Adjustable bias mod in a Roadster

With about 8 months left of warrenty I decieded to go ahead and Mod my Roadster for Adjustable fixed bias.

The Manfield Guitars Bias Kit went in great. I'm very happy with the results.

Here's some pics during the install:

Here is the open spot where I removed a resistor from the bias circut. This is where the bias PCB will go. Be sure to discharge the caps and check there is no voltage in them!


I mounted the bias PCB on the outside of the chassis so I don't have to remove the chassis to adjust or check the bias. I used a grommet that was already in the chassis that came thru right where I needed it.




You got to have some patience while soldering, it's a tight space. Be sure to secure the wiring.



Here it is complete topside.


And back in the headshell. Take notice where I put the original resistor I removed.


Here I am setting up a quad of SED =C= EL34's at 62% MPD. Piece of cake!


I hope this helps someone considering this mod. It was super easy, but you need good soldering skills.

Can't wait to start rollin' some powertubes!

Dom

Thanks for the post, Dom! I'll probably do this mod sometime in the future with that kit as well.

Yep, done the mod on couple roadsters. Mansfield kit is easiest to route and install. Its funny, the roadster's actually ran pretty well at stock 22k resistance for best headroom output ratio. I think we only bumped it up a couple mA's with some winged c 6l6s.

Nice job! 8)



I wonder is routing the bias voltage outside the chassis will add some unwanted noise or interference? I'm not saying it will...I'm just wondering out loud

RussB said:

I wonder is routing the bias voltage outside the chassis will add some unwanted noise or interference? I'm not saying it will...I'm just wondering out loud

Click to expand...

Negative. No noise added

Awesome post and pics domct203.

My question is, that when you are adjusting the bias what tool are you using to see values?

A DMM or something of that nature?

And sorry to ask but what can I gain as far as tone if I did this mod.

Would I get cleaner cleans or better smoother gain or both or what.

I do not know what is gained or lost by having or not having an adjustable or fixed bias.

Thanks domct203 for your patience on this as I am still learning.

TremoJem said:

Awesome post and pics domct203.

My question is, that when you are adjusting the bias what tool are you using to see values?

A DMM or something of that nature?

And sorry to ask but what can I gain as far as tone if I did this mod.

Would I get cleaner cleans or better smoother gain or both or what.

I do not know what is gained or lost by having or not having an adjustable or fixed bias.

Thanks domct203 for your patience on this as I am still learning.

Click to expand...

I use a Bias Probe that hooks up to my DMM to measure idle cathode current and plate voltage.
I use something similar to this: http://www.amp-head.com/product_info.php?cPath=21&products_id=70

The reason I did the bias mod was to be able to run any matched set of tubes (NOS etc.) not just "Mesa spec" tubes.

For my tones I really like to keep the power section clean, and I usually run =C= 6L6GC at about 50% MPD. I have also tried JJ E34L's, and for me they sounded best around 60%.

Dom

Cool, and thanks for the information.