Got the little beastie together and waiting for paint to dry completely. My switch doesn't look as nice as the PedalPCB version (the painted part) but will touch it up tomorrow.
I'm noticing a slight volume drop at max volume (which, again, is true to the original). Any thoughts on getting more signal out of this puppy? Was hoping to hit unity at about 3/4 or so.
@Gordo did you figure out how to squeeze a little more volume out of it after all?
I was looking through the schematic after seeing these two posts and here's my take (could be way off here)...
Not knowing a ton about the univibe architecture, I assumed that the switch right before the Volume control is the SPST for Chorus/Vibrato. I ended up pulling R.G.'s "Technology of the Univibe" to confirm:
Again, I'm taking a shot at this but could be wrong. I'm assuming most people don't actually use Vibrato mode here but I'll crunch the numbers anyway.
In Chorus mode, you take the output off the 100k/100k voltage divider into the Volume control. I assumed the Volume control in parallel with R17 here and then calculated the output signal as a percentage of the signal hitting R16 and got 33%. This should be similar to those who have ever modified an Epiphone Valve Junior, as that circuit has a similar arrangement but with 1M.
So if we simulate an infinite impedance by using 5M for R17, we'd get 50% signal through instead of 33%. What I'm not sure of is how this affects the rest of the circuit. A less invasive measure for the actual circuit topology might be to swap the Volume control out for a higher value.
If we swap the 100kB for a 250kB and 500kB, respectively, the available amount of output signal goes up to 42% and 45%.
For Vibrato mode, the output is taken off the R20/R18 divider. There seems to be considerably more signal getting through to the Volume control here so you'd have to adjust the Volume control in this mode or tweak the resistive divider, which again, may have downstream impacts.
A simpler way might be to change R4 and get more signal into the circuit in the beginning:
Again, fans of the Epiphone Valve Junior will remember that one of the earliest mods was correcting the input voltage divider that was present on the V1 and V2 circuits. In this configuration, the 1M resistor originally found across the input jacks in the Tweed Champ circuits was placed AFTER the input resistor in the early Valve Juniors. This formed a resistive divider at the input.
Here, the R1/R4 divider can be tweaked to allow more signal to pass into the circuit. Stock, it lets 41% of the signal get through. So if you find that you are having volume issues, you can tweak R7 up. For example, 100k goes to 45%, 250k to 48%, 2M to 50%, etc.
Here, the R1/R4 divider can be tweaked to allow more signal to pass into the circuit. Stock, it lets 68% of the signal get through. So if you find that you are having volume issues, you can tweak R7 up. For example, 100k goes to 82%, 250k to 92%, 2M to 99%, etc.
(Edited because my simple voltage divider calcs were off because of a copy/paste error in Excel)
Not sure if I'm way off base here or not.