The Mark IIa has a bias pot installed from factory!

[breogan]

Yes it does!

If you check the schematics it can be seen that is the only model in the Mark series that has this feature. The Mark I and the Mark IIb doesn't has it... This is funny... It is probably the only model in the Mesa history with bias pot.

Yesterday I've been readjusting the bias in my MarkIIa, it was very very cold, about 35%. I raised it to 55%, which is still a bit cold, and things improved considerably. The tone is liver and has more definition on the clean channel and punch in lead mode.

Have it a try!

Anyway, I just wanted to share this finding.

 

[carlosasi]

Where is this little darling hiding? And how do you go about making the adjustment?

Thanks,

And don't you just love this Mark IIA?

 

[lesterpaul]

Yes it does!

The tone is liver

...haa..when I first read this, I was thinking you meant "liver" as in the organ...whch is somewhat brown...maybe a new way to describe the "brown sound"....

 

[breogan]

Carlosasi,

The trim pot is a blue cylindrical component that is located next to the power transformer. In the same board as the filter caps.

I'll try to post some pictures.

About bias adjustment, I think is a good idea to read this document. It is a bit long but is very well explained and has a lot of useful information. I performed the adjustments following the info here.

http://www.duncanamps.com/technical/lvbias.html

In this document some mods are recommended to bias the amp. You don't really need to do that if you have the right tools. This replaces the mods needed for the "cathode bias resistor method".

http://www.tube-town.net/ttstore/product_info.php/info/p471_Kit-Bias-Probe-Octal.html

If you have any doubt, please feel free to ask.

Yes it does!

The tone is liver

...haa..when I first read this, I was thinking you meant "liver" as in the organ...whch is somewhat brown...maybe a new way to describe the "brown sound"....

Oh my, please pardon my poor English... I was trying to say that tone was more exciting with more harmonic content.

 

[carlosasi]

I found it by looking at pictures I took of the guts. NICE. Now I need to go to the school of biasing and give this a try. Of course I could just turn it clockwise until i hear the tone I ZZZZZZZZZZZZZZZZZAAAAAAAPPPPPPP! POOF!

I think this is the Bias pot in this photo. Round blue item under caps. I hope you can see this picture? PS This is an older photo and all the caps have been changed.


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