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813 amp project




 
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Author Topic: 813 amp project  (Read 4987 times)
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Patrick J. / KD5OEI
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« Reply #25 on: February 07, 2020, 01:20:02 AM »

Looks really great. I love the pi coil.

Are you planning a tune/operate switch? For some reason the power supply author suggested a 250V one on the secondary. I hope a better scheme will be used if wanted.

Using 100K resistors for the capacitor equalization gives only a very low current (5mA@3KV). Should a capacitor get too leaky, the voltage across it will drop quite a bit because the resistor string can't give up enough current. The remaining voltage will appear across the others. It should not be a huge issue since everything is new but at some point there could be a surprise if they don't all age the same.

Looking forward to more and completing, testing, etc!
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« Reply #26 on: February 07, 2020, 10:32:35 AM »

Very good Chris. As a rule of thumb and the experts can correct me on this, you need about a 1/4 inch separation/1 kV between the hot potential and the flash over point in air. HV insulation will certainly improve that number. In any case at 3 kV without any added insulation sheets, you need an inch to be on the safe side. I recently got zapped on a 1 kV supply I am working on for a small amplifier. Did not observe the rule and had several arc over points! Had to re drill out the card via holes a bit and added kapton tape as an extra measure.

Placing equalization R's in parallel is a good idea. Although in the 1-to- 1.5kV supplies I made and did not do that, but wish I did. On a 3 kV amplifier that I worked on for a ham colleague, HV caps went because of long term aging and heating of the equalization R's! Several of them failed OPEN. We replaced the caps, and soon afterward, the new caps blew again! Lesson learned, replace the HV caps AND the equalization R's.

Alan
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« Reply #27 on: February 07, 2020, 11:27:01 AM »

Chris, some additional comments and again, the experts are here to add or correct.

On this 813 amplifier as published, there is no means of placing the tubes at cutoff or into standby mode. A grid negative supply V is not shown as well no means of monitoring grid current as well relative Pout. Tune up of a GG amplifier is facilitated by those measurements. Plate current alone is not sufficient and in looking at the paper again, is not monitored.

On the PS, he uses the secondary to provide reduced HV out for the voltage doubler. I think the concern here is the transformer insulation breakdown voltage where the CT is not at ZERO potential. Is that a risk? I've used the voltage doubler successfully but it was driven by necessity, did not have the desired size xmfr on hand. However, the center tap was not used and I think that raises a flag.

Alan
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n4joy
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« Reply #28 on: February 07, 2020, 12:15:07 PM »

Thanks for the added information and comments.  I did not incorporate a tune/operate switch for reduced output as the author recommended.  I am not a fan of hot switching and, as you suggested, not having the CT at zero potential.  Yes, there is no grid meter as I will tune by monitoring plate and maximum output via a separate watt meter.  I plan to use the amp with my Lettine 240, which has an output of no more than 25-30 watts.  I can't imagine over driving it at these levels.  The transformer is an Antek toroid with the primary and secondary taps wired in parallel for 800V at 1 amp.  I am guessing I should see around 2100V to 2200V no load.  I would honestly be happy with just a 90-100w carrier.

I will be cutting the phenolic sheet today and, with spacers, raising the capacitor string up as high as possible.

I also used the doubler as it has been impossible to find heavy iron, which is why I settled on the Antek.  The good iron are either in land fills or in the basements/garages of elderly hams who have hoarded them.  I certainly searched and searched but simply wanted to move forward with the project.  

The continued suggestions are most helpful!  I wanted to make this a true junk drawer project and utilize as many resources as I already had on hand (e.g., the modified cake pan for a chassis).
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n4joy
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« Reply #29 on: February 10, 2020, 09:11:47 PM »

Addressing Alan' concern, I custom cut a piece of pheonolic plate , drilled holes to match the screw pattern on the chassis, and added nylon spacers to lift the board.  Let me add that doing this without unsoldering the capacitor string was not easy!  It took me at least an hour to do it properly!  I still have to cut the resistor leads and add some lead to keep them further away from the caps.  The dangling grey wire in the photo is for the blower, which had been mounted but not connected.

I am almost done and waiting on the cabinet and front panel to be powder coated.



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n4joy
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« Reply #30 on: February 17, 2020, 04:54:48 PM »

I had the front panel and cabinet sand blasted and powder coated (hammertone gray/silver).  The most challenging part of the project, so far, was measuring and drilling out the front panel!  It took hours!  I used step bits of various diameters for the drill and the holes for the meters required a fairly large step bit.

I am waiting on a variac I ordered from eBay and will begin testing soon!



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n4joy
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« Reply #31 on: March 23, 2020, 10:19:09 AM »

I'll make one more update as there doesn't seem to be much interest.  I have been searching for some nice 4" plus skirted dials for the plate/load caps.  Otherwise, all has been completed.  The bandswitch and meters were installed.  I will be applying power in a few days and have strict instructions from my wife to only do so when she's home!  Roll Eyes     

I have used Carolina Laser to make phenolic labels for past projects.  They do great work and their online creation tool is easy to use.

I appreciate all the past assistance/guidance. 


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N0WEK
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« Reply #32 on: March 28, 2020, 02:55:56 AM »

That's a very nice looking job!

I'll be interested to see how it runs and how the power supply design works out.

Greg
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K8DI
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« Reply #33 on: March 28, 2020, 10:59:26 AM »

Looks nice!

Where are you (and lots of other builders whose rigs show up here) sourcing the shaft couplings, extensions, flex joints, bushings, etc.?  When I look to typical suppliers I don't find much, and what I do find seems overpriced...

Ed
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AG5UM
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« Reply #34 on: March 28, 2020, 11:36:10 AM »

I'm interested and watching your progress, Keep us updated...now the real fun starts......PowerUp time...
It looks very cool, nice job and great idea for a project..
I'm interested!!!
AG5UM
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KB2WIG
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« Reply #35 on: March 28, 2020, 11:53:47 AM »



I don't let the Warden know when I do 'things'.

Life can be a dream when she's not around for the sh-boom, sh-boom.

klc
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What? Me worry?
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« Reply #36 on: March 28, 2020, 12:22:50 PM »

Two schools of thought on working alone....
1. Alone means alone....no help-on-the-way....no-one to call for help....
2. It's often dangerous to have someone distracting you, asking questions, telling jokes...
    or worse-yet.. startling you!! , make you jump...
    and of course ..if you grab onto 7000 volts, 600 amps... are you really going to need any help??
     or just your burial plot paid up???
3.  maybe your wife could get one of those "Baby monitors", and watch from the house?? just kidding..
Anyway.....
Great project, great work, and like was said... do you really want her to know Everything your doing???
If this is not funny, sorry, it was meant to be, just kidding...
AG5UM
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KD6VXI
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« Reply #37 on: March 28, 2020, 08:38:12 PM »

I dunno.  I had 6kv at 2a ccs xformer driven, 180uF cap bank hit me.

Obviously I'm still here lol.

--Shane
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ka1tdq
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« Reply #38 on: March 28, 2020, 08:42:14 PM »

She's very wise!

When I fire up high voltage projects I have my wife standing by with the phone to dial 911 just in case.

Excellent workmanship! I envy your abilities.

Jon
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