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Vintage threads from the first ten years

vibro-champ- observations

4/26/2000 9:20 PM
Joe Z.	vibro-champ- observations Ok, I changed all the caps last night save for the two .01's that connect to pins 1&7 of the 2nd 12ax7. As to be expected, the sound is fuller and louder. I also changed the 10k, 1k, and 470 ohm resistors in the power sections as well as the filter cap can. Here's the hitch, this morning, the new 2watt 470 ohm bias resistor was sizzling, and the amp would throb (like tremelo) when both the Bass and volume were full up. I checked some connections and installed a !! 3WATT !! 470 ohm resistor. I played a good hour, the resistor is now seemingly O.K. but the amp will still throb a bit ONLY when the Bass is on 9-10, and the volume is near or at full. I will also mention that I bridged the first two sections of the 20/20/20/20uf filter cap. Any help, will be greatly appreciated.

4/26/2000 9:20 PM

Joe Z.

vibro-champ- observations

Ok, I changed all the caps last night save for the two .01's that connect to pins 1&7 of the 2nd 12ax7. As to be expected, the sound is fuller and louder. I also changed the 10k, 1k, and 470 ohm resistors in the power sections as well as the filter cap can.

Here's the hitch, this morning, the new 2watt 470 ohm bias resistor was sizzling, and the amp would throb (like tremelo) when both the Bass and volume were full up. I checked some connections and installed a !! 3WATT !! 470 ohm resistor. I played a good hour, the resistor is now seemingly O.K. but the amp will still throb a bit ONLY when the Bass is on 9-10, and the volume is near or at full.

I will also mention that I bridged the first two sections of the 20/20/20/20uf filter cap.

Any help, will be greatly appreciated.

4/26/2000 9:39 PM
Don Symes	Are you _absolutely_ sure of the value of that 470? 470 Red-(something)-Brown, not 47 Red-(something)-Black? After nearly 20 years, I still boob this up on a regular basis.

4/26/2000 9:48 PM
Don Symes	See? I even boobed up the color code! Yellow-Violet-Brown instead of Y-V-Blk? The remark about 'bridging the first to sections' - you're saying you have: B+ - 40uF (20+20) - 1k/1W - 20uF - 10k/1W - 20uF with the OT connected at the 40uf/1k node and the screen at the 1k/20uF node, right? There's too much current through the 6V6's cathode (duh), are your voltages on that tube close to right?

4/28/2000 4:23 PM
Joe Z.	Re: vibro-champ- observations (Long) Thanks everyone, I finally got it right. It seems I had at least one bad ground connection in the preamp section, and a few cold solder joints elsewhere. Before the fix, all the voltages were low to the tune of 40 volts on the power tube, except for the voltage on the 6v6 cathode which was more than double (56v instead of 21). The amp seems to be working fine for now,(crossed fingers) the voltages on the 6v6 are now a little higher than the schematic, but no big deal. The cathode is dead on. Now some more questions: 1) Was the original speaker a 3.2 or a 4 ohm? 2) With the volume on 10, the amp is still clean, only crossing into distortion when I strum a chord fairly hard. I can't get single notes to really soar like a tweed. Was this intentional? I know they tried to clean up some of the rawness of earlier models with the blackface amps. How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ. Thanks again. Joe

4/28/2000 4:23 PM

Joe Z.

Re: vibro-champ- observations (Long)

Thanks everyone, I finally got it right. It seems I had at least one bad ground connection in the preamp section, and a few cold solder joints elsewhere. Before the fix, all the voltages were low to the tune of 40 volts on the power tube, except for the voltage on the 6v6 cathode which was more than double (56v instead of 21). The amp seems to be working fine for now,(crossed fingers) the voltages on the 6v6 are now a little higher than the schematic, but no big deal. The cathode is dead on.

Now some more questions:

1) Was the original speaker a 3.2 or a 4 ohm?

2) With the volume on 10, the amp is still clean, only crossing into distortion when I strum a chord fairly hard. I can't get single notes to really soar like a tweed. Was this intentional? I know they tried to clean up some of the rawness of earlier models with the blackface amps.

How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ. Thanks again. Joe

4/28/2000 4:57 PM
Steve	Joe, I'll take a stab at these: 1) Was the original speaker a 3.2 or a 4 ohm? I believe it is both: the 3.2 ohms refers to the DC resistance of the voice coil as measured with a multimeter. The 4 ohms is the AC impedence of the voice coil. 2) With the volume on 10, the amp is still clean, only crossing into distortion when I strum a chord fairly hard. I can't get single notes to really soar like a tweed. Was this intentional? I know they tried to clean up some of the rawness of earlier models with the blackface amps. I believe it was, the voltages in BF champs are higher than in tweed Champs, I think Leo was trying to get more clean headroom out of it. Also the tonestack in the BF Champ soaks up a lot of the signal, where the tone knob on a Tweed Champ loses little signal. How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ. Try experimenting with different values on the cathode bypass caps, try disconnecting the tone controls to get a more Tweed-like grind out of it (I believe this is what Weber's mod does). You can also try disconnecting the negative feedback or subbing different values for the resistor. Also you might try lowering the B+ with a Zener diode. Also, to appease the vintage amp police, don't cut or drill any holes, don't do anything that can't be reversed. YMMV. Steve

4/28/2000 4:57 PM

Steve

Joe,

I'll take a stab at these:

1) Was the original speaker a 3.2 or a 4 ohm?

I believe it is both: the 3.2 ohms refers to the DC resistance of the voice coil as measured with a multimeter. The 4 ohms is the AC impedence of the voice coil.

2) With the volume on 10, the amp is still clean, only crossing into distortion when I strum a chord fairly hard. I can't get single notes to really soar like a tweed. Was this intentional? I know they tried to clean up some of the rawness of earlier models with the blackface amps.

I believe it was, the voltages in BF champs are higher than in tweed Champs, I think Leo was trying to get more clean headroom out of it. Also the tonestack in the BF Champ soaks up a lot of the signal, where the tone knob on a Tweed Champ loses little signal.

How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ.

Try experimenting with different values on the cathode bypass caps, try disconnecting the tone controls to get a more Tweed-like grind out of it (I believe this is what Weber's mod does). You can also try disconnecting the negative feedback or subbing different values for the resistor. Also you might try lowering the B+ with a Zener diode.

Also, to appease the vintage amp police, don't cut or drill any holes, don't do anything that can't be reversed.

YMMV.

Steve

4/28/2000 8:00 PM
bronco	How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ. Remove the tone controls, this is a very simple mod and is very easily reversed. I will draw you a diagram showing you what to do if you would like. BTW I have a Silverface Bronco which is the same circuit as a BFVC. I did this mod and it sounded great but the OT was messed up from previous owner using 16 ohm speakers in it and it really farted alot so I reversed it....

4/28/2000 8:00 PM

bronco

How about some well known mods? Without starting some kind of flame thing, I know Torres sells a kit to tweedify a blackface champ.

Remove the tone controls, this is a very simple mod and is very easily reversed. I will draw you a diagram showing you what to do if you would like. BTW I have a Silverface Bronco which is the same circuit as a BFVC. I did this mod and it sounded great but the OT was messed up from previous owner using 16 ohm speakers in it and it really farted alot so I reversed it....

4/28/2000 11:58 PM
MBSetzer	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

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

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