Back to basics - 5f1

Kevin Wolfe

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I think you can end up making amps almost too clean with all the mods to smooth things out, but I think I like them better that way in the end. Take a 5e3 or other 2x6v6 tweed amp - by the time you add the phase inverter grid stopper, power tube grid stoppers, screen resistors, and reduce the phase inverter input capacitor you can end up with an extremely smooth & clean amp and nice, tight overdrive. But, I'll admit that it loses some of the vintage vibe in the end as well.
I never thought of that process quite like that, but that’s exactly what’s happening. I like it too!

And a beautiful build @theprofessor.
 

screefer

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So tidy and love the mods!

I referred extensively to your 2017 thread when I built my 2a.

This is also timely as I plan to do your fuse/switch mod in a planned GA-5. Wasn’t sure how close to the rectifier I could be Re the fuse. The chassis tab eats up a lot of space.
 

theprofessor

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So tidy and love the mods!

I referred extensively to your 2017 thread when I built my 2a.

This is also timely as I plan to do your fuse/switch mod in a planned GA-5. Wasn’t sure how close to the rectifier I could be Re the fuse. The chassis tab eats up a lot of space.
So glad that thread was helpful! I figured that if I had the questions, others would too. My fuse holder is pretty close to the rectifier socket, but you've got to allow for clearance for the nut to turn and not run into the ledge where the side and back of the chassis are tacked together. Yes-- the chassis tab. That is why the fuse holder isn't farther over. But I'd think the fuse is far enough away from the heat of the tube.
 

theprofessor

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I never thought of that process quite like that, but that’s exactly what’s happening. I like it too!

And a beautiful build @theprofessor.
Yes, I've come to realize over the course of building a few amps how much they can end up sounding the same. That's because I select certain components and even certain tubes for a tonality I like. And I realized that, on the whole, that tonality is a blackface Fender tonality. But! I do love the 5f4 Super, which I still have (though of course that's one of the closest to the "bright lemonade" tonality that Leo himself tended to favor, which is exemplified especially in the BF amps. So I built this in a way that tried to mitigate against me turning it into something else. It is a Champ, and it sounds like a Champ. And that's what I wanted it to do. I have my Princeton Reverb and Vibrolux Reverb for a reason.
 

theprofessor

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I ended up changing the output transformer secondary over to the 4R tap to use with a Weber CVA8 that I got from Chicago Music Exchange on Reverb. I also changed the cathode bias resistor to a 360R. And I got this 5A1-style TV front Champ cabinet this afternoon from Guitar Cabinets Direct. I spent the afternoon shellacking it and lightly aging it. It sounds superb!

IMG_3660.jpg


IMG_3658.jpg
 

theprofessor

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So this is where I've ended up after it's all said and done:

(1) I could not resist using one of my RCA 6V6 black plate singles.
(2) I changed the bias resistor to a 360R.
(3) Under load where the supplied voltage is 118-119ACV, there are 299 DCV on the plates of the 6V6.
(4) Using my Eurotubes Pro One bias probe, the readout on the plate voltage (I suppose this is plate-to-cathode) is 281 DCV. The current to the 6V6 is 42mA, which means that RCA is only dissipating about 11.8W. It sounds good, though, and I'm not going to change anything. If anything, the increased current only makes the amp break up faster and sound slightly cloudier (less clarity).
(5) Pulling 118-119ACV, the heaters are at 6.5ACV.
(6) The Weber CVA8 is a magnificent speaker for this amp.
 

OldPup

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Very nice build and report, Professor! I always envision work that looks as tidy as what you have shown here - but somehow it's never quite that nice. I have a question and a comment. Is there any solid guidance on how close your bias resistor should be to the bypass cap? In these champ style amps I hear it's good to have some space because the resistor can heat up enough to cause the resistor to fail. I had one where I *think* I had them too close together in a SFVC. Something caused the bypass cap to blow apart and it smoked the vintage RCA 6V6. That was not a fun feeling! Live and learn. I subsequently built a 5f2a and was sure to have some air around the bias resistor.
 

theprofessor

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Very nice build and report, Professor! I always envision work that looks as tidy as what you have shown here - but somehow it's never quite that nice. I have a question and a comment. Is there any solid guidance on how close your bias resistor should be to the bypass cap? In these champ style amps I hear it's good to have some space because the resistor can heat up enough to cause the resistor to fail. I had one where I *think* I had them too close together in a SFVC. Something caused the bypass cap to blow apart and it smoked the vintage RCA 6V6. That was not a fun feeling! Live and learn. I subsequently built a 5f2a and was sure to have some air around the bias resistor.
Thanks, @OldPup ! Yes, it's definitely a good idea to have space between them for the reason you noted. You can do that either horizontally or vertically. It will help, too, if the resistor is a wire-wound cement 5W resistor like this one. Should be able to handle more heat.
 




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