Last week I finally had the energy to debug the Beamer. It turned out there was a cap on the the Treble control that was only conected at one end. Doh!
So now I’m making noise again, and ready for some tuning. The first order of business is to try a little negative feedback from the output transformer. As the master volume goes up the low end gets a litttle too rough for what I want from the Nice channel. I might also need to reduce the output from the preamp as I suspect it gets too hot for the phase splitter at high Preamp volumes.
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.
Here’s the layout plan (for everything except from the OT secondary to the outputs, which are trivial to do):
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).
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:
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.
First: I give up — The Beamer is gonna be the name.
Second: In addition to the Nasty/Nice channel switch & LED, the new schematics include circuitry for this IN-13 linear indicator tube I’ve always wanted to use:
The active length (between the two red lines) is 4 inches, so it should look great if I can come up with a good way to mount it (horizontally). The schematic currently has it getting the signal after the Master volume, but I’m thinking it’ll probably end up tapping the signal just before that pot, so it’s not boring at low volumes.
The spec for the IN-13 has me thinking a standby switch is probabaly the simplest way to (mostly) adhere to the startup recommendations, so that’s back in.
I haven’t done any detailed layout yet, but I have decided that – for both electrical layout and control clarity reasons – it will be best to place the input jack between the Nasty & Nice volume controls. I’m also planning on using different/larger knobs for the Nasty, Nice and Master volumes:
Drive Tone NASTY jack NICE Bass Treble MASTER lamp
The red LED will go above the Nasty knob, and the channel switch will go above the input jack.
While avoiding the layout of the next version of the prototype, it ocurred to me that I haven’t even shared that schematic yet.
So I’ve started a Beamer page [<– over there] with PDFs of the three pages of the current schematic. This is essentially the current prototype as it will be built into a fresh box, with something close to a final layout. Be aure to read the notes…
Then I’ll play with a few Rs and Cs to perfect the sound!
I’m not crazy about “Beamer” as an amp name (derived from the push-pull 6BM8 design), but that’s all I’ve got. Suggestions are welcome!
The more I look through my schematics & notes from 15 years ago, the more I realize that my Beamer project really wants to be a stripped-down Verberator II: no reverb or tremolo (we have boxes & software for that these days), just a wide tonal range, from clean to dirty.
Today I dug out my old Verberator II prototype (grrrr: link not working) chassis and immediately realized that I’d already done much of the work toward a Beamer. I suspect I gave up on it (and amp building) because the tremolo was more than I could master. With all the crazy shit you can do with software these days, it makes sense to leave that to post-production and concentrate on the core function (TONE) with electric guitars. So we’re talking about a gain pot, a foot-switchable ‘boost’ stage, and a master volume, with one or two tone stacks (or single-pot controls).