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Vibro Champ Questions

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4/24/2000 1:31 AM

Vibro Champ Questions

I had the good fortune of picking up a 65 (OJ) Vibro Champ in good cosmetic condition but with a few electronic glitches. Namely, it begins to lose power about after 3-5 minutes. It is also shy on bass. The speaker is a reissue jensen CR8.

I ordered a new multi-section cap for the power filters, as well as a few new resistors for the power supply. Aside from the cap can which looks like it was replaced in '74, everything else is orginal.

This is my first "vintage" amp, so what is the general procedure. Should I replace ALL the caps and resistors being such a simple circuit? Also, what should I use to re-apply lose Tolex.

Lastly, any suggestions to improve bass and mid response? Thanks. Joe

4/24/2000 1:53 AM

quote:
"This is my first "vintage" amp, so what is the general procedure. Should I replace ALL the caps and resistors being such a simple circuit? Also, what should I use to re-apply lose Tolex."

I would definately recap the entire amp. It will make a pretty big difference ;-)

;-)

The value usually drops when you retolex the amp so if you ever want to sell it I would advise against it. If you plan on keeping it then run with it! (ha, ha)

quote:
"Lastly, any suggestions to improve bass and mid response? Thanks. Joe"

Joe, I'd recap the amp first and I think you'll be surprised at how the bass and midrange will react. If you still want to mod it after the recapping then just hollar! I have some cool tips and I'm sure everyone here does as well.

Trace

4/24/2000 4:04 AM
Joe Z.	Trace, thanks, a recap then it is! No, I don't want to re-tolex, just glue down some of it in places where it has loosened. You guys would no doubt get a kick on how I brought back the exisiting tolex to its former glory. Lets just say it involves Kiwi Black Shoe Polish, a can Pledge and some elbow grease. When the time comes I would love to get the low-down on those mods. Joe

4/24/2000 5:48 AM

quote:
" No, I don't want to re-tolex, just glue down some of it in places where it has loosened."

Smart man *laugh* against the advice of many a wise man, I did my own tolex job... I"m about 95% finished, and I swear with all of Ampage as my witness, I will NEVER tolex an amp myself again!!! *laugh*

quote:
"When the time comes I would love to get the low-down on those mods. Joe"

Not to step on anyone's toes, I have a few mods i'd b willing to share as well... what the hell, I'll share my favorite right now. . solder a 1k resistor in place of the 22k NFB resistor there, also in series, bput in a 250k pot. NOW turn the pot till you find the sweet spot that you like. Measure the value of the pot + the 1k resistor, nwo replace the mess with a resistor of the same value. I have more, but I have to be fari and let others share as well

4/24/2000 5:28 PM

quote:
"solder a 1k resistor in place of the 22k NFB resistor there, also in series, bput in a 250k pot. "

When the pot is set for minimum resistance, that will really screw up the bias of the triode unless you use a blocking cap.
With the pot set to zero, the FB voltage is seeing only the 1K and you could get over coupled NFB if you also don't use a tone stack and go Tweed style.
Try a series RC of 4k7 and a .33uF to .68uF cap.

Biggest difference you'll find with this amp, after you've recapped it as needed, is a good speaker.

Bruce

4/24/2000 5:43 PM

quote:
"When the pot is set for minimum resistance, that will really screw up the bias of the triode unless you use a blocking cap."

True, but remember, some of the tweeds used 820ohm NFB resistors. WHat I've fond, and i'msur eothers will agree, settings between 15k-50k will sound the best.

quote:
"Try a series RC of 4k7 and a .33uF to .68uF cap."

Those will give other otions in the sound as well and could be determined.

quote:
"Biggest difference you'll find with this amp, after you've recapped it as needed, is a good speaker"

I agree here, the old champ speakers were not the best in the world *laughs* I'd personally put in the newspeaker BEFORE i messed with the NFB settings or any other mods, since changing the speaker may nullify your changes, or overemphasize them in a bad (or possibly better) way, it WILL Be different

4/25/2000 6:55 AM

I guess I don't know my Tweeds as well as I could but, what tweed amp has it's NFB hooked up like this with an 820 ohm resistor?
The smallest resistor I can think of is in a BF Fender...Vibro Champ, Princeton, etc.. I think they are 2K7, and the RkCk is held above ground by a 100 ohm resistor with the NFB applied between the 100 ohm resistor and the bottom of the 1500/25uF RkCk combo.
If it was not done this way, you wouldn't be able to use a very large cathode bypass cap because it would bleed the NFB AC voltage to ground and the amp would sound like there was nearly no NFB.
And also. keep in mind the other side of that OT secondary is dead shorted to ground, so the NFB resistor is in parallel to ground with the cathode resistor of the triode in the tweed champ... if the NFB is feed at the cathode.
I'm with you on the value being larger more then smaller.
WeberVST has some nice sounding small guitar amp speakers.

Bruce

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