MusicMaster Bass Amp Power Supply

[swampwood]

I'm looking to make some modifications to my 6V6 Fender MusicMaster Bass Amp and I found a link on the Fender Schematics page to a modified circuit that's pretty much exactly what I was planning to do with the preamp. I'm less clear on the changes to the power supply, though, and wondering if someone who understands the changes there could walk me through why one would use two filter caps with a resistor between them instead of the original circuit's single filter cap.

[pompeiisneaks]

My first thought on that is they just wanted you to add more capacitance and rather than remove the resistor, and cap and add a new one, would just be to add the cap at the end with the additional needed capacitance, but I'm no expert, so I'd wait for more consensus on that. 

I.e. it was say 22uF and needs 44uF after the change, but instead of removing the 22, and adding a 44, just add another 22uF in the line.  Where it sits before/after a resistor, I think, is irrelevant for total capacitance in the line. 

I'd love to learn otherwise though.

[swampwood]

The original schematic for somparison: http://el34world.com/charts/Schematics/files/fender/Fender_musicmaster_bass_schem.pdf

[pompeiisneaks]

Oh I see what you mean.  It looks like they're not adding a cap per se, but doubling the first filter cap to 40 uF to give more filtering.  The addition of the 1k resistor, though, isn't as clear to me, maybe just to modify the output voltage a bit to also adjust it more to what they find is a 'nicer' less hot tone?  The gurus will likely have a more intelligent answer on that part.

[Willabe]

The 1st drawing is drawn in an odd way.

The 1K R between the plate node and the screen node is there but is located in the drawing up between the OT CT and the power tubes.

All they did was change the 1st filter cap from 20uF to 40uF. They moved the 1K R in the 2nd drawing back to where it normally would be found.

[PRR]

They are equivalent except the first cap value (and 40u makes more sense if you are not gasping for pennies). The 1K is the "same" place. Cut it off the paper, tie long strings to it, slide it along the PT CT and screens lines, nothing changes.

And the tone control, of course.

And the cathode network on the driver, though this may make little difference and could be a factory running-change.

If you have a real speaker, you'd consider un-grounding the side of the heater chain and using a center-tap affair.