High Power Vibro Champ build

[jsnwhite619]

Yeah man, speakers are king. If you're a solo act looking for a lot of low end out of a small cab, I still think the GB128 is your number one. That's my opinion.

I wanted to have a 12" option on this,but the freaking OT is too big. 125DSE. A vintage style might fit, but I've not better 10's than a small fre 12". Would need a out 1" taller to easily fit a 12" I think with this OT.

 

[jsnwhite619]

Well, it wouldn't be one of mine without something to "fix" with it. I figured out why it was SO clean and the bass wasn't strong enough, and it sounded really good, but just not right. I was trying the #2 input, and I just couldn't tell any difference between the two. Measured the resistance and the meter was looking correct - plug into #1 and get 34k, #2 was 68k between the jack and tube pin. And then I saw it - the 1M grid resistor on the input jack was on there & in place, but it wasn't connected to the ground lug on the jack. I soldered everything else but missed the ground connection and it was run through just right so as not to make any contact at all. One drop of solder later and THERE is the gain, more output, more bass, and no more super bright treble! The Tele on 10 has all the crunch I want, sounds fatter, the bass response is better. Even the treble is smoother and saturates now - cranked it to 10 and bass 0 for some Johnny Winter tones and it wasn't painfully bright. The Les Paul is a real monster now!

 

[cobaltu]

Well, it wouldn't be one of mine without something to "fix" with it. I figured out why it was SO clean and the bass wasn't strong enough, and it sounded really good, but just not right. I was trying the #2 input, and I just couldn't tell any difference between the two. Measured the resistance and the meter was looking correct - plug into #1 and get 34k, #2 was 68k between the jack and tube pin. And then I saw it - the 1M grid resistor on the input jack was on there & in place, but it wasn't connected to the ground lug on the jack. I soldered everything else but missed the ground connection and it was run through just right so as not to make any contact at all. One drop of solder later and THERE is the gain, more output, more bass, and no more super bright treble! The Tele on 10 has all the crunch I want, sounds fatter, the bass response is better. Even the treble is smoother and saturates now - cranked it to 10 and bass 0 for some Johnny Winter tones and it wasn't painfully bright. The Les Paul is a real monster now!

Clips please...

 

[jsnwhite619]

Clips please...

I didn't get a chance yet since I fixed it. A friend of mine was playing out this weekend, and I let him borrow it and try things out. I only ever play solo acoustic these days, so he can give me a better performance review. ;-)

I did wonder about playing with that 1M resistor value though, maybe not on this one, but something to keep in mind. Since I heard the drastic difference in effectively ZERO and 1M, I wonder what in-between values could do? Amp is too dark & muddy/dirty - what would a 470k resistor do vs the 1M? The single coils with this went from "barely noticeable breakup" on 10 to straight up rock & roll crunch, and that was the only difference made.

 

[Michael Smith]

I haven't scratch built any amps myself, but I did build a new 2 x 10 cabinet for my 1979 Vibro Champ. The speakers are Eminence V1030's. Nothing special but they sound so much better than the Oxford R8EV alnico that is in the original cabinet. The only modification I have done to the Vibro Champ was to increase the value of the bias resistor from 470ohm to 1kohm and upped the bias cap voltage. Before the idle plate dissipation was around 130% of "maximum" for the 6V6, using the original 5Y3, even higher with a new (old stock) 5Y3. Now I'm at around 100% idle plate dissipation. (Ignore the fact that I installed the grill cloth with the wrong side facing out, and melted holes in it before installing the Fender logo). The grill cloth was left over from a 4 x 10 cabinet build I did last summer, so I don't feel too bad about installing it wrong side out.

 

[King Fan]

he only modification I have done to the Vibro Champ was to increase the value of the bias resistor from 470ohm to 1kohm...

Nice amp. A 1K bias resistor is a pretty common 'fix' for the VC's notoriously high B+, but pros point out it just pushes more voltage into the preamp. Experts I trust suggest a dropping resistor (eg, 470Ω 5Wwire wound) between rectifier and cap can instead, or a bucking transformer. Also, you may have tested different bias points by ear, but if not, some folks like dissipation as high as 110-115% on these amps, which is still safe in self-bias,.