Does heat change with amp volume settings on VVR?

[doveman]

Dumb question- that has come to mind about Variable Voltage Reduction:

Is the heat that is transferred to the mosfet related to the volume pot setting ... or ... is it just related to the voltage that is reduced on the B+?

In other words, when testing the VVR will the heat on the mosfet stay the same for any volume and tone settings? I suspect the heat changes based on the other amp settings but I'm not sure.

[markr14850]

I think the the pertinent equation is P=I*V, power equals current flow times voltage drop.

At idle, I assume that the voltage drop across the mosfet goes from zero to full B+, based on the regulator's control pot. I also assume that the current decreases linearly with the voltage, but I haven't done the math to see whether this is really true. If you sample I*V across a few points (with these assumptions), you'll see that it peaks at 50% voltage reduction.

When not at idle, the current drawn by the power amp will increase. So, the power dissipated will increase as well. How much so depends on how far towards class B you are biased. A "mostly class A" biased amp might not change much. But an AB biased 4xEL34 amp could have a huge change in current.

Anyhow, at idle, max dissipation occurs at 50% voltage, and while playing max dissipation still occurs at 50% - but the dissipation while playing will be higher.

If you want to make comparisons between levels, the exact details will matter, and you would need to measure the average current that this amplifier is drawing under the different settings.

Also, the setting of the volume pot doesn't directly matter. What matters is how much current the power tubes pull. If you dime the amp's volume pot but don't plug in your guitar, you're still at idle as far as dissipation through the mosfet is concerned.

[doveman]

So basically:

If I set the VVR pot and play the amp at half volume ... then without moving the VVR pot play at full volume ... will the heat on the mosfet increase? I think it will but not sure.

[markr14850]

If I set the VVR pot and play the amp at half volume ... then without moving the VVR pot play at full volume ... will the heat on the mosfet increase? I think it will but not sure.

Yes, it will increase. But the magnitude of the increase depends on the exact details of that particular amp. It might be barely noticeable, or it might be huge.

Do you have cathode resistors on the power tubes? Measure the voltage drop across one at idle, at half volume, and then at full volume. That should give you a feel for the change in current as a function of volume.

[doveman]

That really answers my question. What I was after is that I still have to test the amp carefully and might lose my second mosfet in the process. I do have a 3rd already in my possession just in case.

But I though possibly the only thing the mostfet was seeing was the voltage that has been reduced from the tubes at idle. Didn't think so but it was just a thought.

The fact that it's different at all really answers my question. Thanks.

[Colossal]

Doveman,

Out of curiosity, do you have a reservoir capacitor IN FRONT OF the VVR board? In other words, did you add an additional cap so the order would be:

1) rectifier
2) Cap
3) VVR
4) B+1 reservoir cap (B+1)
5) Choke/resistor
6) etc

or do you have:
1) rectifier
2) VVR
3) B+1 reservoir cap (B+1)
4) Choke/resistor
5) etc

If I recall, I think you may be regulating the output tubes and maybe phase inverter, but the question above still stands.

Thanks,
Dave

[doveman]

Doveman,

Out of curiosity, do you have a reservoir capacitor IN FRONT OF the VVR board? In other words, did you add an additional cap so the order would be:

1) rectifier
2) Cap
3) VVR
4) B+1 reservoir cap (B+1)
5) Choke/resistor
6) etc

or do you have:
1) rectifier
2) VVR
3) B+1 reservoir cap (B+1)
4) Choke/resistor
5) etc

If I recall, I think you may be regulating the output tubes and maybe phase inverter, but the question above still stands.

Thanks,
Dave

I did not add a cap. I also decided to try scaling the whole amp first. It was simpler and seems to work great.

[UR12]

Here ia a excerpt from a post a few years ago on this board where we discussed power and heat.

Take an "18W". Cathode-biased, it'll idle around 36 Watts heat. It may run 300V or 350V, but take 300V and 120mA as an example.

18W out - 300V 120ma = 36W tube dissipation - 0V 120ma = 0W regulator diss - 36W total
9W out - 210V 84ma = 18W tube dissipation - 90V 84ma = 7.5W regulator diss - 26W total
4W out - 150V 60ma = 9W tube dissipation - 150V 60ma = 9W regulator diss - 18W total
2W out - 105V 42ma = 4.4W tube dissipation - 195V 42ma = 8W regulator diss - 13W total
1W out - 75V 30ma = 2.3W tube dissipation - 225V 30ma = 6.8W regulator diss - 9W total

The maximum heat in the regulator is one quarter the power in the output stage when working full-up. For a cathode-biased amp, this is pretty-near the idle heat; round-up about 20% because we let self-bias amps slide a bit into Class AB. For a fix-bias amp, look-up the operating conditions for a similar application. 2*6L6GC ala 5F6A will idle near 400V 100mA but roar to 400V 220mA, so at half-voltage there could be up to 200 110mA or 22 Watts heat in the regulator.

The maximum heat happens at half-voltage, which will give something less than quarter power output.

The heat in the regulator is fairly constant for all outputs from 50% power to 5% power.

But the total heat in the box falls. Adding power reduction will not blister your Tolex any more than before, and generally less if you actually use the power reduction.

[Colossal]

I did not add a cap. I also decided to try scaling the whole amp first. It was simpler and seems to work great.

I would add a reservoir cap in front of the VVR (and in addition to the normal B+1 reservoir cap). This will not affect the tone or feel of the amp; try 16-47uF. The VVR needs somewhere to source current (not just right off the rectifier) and I'm wondering if that is what is stressing your MOSFET. So the order would be:

1) Rectifier
2) 16-33uF cap / 220k-1M bleeder resistor to ground
3) VVR
4) B+1 Reservoir cap (B+1 plates)
5) dropping resistor or choke
6) B+2 cap (B+2 screens)
...and so on, as normal

[katopan]

And you don't have to have the normal cap after the VVR if you put one before it. The VVR is a linear regulator with pretty much a fixed voltage drop. It will supply nice DC without an output cap if fed DC from a filter cap.

[doveman]

This is the way I installed it ...

Plan:
[IMG:639:423]http://i104.photobucket.com/albums/m168 ... C_0293.jpg[/img]

Installed:
[IMG:639:423]http://i104.photobucket.com/albums/m168 ... C_0298.jpg[/img]

Rectifier is the tube on the far left.

[markr14850]

In the plan image, what is the line labelled "Out B+ in" connected to?

[doveman]

In the plan image, what is the line labelled "Out B+ in" connected to?

On the photo: The two red wires go to the standby switch (see it labeled at the top) ... one wire is the one I labeled going to the cap can ... the other is going to the rectifier (see it labeled on the red wire).

[IMG:639:406]http://i104.photobucket.com/albums/m168 ... C_0296.jpg[/img]

On the schematic: The orange highlight represents the (in>B+>out ). The missing resistor note represents a slight difference in the wiring on my amp and the schematic that I think I figured out with the guys on Dr Z forum. All seems to be in order with that but the note is on the picture I have available. But the schematic helps I think.

[markr14850]

I think it should be like this:

[doveman]

I think it should be like this:

Interesting ... what would that do? Is this possibly stressing the VVR?