How to install a bias pot in a Rectifier

[Gnarkill3k6]

@ mikey383

What kind of 50k pot did you use? I question this because of the price... here (Netherlands) a 50k pot costs like 0,36 cents.

 

[deeman]

Instead of starting a new topic I figured I'd bump this old one.

I have my 3 channel dual rec at a shop right now for a cleaning and check up. The tech offered to perform the bias mod for $40 bucks. Price is not a problem for me but i'm just not sure if it is worth it to me to get this mod. I play mainly rhythm guitar on raw mode in channel 2 and have become quite fond of the tone i get out of the amp. I am feeling like if it ain't broke don't fix it, plus i'd like to keep the amp original if at all possible so I'm not going to do this mod if it really won't make a difference for me.

Any thoughts from those of you who have done this? I saw there were some people who commented earlier in this thread and said it improved the sound but it wasn't very clear what the changes were (except for the one post by someone that seemed to be advertising for a particular business).

Thanks!

 

[Gnarkill3k6]

@deeman

In theory (and in practice I think) you can set the adjustable bias exactly the same as you amp is now.
The only thing that changes is that your bias goes from fixed to adjustable. People say Mesa amps are biased 'cold' and why people mod their amps is to run the tubes a bit hotter and to use different tubes then the Mesa-specced ones.

If you do the mod, measure the fixed bias first and adjust to that value after the mod. I think you get the exact same sound as you have now plus the option to run hotter plus the option of different tubes then Mesa's.

 

[RussB]

@deeman

In theory (and in practice I think) you can set the adjustable bias exactly the same as you amp is now.
The only thing that changes is that your bias goes from fixed to adjustable. People say Mesa amps are biased 'cold' and why people mod their amps is to run the tubes a bit hotter and to use different tubes then the Mesa-specced ones.

If you do the mod, measure the fixed bias first and adjust to that value after the mod. I think you get the exact same sound as you have now plus the option to run hotter plus the option of different tubes then Mesa's.

The amp is fixed bias whether there's an adjustment pot or not.


Having the ability to adjust the (fixed) bias circuit allows you to use a wide variety of tubes that would otherwise run too hot or too cold.

 

[Gnarkill3k6]

When you're able to change something that was fixed... that means it's variable/adjustable and not fixed anymore, right?

 

[nakedzen]

When you're able to change something that was fixed... that means it's variable/adjustable and not fixed anymore, right?

Fixed bias = it stays where it's set. The other variety are the cathode bias amps that adjust the bias automatically.

http://www.thetubestore.com/Resources/Matching-and-other-tube-info/Bias-Types

 

[mat.corona]

Hi,
Has anyone found the Mouser part number of the Bourns pot (pannel mount)?
I'm writing from Italy, I wish to order it in europe if it's possible.
Anyone can help me?
Any suggestion of other brands good quality pots?
Best regards.
Matteo

 

[mat.corona]

For exemple. Is this one a good choice?
http://www.tubesandmore.com/products/R-VPEC-50KL
Thanx
Matteo

 

[afu]

For exemple. Is this one a good choice?
http://www.tubesandmore.com/products/R-VPEC-50KL
Thanx
Matteo

Yes. The power dissipated is about 144 milliwatts. So a 2W pot will be fine and that one looks like a quality part.

 

[RussB]

I notice that photophuckit hijacked my pictures, so I'll repost...


I installed a bias pot in my Single Recto today. I used a 50k pot with a 1/2 watt 47k resistor in parallel with the two outside legs. Leg 1 and 2 of the pot are tied together. I used the legs of the resistor to attach the pot to.

I removed the factory 22k resistor (to the right of the 220uf/63v bias cap, just above the 30uf/500v blue cap for the screen supply) and installed the pot assembly in the next "spot" over because it's easier to solder there... :lol: Electrically, it's the same connection.

I have EL-34's in my amp. For really good reason, I "hardwired" the power tube socket board.

 

[BjornLaSanche]

Just curious about this mod. What has been the average B+ you guys have recorded off the dual rectifiers (both 2&3 channels), and what percentage plate dissipation are the amps coming from the factory stock? I know they are biased colder than say Marshall’s, but there has to be a general parameter yes? Like B+ avg is 485-495V and approx biased at 45-50% dissipation? I’ve always been curious about that I’m sure Mesa isn’t having Transformers that are reliably consistent at say <5% on the B+ from amp to amp. My gurus would be closer to 10-15% though I could be wrong.