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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 3:20 pm
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I originally answered this post as a Cathode biased, no adjustment (VC, duh!), but then remembered that same thing in the description, and deleted my post, because I couldn't verify exactly what kind of circuit that it has. We don't see many single-ended amps with a fixed bias, but not out of the question.

If it really does use a "bias wiggle", then it would have to be a fixed bias wouldn't it? If so, bias adjust might be needed with new output tube. It wouldn't necessarily have to have a pot. Of course when we think VC, we instantly think Cathode bias, but...

I would have to see the inside of the amp to see exactly what the circuit is. It's not like Fender has ever been vague with us before. :lol:

Perhaps the OP could post a clear picture of the inside. :D

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 3:36 pm
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I have never seen a single-ended amp with fixed bias. This would defeat some of the simplicity and tonal qualities of the SE, cathode-biased amp. It would NOT sound vintage VC.

In fact, nearly ALL Class "A" amps, push-pull or single-ended are cathode-biased. From a lowly AC15 to a $$$ Matchless Chieftain.

Why are all guitar SE amps cathode-biased, Class "A?" Because of the way a SE amp works. Its output tube must be "on" 90% or more of the time. With the positive and negative sine waves being generated by it and the tube's interaction with the OPT. If you bias the output tube to AB1 (like a push-pull), it is going to sound really cold and sterile. With very heavy crossover distortion. This is why a good quality output tube makes a world of difference in these amps. A lot is asked of them.

Why are most BF and SF push-pull Fender amps fixed-bias, Class AB1? Because there are two output tube pairs (at least) to do the work of generating the sine wave. You can then have one doing the upper half and one the lower half (positive and negative) of the sine wave. This allows greater output power, as one tube is virtually "off" most of the time (and thus cooling). "Off" to the point of the idle bias point, to avoid crossover distortion.

This is why many vintage tweed push-pull paired amps (that are Class "A" cathode-biased) make less power than BF push-pull amps (Class AB1, fixed bias) with the same tubes. MAN! Someone at Fender needs to do their homework on their own history and topology of their amps.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 3:45 pm
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It is possible for fixed bias to run in class A. Not typical but possible. In order to be a class A amp, the output tube has to be on (conducting) 100% of the time, always conducting even on low signal swings. That's all that class A means, output tube always conducting, never shuts off. :idea: I don't remember any references from Fender that this amp is class A, or not. :?:

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 3:57 pm
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Shimmy, yes I agree, in theory--- one can run a fixed-bias, push-pull amp in Class "A" mode. But, can you name one? Not guitar amps. Why would you? When an entire bias supply circuit is virtually doing nothing (add very little negative voltage to the control grid)---as the output tubes are going to be "on" most of the time. This is much easier, cheaper, and reliabily done with cathode resistor(s) and bypass cap(s).

For an experiment... try running your DR in Class "A." Without the aid of a cathode-tied resistor or a lot of local and global FB--- your output tubes are prolly going to go ballistic.

As for SE amps... you cannot generate a close-to-symmetric sine wave, if that output tube is biased in Class AB1. SE amps work much different than push-pulls.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 4:02 pm
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you guys have lost me!

let me know if you'd like to see a photo of the back of the amp.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 4:11 pm
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You're right BMW2002Ti, very odd. And yes, easier to Cathode bias, but then you couldn't have the "bias wiggle" as claimed. I do remember seeing a single ended amp with the Cathode tied directly to ground, will peruse my schematics, probably not a Fender amp.

nordiquemusic wrote:
...let me know if you'd like to see a photo of the back of the amp.


We need to see the components inside. Is that possible for you?

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 4:21 pm
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shimmilou wrote:
We need to see the components inside. Is that possible for you?


here it is.

Image


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 5:51 pm
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Well, as near as I can tell, looking at the output tube socket, if the blue wire on the left is pin 3 (Plate), then it appears that a yellow wire is coming from pin 8 (Cathode) on the other side of the socket, and going to a cap and resistor in parallel, which would seem to indicate a Cathode bias (of course we can't see under the board to see where those components are connected, could be to ground). I sent an email to Fender, requesting a schematic, maybe we'll see. I am not sure how this amp can have a "output tube bias wiggle" tremolo, maybe that is just an error.

Interesting to note what appear to be diodes on the rectifier socket, the same mod that BMW2002Ti posted about recently. :idea:

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 5:52 pm
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nordiquemusic, see that green resistor behind the green, orange, and gray wire (connected to the 6V6GT by a yellow wire)? It has a black with green lettering Sprague Atom cap bypassing it. I bet the bands are yellow, purple, brown, gold = 470-ohms, 5% tolerance. This is the cathode-tied, cathode-biased resistor. That yellow wire should be tied to pin #8 on the 6V6GT socket.

If so, my bet that this is a simple cathode-biased amp. And no bias adjustments are needed AS LONG AS the resistor and bypass cap are within tolerance. The far end of the resistor and cap should go to ground.

BTW---the yellow-purple-brown resistor soldered from tang to tang, on the socket is the screen resistor.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jun 23, 2012 8:15 pm
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That's what it looks like.

Apparently they are using something similar in the Excelsior, Cathode biased, but using the tremolo varies a grid bias on the output tubes. :?: The Exc uses a transistor for the tremolo, while the EC VC uses a tube. Really curious about this circuit, still.

nordiquemusic,

Thank you for posting the excellent picture!

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Mon Aug 27, 2012 7:06 pm
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The Exc uses a transistor for the tremolo, while the EC VC uses a tube. Really curious about this circuit, still.

Nope. The Exc uses the output tube for the trem.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Wed Aug 29, 2012 9:03 am
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Not sure what you're basing that on, but it is just as I said. "The Exc uses a transistor for the tremolo, while the EC VC uses a tube."

Neither amp uses a "bias wiggle" for the trem, nothing to do with the output tubes at all in either amp. Even Billm was a little off on his description of the trem circuit in the Excelsior, it is not a combination of fixed-bias and Cathode-bias, it is simply Cathode-bias with a simple oscillating trem circuit tied to the PI.

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Thu Oct 25, 2012 5:36 pm
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nordiquemusic wrote:
shimmilou wrote:
We need to see the components inside. Is that possible for you?


here it is.

Image

Hello ! The picture is gone :( ( http://www.nordiquemusic.com/dl/IMG_0433.jpg )
Can someone post it again or PM it to me please ? I was also wondering about this output tube tremolo on this SE amp. I doubt there is such this kind tremolo here. And I was also interesting in the attenuator used here too !
Thanks.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Thu Oct 25, 2012 6:49 pm
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After examining the schematic for the EC VC, there is no "output tube bias tremolo", it is the same tremolo as in other VC amps, signal modulated, definitely not output tube modulated. Also after examining the Excelsior schematic, it seems that Billm has it wrong on his site about the Excelsior tremolo, it is also signal modulated tremolo and definitely not output tube modulated, although the Excelsior does use a transistor for the tremolo, whereas the VC uses a tube for the tremolo. The EC VC is Cathode-biased, no bias adjust needed when changing the output tube.

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Fri Dec 28, 2012 4:05 am
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Thanks shimmilou for the info.

Where did you get the EC VC schematic ? Can't find it on the Fender website.


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