Modding the mark 3 from fixed bias to adjustable bias

[Paul Secondino]

I finally found a good amp tech here in Connecticut and want to pursue modding my Mark 3 blue stripe from fixed bias to adjustable

I had tried to have another tech add a trim pot to my old Mesa 290 pwoer amp and to make a long story short, the guy couldn't figure it out, even with a schematic in front of him.

After speaking with the new tech, he warned me that modding the mark 3 isn't all that simple and he wants to do some research first.

Any advice about making this happen ? Have you modded yours yet ? Thanks

 

[LUCRETIA]

http://forum.grailtone.com/viewtopic.php?t=84&highlight=markiii+mod

 

[Paul Secondino]

Thanks for the link

 

[Boogiebabies]

Russ is awesome.

 

[Paul Secondino]

What I learned from my engineer friend in France is that location R117 as in Russ's pic comes empty from the factory ( maybe)

A 50k trim pot can be installed there in paralell. It's safer according ot my frined

Just installing the 50k pot works but if oyu aren't careful and turn the pot down too far, you can bring the resistance value down to zero and create a short that will hurt the amp. He suggested a 33k pot in series with a 10k fixed resistor all in paralell with the 6k8 resistor.

Still reading up on the stuff but figured I'd pass on what I learned so far in case anyoen else is looking on.

 

[disassembled]

Check the non-boogie classifieds for my Bias meter I am selling. Which will allow you to set the Bias yourself.

Oh, I wouldn't trust a "tech" that can't figure out how to do an adjustable bias. It's a simple mod and should only be a head scratcher for someone that doesn't understand tubes well or, they fell asleep during that section of class Either way, find a competent tech that is familiar with this stuff.

 

[Paul Secondino]

This thing will give me a voltage reading but it wont set anything for me.I already have a Weber bias rite that tells me how my tubes are running. I am looking for info on how to modify the amp. Better to be safe and ask questions than to destroy a perfectly good amplifier

 

[disassembled]

This thing will give me a voltage reading but it wont set anything for me.I already have a Weber bias rite that tells me how my tubes are running. I am looking for info on how to modify the amp. Better to be safe and ask questions than to destroy a perfectly good amplifier

I guess I wasn't clear enough. The Bias meter I am selling will NOT set anything for you, just like any other bias meter, BUT it will read the current on your plates. Seeing as how you already have the bias rite then you should be good to go for rebiasing. Good luck. Be careful inside the amp!

Oh yeah, if you install the pot in series with the final bias resistor in place then there is no worries about having zero resistance. That way you can also compare stock bias settings to modified. This is what I tend to favor doing if I mod an amp since it is easier.

 

[pfflam]

This thing will give me a voltage reading but it wont set anything for me.I already have a Weber bias rite that tells me how my tubes are running. I am looking for info on how to modify the amp. Better to be safe and ask questions than to destroy a perfectly good amplifier

I guess I wasn't clear enough. The Bias meter I am selling will NOT set anything for you, just like any other bias meter, BUT it will read the current on your plates. Seeing as how you already have the bias rite then you should be good to go for rebiasing. Good luck. Be careful inside the amp!

Oh yeah, if you install the pot in series with the final bias resistor in place then there is no worries about having zero resistance. That way you can also compare stock bias settings to modified. This is what I tend to favor doing if I mod an amp since it is easier.

If you do this, doesn't that mean that you can only bias your tubes hotter?

 

[disassembled]

This thing will give me a voltage reading but it wont set anything for me.I already have a Weber bias rite that tells me how my tubes are running. I am looking for info on how to modify the amp. Better to be safe and ask questions than to destroy a perfectly good amplifier

I guess I wasn't clear enough. The Bias meter I am selling will NOT set anything for you, just like any other bias meter, BUT it will read the current on your plates. Seeing as how you already have the bias rite then you should be good to go for rebiasing. Good luck. Be careful inside the amp!



Oh yeah, if you install the pot in series with the final bias resistor in place then there is no worries about having zero resistance. That way you can also compare stock bias settings to modified. This is what I tend to favor doing if I mod an amp since it is easier.

If you do this, doesn't that mean that you can only bias your tubes hotter?

Yeah isn't that what everyone wants? You can do it anyway you choose I just told you how I like to do it.

Greg

 

[pfflam]

So, then all I would do is find the bias resistors snip the out-end then put in a bias knob? (of what value?)
. . . i could even run wires to outside of the chasis for that pot, I suppose, and even maybe hook up DMM reading ports or something
. . . maybe a little meter that would read the current.

hmm?!

 

[Restless Rocks]

A 50k trim pot can be installed there in paralell. It's safer according ot my frined

Just installing the 50k pot works but if oyu aren't careful and turn the pot down too far, you can bring the resistance value down to zero and create a short that will hurt the amp. He suggested a 33k pot in series with a 10k fixed resistor all in paralell with the 6k8 resistor.

.

I think last couple times we've been on this subject a few of us agreed that using a 100K cermet trim pot would give you a much broader range of adjustment as well as a smoother transition. The 10K series resistor is not a bad idea either for those who could use a safety net.

 

[vcamargo99]

Do you use just a single pot to adjust the 6L6's and EL34's?

 

[Restless Rocks]

The "adjustable bias mod" discribed here acts on the bias power source for both pairs of tubes. To set up a seperate adjustment for each pair (class A and class AB) would be a bit more complex. Many have done this basic mod with success.