VVR not VVR-ing

[theundeadelvis]

Happy papa day! Soooo, I decided to give VVR a try for the first time. I built it on perf, used an IRFP460PBF, and an aluminum oxide thermal pad, otherwise I used the BOM from the VVR thread. To keep it simple I'm scaling the entire amp, so I inserted the VVR between the standby switch and the first cap of my power supply.


My problem is, it won't drop the voltage? It passes full voltage no matter where the pot is turned. If something were to fail, like the fet, would it fail open, passing all voltage? I've checked the pot, and it's fine, I've checked for any solder bridges and found none. I've triple checked my layout (of course, it still could be the issue I'm sure).


Is there anything obvious I'm missing, like does the pot need isolated? Any suggestions are greatly appreciated. Thanks guys!


Here's a schem of the amp: http://i.imgur.com/4J75ZMc.jpg

[shooter]

I’d verify zener is in correct.

Power off, discharge everything, turn the standby switch to standby, that will/should open the the drain side of the fet, then you should be able to ohm between pin 2 to pin 3 of the fet.  I believe it should be approx same ohms each direction but NOT short.
 
I'm going by the schematic below

[theundeadelvis]

Thanks shooter, One direction I get about 9 meg ohms, the other 20 meg?

[HotBluePlates]

Are you sure you know the pin-out for the FET you're using? IRFP460

Thanks shooter, One direction I get about 9 meg ohms, the other 20 meg?

Probably okay, as there is an internal drain-source diode.

[PRR]

> If something were to fail, like the fet, would it fail open, passing all voltage?

Semiconductors often fail SHORT. Which would be the same as no VVR at all: full B+.

Yeah, check the diode.

I would *carefully* verify that the pot wiper gives a voltage varying from full B+ to very-small. This is a power-ON Fatal Voltages check. Use clip-leads.

[theundeadelvis]

HBP - I'm fairly confident I got the pinout correct, but I will double check the datasheet.


PRR- I'll check those voltages when I get home.


Thanks again for the suggestions. Also, not that I think it will be of any help, but here's a pic of VVR: http://i.imgur.com/HznLU2C.jpg

[theundeadelvis]

OK, finally got around to checking the voltages at the pot wiper. I'm only getting a range from 340 volts to 390 volts?

[HotBluePlates]

What's the value of the pot? What is the value of the resistor between the pot and ground?

[PRR]

> Is there anything obvious I'm missing

Pointer to the actual plan you used. "the BOM from the VVR thread"..... well, there's a lot of VVR threads.

[JB]

You said you used a thermal pad on the mosfet, presumably you mean an insulating pad between the transistor case and the amp chassis?  If so with everything off and discharged check for continuity - or rather lack of - between the tab of the mosfet and the chassis.  It's very easy to create a short there that will kill the device.

Also check the ground connection to your board.

[theundeadelvis]

Ok, here's thread I got most my info from: http://el34world.com/Forum/index.php?topic=6899.0


I used the image in the upper left corner of this jpeg to get my layout: http://el34world.com/Forum/index.php?action=dlattach;topic=6899.0;attach=10391;image

The pot I used was a linear taper 1 meg Alpha, and the resistor between the pot and ground is 100k.


And, I have checked to make sure the mosfet is isolated from the chassis.

[HotBluePlates]

I'm only getting a range from 340 volts to 390 volts?

The pot I used was a linear taper 1 meg Alpha, and the resistor between the pot and ground is 100k.

That small range of voltage variation would happen if you had a 100kΩ resistor to ground and a 1kΩ pot. Or possibly resistors in the wrong place, or maybe bad solder joints.

Do still check that with power on you get the correct voltage across the zener.

[PRR]

> 340 volts to 390 volts?
> small range of voltage variation would happen if

Note that if he has a 1Meg pot, a 10 Meg volt-meter, and NO 100K to ground, he will read from 390V to (10/11)*390V or 354V as pot goes zero to 1Meg.

Or a 6.7 Meg meter will give 340V; and some DMMs have such odd-value loading.

The voltage at the junction of the 1Meg and the 100K to ground needs to be pretty-close to 35V or else the 100K-to-Ground aint doing its job.

[theundeadelvis]

Ok, took a few more measurements.


HBP, I get 390 volts on one end of the zener, and 340 at the other.


PRR, I'm getting 340 volts where the 100k to ground meets the 1 meg pot lug. 

[PRR]

> 340 volts where the 100k to ground meets the 1 meg pot lug

Then perhaps ask what the voltage is on the OTHER end of that 100K?

I got 2 cents on a bad solder joint.

[theundeadelvis]

Well, PRR I'm going to need an address where I can mail a check for 2 cents to. You hit the nail on the head. I had a ground wire soldered to the pad next to the end of the 100k resistor, but I never connected the two. I'm using cloth covered wire, and every time I looked at it, it looked fine, but it wasn't until I pulled the cloth back that I saw my blunder.

Thanks a million!

[Willabe]

So it's working now?


        Brad   

[theundeadelvis]

Yep! It's amazing what the people here are capable of. It blows my mind. Thanks again!

Up next, a ReVibe resurrection. ..

[Willabe]

Great, but wait a minute now, how does it sound? (The VVR.)   

We need to hear at least a little bit of what you think of it.


                Brad     

 

[theundeadelvis]

It's ok. Not great, but I knew scaling the entire amp was not the ideal method, but it's all I was wanting to try with this particular amp build. I've never used VVR, so I wanted to give it a try in its simplest form first. It does help tame the 26 watter. Knocking a little volume down when it's cranked, and it sounds pretty good. But if you take it down to bedroom levels, it's not that great. I haven't spent a lot of time with it, so I may change my mind after I've tweaked it a bit.

Great, but wait a minute now, how does it sound? (The VVR.)   

We need to hear at least a little bit of what you think of it.


                Brad     

[dude]

The VVR isn't "really" made for bedroom levels. It's basically cuts just enough when the owner says "Turn it Down"

As always,
al

[Willabe]

Thanks for letting us know what you think about it's sound/tone.

I've heard examples where it did take it all the way down to bedroom levels and examples where it did not.

The examples I've heard that did  take it all the way down and still sound very good did not VVR (Power Scaling) the preamp.


                  Brad     

[tubenit]

IMO, the whole idea behind the VVR is to get an overdriven tone at a bedroom level ................ not a clean tone at that level which the amp can already do.

Here is an overdriven sound clip of an amp I built at a bedroom level where we could've easily conversed. It was a 23 watt 6L6 amp.

http://www.soundclick.com/player/single_player.cfm?songid=9488477&q=hi&newref=1

The LTPI and power tubes alone were VVR'd.

With respect, Tubenit

[tubenit]

I'd try this.

Tubenit

[theundeadelvis]

Tubenit, scaling the PA and PI was what I wanted, but for this build, I didn't want to redo the layout in order to do so. As it sits now, it will get the volume to a tolerable level, and I'll actually plan my layout for the next build around VVR-ing the PA and PI.

[tubenit]

It's ok. Not great, but I knew scaling the entire amp was not the ideal method,

So, you're going to leave it "ok" and "not great"?   

I think you should approach this however you want & if that's what you want, then it's totally a reasonable choice.

Having said that,  honestly ........... you're looking at a couple of filter caps, a resistor or so and moving a couple of wires for the standby switch.  Not much of a layout change really from what you already have I would guess?

With respect, Tubenit

[theundeadelvis]

Yeah, my problem is, my chassis is pretty crammed full. Not sure where I'd fit those extra caps. I may take a look at it again.

[PRR]

Should be pretty much able to move the preamp B+ feed over to the raw (pre-VVR) B+.

Yes, might need another cap filter.

[kagliostro]

May be something like this can solve your problem ?

http://www.partsconnexion.com/capacitor_ele_mundorf_hv.html




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