Vibro Champ Mods?

Are you sure it hasn't been modded already, or is maybe not working properly? All the Vibro-Champs I've ever heard have been very sweet little amps with a surprising amount of bottom end for a little amp with an 8" speaker. I strongly dislike over-bright guitar tones, and even at that I would never run one with the treble on zero I don't think.

 

Ah - if it's dead stock it will probably be biased wrong. They did that, except for the very first ones. After a short time, they replaced the power transformer with one from a Princeton, which increased the output power to 6W (from 4), but they didn't change the cathode bias resistor to account for the higher voltages. Later BF and all SF Champs/Vibro-Champs run far too hot because of this - probably no-one noticed because the 6V6s back then were good enough to take it... typically they run at 15W or more. But this also makes the tube clip too soon on the forward end of the waveform, which lowers the clean headroom and makes the distortion quite harsh - kind of like overdriving a mixer , it's like a clean tone underneath with a farty, buzzy distortion riding on top.

To fix it, you need to raise the value of the cathode resistor from 470 ohms to 1K, and change the bypass cap from 25uF/25V to 50u/50V (the voltage rises to about 35V, and you need to double the value to maintain the same bass roll-off since the resistor value has doubled). It's also a very good idea to fit a screen-grid resistor (move the wires from pin 4 to pin 6, and fit the resistor between 4 and 6). I'd use wirewound types for both, 5W for the cathode resistor and 2 or 3W for the screen.

I don't know whether that will cure the 'ice pick' you're hearing, but it does make the amp sound better.

Check also that it does have the 330pF cap from the power tube grid to ground - some people take these off as 'tone killers', but for what you want it's better to have it there I think.

 

John's right.
First get it checked out to make sure it's stock and in good working order. If you like it then fine.

I own a '67 and am not really a big fan of BF Fenders. I had Jimmy Somma of Sommatone (www.sommatone.com) modify it slightly (and completely reversible) by putting in a switchable tone stack bypass. This has the effect of increasing gain, making it more tweedy like but you can switch back and forth. Also changed speaker because the original was not fit enough for a radio.....

Best, Pete.

 

Another popular mod you'll see with champs is a switch to remove the global NFB (goes from the speaker thru a 22K resistor back to the 2nd gain stage). If the amp is too bright for you already, don't do it.

Instead, you might consider a bypass cap around the 22K (or a combination bypass cap and resistor) to emphasize the highs in the NFB circuit -- effectively cutting the highs in the output. Alternatively, if you can find a place to mount the pot, you can install a presence control to achieve the same ends but be able to adjust the effect.

 

I thought when you increase the cathode resistor you have to decrease the capacitor. Maybe I did my math wrong, but I have the parts to mod my SF champ, but haven't had the time

When I was poking around on the Fender Forum and doing some math, I came up with 10uF cap to get the same bass rolloff. Am I mistaken?

 

If you raise the value of the resistor, you are increasing the resistance in the signal path which will reduce the output power (this isn't the same as in a simple gain stage where it's only the voltage gain that matters). This mostly affects the bass since since the cap allows higher frequencies to bypass the resistor. You need to lower the resistance of the cap at low frequencies in order to retain the same overall resistance in the bass - which means increasing its value.

 

Screen grid should be 1K also , John ? I just recapped (the 25's @50 anyway , the can still seemed fine) a Champ , and replaced the 1K voltage divider (it had dropped to 350 or so ) and the 470 bias (it had climbed to 750 ) with 1K's@2watts (wirewounds) . It's WAY mo'betta now all around as far as controls doing what they're labeled , and sounding good thru a 10" G10D 25 . 8 ohms seems fine with it too , going from 4-8 just loses a little volume ??? "Enough bass" doesn't seem to be an issue , at least to my ears .

 

Sorry John to bring this back from the dead as my introductory post.

I got a homebrew Vibrochamp that was stock until last night. I have been modding the crap out of it for the last 24 hrs. I removed the tonestack and also removed the NFB. I'm thinking of just adding a pot for the NFB sometime soon too.

Anyway I was reading your post and noticed that my VC is biased a little high so I will replace the 470K resistor and up the bypass cap I will up to 50uf/50v.

My question is what value should the screen-grid resistor be?

Here's a few pics of my amp.

Cheers

 

Typical Fender value is 470 ohms. Go with at least a 2 watt. The 5 watt John P recommends is better.

Mike

 

thats one sweet vc
i did all of the kendrick mods. rectifier, tweed specs etc. i put a ceramic weber in it which i dont think was a great match, should have staye with an alnico.
after all said and done, the stock circuit with a heavy duty weber alnico (to handle the thnkg cranked) is what i prefer

 

It is our misfortune that John Phillips no longer attends this forum.

 

Thx guys,

Mike,

Are you talking about the cathode resistor or the screen grid resistor?

Cugel,

I actually did all the Gerald Weber mods on mine. I still need to change a cap or two to make it complete but I like the sound so far.

The speaker I got is a Weber 10A125. The Alnico does sound really good.

Removing the tone stacks gave it a pretty nice boost.

 

Connected my new Texas Heat to my VC which had been sounding a little fizzy-er than I liked (through 2xJensen Mod12-35, 4 ohm load), now sounds smooth and balanced.
Not a bad fix for 70$.
Mike

 

Wow. Thread revived after almost 5 years. I wonder what the record is?

 

I recently got an SF Vibro Champ in a trade. Did a cap job and installed a new speaker which helped a lot. Then I got looking at an empty SCXD cab I have. So I ordered a Jensen 10" P10R, did a little work on the cab and now have the Vibro Champ installed in the SCXD cab. What a great sounding little amp this thing is now. Much fuller sounding and louder than stock.

 

I did the tone circuit bypass and the switchable nfb.when both are bypassed And with the amp dimmed I fried 6v6's.
It seems happier with the cathode resistor switched from 470 to 1k. At 470 ohms it 16 watts dissipation!
At 1k it's about 10 watts.
Tonewise and when not dimed, I think the amp sounds better at the 1k.

 

a good running sf champ or sf vc is a killer amp, no question. my sf champ roars. i did a few simple mods (as suggested by john phillips above) and re capped and it put the loudest 8 weber ceramic they make, and honestly i couldnt be happier. i personally do not like the tweed mod. but i did it as well as the nfb lift. my nfb mod actually gets rid of it completely and is not to champ specs. dig the tone though. i also only run nos tubes. the tube selection makes a huge difference