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Post subject: Vibro Champ ; Class A or AB ?
Posted: Wed Jun 10, 2015 4:23 am
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Location: Province de Québec, Canada
I just receive a blackface Vibro Champ AA764 with NOS tubes

The RCA 6V6 GT original plate resistor ( 470 ohms ) was replace by a 1.5K
Bias was ok for a class AB amp at 122 volts modern input wall voltage at 70% plate dissipation.

With 117 volts period input wall voltage and original 470 ohms resistor, bias was very very hot , more than 100 % plate dissipation .

What Leo Fender want for this amp ? A class A amp at 90 % plate dissipation or AB class at 70 % ?

Any idea.

Thank's


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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Wed Jun 10, 2015 5:17 am
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I found my answer ; Class A
From the book ; Sound Advice from Gerald Weber , page 148


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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Thu Jun 11, 2015 5:59 am
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Stratele, I like the tone of many Class "A" amps. Whether single-ended or push-pull.

Anyhow, you need to keep most single-ended circuits (including the gain stage 12AX7s) in Class "A." If you drop the tube into Class AB or Class B, the tone will suffer.

A well-known amp tech (who must have only worked on mid-to-big Marshall and Fender amps) biased a friend's Matchless Chieftain into Class AB1. The tone was terrible. Like when you bias a typical Fender push-pull into Class B. After going back to OEM cathode resistor and cap bypass values --- the amp returned to its sweet tone.


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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Thu Jun 11, 2015 9:00 am
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You can't have AB with only one output tube, only A or B.

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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Thu Jun 11, 2015 2:56 pm
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Thank's gentlemens for your answer .

I bias it in class A and amp have a really sweet tone .


Class B like AB need push pull

http://www.pentalabs.com/audio_help6.html


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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Thu Jun 11, 2015 7:46 pm
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Yeah, I meant that the idle bias dissipation should be 85% or more of max, for the tube. Single-ended setups sound incredibility bad at 65-70%.


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Post subject: Re: Vibro Champ ; Class A or AB ?
Posted: Fri Jun 12, 2015 1:38 am
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BMW2002Ti wrote:
Yeah, I meant that the idle bias dissipation should be 85% or more of max, for the tube. Single-ended setups sound incredibility bad at 65-70%.



When I receive the amp with a colder bias close to 65% ,it sound "very" good to me and for my customer ( same customer I fix Deluxe Reverb with zeners , see my other post ) .

He bring me the amp just because I do so good job this DR and week after on his Pro Reverb amp and want a check up .

So I don't know what to think about different class ... :?


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