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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 8:53 pm
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Aspiring Musician
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Guys, Guys - please read the specs on the Fender site of the Supersonic head and twin. IT IS SELF BIASING - no biasing by the owner required. I OWN The TWIN so I am telling you from first hand experiance. MAYBE BRAD NEEDS TO CHIME IN TO SET YOU ALL STRAIGHT ON THE SUPERSONIC TWIN & HEAD 100 watters. You can put ANY grade tube in and it will adjust the BIAS for you to run the TUBE at the bias required.

BlueSky636 it monitors the milliamps/millivolts like your probes do and maintains that value via a microcontroller which pulswidthmodulates a transistor to generate the bias voltage that you would normally do via a potentiometer. UNDERSTOOD. Fender would have pre-selected their optimal bias values for the 3 bias selections, but the amp will maintain those values automaticaly what ever they are. If the NORMAL setting is somewhat "COLD" based on what you would like then there is a "HOTTER" setting. Ofcourse wether it is hot enough would have to be measured. But assuming the desired bias can be achieved via the selector switch, then the "Autobias Unit" would constantly maintain that bias regardles of tube grade. If the bias is too cold and cannot be made hotter, then a "Potential Devider Mod" would be required in the "BIAS FEEDBACK LOOP" to trick the processor into running "HOTTER", but once done the BIAS will always be maintained.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 9:05 pm
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Aaron, Wow! What a circuit! Darn auto-bias runs off a micro-processor. As sophiscated as several high-end hi-fi amps, I've seen.

Thanks!


http://support.fender.com/schematics/gu ... _Rev-B.pdf


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 9:16 pm
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Please show me where it says all that:

http://www.fender.com/products/super-sonic-100-head

http://www.fender.com/products/super-sonic-twin-combo

http://support.fender.com/manuals/guita ... manual.pdf

Looking at the schematic, the "normal" bias appears to be 33 ma cathode current at a plate voltage of 427 VDC. That translate to an idle plate dissipation of 13 watts (using the Weber bias calculator) which is only 43% of maximum plate dissipation. Unacceptable in my book.

By the way, Brad will be the first to tell you that he is not technically knowledgeable in areas like this.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 11:37 pm
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BlueSky - the manual does not delve into the ins and outs of the "AUTOBIAS" and the schematics are very hard to follow if you just brush through them, (I painstakingly mapped out the schematic to work it all out so that one day I could replace it if neccessary), so to make it simple watch the YOUTUBE clip with Damon from Fender UK around the 5:00 minute mark.

http://www.youtube.com/watch?v=07hB-WRlDu4

Whether the Fender Bias Point is too "COLD" or too "HOT" for you is irrelevant as firstly that is a personal preference and secondly this amp actualy has 7 bias selections. COLD - * - * - WARM - * - * - HOT. The normal setting being WARM which is probably where the standard values on the schematic are based apon. But as I said earlier, a little mod can be done to have this amp BIAS range running alot hotter and then you would not need to adjust it ever again. Just think of the AUTOBIAS as a little "you" continuosly checking the bias to keep it spot on even while your playing.

It would be interesting to see where Fender has set those Bias points though.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 12:30 am
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My apologies to the origional poster, this "AUTOBIAS" discussion has hijacked his origional questions.

If you are concerned about having to bias your valve amp then you have 3 other options.

1. Get an amp that requires no actual biasing on its output tubes as it is a fixed design.

or
2. Replace your tubes with the identical grades.

or
3. Get a Fender SUPERSONIC Amp.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 3:18 am
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AaronK wrote:
My apologies to the origional poster, this "AUTOBIAS" discussion has hijacked his origional questions.

If you are concerned about having to bias your valve amp then you have 3 other options.

1. Get an amp that requires no actual biasing on its output tubes as it is a fixed design.

or
2. Replace your tubes with the identical grades.

or
3. Get a Fender SUPERSONIC Amp.


My opinion to ;
1 - Buy a solid state amp . These are the only amps never need bias.

2- This is not a warranty new tubes ( same grade or other ) won't need bias. Probably yes if you buy the same brand AND same batch of tubes AND same years of the original tubes in the amp. 2012 tube could be not the same.

3 - Supersonic need bias .


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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 3:58 am
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StratTele - if you read further back I was implying SUPERSONIC 100 watt Head or Twin as focused on earlier. Those 2 do not require you to bias them as they get biased electronically. However the SUPERSONIC 22 & 60 DO need you to bias.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 4:41 am
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AaronK wrote:
StratTele - if you read further back I was implying SUPERSONIC 100 watt Head or Twin as focused on earlier. Those 2 do not require you to bias them as they get biased electronically.


I don't have time to check the Surpersonic 100 to look how it work. Yourself did you know if it really work GOOD this electonicaly bias ? Or it is only advertising say that ?


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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 7:49 am
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AaronK wrote:
1. Get an amp that requires no actual biasing on its output tubes as it is a fixed design.

or
2. Replace your tubes with the identical grades.

or
3. Get a Fender SUPERSONIC Amp.


1. Totally false statement. Fixed bias only means that the bias voltage is supplied by a separate power supply rather than depending on the voltage relationship between the cathode and grid of the tube (cathode bias). Fixed bias amps either have a pot for adjutment or the resistor values in the bias power supply are adjused to change the bias voltage supplied to the tubes. Read this:

http://www.aikenamps.com/WhatIsBiasing.htm

2. Replacing tubes with identical grades is no guarantee that the bias will be optimum for different tubes. It has already been shown that different tubes can draw different amounts of plate current even for so-called "identical grades" of tubes because of manufacturing tolerances. The only thing that is "guaranteed" is that the tubes will operate within a safe range of bias values.

3. The Supersonic Twin/100 is a fixed bias amp. Just because it has a fancy microprocessor to set the final value of bias voltage does not change that fact. The amp must still be biased properly. Unfortunately, the actual bias set point is totaly unknown to the technically minded user.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 8:16 am
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AaronK wrote:
BlueSky - the manual does not delve into the ins and outs of the "AUTOBIAS" and the schematics are very hard to follow if you just brush through them, (I painstakingly mapped out the schematic to work it all out so that one day I could replace it if neccessary), so to make it simple watch the YOUTUBE clip with Damon from Fender UK around the 5:00 minute mark.

http://www.youtube.com/watch?v=07hB-WRlDu4

Whether the Fender Bias Point is too "COLD" or too "HOT" for you is irrelevant as firstly that is a personal preference and secondly this amp actualy has 7 bias selections. COLD - * - * - WARM - * - * - HOT. The normal setting being WARM which is probably where the standard values on the schematic are based apon. But as I said earlier, a little mod can be done to have this amp BIAS range running alot hotter and then you would not need to adjust it ever again. Just think of the AUTOBIAS as a little "you" continuosly checking the bias to keep it spot on even while your playing.

It would be interesting to see where Fender has set those Bias points though.


Statement at around 7:10: "If you want to crunch it, you can warm it up and it will run more voltage across the power tubes" and it will break up earlier. Totally bogus and shows that Damon has no idea what biasing an amp means.

Amp bias is not adjusted while you are playing as you imply. Bias is set at a static, idle state with no signal running through the amp. Once you start playing, plate current is all over the place.

Look, I am not saying that this is not a cool little tool to simplify amp maintenence. For the non-technical user it is great. But the fact of the matter is, there is no real insight into the real operation of this circuit. If Fender considers running the amp at less than 50% of maximum plate dissipation to be "normal", that shows that their concern is only for tube life, not optimum performance. If this auto bias circuit is so great, why should one have to mod it to get the bias values that they want? Modding a circuit like that is certainly going to be more complex than adjusting a pot or changing a couple of resistors. How many people are going to have the technical knowledge to do that?

This automatic control of something as simple as amp bias reminds me of the change from carburettors to computer controlled fuel injection in a car. If my 1964 Ford Galaxie was running rough, I would throw in a new set of plugs and maybe adjust the idle speed/mixture. In my neighbor's brand new BMW 550IL with twin turbos, if it starts running rough, the mechanic will probably have to swap out the computer system at the cost of $1000 or more. Same thing with this amp.

You want to see what Fender has actually set the bias values to? Stick a bias probe on the output tubes and use a DVM to measure the actual bias point. You might find it surprising.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 8:24 am
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Bluesky, isn't running the output tubes in push-pull, at less than 50% max idle dissipation getting very close to cutoff and associated crossover distortion? Really, close to Class B operation.


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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 8:51 am
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The bias is a very mysterious concept for many musician who would like to change their amp's tubes themselves without going to see / pay a technician.

This desire is so strong that many come to refuse to accept reality.
I see all over the forums seminar I participated.

I think I'm going to get an answer that I will has copied both arguments are always the same.

I must admit it is not easy to find a qualified and honest tech .
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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 9:02 am
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BMW2002Ti wrote:
Bluesky, isn't running the output tubes in push-pull, at less than 50% max idle dissipation getting very close to cutoff and associated crossover distortion? Really, close to Class B operation.


Yep.

Looking at the schematics of several modern Fender adjustible fixed bias amps we see:

S/S Twin: 33 mA cathode current at 427 VDC plate voltage = 13 watts at idle (6L6GCs)

Blues Deluxe Reissue: 30 mA cathode current at 431 VDC plate voltage = 12 watts at idle (6L6GCs)

Blues Deville Reissue: 30 mA cathode current at 485 VDC plate voltage = 14 watts at idle (6L6GCs)

59 Bassman LTD: 30 mA cathode current at 476 VDC plate voltage = 14 watts at idle (6L6GCs)

I used the Weber bias calculator to derive plate current (not shown) and used B+ = plate voltage, ignoring any voltage drop due to the OT.

Fender's "normal" bias is very cold. I run my BDRI at about 20 watts at idle and my 5F6A clone at about 21 watts at idle (67% and 70% max plate dissipation respectively).

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 10:45 pm
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Bill your answer for every other post is in this thread is probably on the mark, but I will tell you that the Bias setpoint that you choose is not written law and it is based on what you feel and hear is good for you. My Supersonic runs pretty sweetly to me with a Bias of 33ma if even that. And I know that my bias will never need checking by me for a long time because it is taken care of electronicaly. So while you are twiddling and turning and checking your bias I will be playing and playing. Oh and another thing my AUTOBIAS does continuosly check while I am playing. When the guitar signal is passing through it checks that my valves are OK and when my guitar goes idle it monitors and checks the Bias. I have tested this by pulling out one valve during playing.

I do agree that physicaly checking the bias every couple years to check that the auto system is functioning correctly is a good idea, but its not required when swapping tubes on the SS Twin amp.

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Post subject: Re: fixed bias amps.
Posted: Wed Nov 14, 2012 11:13 pm
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I check my bias every few months. Takes less than an hour. Not a big bite out of my life. Hopefully that autobias circuit never fails cuz you will be out of luck. I may be a dinosaur, but I prefer to maintain my amps myself and not trust them to the whims of a computer. :lol: :lol: :lol:


P.S.: I am glad you like your amp so much. They are pretty nice. :wink:

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