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Old 21st February 2010, 07:08 PM   #21
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Decided on SiC Schottky. Left snubber caps in for now, I don't think it would hurt.
TOTALLY unnecessary! Being majority carrier only devices, Schottky diodes do not exhibit a reverse recovery spike.
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Old 21st February 2010, 08:00 PM   #22
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Time to read some more on these things.
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Old 21st February 2010, 09:10 PM   #23
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MJ puts snubbers on Schottky's too. I dunno why...seems redundant. I've been getting lazy about putting snubbers on UF rectifiers too, but those have been lower current supplies.

Last big supply I built used HFA08PB120 HexFREDs. I have not blown them up yet.
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Old 22nd February 2010, 01:07 AM   #24
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OK. All parts that can be pretty much pinned down have been ordered.
The rest of the parts should be in my parts bin(s).


Now I just need to finalize the iron and address any comments, suggestions, or errors in design based on your input(s).

I figure I can do alot more reading, studying, and analysis. I beleive that the school of solder and measurement is where I am going to learn the most.

FYI. I will not use the big caps during the initial construction and proof testing. Even with lineman gloves, there is too much risk to be prodding and probing during the inital breadboard phase.

Thanks for everyones help so far. Especially Eli's
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Old 22nd February 2010, 01:29 AM   #25
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FYI. I will not use the big caps during the initial construction and proof testing.
I have some 2400 uF 450 volt monster caps. They may not be in the same league as yours, but I was unimpressed with them. I needed some big caps to put in series for my 845SE amp, so I got those. I ran 4 of them in series in an 1100 volt supply. Talk about big, and serious energy storage too. The deal buster came when I could see several volts of ripple across them when the amp was driven to clipping with a 5KHz square wave. Some "small" 220 uF 450 volt Panasonic snap in's worked much better even though they were less than 1/10 the value. I added a 4 uF 2000 volt oil cap in the amp chassis right at the red wires of the OPT, and I couldn't see anything on the B+. I am guessing that the ESL (inductance) of these caps is high enough so that they work well at 60 Hz, but suck at 5KHz and higher.

Further experiments with these caps and a Simple SE led me to realize that a film based motor run cap worked far better, so the big electrolytics are in the closet, and likely will get voted off the next time I do a clean up of the closet.
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Old 22nd February 2010, 01:31 AM   #26
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Thought I would put the latest schematics up for the amp and power supply as it sits now.

I should also than George at tubelab for sharing his power drive



George, you may be right in the big caps. I guess it will be interesting to see what these do. If I don't try I will always have these things in the back of my mind bothering me..... I will run the experiment
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Last edited by SGregory; 22nd February 2010 at 01:36 AM.
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Old 23rd February 2010, 02:36 AM   #27
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Thought I would put the latest schematics up for the amp and power supply as it sits now.
Its hard to keep up with all of the KT88 P-P threads. I looked at your schematics and noticed one thing. Your negative power supply is -90 volts. In my KT88 test amp I needed to increase my negative supply to -105 volts when cranking KT88's hard in triode mode. I was using resistors in the source leads of the mosfets. The 10M45 needs about 20 volts across it to be happy. You may need a bit more than 90 volts in your case.

Also putting the entire B+ on the drain of the mosfet makes it rather warm but it wont be a problem with a good heat sink. I use a rather low +25 volt supply during my testing, since one of the power supplies on my bench only goes to 25 volts. Another very valid reason is that if a mosfet ever decides to short out it will put the full B+ on the grid of your KT88. This will make them very unhappy, and I think you are planning to use some rather expensive tubes.

What are you planning to use for your power transformer?

I built an experimental amplifier that made 75 Watts in triode mode using EH KT88's. That design was detailed from the paper napkin scratchings up to a working prototype. Ignore the title of the thread. It might be interesting reading if you have a few hours to burn.

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Old 23rd February 2010, 03:05 AM   #28
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I looked at your schematics and noticed one thing. Your negative power supply is -90 volts. In my KT88 test amp I needed to increase my negative supply to -105 volts when cranking KT88's hard in triode mode. I was using resistors in the source leads of the mosfets. The 10M45 needs about 20 volts across it to be happy. You may need a bit more than 90 volts in your case.

Scott,

IMO, you should heed the voice of experience. With that in mind, I suggest you phase up the 75 V. and 5 V. windings, wire them in series, and rectify the composite. R102 becomes a simple bleeder, not a voltage set part. George has made clear that too few Volts is the issue with 10M45Ses, not too many. Insert a RC decoupling network between the point the B- (FETs/CCSes) is taken and the point C- (bias) is taken. Given the bias set trim pots., I don't think that any particular "locked" value for the C- voltage is necessary.
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Old 23rd February 2010, 03:13 AM   #29
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George,
Thank You for your comments. Very Good Points.

Power Tranny will be either a Edcor or electraprint. Jack is working on quoting my choke and OPT's right now. I havn't ordered the PT yet as I am waiting on some feedback like yours.

Your point is well taken on negative bias, easy enough to change at this point. I can spec 85V on the bias winding and hit 105V. I was thinking about what would happen if the CCS or the FET fails. Certainly tube failure is one real possibility. Another one is what happened on the tubelab SE. When a FET failed, (I didn't tighten my heatsink well enough), it took out a bias rectifier and blew the power fuse. (I won't count on that to happen).
I will have to think about adding more to the PS to give an intermediate voltage to tie the FETS to. It is already getting complex. I calculate about 6W dissapation for each which is easily handled with the heatsink I was planning to use. Something to ponder......

I started reading that thread a few days ago and haven't got to the point of you needing to drop the rail. I'll keep reading.

Non of the kt88 tubes are cheap!!!! For testing and initial proofing, I will use the cheaper EH.


Eli, I am all ears on this stuff. Not sure what you are saying. Let me see if I understand, put a R and C in parallel to each other between B+ and the drain of the FET?

Nope re read what you were saying and am not shure what you mean.
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Last edited by SGregory; 23rd February 2010 at 03:19 AM.
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Old 23rd February 2010, 03:24 AM   #30
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At least I do understand what you are saying about C- having to be locked down.

And after a moment of silence, it clicked what you said regarding a RC decoupling network between the bias set voltage divider and the CCS returns
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Last edited by SGregory; 23rd February 2010 at 03:27 AM.
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