Mosfet VVR question

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flood
Posts: 212
Joined: Wed Jan 30, 2008 3:24 pm
Location: Bombay, India

Re: Mosfet VVR question

Post by flood »

Tillydog wrote:
flood wrote:I've been using IRF840Bs in my VVR installations, and none of them have lasted particularly long.
It would help to know voltages & current requirements of the circuits that you're building.

As long as you don't exceed the voltage rating of the IRF840, and it's adequately heat-sinked, it's hard to see how it could be killed by almost any conceivable VVR type application.
Tillydog, thanks for writing back. Both installs were in marshall 18W circuits - in one case the 1974x and the other was a 2061x style build.

The IRF840B is rated for 500V and 8A - I'm wondering if the lowered voltage may be a problem, but am not convinced this is the reason. It could be my construction - my VVR circuit is laid out turret-board style with wires to the FET and the pot. I'm aware this is uncommon, as most VVR circuit boards are fairly compact - so it could be the source of the problems as well.

The ST part looks like a heavy-duty part that may just work well as a drop-in replacement, and is better priced than the NTE2973.

A final consideration - does a tube rectifier impact the VVR circuit in any way? both amps have a tube rectifier, the 2061x has a switch that selects between tube or diode rectification.

Thanks :)
In the interest of full disclosure, I am Animal Factory Amplification.
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UR12
Posts: 1570
Joined: Mon Nov 28, 2005 7:22 pm

Re: Mosfet VVR question

Post by UR12 »

Tube rectos needs to have a filter cap on the output of the tube and before the VVR. If you are installing the VVR right after the recto and before the first fiter cap then that COULD be your problem.
Tillydog
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Location: Wales, UK

Re: Mosfet VVR question

Post by Tillydog »

flood wrote:It could be my construction - my VVR circuit is laid out turret-board style with wires to the FET and the pot.

...

A final consideration - does a tube rectifier impact the VVR circuit in any way? both amps have a tube rectifier, the 2061x has a switch that selects between tube or diode rectification.
Do you have a 'gate stopper' resistor at the MOSFET, or is this on the turret board? This needs to be as close as possible to the MOSFET for all the same reasons that grid stoppers need to be right on the valve sockets. Could be your problem. If it is the problem, it will be a problem for a different part#, also.


I can't see how a tube rectifier would make much difference, other than to be careful of the voltage rise on the rectified output when the voltage regulator is turned right down, but I guess you should be well clear of the 500V limit if you're running EL84s. [Edit: Post crossed with Dana's - Just my 0.02p - not arguing with him!]
Last edited by Tillydog on Thu Jan 24, 2013 4:58 pm, edited 1 time in total.
flood
Posts: 212
Joined: Wed Jan 30, 2008 3:24 pm
Location: Bombay, India

Re: Mosfet VVR question

Post by flood »

Tillydog wrote: Do you have a 'gate stopper' resistor at the MOSFET, or is this on the turret board? This needs to be as close as possible to the MOSFET for all the same reasons that grid stoppers need to be right on the valve sockets. Could be your problem. If it is the problem, it will be a problem for a different part#, also.


I can't see how a tube rectifier would make much difference, other than to be careful of the voltage rise on the rectified output when the voltage regulator is turned right down, but I guess you should be well clear of the 500V limit if you're running EL84s.
:!:

I'm not completely sure, but i think the MOSFET is distanced from everything in both builds. will have to check again.

Dana, there is a cap between the EZ81 and the VVR circuit.

Thanks again :) you people rock.
In the interest of full disclosure, I am Animal Factory Amplification.
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