Adjustable Bias mod on roadster..

[cjae]

Hi,

I own a roadster and was wondering what change I can expect from an adjustable bias mod. Do you think its worth it? I am quite satisfied with most of the tones I get from the roadster except lead tones. I have JJ pre/power tubes and will a biad mod improve my lead tone(less fizzy)? Also, any technicians to recommend in philly?

Thanks,
Jae

 

[Juancito12]

The bias mod will help ensure whatever JJ output tubes you use can be adjusted to idle correctly. Think of it like an idle on your car. The higher the idle the easier it is for your care to get going... if its too low it will hiccup at low speeds... If its way too high you can burn out the engine faster or worse... same thing in an amp. You'll read on these pages that Mesa's are biased cold. If you're amp is idled low you'll get constipated sounds at low volumes. At higher volumes it sounds way better but you may never reach the tubes sweet spot depending on the amp and definition of "sweet spot" (which will be different to each players ear anyway).

As far as fizziness the bias mod won't fix it. Rectos are built to sound like that. I've read in some Gerald Weber books (some amp tech in Tx) that Mesa did some things to the Mark I - IV to clean up some of that fizz and it can be done on a Rectifier. Talk to a Tech about it (don't know any in Philly). Voodoo amps in NY also has a standard mod that will take the fizz out of it. I had it done and it sounds great.

Remember the Preamp tubes are self biased so the mod is only for Output tubes.

I've bias modded my amp and I think its a must if you want to try/use output tubes other than Mesa's or Groove Tubes. In its most basic form, the bias mod is nothing more than an adjustable resistor/pot that allows you to increase or lower resistance. Making it so you can access it safely from the outside of the chassis requires drilling that I don't like. If you really like your JJ's you may just want to take it to a tech and have them check the bias and adjust the bias resisitor to an appropriate value. It may be quite a bit cheaper than the bias mod and once output tubes are biased correctly the tubes should last a while.

 

[cjae]

thanks for the imformative post.

Anybody know any good techincians in philly?

 

[diamondschwin]

heres the deal... order tubes from dougstubes.com or eurotubes.com and tell them what amp they have they can select tubes that will run warm in your amp..


mesas might be biased cold, but some tubes can get warm with the cold bias while others need way more..


dont change the amp, just get tubes that will run as hard as you want but at the bias of the amp.

 

[domct203]

heres the deal... order tubes from dougstubes.com or eurotubes.com and tell them what amp they have they can select tubes that will run warm in your amp..


mesas might be biased cold, but some tubes can get warm with the cold bias while others need way more..


dont change the amp, just get tubes that will run as hard as you want but at the bias of the amp.

+1 for Doug. For my first set of SED =C='s I asked for a quad that would bias up the same as the stock STR-440 GRN's (25-28mA.) and the tubes he sent came in at 26-27mA.

I'm now waiting on a Quad that should come in around 35mA. (just over 50% MPD).

Dom

 

[zeppelin]

The best way is to be able to adjust the bias and set it accordingly for the tubes and your taste. Does anyone know where to find a schematic for the Roadster or some pics showing one modded? Sounds like a easy mod. Thanks

 

[212Mavguy]

Send a PM to Stokes, you'll find him in the tubes section. He's a tech and has installed the bias mod quite a few times.