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Post subject: '57 Pro Amp fixed or internal bias and what does means?
Posted: Fri Jun 28, 2019 2:53 pm
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I am trying to understand if my 57 Pro uses an internal or fixed bias. First, I am not sure what the difference is and second I was told that it was fixed bias and and that it meant that the 6L6 tubes could be chanced without having to adjust bias. I read that it was internal bias however and I am unsure which is true and what this means.

Thanks for any info in advance.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Sat Jun 29, 2019 2:42 am
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Did you talk about the 57 Custom Pro amp ? Or a original old vintage Pro ?
I guess it is the 57 Custom Pro.


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Google;

.... it replicates the important details of the Eisenhower-era model in grand style via an original-style handwired 5E5A circuit, custom-made vintage-style Pure Vintage yellow capacitors, and Schumacher transformers. The tube complement consists of two 12AX7s and a 12AY7 in the preamp, a 5AR4 rectifier, and a pair of 6L6s. Cathode biasing (along with lower B+ voltage) means less power than two 6L6 tubes are capable of, but the advantages are that you can change power tubes without having to re-bias ......

https://www.guitarplayer.com/gear/revie ... om-pro-amp.
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Original 5E5A circuit is a a fixed bias and need adjustment with new 6L6's
A amp with fixed bias need always a bis check.

The 57 Custom Pro use a cathode bias * wich do not need (theoretically ) a bias adjustment .
And most of the time, tubes are safe and sound is good.

I always check bias to have the better tone, it is so easy for me.


* If guitar player is right. I can't find the schematic of the 57 C Pro


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Sat Jun 29, 2019 2:47 am
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You want to learn more ?

Google ; fixed bias tubes amps

http://carlscustomamps.com/cathode-vs-fixed-bias


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Sun Jun 30, 2019 10:38 am
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Thank you for the input. What is the benefit of having it cathode biased other than not having to worry so much about the power tubes? is it the desire to have a lot of sag in the sound? I like the sound of it all the same - just curious.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Sun Jun 30, 2019 1:10 pm
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backbreakerjr wrote:
Thank you for the input. What is the benefit of having it cathode biased other than not having to worry so much about the power tubes? is it the desire to have a lot of sag in the sound? I like the sound of it all the same - just curious.


Cathode bias amps are cheaper to built; I need less parts and less expensive power transformer.
Most of the low budget amps are cathode biased.
This do not mean these amps did not have many quality.
Beatles's Vox amps are cathode bias.


Gerald Weber book ; All About Vacuum Tube Guitar Amplifiers page 274

" The cathode bias amp have less punch, less volume but more sustain and more signing quality in general.
...since is more compressed it work well at low volume

When playing with a pounding drummer or a loud band, the fixed bias is better
Fixed bias has the attack and punch needed to cut through a mix
Where it lack of sustain, it more than makes up in edge and dynamics.
They are much louder than their cathode biased counterparts.

No one of these amps are better than the other
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More if you read the book;

https://www.amazon.com/About-Vacuum-Tub ... 0964106035


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Tue Jul 02, 2019 3:51 am
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More ;Cathode bias amp ( most of the time) work in class A
Class A ; Output tubes operate at full power all the time = shorter tube live and tubes are very hot.

Fixed bias amp work ( most of the time ) in class AB. Tubes run "cooler" as they do not operate at full power all the time.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Tue Jul 02, 2019 8:22 am
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Stratle, you gotta get that Class A, cathode bias, BS outta you're head.
You can bias a set of power tubes to burning hell and they still won't be conducting for more than 180 degrees of phase.
Even if the grid is hot enough to conduct at zero signal, it's cathode is essentially turned off because of the high cathode voltage of the other tube that is passing signal. It generates this AC voltage at the cathode resistor and pinches off current flow of the low side as long as it is higher.
If it was conducting signal, it would lower the volume significantly because its out of phase.

In my experience, cathode bias amps are gentler on power tubes because of their higher series resistance and current limiting created by the cathode resistor.
Their compressive effect at high power is a feature not available with fixed bias amps.

It is a myth that you can put any tube in a cathode bias amp and it will bias properly
The gain of the tube must be selected for the rated DC voltage spec at its cathode. Otherwise, the current flowing through the tube will be as variable as if you took a bias control and rotated it randomly.
The sound will suffer the same as an incorrectly biased amp.
Its harder to bias correctly and more expensive because you have to buy a new tube or tubes to achieve the right bias.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Tue Jul 02, 2019 8:38 am
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" It is a myth that you can put any tube in a cathode bias amp and it will bias properly"
- TimAudio

That's why I wrote; "I alway check bias on cathode tubes amps.

You can give your opinion without having to insult me (correct me if I read wrong ) and members of the forum will understand.

People as knowledgeable as you do not quite have the same way of seeing things.
Otherwise there would not be so many questions on the forums


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Tue Jul 02, 2019 6:47 pm
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I'm sorry Stratle52.
I know you are a careful tech.
Because there are so many questions, we need to be giving accurate answers


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 12:47 am
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TimsAudio wrote:
I'm sorry Stratle52.
I know you are a careful tech.
Because there are so many questions, we need to be giving accurate answers



Thank you Tim
I still have to learn from competent people like you.
The incident is closed.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 5:40 am
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TimsAudio wrote:

It is a myth that you can put any tube in a cathode bias amp and it will bias properly
The gain of the tube must be selected for the rated DC voltage spec at its cathode. Otherwise, the current flowing through the tube will be as variable as if you took a bias control and rotated it randomly.
The sound will suffer the same as an incorrectly biased amp.
Its harder to bias correctly and more expensive because you have to buy a new tube or tubes to achieve the right bias.


Or you could just change the cathode resistor.

Not much harder than fixed bias amps that have a resistor instead of an adjustment pot.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 8:23 am
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You know, I've tried that without much luck.
But then again, I was usually trying to lower the bias of a worn out tube.
It would be worth trying if you have a new tube. Just don't exceed the voltage rating of the cathode cap.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 3:47 pm
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Okay, so why would fender make this a cathode bias amp if the original was not that? I believe the 57 bandmaster was fixed bias (at roughly the same era and roughly the same price). The transformer in my 57 Pro are a "boasting point" so I am assuming it was not to do with a cheaper transformer. I am now curious. And other than the post that was originally referenced (I went to the link that was given), I don't see where it is specified that the amps is cathode biased (not in the manual nor on the website). So... questions answered and new ones fill the space.... sorry.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 6:19 pm
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The referenced 5E5 circuit is a cathode bias. Most amps of that era were cathode biased. It was felt that they were more musical because of the compression that occurs at maximum power. I
It was only the demands of more power that swayed designers in the other direction. The cathode resistor limits max power with higher series resistance.

You can confirm your amp bias by pulling a power tube.
Measure from pin 8 to ground.
Zero ohms is fixed bias
250-500 ohms Cathode bias.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 7:43 pm
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They call it the "Custom Pro", not the "Pro Reissue".

As the article mentioned, the original Pro is one of the less popular tweeds. Since there aren't a lot of people looking for originals, they customized it for modern times.

There's a lot of demand for low~medium power amps. The lower power Custom Pro is better suited for most people. People who need more power have the Bassman and Twin, people who need even lower power have the Champ and Deluxe. The Pro and Bandmaster fill in the middle range.

The specs sound really appealing to me. I think Fender made a good choice in "re-imagining" it instead of reissuing an exact reproduction.


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