Saturday, August 6, 2011

The Grail is in its cab!

Been awhile since I've posted an update, but I've been busy.

The Grail is coming along nicely.  There's still some tweaking to do to get it just so  -- but everything except the "Watts" variable voltage control is up and running.   Version 1 of the VVR circuit ran into a bit of trouble with the small signal transistors getting reversed biased, but I think a fix is in hand... just haven't had a chance yet to redo that yet.  In the meantime, the basic amp runs fine without VVR.

I had it out for its first full-volume test drive with talented Richmond musician Marshall Costan last week.  First impressions were very favourable! Nice cleans, and overdrive that goes from fairly subtle all the way to that full-on sustain-forever buttery drive that I was hoping for.  Still the session pointed the way to a few areas for further attention.

Since then, the main changes to the "build version" of the schematic involve alterations to the reverb circuitry. I increased the drive slightly.  But in particular, I had to significantly change the ratio of dry to verb signal in the reverb recovery mixer in order to get the verb wet enough at max.  In fact, I totally eliminated the verb mix resistor, and am now just using the output impedance of the first recovery stage as the series resistance in the VE mixer.  The dry resistance was bumped up to 750K.  Finally, the 1Meg reverb control was replaced with a 500K linear pot, in combination with a 470K fixed resistance in the ground leg. I may tweak these values a bit more yet, but it's pretty good as is.  I'm probably going to drop the first recovery plate resistor to 100K.  An LTSpice sim suggests that any gain realized from the larger 150K plate resistor is immediately neutralized by the increased output impedance of the stage... which interacts with the VE mixer to drop effective wet level... so there's no point in adding the noise that comes along with the larger plate resistor.

But the big news is that I slapped on the faceplate and dropped it into its case for the first time today.. and it looks gorgeous. It's always a relief when the custom faceplate actually fits perfectly!

What do you think?  I'm taking it out for its second spin on Sunday with my friend Andy.

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