Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: phsyconoodler on August 13, 2010, 03:30:31 pm
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Is it ok to use an N-channel mosfet that is 11 watts when the schematic calls for a 9 watt?I'm not familiar too much with solid state stuff.
It's for a VVR. The rest of the mosfet specs are the same.
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Well I stuck one in and fired it up.It seems to be working fine.I just was wondering if the 11 watt one was way overkill or is it like filter caps:as long as the voltage meets the 'minimum' it's ok to use larger?
I just don't have that info at my fingertips.
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Speaking of the correct Mosfet for a VVR
I've been using this one NTE-2973
http://www.nteinc.com/specs/2900to2999/pdf/nte2973.pdf
It's $9.69 at Mouser, I've had good luck with this one but also have had a few of my amps blow this mosfet.
A 5E3 with up-graded iron and 6L6's blew this NTE-2973 (after six months), also I had one go in an 18 watt Super Lite (again lasted over six months). Both of these amps are under 25watts output. I also used a CP processor fan at 6.3 volts too. Seem to me the install is usually near the power supply (PT) and it's hot in there.
I have another 18 watt amp with one of these mosfets that has been running for over a year (played for hours on end) and no problems, go figure why the other two blew.
Anyone have better results with another mosfet for amps under 25 watts and low in price? I believe Dana Hall used this one at one time but don't know what he's using now.
al
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solid state, on a tube forum?
noodler..................!!!
I love the simplicity (relative) of tube amps and don't understand solid state at all. However, it's becoming obvious that there are places where solid state is just plain better. Switching for example. Voltage regulators. Buffers. Bass power amps. Graphic equalizers. Fuzz Face pedals :wink: I've even been thinking that a solid state version of a concertina splitter might be cool to fiddle with and makes a lot of sense. It's a whole new world, and some of it is pretty cool IMHO.
YMMV
Chip
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Is it ok to use an N-channel mosfet that is 11 watts when the schematic calls for a 9 watt?
Yes. Silicon is rated for maximum voltage, current and power. You should *never* exceed any of these ratings. Say you've got a 200V, 1A, 12W-rated three-legged bug, you can use it at 200V x 60 mA (max voltage) or 12V x 1A (max current), but *not* at 200V x 1 A (fried bug).
As silicon is cheap, you should always derate parts by a factor of 3 or more when it comes to current or max power. If you need a 2A mosfet, get a 6A or 8A part (provided that other specs are OK), if your amp calls for a 40W part, get a 120W version. Etc.
Regarding VVR's and similar stuff, worst case dissipation in the pass element is roughly 50% of max available power. Say your supply can deliver 150 mA @ 400V => 60W max and 30W dissipated by the mosfet, get a 120W part, suitably heatsinked.
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go figure why the other two blew.
Most VVR's die because either of poor heatsinking - a toob steel chassis isn't a proper heatsink ! - or oscillations - mainly because the stopping resistor is located too far from the gate.
Anyone have better results with another mosfet for amps under 25 watts and low in price? I believe Dana Hall used this one at one time but don't know what he's using now.
Check the archives, this subject has been discussed quite in depth. I use IRFP450, -460 and -E50 mosfets which are every bit as good as NTE-branded parts and come at a quarter of the price.
I believe Dana Hall used this one at one time but don't know what he's using now.
AFAICT Dana still uses NTE parts.
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phsyconoodler
Sorry for hijacking your tread.
al
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POWER is the key here. Just like resistors.... Just like the dissipation at idle on your PP Output tubes...
Some parts will sink enough power without extra heatsinking (i.e. using a 7805 for most switching circuit regulation) and some will need all the heatsink you can get.
Another example, is a power supply for a PC. Yeah, you might get a 500watt model for $10, but can it deliver? are the heatsinks big enough? I'll always go with the heavier power supply for a given power output as it usually means they didn't cheap-out on the heat sinks. (In my example too, some brands are less cheapo than others. Bestek, for example, is garbage, Corsair or Seasonic are usually good bets.)
Too small of heatsink(s) = less able to deliver the power as advertised = FAIL.