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Post subject: Installing a Bias switch in a BD
Posted: Tue Sep 11, 2012 3:11 am
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Has anyone heard of installing a Bias switch pot in a early 90's Blues Deluxe? I am having it done tomorrow as the only trusted/Licenced fender amp tech in my area has said it will be easy to then Bias it to a set of matched tubes now and in the future. I obviously put faith in him as i am not electronic savvy at all really i just am starting to know what sounds good to me. I realise over the ponds you would not prolly go to this trouble for a BD as vintage amps are the same dollars but does anyone have experience with wiring a bias pot into the board? Will it change anything sound wise in the amp??

Thoughts on matched JJ millitry spec tubes also????

Bit of speaker porn too and a visual of the amp....A new Tone Tubby red go's in at the same time.....Suffice to say i'm pretty excited... :|

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Post subject: Re: Installing a Bias switch in a BD
Posted: Tue Sep 11, 2012 3:35 pm
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The sound after installing a bias pot can be exactly the same as before, or adjusted for earlier or later breakup. The bias pot itself does not affect the sound of the amp, as you are basically replacing a fixed value resistor with an adjustable resistor to be able to change the idle current. Nice to have the option to be able to adjust the bias correctly for any set of tubes that you might want to use. Adding a bias pot is a common modification that I've done many times on many different amps, including a BDlx. I would say that it is worth it for just about any fixed-bias amp.

I don't know what country is using JJ tubes for their military though. Slovakia?

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Post subject: Re: Installing a Bias switch in a BD
Posted: Tue Sep 11, 2012 6:27 pm
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shimmilou wrote:
The sound after installing a bias pot can be exactly the same as before, or adjusted for earlier or later breakup. The bias pot itself does not affect the sound of the amp, as you are basically replacing a fixed value resistor with an adjustable resistor to be able to change the idle current. Nice to have the option to be able to adjust the bias correctly for any set of tubes that you might want to use. Adding a bias pot is a common modification that I've done many times on many different amps, including a BDlx. I would say that it is worth it for just about any fixed-bias amp.

I don't know what country is using JJ tubes for their military though. Slovakia?



Thank you for your valued opinion Shimm!! Good to know..... I can rest easy knowing i have a very experienced and un "biased" Ha ha see what i did there........That was terrible.. :roll: ..Opinion....

I am going to drop it off in two hours so i will ask why he is calling the JJ's military spec and what country uses them and get back to you....

Thanks for your time again..... 8)

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Post subject: Re: Installing a Bias switch in a BD
Posted: Tue Sep 11, 2012 9:01 pm
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ozrv wrote:
shimmilou wrote:
The sound after installing a bias pot can be exactly the same as before, or adjusted for earlier or later breakup. The bias pot itself does not affect the sound of the amp, as you are basically replacing a fixed value resistor with an adjustable resistor to be able to change the idle current. Nice to have the option to be able to adjust the bias correctly for any set of tubes that you might want to use. Adding a bias pot is a common modification that I've done many times on many different amps, including a BDlx. I would say that it is worth it for just about any fixed-bias amp.

I don't know what country is using JJ tubes for their military though. Slovakia?



Thank you for your valued opinion Shimm!! Good to know..... I can rest easy knowing i have a very experienced and un "biased" Ha ha see what i did there........That was terrible.. :roll: ..Opinion....

I am going to drop it off in two hours so i will ask why he is calling the JJ's military spec and what country uses them and get back to you....

Thanks for your time again..... 8)


Is your tech also installing a 1 ohm resistor in the power tube's cathode circuit so you can measure the cathode current? Don't forget that you also need to measure plate voltage. You might want to consider buying a bias probe to do all this safely. I use this one along with a DVM:

http://www.amp-head.com/product_info.ph ... ucts_id=70

As far as the JJ tubes go, this is where they come from:

http://www.jj-electronic.com/

I use JJs in all my tube amps and like them a lot. Never heard them referred to a "Mil-Spec" though.

I'd be interested in knowing what your cathode current and plate voltage are after you have the amp biased. You might want to read my thread here on trouble shooting a bad tube and the mods I made to set the bias where I wanted it to be in my Blues Deluxe Reissue.

viewtopic.php?f=13&t=74557

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