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Discussion in 'Shock Brother's DIY Amps' started by Phrygian77, Jun 26, 2019.
By the way, this is first time I'll be using the GA-SC64 and Tung Sol 6V6GTs. Very curious to see how much power it puts out using the scope, and how good it sounds. I've normally used the JJ 6V6s and Eminence GB128 for these builds. I'm betting on a plate voltage around 420-430VDC, which should be okay for those tubes even though it's a little high, and I'll bias around 60-65%.
I use 3/8 inch zinc plated steel internal tooth lock washers on the 1/4" jacks and RCA jacks. The flat washers go bye bye.
I highly recommend these Carling parts for the switch, at least the lock washer, since you won't get one when you purchase a switch. The hex panel nut, rather than the dress nut, makes it look more vintage.
What's the plan for that unused ground switch hole?
None, it's plugged. I did a NFB cut in my first Princeton Reverb build, but the guy I'm building this for wasn't interested, even though he wanted the bright and raw switches. Less work for me.
This is the first time I'll be using a Switchcraft 13A for the external speaker jack to switch the output secondary to 4 ohm when it's plugged into.
Wonderful looking build, mate!
I haven't done a vulcanized eyelet board build before (all been turrets thus far). What do you use to standoff the boards from the chassis and do you use an insulating piece underneath the main board? My current practice with glass turret boards is to standoff with Delrin spacers that I make in the lathe (generally 10mm or 3/8") with no insulating piece underneath.
I'm using 1/16" Garolite XX. It's phenolic. Really easy to cut and drill. McMaster-Carr calls it, "Economical Electrical-Insulating Garolite XX." It's not as flexible as fiberboard. I use 1/8" tall hex standoffs between the insulator board and the eyelet board. Gives it the classic look without using the same material. It seems to hold up well. My Blackface Bassman has it for the filter cap board, and it's been going two years of constant gigging and practicing, not to mention sitting in the trunk of the car on hot humid July days here in Florida.
I've thought about using 1/8" Garolite without the insulator board. My issue is that I like to get everything as close to the chassis as possible. Aside from the heater wiring, all of it gets pushed down to the chassis. Wires cross as close to 90 degrees perpendicular as possible, with the upper wire(s) separated slightly in a hump (inverse square law, any little bit helps). Check my last builds to see what I mean.
Thanks - I see what you mean re the wiring.
So the insulator is sitting directly on the chassis with the 1/8" gap between it and the Garolite? I like the idea of this material - will have to see if we can get it over her in Australia. Do you use the same material for the insulating board?
Same stuff, Phenolic Garolite XX, basically made from phenolic resin and paper. Much easier to work with than the fiberglass stuff. Electrical insulating properties are just good. Fire resistance on the other had, I don't know, but they use this stuff for electrical purposes all the time. Those little terminal strips you see like this...
Here's the 1/8" tall standoffs...
Hopefully this works. Switchcraft 13A on the left, and a standard 12A on the right. This should switch to the 4ohm secondary when an extention cabinet is plugged in.
Primary wiring also done...
Man, I'm exhausted. I was hoping to get the heaters and mains wired up, but I'm calling it a night.
Very, very nice build. My only comment is it doesn't look like your ground bolts have a star washer between the chassis and terminal.
Rob, those lugs have an internal tooth lock, and there's a 8-32 Keps nut on the other side. The safety ground however, is a crimped and soldered tin plated copper ring terminal with an 8-32 Keps nut inside the chassis. I have some tin plated steel ring terminals with a lock washer that I thought about using, but then I realized that putting an 8-32 nut on the outside so close to the chassis mounting nuts wouldn't be a smart idea. Someone could easily mistake the ground nut for a chassis nut.
Here's the completed chassis...
By the way, I couldn't get those Tung Sols to bias any less than 70% at 413VDC.
That's a beautiful amp, congrats.
So you tried fiddling with the tail resistor value in the bias supply?
(Or you could make a voltage doubler to get more negative bias voltage)
No, I was under the gun to get the amp completed. It's 10k bias pot, with a 22k to ground. I've always had enough range with JJs. Never used the Tung Sols before. These were marked 31 on the pair, so I'm thinking that's fairly hot.