Vibro Champ back to normal

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CJ

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ever since 64 this poor amp has been changed to a kt88, el 34, 6v6 push pull, fixed bias, self bias, solid state rect, tube rect, sears, harmony, gibson, vox, early fender, late fender, rivera fender, i think you get the concept,

so what a shame to drill thru a nice retro chassis, so we soldered up some holes and got some new iron, built a non conductive black bakelite pc board from a 50's drive in circuit box, and managed to get it back to it's old self again,

tried a 5v4 rectifier tube  but the new pwr trans from antique electronics, , which dropped right in, had a slightly higher plate voltage, and with the lower foward drop of the 5v4, the plate of the 6v6 was seeing about 410 volts, which gave a def tones type of crunch, which was not exactly the sound we were trying for with a 2 stage SE amp, so the 5y3 is back in there,

no mods except some heavier wattage 100k plate resistors, because everybody has heard 1/2 watt resistors making pop corn at the worst moments, right?

there was one mod, a .02 cap in place of the .01 cap in the vibrato circuit, this slows down the vibrato real nice, i bought a vibrato bug, the optp coupler, but this circuit does not use it, and i thing it sounds cooler than the opto version in the big amps,

note that with the non opto, putting in a tube with a different cathode voltage, like a 6072/12ay7 in the preamp slot, the vibrato will not work! the voltage on the cathode is too high to swing, so you are stuck with the 12ax7,

there was a problem with the new output from antique, it was wired out of phase, so i had some nasty squeals coming from the output, which zapped me a few times, so i reversed the black and yellow speaker leads and every thing is cool,

some folks say to dis connect the feedback, but i like the mellow sound these days, so let mr fender decide what the world sounds like,
oh, i did change the heater circuit from grounded to balanced, and there is a lot less hum than stock, no 100 ohm resistors to ground needed,
 

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CJ

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ready to bolt up and plug into this extension speaker cab,

there is a Kendrick black frame 10 inch , so maybe a resto back to the 8 inch, Weber has a ton of champ speakers, Jensen has the ceramic and alnico versions, i have heard good things about Warehouse speakers, anybody tried them?

popped in a blue jewel pilot lamp lens to up the cool factor,  8)

the amp sounds great, very touch sensitive, you can not get away with anything, mistakes are right there, so play clean and you will sound mean.

and every guitar will sound slightly out of tune and slightly like matchbox 20,



 

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okgb

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Interesting , I bought a 70's silverface vibro champ and got a retro black faceplate for it .
someone had been inside messing around but it still sounds good as is , but I've been
meaning to get in there and clean it up , still to make a list of things to do for it ., Nice !
makes a great t.v. practice amp  ,  play by myself amp and haven't recorded with it yet .
 

CJ

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i hope everybody had a safe 4 of july,  :eek:

was not happy with the standard vibro champ output xfmr so i wound a new one with twice the core and multi-section winding instead of pri-sec,

i think Fender wanted to save the Oxford 4 ohm speaker the heavy bass, so the small core with no gap limits low end,

stock fender vibro champ output saturates at 40 volts 6v6 plate voltage output rms at 80 hertz, and- 3db bass roll off is about 60 hertz, less hum if you roll off early, and hum is a problem with SE amps, so another smart move,

air gaps are fragile, with the small cores, you move the gap .001 inch and you have a different transformer, so we are using a 10 x 10 core lace up which creates a natural gap which is much more stable with temp and mechanical stress,

clue to winding structure can be seen at right, looks like bi-fi middle sec,

threw 10 nickel lams in there for low level inductance, once it saturates, the silicon takes over,  Neve specifies rusty lams, so we thru a little neve in there,  :D




 

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CJ

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custom stacks mean you can not use the square stack bracket, but with a pair of snips and the old bracket as a model, we can fab a new one from lowes sh*t metal, i mean sheet metal,  :D

the end bells do not care about stack size so we can use them over,

lacquer dries quick, and it huffs the best,  ;D

 

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CJ

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how does it sound?

fantastic!  more bass with less distortion, and instead of cranking the treble to 10 as usual, i only have to crank it to 4 with the extended hi end of the new transformer.

now the new transformer is nice, but the speaker choice for this amp will have the most effect on the sound,

crunch and munch thru a wolfer type speaker with a big magnet does not sound good, you need a small voice coil with a medium magnet for that original sound.

somebody brought over a bunch of speakers that they pulled from goodwill thrift shop cabinets and kept them in the closet, i do not know the name of the 8 inch i put in there, it has an old dryed up cone and a bit bigger magnet than the stock Oxford, and it has the sound we were trying for so this project is finalized.

new xfmr next to the old DIY attempt,
 

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