The Preston amp tech: January 2013

Saturday, January 12, 2013

Building Valvolux's home - Part 2

Part two

Next up came installing a new window frame in order to replace the awful louvre window that was present. I had the window measured up and took delivery of the replacement in Western Red Cedar with 10mm panes. The old louvre window that was in there was levered out. When we unscrewed one of the metal louvre frames out popped this spider almost as big as my hand. It must have lived almost its whole life in there, there was no way it could have gotten out without us removing the panel. Scared the bejesus out of us!

The frame it is sitting on is approx 100mm wide

Old window frame is gone

New frame and panes installed


Starting to frame

Stud dividing wall goes up


Rear of the dividing stud wall before final sheeting

The framing was going up at the same time. I decided to divide the shed into tw parts, about one third of the way along in order to leave me a space for gardening tools, bikes and assorted junk so I constructed a stud wall across the width of the room.

Whilst it may appear that all this stuff is happening pretty quickly, don't be fooled, it is a trick of the interwebs. It took months and months of hard slog to get this all happening and writing this up for the blog makes me realise I could have taken a lot more photos in order to avoid apparent gaps in continuity.

It's also pretty lame that I am finishing off these blog posts about three years after completing the build!

Repair Job #318 Steelphon GA810

I had this very unusual Italian Steelphon GA810 come in for repair and couldn’t seem to find too much info online about it, particularly in English. So I thought I better write up a blog post on it with some photos and take some voltage readings etc that might be of use to somebody out there.

When it came in the amp was humming dreadfully when powered on. This later turned out to be a faulty Electrolytic can capacitor. These types of can caps that have a single plastic thread at the bottom for mounting are not readily available so I had to widen the existing mounting hole and install a clamp style can capacitor in its place.

The output stage of this amp is a pair of 2N3055’s with rails at +40VDC and -40VDC. This particular example seems to be missing one of the output transistor PCB's as can be seen in the photos.

The preamp is driven by an ECL86 and 2 12AX7’s.

12AX7 plates measured at 182V, 139V, 100V and 101V.
ECL86 triode plate measured at 80V and pentode plate measured at 149V.

Overall, this amplifier sounds very nice, jangly and sparkling with tons of reverb on call. There is even an internal reverb depth control so you can set your max reverb where you like it. The tone turns to a nice crunch at higher volume.