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Post subject: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 1:49 am
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Hi all

I have been looking at my Super Sonic Twin Circuit Diagrams to work out scenarios for replacing the fancy auto-bias PCB one day if it fails to a manual POT bias setup. The Twin uses 4 6L6 tubes so I would naturally want to Bias them each individualy with 4 seperate POTS. However looking at the 60watt Combo SS Fender uses 1 POT to bias 2 6L6 tubes. Is this just a cost saving than rather putting in 2 POTS to bias both Tubes seperately. Surely using 1 Reference voltage will not get both tubes to the exact same bias? and is it common practice to bias the 6L6 Tubes by applying an OFFSET DC bias on the tubes incoming audio signal?

My next question is that when I look at the SS Twin Circuit Diagram it shows the 2nd Push/Pull 6L6 tube pair is somehow tottempoled off the First PUSH/PULL pair. Is this correct or is the Diagram incorrect and both PUSH/PULL pairs should run in parrallel to the output transformer?

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 6:07 am
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Yes, typically one pot to adjust the bias for two, four, or more tubes. Use matched pairs or matched quads for equal bias on all tubes. I don't think that it's worth the effort to have more than one bias pot, and having more than one pot increases the chance of failure, one pot and one reference voltage is sufficient. If tubes are so unmatched as to need separate bias pots, the sound wouldn't be the best, although a few might enjoy the sound of mismatched tubes. Look at just about any modern Fender "fixed bias" amp schematic to see the bias circuit, they are all basically the same circuit with minor variances. Yes, a negative DC voltage is applied to the tube control grid for bias in a fixed bias amp, whether 6L6, 6V6, EL84, EL34, or whatever tubes are used.

With two output tubes, one on each half (side) of the output transformer (OT) primary, each side handles half of the signal feeding the OT, one "pushes", while the other "pulls" and vice-versa. For more power out, simply parallel another tube on each side, so that two tubes now handle each half of the signal. With added tubes, the other amp components will have to be heavier duty also (transformers, power supply, etc). For even more power, parallel a third or even fourth tube on each side.

BTW, the output tubes are in series with the OT primary. When using more than two output tubes, the additional tubes are paralleled to each other on each side, but all tubes on each side are still in series with the OT.

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 8:33 am
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Thanks Shimmilou, your insight on the biasing of these amps is most welcome. As I am sure you are aware the Supersonic Twin has an auto bias module which individualy bias each of the four tubes seperately and automaticaly and this way Un-matched quads can be used. It will be an interesting debate as to whether I would maintain that individual bias of each tube should this Autobias module fail. But it is similarly reassuring to know I can also go with the tried and proven common bias method.

With regards to the series/parrallel tube configuration, I would expect the pairs would be in parrellel to each other, but unless there is a mistake on the Circuit Diagram, they are not. I am going to go with the notion that there is a mistake on the Circuit Diagram.

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 9:23 am
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Wow, good eye! I had not noticed that before, but yes, the drawing is off just a little. Notice the small dot on the line coming from the Plate (pin 3) of V11, it is supposed to line up with the crossing line that feeds the Plate of V10, and would indicate that those two lines are connected. In fact, the dot is labeled; "3 (V10/11)" on one side, and "3 (V12/13)" on the other side. Same mistake on the other side with V12 and V13, the dot isn't lined up. If the drawing were correct, V11 and V12 would have no Plate voltage.

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 11:22 am
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shimmilou wrote:
Wow, good eye! I had not noticed that before, but yes, the drawing is off just a little.


+1

Something definitely wonky there, shimmilou.

:?

Arjay

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Wed Jun 13, 2012 10:59 pm
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Thanks again, Shimmilou. :D That's what I was hoping and expecting the case would be.

*Note To Fender Drawing Office - pay more attention to detail please.

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Post subject: Re: Fender Power Amp Design and Biasing methods - Questions
Posted: Thu Jun 14, 2012 7:40 am
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Look around the site long enough, and you will see all kinds of anomalies, including guitar pictures with the wrong necks, headstock decals that are wrong for a particular model, guitar wiring diagrams with the pup selector switch positions mislabeled, and other misinformation. But finding incorrect diagrams/schematics is more common than you think, although for the most part, the amp schematics from this site seem to be OK except for this one. There must have been dozens of different people managing and making these drawings, so mistakes are understandable.

But, if you are paying attention enough that you actually spot the mistakes, you are in good shape. 8)

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