VVR questions

[bluesbear]

Okay, I've been looking at the VVR's and I have a few questions. I'm putting one in my cathode bias amp and one in my son's 5F6a/Bluesbreaker.

First is the heat sink the directions say I'm going to need. You'd think they'd either incorporate it into the kit or at the very least, sell it separately on their site. They don't even tell you what to use or where to get it. What would I need and who sells it?

Another question I have is, is it essential the the master volume be a post PI MV? I prefer the MV to be just before the PI. I can move it if I have to but I don't know where I'm going to put those extra caps!

Lastly, the exhaust fan some have used. Are you putting it inside the chassis?!? I can't imagine a fan small enough to fit... unless you've got a 5 or 6 inch chassis. How are you working that?

Thanks,
Dave

[FYL]

No heat sink is required for a small amp (say up to 30 W) if you use an aluminum chassis, or up to 8 W or so for steel. Just add suitable insulation and bolt the mosfet to the chassis. Short leads are a must if you fancy stability.

Else you'll need an heat sink. It's shape and size are determined by your amp: it needs to fit in or on your chassis and should keep the mosfet in it's safe thermal operating range - die below 150°C, which very conservatively translates to case below 80°C.

Most manufacturers properly spec their heat sinks, look at the °C/W, or Celsius degrees per watt. If the amp can put out 50 watts, the mosfet will have to dissipate around 40% of it => 20 watts - the actual figure is lower but you should always play safe. The amp innards will be at, say, around 40° C, max case temp is 80° C, your budget is 40° C for 20 W => you need a 40/20 = 2°C/W heat sink.

Re. the MV you may use any type.

There are some very small fans, 2" or less, but they aren't very effective at moving air and can be quite noisy.

[Willabe]

First is the heat sink the directions say I'm going to need. You'd think they'd either incorporate it into the kit or at the very least, sell it separately on their site. They don't even tell you what to use or where to get it. What would I need and who sells it?

I never was able to get to the bottom of that, at least in enough detail to make me feel comfortable, but than again I worry to much and tend to over build most of the time.

Most stuff I've read boiled down to, if you are using an alum. chasiss and are under 50w. or so you don't need one, and most guys (at least the threads I've read) seem to not use them.

I have seen a few pictures of a VVR that was heat sinked and the HS looked to be about 1.5"W x 2"L x 2" T, it's kinda hard to tell by lookin at a picture. Mouser and Digi Key have them, altho I have'nt figured out which part exactly. Seemed like not many guys were using one and my amps are all lower wattage in alum. chasiss. If you use a lower/border line mosfit you might need it more for safty, but with a higher raited mosfet and in the larger TO-247 package (not the smaller TO-220 package, which I think Dana uses, but maybe he's changed that) it does'nt seem nessary.

Another question I have is, is it essential the the master volume be a post PI MV? I prefer the MV to be just before the PI. I can move it if I have to but I don't know where I'm going to put those extra caps!

You can use a bootstraped MV on the PI input, some guys like it better.

Lastly, the exhaust fan some have used. Are you putting it inside the chassis?!? I can't imagine a fan small enough to fit... unless you've got a 5 or 6 inch chassis. How are you working that?

It should go on the outside pointed at the area where the mosfet is bolted to the chasiss.


            Brad      

[Willabe]

Most manufacturers properly spec their heat sinks, look at the °C/W, or Celsius degrees per watt. If the amp can put out 50 watts, the mosfet will have to dissipate around 40% of it => 20 watts - the actual figure is lower but you should always play safe. The amp innards will be at, say, around 40° C, max case temp is 80° C, your budget is 40° C for 20 W => you need a 40/20 = 2°C/W heat sink.

So thats how you figure that out. Thank you FYL.


               Brad           

[FYL]

So thats how you figure that out.

Basic thermal equations are pretty much like their electrical equivalents. For instance, elements in series behave like resistors. Say the mosfet has a case to sink thermal resistance of 0.6°C/W, the insulator is rated at 0.3°C/W and the heat sink at 2°C/W, the total thermal resistance will be 0.6 + 0.3 + 2 = 2.9°C/W.

[LooseChange]

I use steel chassis and add a heat sink to the outside.
A MV is very usefull to dial in the sound with the VVR turned very low... Too much drive into the dialed down power tubes does not sound very good at times.

[tubenit]

http://www.el34world.com/Forum/index.php?board=11.0

Maybe you've read it already!  Good information in ARCHIVES.

With respect, Tubenit

[bluesbear]

I've read that. Okay, I guess I have to give it a try, in both amps. I'll know how I like it when I've tried it in a cathode bias 6V6 amp and a fixed bias EL34 amp.
Thanks!
Dave

[dude]

Since I was the one that recommended a small fan and heat sink I'll tell you where I got mine.

I tried to take a few pic's of one I have lying around but of course, dead batteries.

The Mosfet comes with an insulating pad, insulating screw cover and small screw.

to get a great small fan, get an old PC, about 5 yrs old, there all over the place. Open it up and take the processor heat sink out with the small 2" square fan attached. These fans are 2" X 2" and about 1 1/2" high with four small screws holding the 12 volt dc fan. Opposite the mosfet attach the aluminum heat sink with the fan, use a small screw to hold the heat sink in place. Might be a good idea to use a heat sink cream (Radio Shack). That's it, 2" X 2" is small. As far as the 12 volts dc on the label you can run these fans at 5 or 6 volts dc and they will work fine, less noise and the cooling will be plenty.

To get the dc voltage use either the heater supply 6.3 ac volts, or the 5 vdc. As long as you have 3 volts dc or so the fan will spin. Attached are a few methods to get the voltage off the heaters.

Let me know and I'll open up an amp with this set-up and take pictures if needed.

This extra measure of safety would be good for the 35/40 watt fixed biased but he lower 25 or under cathode biased with aluminum chassis probably doesn't need the fan.  

Note: don't tighten the screw on the mosfet tight, just tight enough to keep it there. For some reason tighting this screw too tight causes squeal. Many people had this problem in the early stages of the VVR. I think it may have something to do with a slight short from the screw touching the metal in the mosfet but know one could figure it out but losing the screw slightly stopped the squeal.

al  

[bluesbear]

That'll help!
Thanks,
Dave