Got it wired up and fired up. I have not secured the wiring yet, but I'm pretty happy with the performance! The PT was putting out voltages dead nuts to the 105, and the OT sounds right on the money. Huge low end on the transformer just like the original. I have a couple minor things to talk with the MFG about but these will be ready for production release soon.
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The extreme mode cap does its thing, and the 3.3k bypass is very subtle as expected. The effect is more pronounced at higher volumes. Although it technically cuts highs due to its arrangement, it sounds like a slight mid push when you activate.
I hardwired in this 100n cap shown in the Mark IV schematic below so I could experiment with it, it's the yellow IC cap sitting on the 8 ohm speaker jack in the photo above.
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I think the reason Boogie added it was to avoid a pop when changing channels, but I always thought they used way too low of a value. In theory it will reduce low frequency feedback, which will result in a bigger low end. I took it in and out of the circuit by jumping across it with alligator clips and it does in fact have a big impact on the sound and feel. I liked the low end and the high gain pick attack the best when I bypassed it, this was the tightest and most articulate sound to me. Fun stuff to play around with, there are really a lot of tailoring possibilities.
I haven't touched the preamp yet, but I might start messing around over the next couple days. One thing I noticed when play testing is that my EQ Q seems lower than other amps I've played. I measured the resistance of the VTL5C4 when it was on and it was higher than others I've measured at 190 ohms. I may change this out for NSL-32SR2 which will have significantly lower on resistance in the circuit, and consequently higher Q, and add some resistance to get it down to 150 ohms or lower which seems more normal.
