Tweaking and a second go at power scaling

I wanted to spend some time fine tuning and tweaking the Ceriatone DC30 this weekend, and hoped to have power scaling installed and working as well.  I some of Saturday and most of Sunday to work out any issues, so I figured I’d be in good shape.  I hoped I could spend next week working on the box for the head.  It didn’t work out quite as I planned.

The new turrets and turret staking tool from Turretboards.com really paid off—the turrets are straight and look nearly professional.  Having the right tools and a bit of experience helps.

I did notice one mistake with my layout that I corrected before I started mounting components.  It was easy to add an additional turret now that I have the staking tool and turrets.  (Don’t bother trying to copy this layout—it’s flawed and doesn’t work as I soon discovered.)

Mounting the new turret board was relatively straight-forward, even if it took a bit of time to do.  I threaded 3mm machine screws through short bits of the standoffs that normally hold this board in place.  The are basically short standoffs between the chassis and the bottom board holding the standard filter caps, with enough space to route wires beneath the board, and also to clear the 10-24 nuts holding the two chokes in place.  A 3mm nut on the machine screw holds the bottom turret board in place, and there’s enough thread left to attach full length (about 3/4”) standoffs to mount the upper turret board with the power scaling components.  Like this:

The three twisted wires—white, purple and white/green—that start at the far end of the turret board and disappear out of view are attached to the pot which should control the power scaling level.  The black device which appears to be floating mid-air is the IRFE50 MOSFET included with the power scaling kit; I used this for testing, even though it meant being extra careful as ~400V was on the uncovered metal back!  At the near end of the power scaling board (bottom of the picture), the yellow lead connected to the 33u cap is the scaled, filtered B+ going to the output tubes.  One lead from the choke is connected to the same cap.  The other lead from the choke and the scaled B+ to the output transformer are attached to to the first 33u filter cap.  The resistor going from B+ to ground, just before the first cap, is to drain the caps when the amp is turned off.

What’s working: If I have the amp running without power scaling, everything works as it should. It I add in the power scaling and test the B+ voltages with the tubes out, everything measures fine—the scaling goes from max B+ of about 385V (a bit high, but this is without tubes) down to just a few volts. So far, so good.

Here’s the problem: As soon as I install the output tubes, I seem to lose a lot of B+. The scaling still works, but only from 0V - 55V if I have the amp set on full power, or from 0V - 95V if it’s switched to half power.

I’ve gone over the power scaling circuit a few times, checked my solder joints, double-checked my wiring on the output tubes checked the bias and cathode resistors. So far I don’t see anything wrong, though I expect there must be a simple explanation.  Like I completely hosed the layout… or completely misinterpreted the schematics… or missed something even more fundamental.

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