the 4-125a, because the 125s are available and cheap-the 250s not so much. The ability to run both by switching the taps on the mod transformer is appealing.

The thought has occurred to run the 125s in parallel too.
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KU8L
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« Reply #8 on: November 13, 2017, 03:20:34 PM »

Sweetwater Sound has reasonable priced rack rails in a variety of lengths.    Ft Wayne area if I recall.

Curt
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KI4YAN
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« Reply #9 on: November 13, 2017, 11:52:40 PM »

I am familiar with all the rack manufacturers-I used to work with them on a daily basis. The rails in this rack arn't totally ruined...just a little "swissed". I layed the rack down today and started laying in panels to find out how many holes were in the way/not positioned correctly/were correct and only one existing hole was mis-drilled. Won't be a problem. There would be a lot of cutting and grinding to remove the old rails anyway, as they are folded as part of the outer sheet of the cabinet, not a separate part.

However, I also finally got an answer from Aerovox on those capacitors...they are only good for 80 hours of service at the full ratings as filter capacitors. Not good enough. So, since I have no filter capacitors OR filter choke, I'm back to the drafting table on the power supply...the lower voltages are all present and accounted for in the transformer/filter cap stocks.

Assuming I can find a 4-250 in usable condition, I could modulate one with 4-125's and only slightly exceed the mod iron's ratings. The downside is that the 4-65a's will make ALMOST the same power-within 30 watts!-as the 125s at 2500V in modulator duty at the same voltage.

If it weren't for the voltage issues, I could set up a single 125 modulated by two, and swap the 250 in later, by configuring the power supply as a capacitor-input filter. This would produce 3250V, which I'd have to drop down to 2700V for the modulator, so I'd need a 200W, 3000R resistor, and even then the voltages would soar in the modulator on key-up due to the drop in current.

I am currently hunting 4kV rated filter capacitors, and contemplating selling the Eimac tubes or trading them for something happier at lower voltage-I could run the transformer at half voltage by wiring the primary for 230V and feeding 125v to it, producing a 1600V/430mA output, but then I can only get 500VA from the 1000VA unit.
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« Reply #10 on: November 14, 2017, 02:21:29 PM »

If it weren't for the voltage issues, I could set up a single 125 modulated by two, and swap the 250 in later, by configuring the power supply as a capacitor-input filter. This would produce 3250V, which I'd have to drop down to 2700V for the modulator, so I'd need a 200W, 3000R resistor, and even then the voltages would soar in the modulator on key-up due to the drop in current.

Hi Jacob,

Sounds like a fun project.

A series resistor feeding the HV for the modulator would create severe distortion due to the varying drop across it as the modulator tube current swung around under modulation. Using a class B or AB1 audio stage needs as stiff a high voltage as possible to insure cleanliness. A power resistor  would work fine with a class A or class C stage (CW, FM or unmodulated) that uses constant current, but not well for "current swinging" linear AB or B operation.   Plus it would be a huge waste of power in the form of heat.

The usual way is using a big Variac or better yet, use a lower tap on the plate transformer if available.  Even a large 240 to 240 isolation transformer usually has taps to allow the drop you need efficiently.

Also, if you use tetrodes like the 4-125A, etc., be sure the screen and grid voltages are well regulated. The screens do need to draw current for their cleanest service as well as putting out rated power. Usually a simple zener regulated screen supply is OK. For the grid bias, use a string of forward biased diodes or a zener in the filament transformer center tap to get the proper idling current. This gives rock solid bias regulation.

You can also tie the grid and screen together, drive them p-p, and use the bias diodes in the CT as a complete system. This is the easiest and will be very close to the tetrode config for cleanliness.  Your audio driver needs to be pristine with no transfomers, if possible. The modulation transformer has plenty of undesirable phase shift to handle if you plan a negative audio feedback loop, which you should have. (If the mod xfmr is included in the loop)  Usually just one transformer is the max in a NFB loop or it may become unstable.  

T
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KI4YAN
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« Reply #11 on: November 19, 2017, 02:36:02 PM »

Now that I have to find new filter capacitors AND filter chokes, the question of cap input/choke input comes up. With modern diodes and capacitors, is there significant advantage anymore to one over the other? Aside from output voltage, clearly.
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KA2DZT
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« Reply #12 on: November 19, 2017, 02:53:28 PM »

Now that I have to find new filter capacitors AND filter chokes, the question of cap input/choke input comes up. With modern diodes and capacitors, is there significant advantage anymore to one over the other? Aside from output voltage, clearly.

A choke input filter will give you better voltage regulation.  Especially if the rig is plate modulated using class B modulators.  I think your xfmr will produce about 2.1KV with choke input, probably even more with today's 125vac line voltage.

You can increase the output voltage by adding another xfmr in series with the HV xfmr.  Example, using another xfmr with a 400 volt winding and its own set of diode rectifiers, connect the diode output directly to the (un-grounded) negative terminal of the HV FWB  diode rectifiers.  Your output voltage will increase by the 400 volts.  The current rating of the 400 volt xfmr has be the same as the HV xfmr.

The VA load on the PS is divided proportionally across the two xfmrs.  Makes for a much better supply than using a cap input filter supply and a Variac.

You can lower the output voltage by simple shutting off the primary of the 400 volt xfmr without disconnecting any other connections.

The secondary winding of the HV xfmr has to be capable of running 400 volts higher above ground.  Should be zero problem with your Electro xfmr.

Fred
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KI4YAN
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« Reply #13 on: November 19, 2017, 07:06:52 PM »

That presents an issue-2100v would be fine for a plate modulated 4-125a, and would work fine with the class AB1 modulators. But it would be a little low for a 4-250a-but would still work, didn't the globe king 500 run around that?

I can always boost the voltage by using the 140V tap on the input Variac.
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KA2DZT
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« Reply #14 on: November 19, 2017, 07:21:26 PM »

That presents an issue-2100v would be fine for a plate modulated 4-125a, and would work fine with the class AB1 modulators. But it would be a little low for a 4-250a-but would still work, didn't the globe king 500 run around that?

I can always boost the voltage by using the 140V tap on the input Variac.

Don't know anything about GK-500s never had one.  All my xmtrs are HB.  You can overheat a xfmr by applying too much voltage on the primary.  Not sure if this would be the case with the Electro xfmr. 
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KI4YAN
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« Reply #15 on: November 19, 2017, 11:46:16 PM »

Either way-I need to come up with a good filter capacitor solution.

Right now I'm looking at using a series string of 450V electrolytics, 10 caps in series. By using 100uF caps I could make 4kV, 10uf units-although they may not be very compact, they would be very inexpensive. I suspect a 500v leeway in the voltage rating should be enough?

I've also found a few chokes that might be useful, 10h and 12h, both sub 80 ohm DCR. By using them in the ground leg, would the insulation requirements be reduced? (I'm thinking insulation relative to the core)
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