"Power Scaling-ish" Questions?

[The Ballzz]

Hey All,
First, I realize that "Power Scaling" is London Power's proprietary product and name, but there are other similar products under different names, such as VVR (Variable Voltage Reduction), VariWatt, etc. I understand that most installable "Power Scaling" style systems utilize a variable pot, various resistors and a MOSFET and are able to be installed for "whole amplifier" voltage reduction (easier, less invasive, but less desirable) and also for voltage reduction of just the power tubes (more difficult, more invasive, but often better operational results) and I have a few short and fairly simple questions.

These systems all seem to have "variable" voltage control. Am I correct that the use of the MOSFET is solely for making the system variable and that if someone wanted to have only one level of voltage reduction, it could be done without the MOSFET? The reason I ask is that the MOSFET appears to be the main failure point of these systems, due to heat buildup, etc.

It would seem that voltage reduction could be accomplished by simply making a parallel B+ node dedicated to the power tubes and create two separate dropping strings for two different wattages. If this is the case, I suppose we could actually create several different dropping strings for several different wattages? It just seems that having a couple or few multiple switched wattage settings would be simpler and more dependable to implement than a continously variable system.

Am I On Drugs?
Gene

[xtian]

Gene! Way to ask questions, man! I’m no EE, and I had to ask the same things. Other folks will answer with far more precision.

Here’s a quick wake up about the “parallel dropping strings” suggestion you had. You cannot drop voltage without drawing a proportional amount of current. Remember what happens in your power string voltages when you remove all your tubes? B+ at HT is equal to B+ at the preamp nodes—no voltage is dropped because no current is flowing. You have to draw current to drop voltage across the resistors, and the resistors dissipate the power as heat.

MOSFETs are very good at dissipating heat, and you can control them like a valve (because they are) and therefore control them with a variable resistor, which is a nifty trick.

VVR for cathode biased amps is very simple, and easy to implement point-to-point. Fixed-bias VVR requires more components, but still not bad.

I haven’t heard much about failures. You have?

[The Ballzz]

So it seems that your saying that it's not the resistors creating the voltage drop, but actually the MOSFET?

Actually, what I'm trying to figure out is how Marshall is manipulating the power section voltage in their new DSL amp series. On the previous DSL series, they used a fairly simple Pentode/Triode switching scenario, but on the new ones they claim voltage manipulation and the sound clips, reviews and demos seem much improved. The new setup has only two switch positions, full power and half power. The impetus for me is trying to figure out how to take (for example) the 20 watt amp that currently can be switched to 10 watts and modify it so that the low setting is actually more like 5 or 6 watts. It just seems that being switched instead of variable must be a little bit less complicated and/or easier to modify to taste.
Thank You Sir!
Gene

[roberto]

If you just want two configurations, you can use the "Fender The Twin" old trick to lower the voltage (I've posted here before).

[xtian]

So it seems that your saying that it's not the resistors creating the voltage drop, but actually the MOSFET?

No, that’s not what I said. The resistors and the MOSFET both have similar function, reducing power thru dissipation as heat. But the resistor string is fixed, not variable, while the MOSFET is variable.

Can’t comment on Marshall’s trick unless we see a schematic!

Roberto, can you post a link?

[roberto]

Sure, pages 20 and 21:
http://medias.audiofanzine.com/files/tw ... 479723.pdf

[Ten Over]

Resistors and MOSFET's drop voltages in different ways. The resistor drops voltage across it in proportion to the current through it in accordance with Ohm's law. This means that the voltage drop will increase when the power tubes draw more current -- generally considered undesirable for voltage scaling. The MOSFET is set up as a source follower so that the Source will always be within a few volts of the Gate. Then the voltage on the Gate is manipulated, frequently with the use of a potentiometer as a variable voltage divider. The current at the Source (which goes to the power tubes) can vary widely while the voltage at the Source remains constant.

The potentiometer controlling the voltage on the Gate can be replaced with a two-resistor voltage divider to obtain full voltage and whatever reduced voltage you want at the flick of a switch.