Power scaling

Power scaling is Kevin O'Connor's (London Power) term for the ability to variably reduce the output power of an amp. This is done by scaling down the B+ voltage, similar in some ways to what a variac can do. Unlike a variac, Kevin's circuitry doesn't affect the heater voltage, and has the bias track the changing B+ so you can change the voltage without rebiasing. And unlike a power attenuator, this approach actually decreases the stress on the tubes and capacitors while getting a somewhat cranked tone.

Kevin builds this feature into his amps but also sells kits to put in other amps. The kit contains all the parts needed, and instructions. Since it will fit into different amps in different ways, no circuit board is supplied, although there is a drawing of a suggested layout. The instructions show several schematic examples of how the kit fits into common amps, but this is not a drop-in kit, some understanding of the amp's power supply is neccessary.

I installed a PSK1 kit into a Marshall Super Lead. The bias supply of some amps is not up to the level needed for power scaling, so the kit builds a new bias supply by tapping of the secondary. However, the Super Lead uses a full-wave diode bridge, so this isn't possible. However the existing bias winding can supply enough voltage if it is slightly rebuilt. Like many amps, it uses a half-wave rectifier--if this is converted to a full wave bridge there will be enough voltage.

I hit a snag in that the new bias supply didn't work. Kevin provided some ideas via email and phone. It turned out that the bias winding was collapsing under the load of the bias circuitry, and Kevin made suggestions about modifying the circuit to decrease the load. This got it up and running.

My first version of the circuit worked fine for about 10 minutes and then quit working. One of the devices had become extremely hot. Once again Kevin came through with advice, and replacement parts. It seems that at the 500+ volts of this supply, the device might have gone into thermal run-away. I'm now using the chassis for a heat sink instead of the heat sinks supplied with the kit.

I was still concerned about the heat of this device, and Kevin came up with the suggestion to wire in a switch that allows the use of half the power transformer secondary. On the Super Lead, the center tap of the secondary is not at chassis ground. If the center tap is used instead of the full winding, the transformer only develops half the voltage. Starting out with 50% less voltage allows the PSK1 to run very cool. I've now used the amp for about 8 hours of practice sessions with excellent results. I need to play out a few times to get some experience with the full power setting.

How's it sound? Good, but not exactly like the amp when it's wide open. The speakers don't distort, the guitar doesn't hear itself, etc. but there is a nice crunch that I can't get with a master volume. The levels I've run it at are with a B+ between 200-250v, and this is a high enough voltage where it doesn't have that cold starved-tube sound.

The kit allows the B+ to either the preamp and power amp, OR just the power amp to be scaled. At first I tried scaling just the power amp, but have just reverted to whole amp scaling. In one of the updates, Kevin mentions that in a low volume setting, it's possible for the preamp to over-drive the power amp more than it would at full volume (he provided the new parts to adjust for this). I noticed that I could get a lot of preamp fizziness with the preamp running at full B+, and that I had to fuss with the power scaling and master volume to get a good tone. It seems that whole amp scaling keeps things in better balance.

(See the subpage below on cascode.)
Subpages (1): Power scaling cascode