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starting work on the amp




 
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W1ITT
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« Reply #200 on: December 01, 2016, 09:04:43 PM »

All this talk about putting rubber caps on high voltage power supply connections and components scares the heebee-jeebies out of me.  I have designed and built transmitters running up to 20 KV at the anode, and operated quite a few running less than half of that.  I'm not afraid of high voltage, but I respect it.
Why anyone would want to reach into a live high voltage supply completely escapes me. I admit to having put a clip lead across a door interlock, but I always operated the rig with my paws well out of the way of high voltage.  These rubber high voltage condoms would only to serve to convey a false sense of security and lead to less than fully mindful behavior.  There are few enough fellows doing good homebrew work left these days, and it would be a shame to lose one halfway through the project.
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« Reply #201 on: December 02, 2016, 08:33:47 AM »

Sound advice Norm!
I spent a year rebuilding a spent Collins 20V-3 transmitter. In the process I cleaned and tested all of the substantial high voltage interlocks and safety devices. For a 1960's design this transmitter is a marvel of analog logic and mechanical implementation.
I have great confidence in the interlocks but I still use a bang stick first and use good practice meter handling. One nice thing is the large picture window on the front of the transmitter. I can hang a meter inside the TX, run the leads and close the doors, reset the interlock logic circuit and key the TX without putting my hands inside.
On another TX newly delivered. I needed a circuit test on the HV area. I opened the door and was about to reach in. Something stopped me! I rigged a Bang stick and reached in with two fingers on the wood handle. BANG! the spark almost flipped the stick out of my fingers. The rebuilder had stripped the interlocks from the TX. No excuse, I should have done an inspection first AND used proper safety but I was in a hurry.
Don't be dumb like me.
We can see that Patric Knows what he is doing. He is probably passing his experiences on to his apprentice. All I can say is NEVER be in a hurry and always take a moment to THINK BEFORE you reach in.

The spark plug silicone rubber caps are interesting.

KJ4oll/Frank, That is cool shack. with a lathe, mill and brake. That's all I saw. I can only imagine. Git that work of art on the air. How do you put up with the fan noise? Is it made to live outdoors? I saw it sitting outside in one of the online pictures.

Mike
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« Reply #202 on: December 02, 2016, 10:09:35 AM »

Patrick has cats, and they can get into the smallest spaces.  He doesn't want harm to come to them, exploring vs. HV hence the insulator question.

73DG
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« Reply #203 on: December 03, 2016, 12:22:38 AM »

I can't find the post about someone intending to reach into a live HV supply but I think there is one that mentioned  enclosing the HV areas because the door facing the controller may have to be open in case of troubleshooting. The rubber covers are for cosmetic and marking purposes and their use is nothing to worry about. They cost money and so might not be used, in accordance with the 'junkbox' rules.

There are these nice "HV shorter-outers" here. Eventually one should be put inside each of the two rear doors to operate after the interlock switches (yet to be rigged) turns off the HV by opening a contactor. It should normally never be needed to have the HV on with the back doors open although during the building it may be done from time to time.

It has not been decided how to handle the HV-grounding function. Shorting a 31uF cap is not a good idea. There's an idea already for the 'hv shorting stick' that there should be two terminals for it. One through a resistor bank to limit the current to non-fireworks level and discharge in a second or so, and the second terminal direct to the cap to just hang the stick on. This kind of thing also needs to be fused with a HV fuse, maybe the #30 or 28 wire type  at some HV circuit location so that any direct short will not cause carnage. There is room for a few wire-type HV fuses in different areas. I think there are enough insulators left to do them.

One rule here is not to break the plane of the rack enclosure while it is plugged in. It's a pain to have to unplug and discharge, rig the test meters and stuff  then re-plug in and test but that is the only way to avoid accidents.

The cats are not allowed in the building. It's too dangerous a place for them.


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« Reply #204 on: December 04, 2016, 03:37:44 AM »

In the diagram, I am suggesting a simple way to have the cabinet interlock, HV removed, and also provide for the shorting stick's safe use (non-lightning/exploding discharge). To have a 'soft' discharge of the capacitor, a resistor has to be put in series with the discharge switch. The resistor will be subject to the 4000-5000V of the HV supply, so a typical single "200W tubular ceramic" of a low enough resistance to bleed off the supply when used is out of the question. Fortunately the bleeder assembly from the Henry generator is intact. It has what look like four 100W resistors in series, all mounted to a metal plate with ceramic standoffs. The may serve the 'soft discharge' purpose. The diagram shows 3 fuses. I have no objection to more than one if it makes the unit less likely to blow something in case of trouble. They cost so little to make.

It's time to start thinking more the RF section. The basic chassis will be made from a Henry RF generator. It was not quite complete so I can't find a model number. Its power supply is 3-phase, apparently 208V between phases because that is the tap used on the filament transformer.

Looking at web pages, I don't see how to tell which it may be, the 2000D or 2500D. The 3000D uses an HN connector, the smaller units use N connectors.
The RF deck says Henry 13.56MHz, CAT # 9600-0050, SN# 28-E4-90. From all that I don't know. Maybe it is not so important.

The RF deck has already a 13.5MHz grounded grid RF amp installed complete with PI-L tank and PI network input matching. This one needs little but cleaning and the replacement of one of the ceramic standoffs that support the filament 'socket' under the chassis. The unit includes a coaxial filament choke, provisions for metering signals, and apparently the 'standby' bias resistor, cathode fuse (1.5A) and safety resistor, and other goodies. As much of this will be used as makes sense.

There are options for something like this, in order of increasing usefulness and trouble required:

1.) Put on 20 meters (14Mhz), or other single band including 50 or 144Mhz.
2.) Multiband HF amplifier on ham bands.
3.) HF amplifier covering 2-30Mhz in 8 or 9 bands, or better to include 160M.
4.) HF+6 amplifier, 1.7 to 30Mhz in 8 or 9 bands, plus 50-54Mhz in "high gear". -very ambitious.

I want to try for the 4th option. If the 50MHz tank is built in the existing chassis, and the HF tank coil is mounted right above the tube anode, this may be possible. The biggest thing may be keeping lead length short and stray capacitance lowest. I already learned about parasitics from COL Tucker's transmitter and its 20" piece of wire from the anode to the RF choke!

At the least, the PI coil on hand should provide coverage of all the ham bands because it was from a full-coverage hf transmitter.



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« Reply #205 on: December 05, 2016, 01:47:02 AM »

Some pictures of this stuff. 5 pcs. UCX 375pF 14KV caps. The schematic calls for larger sizes but let's see what happens.
 
This transmission, 37 turns of the 1/4" shaft turns one coupler 9 turns and the other 18 turns. It is maybe for using a roller coil and vacuum cap together?


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« Reply #206 on: December 05, 2016, 01:52:11 AM »

some insides of the starting point


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« Reply #207 on: December 05, 2016, 01:53:02 AM »

some insides of the starting point and also the filament transformer. It has 5 taps for 280-240V input


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« Reply #208 on: December 05, 2016, 02:32:36 AM »

A couple of the caps have an unfinished look to the outside of the plates. How to know of this is bad or what? They were cranked open til the inside could just barely be seen and that part looks great. Could they have been overheated? Do some of them have a less than perfect appearance?


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« Reply #209 on: December 05, 2016, 10:52:59 AM »

You should also have a 240 to 120 isolation xformer in there that has a couple taps.

This allows it to run on 'two hots and a ground'  safely.  You can save the expense of the neutral wire run.

I elected to bring neutral over in my runs for future proof.   Never know what will end up in the shack.

I was able to get a 40 meter tank coil in one of those rf decks.

--Shane
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« Reply #210 on: December 10, 2016, 09:28:55 PM »

That isolation transformer looks like it's good for 750VA. Very nice.. Thanks for the tip on it. I thought it was just another multi winding unit for the rest of the guts in the generator - which are mostly missing.

A helper brought it up that a Tungsten type filament should be soft started and gave me some links to papers on this and the chart. It goes for Tungsten and Thoriated Tungsten. "W" being the operative element.

http://www.w0btu.com/miller-larson_effect.html

http://www.r-type.org/pdfs/thoria05.pdf

https://www.photonis.com/uploads/lit...rica-Paper.pdf

http://www.tubecollectors.org/federa...beapps(70).pdf

http://www.cpii.com/docs/datasheets/79/3CX15-000H3.pdf

ITT recommends limiting the current to 150% for medium size tubes and 250% for large ones. Of course this could mean 50-100KW as a medium one.. But the issues outlined in the papers about stresses of inrush should be taken seriously. Smaller tubes like the 813 may not suffer as badly but in the boxes of old tubes I get from time to time there have been have had a few of those and many other old Tungsten type tubes with open filaments.

How can one divine the temperature vs current figures for a Tungsten filament? One answer is a chart in one of the publications. There is also a volume resistivity specification for almost every conductive element.

The region of interest for soft-starting is the warm-up between 250-300K where the filament resistance may be 10% of the operating value and 1400K to 1700K where it may be 1/2 to 2/3 of operating  value.

Any startup that is gentler than just throwing the switch is an improvement. Even the quick turning up of a variac is good. MOVs designed for inrush current limiting are used by some and others use a step start. One question is how to have a gradual startup and avoid varistor type elements as they tend to wear out due to their natural state being one of 'hot' - maybe no worse than a couple of transistors unless transistors are well-oversized.

I think a 75 Ohm resistor on a 200-220V primary may limit the inrush current to around 150%.



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« Reply #211 on: December 10, 2016, 10:00:36 PM »

Soft-starting tube filaments via a time ckt isn't really necessary in my opinion, nor is it done in most commercial transmitters of your power level.

Proper technique is to use a big wirewound pot in the primary of the fil trans, rather than a Variac....the pot senses the inrush current and provides the desirable 'ramp-up'. 

The pot should provide the target voltage at 60% rotation or so, and then you have a bit of leeway to increase emission as time goes by.

73DG
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« Reply #212 on: December 14, 2016, 11:46:36 AM »

I'd like to use a resistance with a more substantial portion of power-handling than the 10 ohm one. I'm stuck with a 200 or 208V transformer tap and an actual 220V line voltage so with about 2A, the max R is 10. The line voltage varies a bit as well.

For fine adjustment I'm leaning toward an Amertrans line voltage adjustment variable autotransformer. It does +/- 15% and has a worm drive so it's easy to set precisely.

It would be very fine to have a smaller handwheel in the middle of that big one. There was a concentric pot setup before.
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« Reply #213 on: December 18, 2016, 10:58:18 PM »

The filament transformer for the 3CX3000 was mounted where the leads should reach across to the RF chassis in the adjacent rack. Its 10 Ohm resistor can be next to it. 
It could have been mounted in that rack but it would have either taken up otherwise free vertical space or had to be between the HV transformer and solid state diodes.

Here's a Jennings VMMC-1000 is a 20KV unit covering 50pF to 1000pF. I hope there is some way to use it for the tuning. 50pF is a lot. There is an older article for an 813 amp on 10M that uses a tapped 10M coil and puts the tuning cap on the tap because the 813 plate capacitance is high on 10M. Wonder if the scheme would work to the reverse.. if the tuning cap's minimum value is a bit large. What to do there?

The two coils there are from a 10KW transmitter and one should be large enough work in a high-C circuit without too much loss. Hope they can be used.


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« Reply #214 on: December 18, 2016, 11:41:52 PM »

The schematic is just to recall that 813 output section.

the pi coils are huge. The 1-turn loop is about 9" diameter.


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« Reply #215 on: December 18, 2016, 11:42:26 PM »

fil choke


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« Reply #216 on: December 18, 2016, 11:47:38 PM »

Patrick,

Figure out how much C you want to take away.

Convert C to Xc.

Xl is given by the product of  Xc*-1.  Convert Xl to L.   Add this between Ctune and the pi inductor.

That's one way.

Method two.   Find Xc and Xl as above.   Tap the pi tank coil at Xl.   Now,  use a GDO or VNA and resistor backfed to design your tank.

Method 3.

Use a L-Pi.  Ua 4 to 600 ohm image impedance.

The last ten  meter 3k I did performed ten to fifteen with a standard 25 pf minimum cap.   I did NOT use any trickery like above.   It will NOT work at the fm portion of ten,  but the owner doesn't go above the ssb portion,  so it fit him fine.  I designed the input and output tanks for 25 mhz.   1 of 2 and 8 respectively.

--Shane
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« Reply #217 on: December 28, 2016, 11:03:57 PM »

Looks like that may work out fine. A small L in series with the slightly too-big minimum capacitance for the highest band will have little effect at lower frequencies right?


So it is coming to the layout. Because the parts are large and cumbersome, the Paint program was used again to made drawings which are then 'pasted' together to get different views.

The first drawing 1 looks OK from a standpoint of short connections and wastes some vertical space, but the antenna matcher stuff should still fit above.

The second 2 is better on space but the pi coil to anode is 11 inches. Maybe the RF chassis can be raised higher to shorten this to something reasonable.

The third #3 is most compact but the anode connection is only an inch from the cabinet wall so that's not a good idea.


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« Reply #218 on: December 28, 2016, 11:08:00 PM »

Putting a bit of Xl in series between Ctune and the pi circuit only serves to cancel out some c.   Xc-Xl=new Cmin.   Subtract the same amount from CMax of Ctune and that is your new max value.  Usually your taking 5 to maybe 15 pf.   Not even noticeable when you try for 80 metros.

Make sure there isn't a resonance.   

--Shane
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« Reply #219 on: December 31, 2016, 03:23:23 PM »

If I understand this correctly,
What I want is to be rid of 25pF.
At 28MHz that 25pF has Xc of 227.5 Ohms
227.5 Ohms  XL at 28MHz is 1.294uH
Because the reactances are equal that is a resonance there but as for where else a resonance may show up due to this added L, it has to be checked for, but it should not get out of the series circuit of just the added L and the tuning C.
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« Reply #220 on: December 31, 2016, 08:40:17 PM »

If I understand this correctly,
What I want is to be rid of 25pF.
At 28MHz that 25pF has Xc of 227.5 Ohms
227.5 Ohms  XL at 28MHz is 1.294uH
Because the reactances are equal that is a resonance there but as for where else a resonance may show up due to this added L, it has to be checked for, but it should not get out of the series circuit of just the added L and the tuning C.


Yes.   

I've not had a problem with resonances on any of the networks I've built.....   But,  I also take a bit more C out than I need.   That way Xc is never = to Xl.   

Use heavy gauge wire.   I used qtr inch tubing.

--Shane
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« Reply #221 on: February 04, 2017, 07:07:01 PM »

Today that old Henry chassis was cleaned out pretty well. I left the two capacitors for now. An insulator had to be replaced on the tube socket as it was broken and that is when I discovered the messy way the socket and grid collet had been assembled onto the chassis.  The filament collet plate is mounted off center of the main hole in the chassis and the grid collet is also off center nut in the other direction, although it can be slid around a little. The tube fits OK and the only contact is with the collet springs but the sloppiness a little disappointing. Must have been made on a Monday or Friday.

The capacitance of the tuning and loading caps in the 13.56MHz Henry chassis was measured. The tuning cap looked like 20-30pF and the loading cap 40-100pF. These values are just the bare caps, without the various doorknob style padding caps that were present. It remains to see what to do with them, whether either can be used.

The filament transformer has several taps on it and a rotary switch was found that should be useful there. It was so dirty but is a kind similar to what is used in RF tank switching so even if it is not good enough for that any more it should be OK for this. Some GC control and switch cleaner with 1-1-1-Trichloroethane cleaned that up right away. The red stuff, hoarded for a reason.
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« Reply #222 on: February 04, 2017, 07:33:49 PM »

This chassis arrangement is looking good. Adding a 4" chassis below the Henry chassis makes room for the large coaxial filament choke that will replace the smaller Henry one. It also puts the anode closer to the PI coil.


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« Reply #223 on: February 04, 2017, 08:01:42 PM »

I found a switch on the fil xformer was unnecessary.

The original generator was designed for commercial service,  so 208, not 220.  On 240, you'll see North of 9.5 volts with 240 on 1-2.  The one I'm working on for a friend now with 244 volts on the 250 volt tap,  still makes 8.8 volts measured at the input to the fil choke.

The fil xformer is tapped for 208, 220, 30, 40 and 50.   I'm using the 250 tap and see 8.0 VAC.  If one wanted more finite adjustments,  you could use the 208 to 110 step-down xformer since it has multiple taps as well.   Series them.

How much does that rack weigh?   I've alway wondered,  they are battleships!

--Shane
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« Reply #224 on: February 04, 2017, 11:25:39 PM »

I found that out.. 10VAC there. so the next tap or two needs chosen as you have done.

The switch is just for variation in AC line depending on season. The line voltage varies a lot in the shack when the a/c kicks on and off, and can be from 210 to 240V at different times. The service there is only 100A and I'm also at the end of the transformer's run in the neighborhood.

I want to find a "230V" output sola type regulator, 500VA, to handle the filament under these wide variations.

The rack itself, the double, is only about 150 lbs. It's very strong and made of steel but its open construction makes it fairly light. The doors and sides add quite a bit more. As it is now with the power supply basically finished, it's probably pushing 900 lbs sans doors.

Spent some time figuring out the existing current metering stuff in the Henry chassis. It's being kept intact and should feed external meters OK.


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