Mesa and adjustable bias mod

[Nomad]

Wanted to make myself a little more clear.


There is no problem with Boogies Bias circuit.

For those of us who like to change Tubes as often as we change our underwear (every couple weeks or so when the urge hits me) :lol: a Bias Pot is nice.

Once I find a set of Tube I really like with an amp then I leave it there and usually dont fool with it anymore.

In the Simulclass Amps its a little more complicated how you would Bias both sets of tubes indepently. So for most I would leave it alone. Mine hasnt been alltered and I know how it can be done. I just opted for a resistor change and found some tubes I liked in both the 6L6's and EL34's.

I do have a bunch of stuff I wouldnt mind trying but it begins to be time consuming so I havent even bothered.

A simple amp such as the DC-5 is an easy canidate for an adjustable Bias Pot. #1 there is plenty of room in the amp to do it with and #2 it only has one set of 6L6's. I have a friend who owns two and both have been converted. He dosnt even know how to set Bias but just brings em by the house for me to do them every year or so.

 

[Nomad]

I just opted for a resistor change and found some tubes I liked in both the 6L6's and EL34's.

this seems like a reasonable alternative...

so when you did this, what was the % MPD and bias reading(s)? how did the sound change?

Dude I dont know its been a few weeks. When I get a chance I can throw the Bias Meter in and write it down. If I remember correctly what I ended up with was a little colder than I normally would leave it at but a lot warmer than it was when I got it.

How did the sound change? Warmer, rounder, less fizzy in both channels.

 

[stokes]

I just opted for a resistor change and found some tubes I liked in both the 6L6's and EL34's.

this seems like a reasonable alternative...

so when you did this, what was the % MPD and bias reading(s)? how did the sound change?

One warning I should tell you when biasing a Boogie,you cant go as hot as you might with a Fender or the like.Your clean channel will kick ass,but you get too hot and your lead or drive channel will get pretty ratty,because of the overdriven preamp.Like Nomad says,you want a little warmer than stock but colder than you may think it should be.A normal,non-high gain amp in class AB would be about 70% MPD,but in a high gain amp you probably want to go lower,of course its a matter of personal taste,but a Boogie does tend to get ratty with too much power tube saturation.I mostly use the clean channel anyway on my IIC+,but when I need the drive,it can be tamed a bit with the lead drive backed off some.

 

[Nomad]

Absolutely it will get noisy too.

I run my Mark III Boogie (as I've plastered this all over these pages) :lol: with the Lead Channel set as a Crunch Channel, no R2 at all and running a Timmy and or an OCD into either my Clean Channel or "my" Crunch Channel and get a Plethera of Tones and Combinations. From Clean Boosted Cleans to Overdriven High Gain Hell and everything in between.

Between my Speaker choice and the settings of my Graphic EQ, on to my Tubes (pre and power), with my Pedals and yada yada yada it all has measured up to be a recipe for some fine tones and a wide aray of colors to choose from.

I am one happy camper.

 

[stokes]

i don't know what you mean exactly by "ratty" but I usually play in classA only and pentode/triode depending. when I crank the output up to 5 or more with the jmp-1 output @ 5, it sounds very very thick.

so i just wonder what it may sound like with the bias mod. the jmp-1 has a lot of gain, if not more than the mark iv preamp.

rat from hell!

By ratty,I mean overly buzzy,thin,more distorted as opposed to a smooth overdriven tone.To me the biggest benefit of the adjustable bias mod is so you can use NOS tubes,which to me have a much more defined tone,more even breakup,just smoother overall.To me,all current prodution tubes have a thinner more harsh tone when compared to some good NOS tubes.

 

[Nomad]

Yeah the only thing is getting a set of NOS GE 6L6's and a set of NOS Mullard XF2's that will all fit under the same hood Bias wise. It would definately in the Simul Amps be a chore to get them to pony up.

I currently have several sets of each that I havent tried in this Amp because of it. I suppose if I had the time I could dive in but that being said I'm getting great results from the "real" Svetlanna 6L6's and JJ EL34's. I have so many different Tubes I could try but right now that quad is gonna set there and play when I get the time.

 

[stokes]

Yeah the only thing is getting a set of NOS GE 6L6's and a set of NOS Mullard XF2's that will all fit under the same hood Bias wise. It would definately in the Simul Amps be a chore to get them to pony up.

I currently have several sets of each that I havent tried in this Amp because of it. I suppose if I had the time I could dive in but that being said I'm getting great results from the "real" Svetlanna 6L6's and JJ EL34's. I have so many different Tubes I could try but right now that quad is gonna set there and play when I get the time.

Exactly why the simul amps are a bit more complicated when doing the adjustable bias mod,but it can be done.