Looking for Supro Thunderbolt 6420 Build info

[eschertron]

Bitch of a thing to do with a hand drill, keeps moving off the punch mark.

What tubeswell said. Also, as extra insurance, you can do what a machinist taught me: Start with a bit no bigger than the mark you made. Step up a size or two, redrill. Take several iterations to get to the final size. Not only does this keep the hole centered, but it puts less stress on the material, and the bonus is that all the bits in the set get to share the wear.

[RANT] I hate to open up my bit set to find that the two (or three) sizes I use most need replacement, while the rest of the set is unused. I feel better at least having used most of the bits in the set before having to shop for new [/RANT]

[Enzo]

I learned that lesson^^^^the hard way. As a kid I had few tools and only hand operated drills. Then I got a 3/8 Black and Decker power drill for Xmas. Trying to drill a hole with a 3/8' bit into a center punch spot doesn't work. SO I'd start with a 1/16, then a 1/8, and so on up my drill collection.

[Bloomfield]

If you can get your hands on a centre drill such as https://uk.rs-online.com/web/p/centr...-bits/4087938/, you'll find it helps a lot for starting holes. The small-diameter end is very short so it won't flex, which is part of what is causing the wandering. Also the flute at the tip is ground straight as opposed to helical, which will also reduce wandering and will stop it being pulled into the work as happens with brass especially. You can drill a little way onto the larger angled section to provide a good starting point for the next, larger drill. Note that the angled section is 60 degrees to suit lathe centres and so is not appropriate for countersinking flat-head screws which are typically 82 or 90 dergees.


Another thing that also leads to drilling problems is using the wrong speed. It's true that steel needs to be drilled at slower speed than wood, but for a drill around 1/8" diameter you still want to be up around 2000-2500rpm. This will greatly reduce wandering and breakage as well.

[mikepukmel]

Mea culpa. (again). I had a teeny drill bit, the head was just small enough to fit into the punch mark. Must have broke that, because the slightly less than teeny bit I have now doesn't fit the punch mark and wanders. or I could have dropped the teeny bit someplace. The bad thing I go through, is taht I buy a new drill bit, and keep it in the little cardboard with shrink wrap plastic as long as I can. Eventually they fall apart, and the bit ends up in the box I have for smallest bits. But they all look about the same and I have no way to store them. My friends father, an amazing craftsman, years ago I was lucky to just wander through his basement, had a piece of wood, like maple or oak, drilled holes, and stored the bits there, with teh size marked in pen on the wood. If I could go back in time, Id do that. Too late now.

[mikepukmel]

The 6420 circuit build is under way. The heyboer PT arrived a while ago. Im using the Weatherford schematic that i understand is very authentic. So I got a few really good photos, gut shots, and for some reason got the idea to match up all the parts in the photo, with the schematic. First, I got a count, and the preamp side is off by 1 part. That is, there is 1 more resistor inside the preamp than on the schematic. Curious, I wrote down all the resistor color codes, in order, went to the web, got the ohm values, going through the photos, putting a resistor code (R1, R2, etc) and seeing if I can find out where the phantom resistor would go in the schematic. Tough part, there are very few photos, and its really hard to see where the connections are on all of those terminal strips.

I got the angelfire site photos, they're awesome, anyone know where I can get a few more good gut shots of the preamp side? I only know its a 6420, not year, or variation. I think there are many variations, by comments on the web.

I'll post what I have so far, later tonight. One of you guys will probably point it out in a second.

[mikepukmel]

Found the extra resistor, which isn't an extra resistor! The 100k dropping resistor and 4uf filter cap, for the preamp is in the preamp! Tough to reverse engineer which part goes with the schematic, without a working amp that I can look at, since the photos are from one angle.

[mikepukmel]

The 47K's there are combined grid stops & mixers. BUT a grid stop located at that remoteness isn't doing much as a grid stop. Even worse, on the other side of the DC blocking cap .005. On this schematic, neither input has a shorting bar to ground, so the 47K's act as mixers only and the 2nd input is not attenuated 6 dB as in the Fender style input circuit.

In fact that .005 cap isn't really necessary circuit wise - the triode has a cathode resistor, so its grid will tend to stay at 0 volts anyway as long as there's a grid leak R. However the .005 does act as a filter for very low frequencies, and that has something to do with the tone of the amp.

A couple refinements: A/ As on Fenders, a signal entered at the #1 input passes thru both (typically) 68K resistors, so the effective value is really 34K. On your amp, 23.5K. So you would be justified in using a 22K, close enough. and... 2: The grid leak resistor in the schemo is 270K. If you want to sound brighter, not loading your guitar pickup as much (assuming you will plug your guitar into the amp without intervening effects/preamps/buffers) you could increase that to the Fender standard of 1M, or even more.

Another mild mannered mod, if you choose to go with one input, you could use the hole for the 2nd input to mount a "brown tone" pot, where you could vary the value of that grid leak/pickup load resistor. Or just forget about the whole thing, and stick with 270K because it's what Supro used. Gotta try to get that Page tone, y'know! Nothing wrong with that.

Thanks again Leo. I haven't had many amps, maybe dozen or so max, and played through a handful of others, mostly junky amps in friends basements. There are some guitar tones I haven't heard from other amps, e.g. typical fender, marshall, ampeg, sunn, vox. Curious if I could get some of those with this amp circuit. I have the parts for a few versions. Stock, and a few mods you guys have suggested. Haven't thought of a speaker yet.