Deconstructing Beamer

I’ve obviously been taking my time with this project, but I never intended to build half of it twice!

I was getting occasional, unexplained changes in B+ voltage readings since the first power-up, but the preamp behaved pretty consistently, so I let it slide. When I started looking at the power amp it was obvious that the signal wasn’t balanced, and I eventually determined that something fried one of my 6BM8 pentodes. After a few fried tubes and much inspection & general poking around, I realized that there must be an intermittent connection to the tube and that the connection was in one of the sockets.

Back in the day, when I was buying tubes & parts that might be useful some day, I found a guy in Lithuania on eBay who had acquired lots of Soviet parts and equipment when it fell off trucks as the Russians left for home (or something like that). I got lots of good deals on parts & tubes from him, but these ceramic sockets are a bust. They might work OK most of the time, but can’t be re-tensioned when the fit gets loose. So, after just a little wiggling (like inserting & removing a tube a few times) the socket holes can — and do! — get loose & intermittent.

You can see here that once the contacts on the left get loose, you have no way to get somethng in there to re-tension (smallify?) them.

So now I’ve replaced all the ceramic sockets & re-wired the heaters, but I need at least a sketch for a new layout of the rest before I finish rebuilding. Fortunately, the replacement sockets fit in the existing chassis holes, but the the orientation of the pins is rotated 90 degress relative to the screw holes so all the point-to-point work around the sockets has to be re-done from scratch.


Almost time to start soldering

Here’s the layout plan (for everything except from the OT secondary to the outputs, which are trivial to do):

Beamer Layout

It’s easier to read when printed, but the different sections of wiring are on different layers, so on-screen I can choose layer colors that emphasize the section I’m working on. I’ll post the LibreCAD file when I have time to work around WordPress (it won’t upload .DXF files).

Finally getting serious about layout

I make a point of getting out in nature for a stroll at least a few times a week, but this &^%$ pandemic and my chemo vacation have given me the time & mental energy to finally get serious about the Beamer layout.

I’ve “finished” a chassis drilling drawing (maybe 2 more holes…) to give to my son & his milling machine, and I’ve been making progress on the electrical layout and one part is done. Knowing that maximizing the IN-13 performance would require some experimentation, I wanted to lay out the circuit on a small prototyping board (and use trim pots in a couple of extra places). This Adafruit board is only 2 x 1.7 inches and looks like it will do the trick nicely:

This is the layout I generated in LibreCAD for the IN-13 circuit at the left side of the power amp schematic page:

Note that the B+IN13 supply rail goes directly to the tube. Did you spot the change from the schematic?

The full chassis layout looks a little different, but uses similar shorthand. Components are just sketched in with rough sizes & only enough detail to identify the part; parts overlap without any indication of final 3-dimensional status. The emphasis is on connections while ensuring enough space.

I’m getting pretty good at LibreCAD, btw…