The Supro 1616T is pretty much wrapped up. I did some work on it sporadically over the last week or so. My stoopid job kept me away from it. But I got some time the last couple of days, finally. This post covers the electronic stuff I did after rebuilding the filter cap can.
This amp is no exception. I got a Weber 6x9 replacement for it. I was pleasantly surprised to find that Weber makes this speaker.
You can see it on the right next to the old original Rola. And if you're paying atttention, you'll notice that the Rola is not a 6x9 as I thought! It's a 7x10.
I'm stoopid. There. I said it.
These are just cut from birch ply. I painted the screw heads and the blocks black so they'll be less visible from the front of the amp when the speaker is installed.
I even beveled the edges. I suppose doing guitar repair work is making me do stuff like this.
One end goes to the factory bolt in the amp and he other end goes to the speaker. Simple and effective. Disaster averted. Or is it inverted?
If you have one of these amps, this is where you can replace one of the .01 ceramic disks with a new cap. Doesn't matter what it's made of. It's a functional cap, no signal goes through it.
Simple, twenty-five cent easily reversible mod.
If you were really slick, you could rig up a 'range' switch to change the speed ranges from one cap to another!
There four caps I replaced. The first is the .005 cap from the "treble" input. I put an .005 Orange Drop there. The purpose of that cap is to cut bass and mids. A guitar sounds thin and trebly on that input.
The next three are the most important. If you refer to the picture with the capacitors and arrows, I'll walk you through it. This picture is after I changed two of the caps.
The first cap on the left is an .005. I put an Orange Drop here, mainly because I don't have any PIOs in that value. I might have gone with a Mallory 150, but I don't have any of them in that value either! An Orange Drop might be the best choice anyway since they have good clarity and detail. This cap is actually part of a high-pass circuit. It's hard to see, but the right-hand side of the cap connects to a 270K (I think...) resistor that goes to ground. The purpose of the cap-and-resistor circuit is to cut lows and pass mids and highs (hence the name "high-pass" - high frequencies are passed).
In this case, the cutoff is about 68Hz. Any signal below that frequency is cut. When you consider that the low "E" string on a guitar is 80Hz, this is a pretty good way to cut any hum or noise in the signal from passing through the amp.
The next cap was a .005 ceramic. Again, this would cut bass and mids from the signal coming off the filter. I wanted to bump up bass and low mids a couple of db, so I upped this cap to .01. This is not an extreme change - we'll just get a bit more 'meat' and 'body' to the tone. My guess is the Valco engineers went with smaller value caps because they figured too much low end would sound mushy though a small speaker.
The silver cap is a Russian K40Y-9 paper-in-oil capacitor. Smooth, rich, with creamy highs. Or something like that.
The last cap in the picture is the coupling cap that goes from the second stage of the preamp tube to the input of the output (6V6) tube. It's the one the red arrow is pointing to in the picture above.
This is the most critical capacitor tone-wise in the whole amp. The value is .01, but I'm going to bump it up. I'm also going to relocate it from its current location.
If you look at the green arrow, you'll see a yellowish wire there - which then goes upward out of the picture. Signal is flowing from left to right in the picture - through the .01 cap, then via the yellow wire to the output tube. I'm going to move the yellow wire to the left terminal of the cap, then move then wire a new cap over on the tube.
There's also a 470K grid load resistor connected to that wire and going to ground. We're a-gonna move that too.
So we moved the wire to the terminal where the left-side (the "input" side, if you will) of the coupling cap was. Now that yellow wire is connected over to pin 1 of the 6V6. See the picture - it's the green arrow. Pin 1 on a glass 6V6 has no connection, so it's a perfect place to use it's tab as a connection point.
The wire goes there, and we also have the "left" side of our new cap there too. The other end of the cap then goes to the grid (input), pin 5 of the tube. This connection is indicated by the right arrow.
All I did was just shift the cap over to the tube. I did this for 2 reasons: first, the new cap is physically larger than the old one and would have been very difficult to mount on the terminal strip where the original was. Second, it's better wiring practice to connect the grid side of a capacitor as close to the input (grid) as possible. This helps keep noise down. So we got 2 benefits from this move.
You'll notice this cap is also a K40Y-9.
Finally, the 470K grid load resistor is reconnected at the grid and goes to ground. You can see this resistor also at the red arrow. I could have left it back on the terminal strip, but I would have needed a wire back to it for it to work. This makes more sense, and in fact, most amps are wired this way to start with.
The value of the cap, by the way, is now .02 instead of .01. Again, we'll get a couple db boost in bass and lower mids, which will give a fuller tone. The amp now sounds fatter and a lot smoother. There's a nice balance of highs and lows and the highs are rich and smooth thanks to the Russian PIO caps. Da!