Bias of a Blackstar Amp !

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Bias of a Blackstar Amp !

Post by MusashiChan on Sat Jun 12, 2010 1:54 pm

Hi,

As i know, and nobody has find another info on this, ALL blackstar amps are cathode bias and NOT fixed bias. So...they all need to be rebiased by a technician (or you if you really know what you are doing...) each time you change the power amp section valves !!!

Understand ?

Here is a good comment on the official forum :

escargotomy wrote:OK, my amp guy, who has been doing this for over 40 years and is actually very well known in the amp community as the designer of several well-regarded tube amps, told me this. There is a tremendous amount of variation of current flow from brand to brand, model to model, and NOS and new generation tubes. Therefore while it is possible that you could be replacing a tube with one of approximately same values in which biasing is not necessary, it is much more likely that the one you are replacing does not have sufficiently equivalent charcteristics and so biasing is necessary. You could actually have a same brand, same class tube which came off the line minutes after the one you are replacing and it could potentially need biasing too. Certainly if you are replacing a new generation tube with a NOS tube, biasing is a given because manufacturing standards have changed significantly over the years. While the Blackstar schematics have not been made available, most amp technicians with even moderate exprience know what they are looking at and don't need them. I guess the trick is to find someone with a good reputation as an amp technician. Incidentally, these guys are usually specialists in a music shop and don't typically do guitar maintenance, just amp work. It is its own discipline, apart from luthiery/guitar repair. I hope that helps.

It means that even if you choose the same brand, the valves can have a different rate so you'll have to do it...

Note that it's not the same thing for preamp valves, they don't need a bias setting.

Wink

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Re: Bias of a Blackstar Amp !

Post by mrelwood on Tue Aug 16, 2011 7:15 pm

Must comment here also. Had to register just to correct some of the false information:

MusashiChan wrote:blackstar amps are cathode bias and NOT fixed bias. So...they all need to be rebiased

Fixed bias means that the amp most definitely has to be biased, while a cathode biased amp will usually run okay without a biasing. Cathode biasing means that the amp shouldn't explode even with an unoptimal bias, while an amp with a fixed bias with a way off bias will fry the tubes and possibly the transformer.

Most modern cathode biased amps will also sound a lot better when biased properly, although the adjustable bias pot might have to be installed.

A fixed bias is a system that was common in older Fender amps, while Vox AC30 is cathode biased.

Must say though, that a cathode biased AC15 doesn't change much with biasing, while a cathode biased Classic 30 will sound quite a bit better after biasing.

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