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previous: Joe Z. Thanks everyone, I finally got it r... -- 4/28/2000 4:23 PM

Re: vibro-champ- observations (Long)

4/28/2000 11:58 PM
MBSetzer	Re: vibro-champ- observations (Long) Looking at the old Champ circuits, some tweed models used a 22K resistor for NFB, where the BF/SF had the 2K7. So there was less feedback voltage making it through that 22K, and therefore would not reduce the gain of the 6V6 as much as the more modern combos. The 5C1 Champ however had no NFB at all, and that's the way I like my SF. I have known for a long time I like the gain tone of the 6V6 to combine with the 12AX7's not just reproduce the triode tones. So the easiest mod is just to lift one end of the 2K7, or disconnect its lead, and test listen. It's already been mentioned how the tweeds used the same tubes without a tone stack in betwen the triodes. This is a regular Fender tone stack even though it's just a Champ, so unless you have a guitar & speaker where you can play with the treble almost all the way up, you will typically lose gain. Plus especially with such a small speaker, you will need to cut back on the bass as much as you can stand without losing gain too. And your guitar should be all the way up. Besides the one-step removal of the NFB, there is also the simple lift of the tone stack from ground, by lifting one end of the 15K midrange resistor, the bass & treble knobs will not then become completely inactive, but to a pretty good degree worthless and best turned all the way up. After this simple test, if you like it better than stock you could then truly bypass the tone stack by lifting the 100K between the 250pf and 0.1mf as well as the previous removal of the midrange resistor. With the treble all the way up you would really only be passing signal through the 250pf, so you might want to try other value PF up to 500pf or 750pf if the sound is too thin. Of course if you are changing the PF capacitor you could always bypass the treble pot completely with a jumper wire. Each of these simple changes will produce a jump in volume, and have been used by some homebuilders as booster switches. And it will make it sound more like what you might expect from a tweed version with your same tubes plugged in to it. Hope this helps, Mike

4/28/2000 11:58 PM

MBSetzer

Re: vibro-champ- observations (Long)

Looking at the old Champ circuits, some tweed models used a 22K resistor for NFB, where the BF/SF had the 2K7.

So there was less feedback voltage making it through that 22K, and therefore would not reduce the gain of the 6V6 as much as the more modern combos.

The 5C1 Champ however had no NFB at all, and that's the way I like my SF.

I have known for a long time I like the gain tone of the 6V6 to combine with the 12AX7's not just reproduce the triode tones.

So the easiest mod is just to lift one end of the 2K7, or disconnect its lead, and test listen.

It's already been mentioned how the tweeds used the same tubes without a tone stack in betwen the triodes. This is a regular Fender tone stack even though it's just a Champ, so unless you have a guitar & speaker where you can play with the treble almost all the way up, you will typically lose gain. Plus especially with such a small speaker, you will need to cut back on the bass as much as you can stand without losing gain too. And your guitar should be all the way up.

Besides the one-step removal of the NFB, there is also the simple lift of the tone stack from ground, by lifting one end of the 15K *midrange* resistor, the bass & treble knobs will not then become completely inactive, but to a pretty good degree worthless and best turned all the way up. After this simple test, if you like it better than stock you could then truly bypass the tone stack by lifting the 100K between the 250pf and 0.1mf as well as the previous removal of the midrange resistor. With the treble all the way up you would really only be passing signal through the 250pf, so you might want to try other value PF up to 500pf or 750pf if the sound is too thin. Of course if you are changing the PF capacitor you could always bypass the treble pot completely with a jumper wire.

Each of these simple changes will produce a jump in volume, and have been used by some homebuilders as booster switches.

And it will make it sound more like what you might expect from a tweed version with your same tubes plugged in to it.

Hope this helps,
Mike

Replies:

Joe Z. Thanks, I tried the amp w/ no neg f... -- 4/29/2000 4:33 AM