Mesa/Boogie Bass 400+ mod question


2010-12-11 2:23 am
I have read about a mod to cut out pairs of power tubes on this amp. I would like to read up about this, and also to add an adjustable bias. I plan to use a cerment multi turn pot and a resistor mounted on a small piece of perfboard to add bias adjust. I could use help finding a schematic on the Bass 400plus that doesn't require a pdf reader as they mess up my mac and also any advice from experts about this power cutting mod. I don't really need all that power, but would like the features of the amp at 200 watts, maybe even 100 or less! Perhaps would be a good guitar amp at 100 watts! Thank you, Roy


2008-08-23 7:28 pm
You can only put out 2 tubes /1 from every shoulder/ and fixed -U bias will remain the same /but all inner tube resistance will increase and output transformers
will little bit doesn't enough, and distortions in max. power will increase./ heaven/
For less output power You need -U about 70-80 v /sound will worst, You will decrease every plate current/, but schematic has only -68v.
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As Azazello said the reduction in number of output tubes will create an impedance mismatch. If you remove half the output tubes, you'd want twice the impedance on the primary. U can compensate a little with impedance mismatch by connecting 8ohm cab into the 4ohm output. Being for guitar, you wont have a problem with low end response due to mismatch.

I'd insert a series resistance with the rectifiers to give some increased power supply sag, and prevent the B+ from rising too much with less tubes pulling current.

But it may be risky messing with this classic? Perhaps better building a clone of one of the smaller Ampegs?