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fenderjazz 16th December 2010 05:50 AM

Mesa/Boogie Bass 400+ mod question
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

azazello 16th December 2010 07:50 AM

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./
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.

SemperFi 16th December 2010 11:32 AM

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?

fenderjazz 20th December 2010 08:22 PM

I see, thank you both for the suggestions. I guess I'll leave it be sometimes folks make their mods seem so easy! Thanks again, Roy

msdava 20th December 2010 08:26 PM

thank you

cyclecamper 10th October 2011 07:50 AM

I see used Mesa 400's up on eBay occasionally going for not much more than the tube complement costs. So are the output transformers any good? Would it be practical to convert a pair of 400's into two lower-powered class-A monobolcks for my stereo? How extensive would the power supply mod be?

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