I was looking at some Boogie schematics, and I seen a switch that lifts the inner pairs cathodes to ground. Is that all it takes for a fixed bias amp?
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You mean at power transformer? Lot's of amps do that. Already a few decades ago that idea even evolved into introducing continuously variable B+ voltage controls.Don't know why more amp builders haven't followed up on the Music Man power switching , that cuts the voltage at the output transformer, simple idea.
Unfortunately, both of those also pretty much eliminate all sag:Exactly, my view is that a key aspect of a great cranked amp tone is that the power supply sags / gets modulated by the signal; drop from four to two power tubes and the current draw will halve, and that modulation will also halve (at least, probably more so if there's an impedance mismatch too).
A (dual track) post phase splitter master volume control is almost as simple to implement, would sound better, and provide an infinitely variable range of control.
And as psychonoodler points out, VVR (rather more expensive and complex) will provide a much better result.
I have a twin reverb head I run with 2 power tubes (= 8 ohm load) and put it on a 1x15 16 ohm cab. The downside is that because its a jbl, in the end the volume is not that much less.Do you guys think pulling the outside tubes and maybe changing speakers to two 30W speakers on each side will be ok?
Everyone seems to pull tubes, but what about changing the speakers to lower wattage as well? Anyone try this?
I am currently running a 135 watt Twin with a pair pulled. Should that fuse value be halved? I don't want to melt my sweetheart.When you remove 2 tubes and leave the mains fuse the original value the amp is now overfused and you leave yourself wide open for a major meltdown.