Tube Amp Rebuild (and Mod)

Step 9 The 'Base Mods'

These modifications are common to both Mods A & B...

--The grid leak resistor on the preamp (12AU6 tube.) Other similar designs for a 12UA6 preamp fell in a range from 2.2 Meg to 15 Meg. The Kay 503 had a 3.9 Meg resistor--I substituted a 15 Meg. This gives the preamp perhaps a bit more 'gain.'

--Input Stage: I removed the 22K resistor tied to GND here, as it wasn't wired correctly, anyway (it was wired directly to GND before the series resistor--effectively making the circuit for inputs #2 & #3 different from #1. Unless that was the plan, but it's not on the schematic....)

One input jack was removed. NO ONE EVER plugs in more than one instrument (unless they don't care about audio quality.) Two inputs jacks were plenty--each one slightly different, more room to work, and less noise. One is wired direct to the coupling cap, the other through a 22K resistor.

A one meg resistor to ground was added for higher input impedance. In this configuration, it's identical to using a one meg POT (at max) as an input volume control.

--The filter cap replacement (See the page Replacing the filter 'can capacitors.' )

--Grounded plug mod (see the page Three-prong plug safety mod.)

Now, on to the real mods....