Fender Princeton Chorus 16v Daughterboard:

While working on my amp during some downtime between runs of shows, (because my chorus stopped working on me), I found out that my amp’s board was pretty charred by these two diodes. After looking through the schematic I found out that Fender designed this ’88 amp with a very rudimentary (and cheap) power supply: a simple resistor on each +27 and -27 volt rails, and a 1n5353 zener diode to ground from each rail. This is extremely inefficient, as the zeners are burning off crazy amounts of power in the form of heat. (since the chips on the 16v rail draw so little power that the zener has to burn off ton of heat to keep the rail at 16v), even the resistors are burning up in heat.

Though, I’m sure it’d be fine for most people, as running it for maybe an hour or two at a time at half volume should be fine. But that’s not what I’m doing, I’m running it at 10, with a maxed out level bd2 going straight into the front of the amp. For many hours (6-8 hr rehearsals). So, it doesn’t really work that great in my case. My amp is burning up already, don’t need this inefficient penny pincher power supply burning up the board too.

The sag

However, what I have found is that a key tone I use for some of my sound (one of the reasons I use this instead of a tube amp) is the sputtering overdrive sound I use on british blues.

There are multiple causes that function together to produce this saturation, however, (with exception of the coupling capacitors DC charge shifts/overcharge), they all are side effects of the DC rail voltage. To reiterate from the initial summary, the DC rails sag when the power amp is drawing crazy power, thus amplifying the hard clipping of the IC trying to output a waveform greater then the rails. Combine that with the rails sagging to lower voltage, and it chokes the ICs, giving a nice sputtering velcro fuzz type tone.
So that means I need to be able to keep the sagging rails for my british blues heavy fuzzed out songs.

HOWEVER, when I play really loud clean songs, I don’t want my chorus acting up. So I need to be able to switch between having rails that can sag or will stay firm. Thus the “FIRM” and “SAG” switch.

“FIRM” - “SAG”

So, I need to have two power supplies on this daughter board. And be able to switch between them between songs. So, that needs some more complicated switching then just a simple switch.

Trying to keep in my analog style of my amp, and prioritizing reliability, I opted to use some more complicated ICs and timing circuits instead of just using a microcontroller. So I apologize for what some might consider, unnecessary complexity. However, my amp is ‘88, and I’m going to keep design parts for it in a similar fashion.

How it works

So this amp has two power supply circuits.

A: The exact same circuit Fender used in my amp, very simple, very inefficient (however I will have it thermally connected to the chassis on my daughterboard, for sake of heat management). This sags

B: This uses ICs to keep the rail FIRM and STABLE, no matter how much the 27v rails sag. Also thermally connected to chassis.

The switching process between A and B is as follows:
Switch is toggled -> Vactrol mutes the input signal, by grounding hot -> short delay (ms) -> relays switch 27v rails to other circuit & 16v output rails to other circuit -> short delay (likely <100ms) while vactrol disengages -> Now on other circuit.