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Post subject: Silverface Champ
Posted: Sat May 14, 2011 10:27 am
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I have a 1979 Fender Champ. I was looking at the schematic for this amp an i noticed 1 thing there is a capacitor running from pin 5 to 8 on the output tube. I was wondering if this was a parasitic suppression cap like on the bigger boys. Has anyone removed this cap?And what were the results?

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Post subject: Re: Silverface Champ
Posted: Sat May 14, 2011 7:55 pm
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this is a arjay question.

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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 1:37 am
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Is this actually a Vibrochamp?

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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 3:22 am
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In my opinion is for removing parasitic oscillation, Disconnect it and heard the sound


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 3:43 am
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No this is a champ not a vibro champ


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 6:20 am
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It's acting like a high frequency crossover to limit high frequency (maybe even above hearing freq or super-sonic). That amp may have had a history of instability and blowing fuses or tubes.

Dressing on the socket and secure grounding points could eliminate the instabilities. Be sure that the secondary winding of the output tranny has a good grounding point.

Maybe replace the 470-ohm cathode tied resistor with a good one (like a 5 watt Mills). And the bypass cap with a new 25mfd/50VDC 'lytic. And this may stabilize the tube's idle enough to avoid the instabilities. And you can toss that disc.

Check out schematic for that particular cap across 5 and 8:

http://www.ampwares.com/schematics/cham ... _aa764.pdf


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 6:31 am
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it's a champ not a vibro champ people

http://www.ampwares.com/schematics/champ_aa764.pdf


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 7:12 am
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But the power tube section is basically the same between the two. Notice the added mica cap on the late model silverface Vibro-Champs? Prolly for the same reason(s).


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 10:06 am
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BMW2002Ti wrote:
But the power tube section is basically the same between the two. Notice the added mica cap on the late model silverface Vibro-Champs? Prolly for the same reason(s).


That 330 pfd cathode-to-grid cap seems to be most prevalent with tremolo-equipped Champs (and the Bronco). Note this original blackface AA764 Champ circuit......

http://classic-web.archive.org/web/2006 ... _schem.pdf

This cap seems to be present in all Vibro Champs and the Bronco. It did seem to find its way into the basic silverface Champ models, but only well into the CBS era. Hazarding a "WAG", I think its original purpose was to provide some stabilized feedback to the output tube when the bias-modulated tremolo was used to increase the life of the tube.

Over to you, Wink......

:mrgreen:

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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 6:18 pm
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Arjay, this is a single-ended amp. I think that the vibrato-tremolo section pulses are being feed into the 2nd half of the 12AX7, just before the 6V6GT.

The section's power comes off the 2nd power tap (+340VDC) of the cap can.

http://www.ampwares.com/schematics/cham ... _aa764.pdf

Just a WAG...


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 7:58 pm
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So why's this cathode-to-grid cap found more prevalently in the trem-equipped Champs (and the Bronco)?

The vibrato circuit is otherwise the only difference between them and a straight Champ Amp.

Arjay

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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 9:13 pm
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Prolly a filter to keep very high frequency from passing into tube. Attempt to lower possibility of parasitic oscillations above 20KHz? Just a WAG.

Maybe the vibrato section induces instabilities more than the non-vibrato? Another bugtime WAG.


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Post subject: Re: Silverface Champ
Posted: Sun May 15, 2011 9:22 pm
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BMW2002Ti wrote:
Maybe the vibrato section induces instabilities more than the non-vibrato? Another bugtime WAG.


This makes as much sense as anything I could hypothesize since I honestly don't know. One thing is certain though -- that cap pre-dates Leo's sale of the company so it cannot be explained away as merely "tampering by CBS dilletantes".

Arjay

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Post subject: Re: Silverface Champ
Posted: Mon May 16, 2011 2:05 pm
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God, I love it when you guys talk like that! Makes me want to run out and buy some old tubers! :mrgreen:

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Post subject: Re: Silverface Champ
Posted: Mon May 16, 2011 5:36 pm
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BMW2002Ti wrote:
Arjay, this is a single-ended amp. I think that the vibrato-tremolo section pulses are being feed into the 2nd half of the 12AX7, just before the 6V6GT.

The section's power comes off the 2nd power tap (+340VDC) of the cap can.

http://www.ampwares.com/schematics/cham ... _aa764.pdf

Just a WAG...
IT'S A FENDER CHAMP NOT A VIBRO CHAMP FOR THE SECOND TIME


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