My power scaling conundrum

I spent waaaaaay too much time today ignoring my wife and kids, and the as-yet undecorated Christmas tree, to figure out how—and where—power scaling would fit in the DC30.  I think I have a plan:  I’m going to build a small turret board to mount the power scaling components between the power transformer and the rectifier tube sockets.  There’s not much room down there—perhaps a space 3” x 1.5”—but it should be enough for the few components that are involved.  Then I can mount the MOSFET semiconductor to the inside of the chassis, with a large heatsink mounted on the outside.  The power scaling control will go in the chassis opening where the speaker phase switch was formerly located. 

This layout has some advantages:  The heatsink should conduct most of the heat from the MOSFET outside the chassis, where it can be dealt with more easily and efficiently using convection and/or a small cabinet-mounted fan.  The control knob will be near the other components, with only a short run of wiring, but still be accessible.  And none of the components will be close to the input and preamp circuitry which is critical for minimizing hum from this high-voltage components. 

So far, so good.  But for me the real challenges come into play with the filter caps. 

Since I’m going to scale only the power tubes, I want to leave the pre-amp tubes and phase inverter unaffected.  According to the instructions Kevin O’Connor included with the kit, I will need to the setup a separate array of filter caps for the unscaled sections of the amp (the existing filter caps will feed the power tubes only).  But there are a couple of challenges with this arrangement.  Logistically, I need to determine where to mount the additional half-dozen capacitors.  These are relatively large 22uF 450V caps and finding a home for them in the chassis is a challenge.  Another problem is the change from capacitor-choke-capacitor (CLC) filtering used in the stock DC30 circuit to capacitor-resistor-capacitor (CRC filtering) provided with the power scaling kit for the unscaled sections of the amp.  CLC and CRC filtering each have advantages and disadvantages, but I’m concerned that moving from CLC to CRC filtering for the preamp will have a profound impact on how the amps sound.  For better or worse, it would change the amp even more than power scaling alone.

In the end, I decided to duplicate most of the power supply filtering.  Going out of the rectifier tubes, one branch will go through the power scaling system then into a CLC filter, then on to the power tubes.  The second branch will bypass the power scaling and go into a second CLC filter, then feed the preamp and phase inverter.  The easiest way I could see of adding the extra components was to use a second turret board with filter caps, mounted just above the existing filter caps using 4-40 stand off posts.  The original filter caps will be moved a bit closer to the bottom of the chassis, then the 4-40 standoffs will hold the new filter caps above the original turret board.  I think will fit inside the chassis without any clearance problems, though it will make replacing the bottom row of capacitors more difficult to work on.  This is just a compromise I’ll have to accept.  I will also need to purchase a second choke and mount it on the outside of the chassis, but that isn’t much of a problem.

No doubt a more experienced builder could come up with a simpler, and likely better, solution but I think this will work well.

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