If you own the SCXD, you should RE-BIAS it (even stock tubes)! TONE!

[Gear_Junky]

I knew this for a while, but being a "non-tech" hesitated to even open it up, until yesterday. Turned out to be VERY easy, actually (BUT BE ADVISED: THERE ARE VOLTAGES INSIDE THAT CAN KILL YOU, but you don't need to touch those parts to re-bias).

Of course, all this I read somewhere else - not my own info/data. It just so happens that many of these amps (SCXD) come biased rather "cold", apparently a move by Fender to allow people to stick in any matched set of 6V6 tubes without re-biasing (they don't want people poking inside to avoid liability).

This means that re-biasing your stock tubes may get you a NICE improvement in sound - a totally free "mod". My amp started sounding richer (more complex) and notes seem to "bloom" more. More tubey. Granted, maybe your tubes will last a LITTLE less, but now that you know how easy it is to re-bias - installing new tubes will be a no-brainer and they'll always sound their best! That's the whole point of tube amps - TONE, not lack of maintenance

To find instructions with pictures simply google "bias SCXD", but it's really simple - inside there's a blue trim pot on the circuit board - it takes a regular flat screw-driver (insulated handle, please!). Set the multi-meter to DC volts (you're measuring 40 milli-volts or .004 volt). Stick the black (ground) probe to the chassis and use the red probe to touch the end of the resistor R20 (the end closest to the printed "R20"). Adjust trim-pot to 40 mV - done.

Again, please don't take this as complete instructions - find the full writeup on google, but doing this helps the amp sound it's best! Highly, HIGHLY recommended. Even if you have a tech do this, it should take a pro maybe 15-20 minutes (most of which is just removing the rear panel and the amp from cabinet) - so this shouldn't cost a lot.

 

[elenore19]

I knew this for a while, but being a "non-tech" hesitated to even open it up, until yesterday. Turned out to be VERY easy, actually (BUT BE ADVISED: THERE ARE VOLTAGES INSIDE THAT CAN KILL YOU, but you don't need to touch those parts to re-bias).

Of course, all this I read somewhere else - not my own info/data. It just so happens that many of these amps (SCXD) come biased rather "cold", apparently a move by Fender to allow people to stick in any matched set of 6V6 tubes without re-biasing (they don't want people poking inside to avoid liability).

This means that re-biasing your stock tubes may get you a NICE improvement in sound - a totally free "mod". My amp started sounding richer (more complex) and notes seem to "bloom" more. More tubey. Granted, maybe your tubes will last a LITTLE less, but now that you know how easy it is to re-bias - installing new tubes will be a no-brainer and they'll always sound their best! That's the whole point of tube amps - TONE, not lack of maintenance

To find instructions with pictures simply google "bias SCXD", but it's really simple - inside there's a blue trim pot on the circuit board - it takes a regular flat screw-driver (insulated handle, please!). Set the multi-meter to DC volts (you're measuring 40 milli-volts or .004 volt). Stick the black (ground) probe to the chassis and use the red probe to touch the end of the resistor R20 (the end closest to the printed "R20"). Adjust trim-pot to 40 mV - done.

Again, please don't take this as complete instructions - find the full writeup on google, but doing this helps the amp sound it's best! Highly, HIGHLY recommended. Even if you have a tech do this, it should take a pro maybe 15-20 minutes (most of which is just removing the rear panel and the amp from cabinet) - so this shouldn't cost a lot.

That sure sounds promising.
Definitely looking forward to trying to get that figured out. Just have to get some tools....

 

[WhiteStrat]

I understand the value of biasing. I also understand that sound & tone are subjective things. But c'mon: more complex? notes that bloom? tubey?

They are english, but I still don't understand them.

 

[Gear_Junky]

I understand the value of biasing. I also understand that sound & tone are subjective things. But c'mon: more complex? notes that bloom? tubey?

They are english, but I still don't understand them.

I'm not sure if you're being sarcastic or wondering about those words for real, so

actually, the first 2 words have a more technical explanation:

"more complex" probably refers to harmonic content - more harmonics means a richer tone.

"bloom" (i think) refers to natural tube compression, which initially reduces the level of the attack, which makes the decay of the note seem to get gradually louder, i.e. "bloom".

"tubey" maybe just refers to those 2 put together, I didn't have enough descriptions

 

[antichef]

I don't have a SCXD, but I got a Blues Jr. - it has fixed bias (unless you hack in a trim pot or two, but I'm not going to try that) - but IMO it was way to hot - I changed the bias resistor to lower the voltage some, and I like it much better - now it's a little cleaner, and doesn't sound like it's about to have a stroke (and the tubes aren't so hot that you feel the radiant heat next door like they used to be). I wonder if they did the same hot bias thing with the SCXD?

 

[WhiteStrat]

I'm not sure if you're being sarcastic or wondering about those words for real, so

actually, the first 2 words have a more technical explanation:

"more complex" probably refers to harmonic content - more harmonics means a richer tone.

"bloom" (i think) refers to natural tube compression, which initially reduces the level of the attack, which makes the decay of the note seem to get gradually louder, i.e. "bloom".

"tubey" maybe just refers to those 2 put together, I didn't have enough descriptions

A tad sarcastic perhaps. I've got numerous tube amps, and I retube and bias them myself, so I'm familiar with the difference it makes. I'm just imagining that if I weren't familiar with it, I'd have no idea what you were describing in terms of changes to the sound.

Just my observation--one I should have kept to myself. My apologies.

 

[xstatic]

Not an uncommon problem. Marshall likes to send there stuff out pretty hot as well. Adds more gain, but tends to reduce clarity and punch. One thing to consider though, I would reccomend not blindly biasing to some certain number, but learn how to calculate what your specific bias needs or wants to be based on your transformer.

 

[Gear_Junky]

A tad sarcastic perhaps. I've got numerous tube amps, and I retube and bias them myself, so I'm familiar with the difference it makes. I'm just imagining that if I weren't familiar with it, I'd have no idea what you were describing in terms of changes to the sound.

Just my observation--one I should have kept to myself. My apologies.

well, no problem and no apologies needed

you're probably right and I didn't think of that, but I figured that those getting the SCXD are either new to tube amps or just getting into them or possibly already experienced, so they'd be familiar with this. It could go either way, really.

 

[Gear_Junky]

Not an uncommon problem. Marshall likes to send there stuff out pretty hot as well. Adds more gain, but tends to reduce clarity and punch.

everything I read about SCXD, everybody's response was that it comes too "cold" - I've even read engineer opinions that the way SCXD is designed it might not need a tube change for a LONG time. just sayin'.

One thing to consider though, I would reccomend not blindly biasing to some certain number, but learn how to calculate what your specific bias needs or wants to be based on your transformer.

i've read about this and i'm sure this is a great point, especially for all-tube, fully analog and high-end amps. Unfortunately I'm a little passive when it comes to learning new things like that and the value of 40 mV (20 mA per tube) was suggested by BillM among others - he's one of the foremost modders and "students" of this particular amp (and other fender amps) and this value reportedly comes from Fender documentation for this amp, so hopefully it's not too "blindly" used

Personally I'd love to have a simple hand-wired classic "retro" stylish cool awesome amp and I'd even learn to work on it, but I can't afford it and don't even have time - hardly even finding time to play, much less mod.

 

[xstatic]

Keep in mind that with cheaper amps come cheaper parts which can often correlate to less consistency. All tube or not, if you have power tubes, then calculating your proper bias range is a good thing. Remember, maybe "BillM's" tranny is outputting a little different. If you have the tools to meter your bias level than you already have the tools to meter your tranny... assuming you are using a multimeter I guess. Just a thought. The sweet spot of an amp is not designated by its actual bias reading but by the interaction of that bias with the rest of the build

 

[Gear_Junky]

I hear you and yes, just using a multimeter, per other instructions I found - measuring voltage at R20. I don't even have an understanding of this process, so it's not likely I'll go further into this. at least, not until i get a simple tube amp I could learn on (which could be never).

All I know is: I love the tone I'm getting and most likely will just spend the time (what little of it I have) playing. And maybe modding guitars/basses

Keep in mind that with cheaper amps come cheaper parts which can often correlate to less consistency. All tube or not, if you have power tubes, then calculating your proper bias range is a good thing. Remember, maybe "BillM's" tranny is outputting a little different. If you have the tools to meter your bias level than you already have the tools to meter your tranny... assuming you are using a multimeter I guess. Just a thought. The sweet spot of an amp is not designated by its actual bias reading but by the interaction of that bias with the rest of the build

 

[ocnor]

I hear you and yes, just using a multimeter, per other instructions I found - measuring voltage at R20. I don't even have an understanding of this process, so it's not likely I'll go further into this. at least, not until i get a simple tube amp I could learn on (which could be never).

All I know is: I love the tone I'm getting and most likely will just spend the time (what little of it I have) playing. And maybe modding guitars/basses

This page has a really good explanation about what adjusting the bias actually does. http://www.aikenamps.com/CrossoverDistortion.htm

 

[Gear_Junky]

thanks for the link, ocnor. i won't pretend i understood everything (and that's the problem - i don't have enough interest in electricity/electronics to put forth the effort to really grasp it, at least not yet).

what I don't see there is how that translates to "tone" - when they talk about "distortion" i know it means something different than what guitar players "want".

how does it sound when you do those adjustments?

 

[xlr8]

Hello folks,

I am new here, but I just performed the bias procedure on my SCXD after swapping tubes. The procedure is as follows:

Place the amp face down on a table. Remove the 4 screws that hold the cage onto the back side of the amp. Remove the cage. Now remove the 4 machine screws on the top of the amp while holding the chassis in place. Once the screws are removed, genly slide the entire chassis downward and out of the back where you can rest the chassis in a position where it will not fall over. Leave the speaker connected. Now plug the amp into a 120v outlet, turn it on and allow it to warm up for a few minutes. After 5 or so minutes, grab a multimeter, set it to VDC and place the black lead on the chassis or any ground point. Place the red lead on pin 8 of one of the 6v6 tubes. Pin one is labeled. Pin 8 is not, but it is the pin just to the right (counter clockwise) of pin one. On my fisrt check, I got .32vdc. If you look at the top open side of the chassis, you will notice a small blue trim pot in front of and just to the right of the output transformer (the smaller tranny). Simply adjust the pot clockwise if yours is low like mine was. THe adjustment is very slight. Just keep bumping the pot clockwise until you arrive at .40VDC. It's as simple as that.

I have looked around and have not seen the procedure spelled out like this. Please share this with others. It is so simple and the sound of the amp really does become a little richer with more of a full bodied sound. It was a significant difference in my amp and best of all, FREE!!