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Old 2nd March 2011, 01:57 AM   #11
rknize is offline rknize  United States
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Whoops...Crss. My bad.
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Old 2nd March 2011, 11:07 AM   #12
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George,

Thanks for the explanation.

What about the current that I have to make available for the powerdrive ? I could not find anywhere this information.

Davide
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Old 2nd March 2011, 01:51 PM   #13
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A related question for mostly educational reasons.......In a Tubelab SE (or similar source follower application) if the rectified off load voltage of the transformer is greater than 500V, is that a problem for 500V rated parts such as the 2SK3563?
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Old 20th March 2011, 05:16 PM   #14
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Looking for elusive MOSFETs... Any chance either one of these would work from Mouser?

FDPF5N50NZ

FDP5N50NZ

Any possible options out there?

Thanks
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Old 26th March 2011, 03:36 PM   #15
beeah is offline beeah  United States
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I'm looking at the ROHM RDX050N50. I can't find any graphs for Crss as a function of Vds, but they spec Crss as 25pF at Vds=25V, compared with 7pF for the 2SK3563.
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Old 26th March 2011, 04:49 PM   #16
beeah is offline beeah  United States
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Quote:
Originally Posted by surface tension View Post
Looking for elusive MOSFETs... Any chance either one of these would work from Mouser?

FDPF5N50NZ

FDP5N50NZ

Any possible options out there?

Thanks
FDPF5N50NZ looks very good - I think I'll go that route. Thanks for the heads up!
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Old 31st March 2011, 03:20 AM   #17
Alco is offline Alco  United States
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Default we'll see

I'm in the midst of building a tubelab SE and decided to try the IRF830APBF-ND. I didn't notice the FDPF5N50NZ until after I placed my orders.

It's hard to tell from the data sheets which one might be better. The graph of the gate capacitance vs voltage only goes up to 30 volts on the fairchild data sheet. It looks flat until that point, but what happens after that one can only guess. Can we assume that it is flat past 30 volts ?

The gate capacitance of the Vishay chip is lower at 4.3pf, but the relation to voltage not near as flat as the Toshiba mosfet, nor as flat as the fairchild seems to be. At 20 volts it's at just above 4pf and by the time it reaches 100 volts its a little under 3pf.

In his earlier post, George mentioned that low gate capacitance is a criteria in this application. Is lower better ? And which is more important, a lower capacitance, or a flatter curve ?

What will be the effect of the decline in capacitance as voltage goes up on the overall performance of the amp ?

Is the capacitance decline so large as to make the vishay chip inappropriate to use here ?


http://www.vishay.com/docs/91061/91061.pdf
http://www.fairchildsemi.com/ds/FD/FDPF5N50NZ.pdf
http://www.toshiba.com/taec/componen...//75/10801.pdf
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Old 1st April 2011, 06:35 PM   #18
rknize is offline rknize  United States
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If you subscribe to the argument about phase distortion, then flatness is more important than the total.
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Old 1st April 2011, 07:54 PM   #19
Alco is offline Alco  United States
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thanks for your thought, Russ. If I were more familiar with the arguements about phase distortion and capacitance slopes I might subscribe to one. But I'm not so...

I looked a little more closely at the data sheet for the Fairchild mosfet and saw that its typical gate capacitance is 4pf, not 8pf. 8pf is the max.

And the scales are so very different on the two manufacturer's data sheets it is very hard to compare the slopes of each chip. Vishay's logarithmic scales exagerate the slope, and Fairchild's chart only goes out to 30 volts, and its vertical scale is linear so the difference between 4 and 3 would be really hard if not impossible to see. Looking closely at the Fairchild chart there is clearly a slope all the way out to 30 volts.

So it looks to me like neither one is the ideal replacement for the Toshiba, neither one has the same flat slope. The two suggested alternatives look very similar to me, the typical gate capacitance is practically the same. I know the slope of the Vishay is only about 25% over the relevant voltage range and I don't know for sure the slope of the Fairchild, so I am going to stay with the Vishay and cross my fingers.

And I guess endure the slings and arrows of the consequences of a -25% slope in my gate capacitance.

I can always rip them out later when we find a better alternative!
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Old 12th April 2011, 02:38 PM   #20
Alco is offline Alco  United States
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Default Success !

Well I finished the Tubelab SE with the Vishay MOSFETs and so far things seem fine. No fires, no detectable leaks of radiation.

And it sounds wonderful.

I brought the amp to life without a choke or oil cap. I wanted both to hear the difference and to keep things simpler. All the voltage readings made sense; my b+ was on the high side at 320 volts so I biased the 45s at around 25 ma. I ran it this way for around 10 hours.

Then I pulled R4, and for it substituted the Triad 150 ohm choke, I changed C4 to 4.7 uf, and added a 50 uf ASC oil cap in parallel with C5. Everything got better: b+ dropped to 308 volts or so, the tiny bit of hum previously present vanished, and the bass response was vastly improved. Where the bass signal distorted and flabbed out it became complete and full. With the lower b+ I reset the 5842 plate voltage again to 175 and biased the 45s at just under 27 ma.

This amp is now absolutely humless and silent when there is no audio signal and the volume is at full. And it sounds great. I had never listened to a SET amp before let alone a DHT SET. Now I know what the hubub is all about. Everything I listen to sounds new again and I find things I hadn't heard before.

I would like to have another Tubelab SE next to this one so I could comment on the difference in sound, if any between the Vishay MOSFETs and the Toshiba's. I can say that the Vishay MOSFETs do sound good and that I have yet to find any bad side effects.

This amp sounds great and is behaving exactly as I expect it to.
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