vvr problem

[andrew]

Hi, I replaced the NTE 2973 mosfet but I still have trouble with it. I get 0 vts with the pot full on. Before the standby switch I get 317vts. and after the standby switch I get 0 vts. With the unit disconnected and the amp rewired I get 450vts before and after the standby switch. I have installed these vvrs before with no problems. Is 450vts too much for it? Any help please. Thank you.

[passfan]

Was the old one doing the same thing ? It almost sounds like the opposite polarity.

[andrew]

Hi, yes it was doing the same. Thanks

[FYL]

Is 450vts too much for it?

The NTE2973 is rated at 900V Vdss and is perfectly OK at 450V.

What kind of VVR are you using? One of Dana's kits?

Check your wiring, heatsinking and insulation. Is the mosfet properly mounted?

Has it been properly handled? Static is a problem in summer...

Maybe the mosfets are or have become defective

[andrew]

Hi FYL, It is one of Dana's units and I have never had this problem before. I am using a 2mm thick ceramic insulator that fits the mosfet. The wiring is correct also but I will check again. Would a bad zener cause this? These nte mosfets are kind of pricey and shipping on top of that. Are there any other suitable mosfets for less?

[ChrisM]

Hi FYL, It is one of Dana's units and I have never had this problem before. I am using a 2mm thick ceramic insulator that fits the mosfet. The wiring is correct also but I will check again. Would a bad zener cause this? These nte mosfets are kind of pricey and shipping on top of that. Are there any other suitable mosfets for less?

I repaired an amp on the weekend. I too thought the $7 MOSFET was dead, turns out it was the $0.03 Zener diode. So yes try replacing that diode first. I found the bum diode by doing some resistance and continuity measurements on it.

[FYL]

Would a bad zener cause this?

That's also a possibility.

These nte mosfets are kind of pricey and shipping on top of that. Are there any other suitable mosfets for less?

NTE is a distrib of replacement semis, not a manufacturer, and their prices are usually quite high.

I use IRFP460 (500V/20A), -E50 (800V/8A) as well as others, depending on the context. The Vishay or Fairchild versions are as good or better as their NTE equivalents and come much cheaper - the IRFP460 streets for less than $1.50 a pop and the IRFPE50 for less than $3.00.

BTW, silicon should only be bought from well established and trusted sources: there are many fakes around and the bargain-priced stuff from China could turn out to be very expensive in the end.

[andrew]

Yep it was the zener. I put in a new one and it works fine. Thanks for the info on cheaper mosfets.

[David Root]

FYL, thanx for that tip about silicon. Last thing I need on a 300W 700V HT supply is a bad rectifier! I have been buying Fairchild Stealth and IXYS, but lower ~500V HT, on which I've been using 1200V rated.

I just bought an Antek toroid PT for that amp, 400VA 500-0-500 720mA, yes I know it's Chinese, but it covers the power tubes and the heaters for 6-KT88s, for less than $100 including a 1/8" thick steel shield pot. Have some 1800V rated IXYS FREDs for that.

We shall see.

[Kagliostro]

I'm a little confused

I am using a 2mm thick ceramic insulator that fits the mosfet

I'm not very experienced

haw can your mosfet dissipate the heat ??

Kagliostro

[FYL]

Mosfet to insulator to chassis, with the latter dissipating heat.

[Cliff Schecht]

Would a bad zener cause this?

That's also a possibility.

These nte mosfets are kind of pricey and shipping on top of that. Are there any other suitable mosfets for less?

NTE is a distrib of replacement semis, not a manufacturer, and their prices are usually quite high.

I use IRFP460 (500V/20A), -E50 (800V/8A) as well as others, depending on the context. The Vishay or Fairchild versions are as good or better as their NTE equivalents and come much cheaper - the IRFP460 streets for less than $1.50 a pop and the IRFPE50 for less than $3.00.

BTW, silicon should only be bought from well established and trusted sources: there are many fakes around and the bargain-priced stuff from China could turn out to be very expensive in the end.

NTE will sell you whatever they are able to find in bulk or surplus, usually for at least 10x the price, and do not give a crap about the "quality" of their products. My dad has always referred to them as "floor sweep parts" and they are notorious for doing just this (apparently..). I only ever use them as a temporary or last resort. I've had more than a few devices from them come DOA. I'm no stickler for ESD, but that packaging they use is in no way a Faraday cage..

I always go Mouser personally. Yeah you pay $5-7 shipping but I can order parts on Monday by 7:00PM and have them on my doorstep the next day by 4:00PM. Ungodly fast service anywhere in Texas using UPS Ground works well for me!

[UR12]

Interesting comments on the quality of the NTE parts I am supplying. With numbers pushing 1000 VVRs out there using the NTE parts. There have only been a handfull of failures and most of those were caused by bad installations. On the other hand, I have personally had nothing but problems with some of the lower rated (Cheaper) mosfets out there and that is exactly why I moved to the NTE mosfet. It works and works great. If you can get something else to do the job for you then great use it, but I will continue to supply the NTE in my kits because simply put, they work great.

[toskrlec]

Hi, I just implementing VVR in my express, but can’t decide between heat sink out of chassis, and cooling direct on chassis. What experience you have? I thing that heat sink have better thermal dissipation. Thermal condition is direct involved in reliability of electric components, so I think it is important stuff.

Thanks

[UR12]

Hi, I just implementing VVR in my express, but can’t decide between heat sink out of chassis, and cooling direct on chassis. What experience you have? I thing that heat sink have better thermal dissipation. Thermal condition is direct involved in reliability of electric components, so I think it is important stuff.

Thanks

I have installed the VVR in a lot of Expresses and have never had any problems using the chassis only for a heat sink as long as you go with the NTE2973. I can't comment on any of the other Mosfets. I have them installed in my personal Express and Liverpool that has been used every weekend for the last 3 years without fail. My Express was used a few weeks back at a gig where it was in the back line for 6 different bands and was given a workout for 8 continous hours. I don't just supply kits, I actually use them in just about all my builds and my amps are played not just used to play bedroom levels in an apartment once a month. So in my "experience" you should be fine just using the chassis on an Express. If you were asking about a 50 watt amp I would suggest using the chassis along with an additional external heatsink.