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Post subject: fixed bias amps.
Posted: Mon Nov 12, 2012 8:01 pm
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i am planning to buy a tube head or combo amp soon. i know some amps are fixed bias, and others have to be re-biased on occasion. is there an advantage to having one or the other?

i do like the idea of just replacing tubes and moving on versus having to take it someplace to be worked on.

how can you tell if an amp is fixed bias? are there key words to look for in the description?

just getting back in the tube world, so any help will be appreciated


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Post subject: Re: fixed bias amps.
Posted: Mon Nov 12, 2012 8:40 pm
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Fixed-bias amps are the ones that need re-biased when changing output tubes, in most cases. Some fixed-bias amps have adjustment pots, some do not. Some fixed-bias amp manufacturers recommend using certain types of output tubes so that no re-biasing is necessary.

The Cathode-biased amps are the ones that do not need a bias adjust when changing output tubes. Typically, many smaller wattage amps (less than 15 watts), and particularly the single-ended amps are Cathode-biased, although there are some higher wattage amps that use Cathode-bias. Most single-ended amps have only one output tube. Always check the specs or schematics to know for sure.

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Post subject: Re: fixed bias amps.
Posted: Mon Nov 12, 2012 9:55 pm
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Here's a good explanation of the two main types of bias, fixed and cathode, in use today:

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

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 1:07 am
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The Fender Supersonic 100watt Twin and Head have an autobias module so you can put in any 6L6 or equivalent of any grade and it will automatical bias them at optimum bias (or alert you that the tube is really worn out). So you can just change tubes, no need for you to bias them. The benifit is also that this module can shut down any tube socket when the tube goes faulty and it will indicate a fault to you so you don't have to puzzle at what just happened.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 2:57 am
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Both amp need to be biased. Cathode bias not need as" often" as fixed bias, but it need a check.
If you look for an amp that you can put news output tubes any time without checking bias , no luck.

I fixed so many cathode bias amps with wrong bias when putting new tubes. Or wrong bias from the builder.

Read what Gerald Weber write about Bias ( Page 101 ) and CATHODE bias amp ( page 108 )

http://books.google.ca/books?id=fvUVBP3 ... mp&f=false


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 4:16 am
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stratele52 wrote:
Both amp need to be biased. Cathode bias not need as" often" as fixed bias, but it need a check.
If you look for an amp that you can put news output tubes any time without checking bias , no luck.


Supersonic Twin And Head Check themselves continuosly, therefore you can put in any grade tube and no need to check the bias yourself.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 4:55 am
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AaronK wrote:

Supersonic Twin And Head Check themselves continuosly, therefore you can put in any grade tube and no need to check the bias yourself.


How does it do that ? I look at the schematic and its is a standard bias circuit with bias adjustment pot . I did not see any "automatic" circuit.

Have you any information I can look at ?


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 6:10 am
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was doing resaerch, and found out that there are amps out there like the peavey valveking that are "plug and play". at my age-anything to make life easier!


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 7:16 am
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phreddybee wrote:
was doing resaerch, and found out that there are amps out there like the peavey valveking that are "plug and play". at my age-anything to make life easier!


Plug and play is like saying amp does'nt need bias. It is compromise, you don't have the better of the amp.

Why buying a expensive tube's amp if it is not perfectly tune ?

That's why I often read people selling their amp because they don't like sound . Seeing a GOOD tech and they will probaly change their mind and keep the amp.


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 7:58 am
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And as Stratele said, only Class "A" amps have a range of "self-leveling" by the nature of the output setup. It's why you don't need to re-bias the 12AX7 and 12AT7 in the input/gain stage/phase inverter stages. Each of these stages are usually setup to be single-ended, cathode-biased.

I recall that amps which actually have circuits to sense and alter bias settings are not the best for guitar amp tones. The amp is going to try to stay "clean." In hi-fi amps this maybe ok (though most purists still go for no auto-biasing --- too solid-state sounding).


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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 8:06 am
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AaronK wrote:
The Fender Supersonic 100watt Twin and Head have an autobias module so you can put in any 6L6 or equivalent of any grade and it will automatical bias them at optimum bias (or alert you that the tube is really worn out). So you can just change tubes, no need for you to bias them. The benifit is also that this module can shut down any tube socket when the tube goes faulty and it will indicate a fault to you so you don't have to puzzle at what just happened.


In my opinion, saying that this amp has "automatic" bias or "auto" bias is a misnomer. The amp is still an adjustible fixed bias amp. What is does have is "selectable" bias. You can select three different bias levels, "warm", "normal", or "cool" to run the power tubes at. However, I see nothing that tells me what those bias values actually are. "Warm" could be interpreted as 70% of maximum plate dissipation, "cool" could be 50% maximum plate dissipation, and "normal" could be 60% maximim plate dissipation. What those terms correspond to in terms of plate current and plate voltage (the values that must be determined to properly bias an amp) are not defined in either the schematic or owners manual that I can find. Because different tubes draw current differently, 70% of maximum plate dissipation for a modern Groove Tube 6L6GC (the tubes Fender uses) may not be the same as 70% for a vintage RCA 6L6GC. So the bottom line is, you still do not know what values the tube is actually biased at when you change tubes and set the bias switch to "normal". I would be willing to bet that even with those fancy terms, Fender still sets the actual bias levels colder in order to obtain longer tube life than what they should be for optimum performance.

Yes, the bias circuit is monitored and can be shut down, but all that means is that Fender has defined a range of values for each of its three bias settings. If the monitor circuit senses the bias has wandered outside of those ranges, it will shut the circuit down. What concerns me is that if you set the bias to "normal" and it wanders into either the "warm" or "cool" range, does the monitor circuit shut the amp down? The tube may still be within its acceptable bias range, but the circuit is outside its narrowly defined parameters.

I'd rather measure power tube bias on my own so that I know what it actually is.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 8:10 am
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Mesa Boogie amps also claims that you do not have to bias their amps. However, they are still fixed bias amps with the bias set cold. In order to "guarantee" their performance, Mesa requires you to buy tubes from them that they have graded to work in their amps. If you use a non-Mesa tube and the amp fails, the warranty is void.

Mesa's bias philosophy:

http://www.mesaboogie.com/US/Smith/biasadjust.html

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Last edited by bluesky636 on Tue Nov 13, 2012 8:21 am, edited 1 time in total.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 8:16 am
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Orange Amps sells an add on module that is claimed to monitor and automatically bias an amp to optimize tube performance:

http://www.orangeamps.com/products/acce ... ement/ov4/

Not a lot of detail on how it works. I did read a review after it was introduced that was pretty good. Here is the demo from the Orange website:

http://www.youtube.com/watch_popup?v=sg ... w4&vq=auto

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 11:15 am
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stratele52 wrote:
Both amp need to be biased. Cathode bias not need as" often" as fixed bias, but it need a check.
If you look for an amp that you can put news output tubes any time without checking bias , no luck.

I fixed so many cathode bias amps with wrong bias when putting new tubes. Or wrong bias from the builder.

Read what Gerald Weber write about Bias ( Page 101 ) and CATHODE bias amp ( page 108 )

http://books.google.ca/books?id=fvUVBP3 ... mp&f=false



Thanks for the pointer.

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Post subject: Re: fixed bias amps.
Posted: Tue Nov 13, 2012 12:47 pm
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bluesky636 wrote:
...I'd rather measure power tube bias on my own so that I know what it actually is.


+1

I can only surmise that these "auto bias" circuits in Fender amps still have the tubes running colder than most of us here would like, considering the factory settings of the previous models with adjustable fixed-bias.

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