I was recently asked to help improve the uncontrolable volume on the a Fender Princeton 122+ amp. The good folks over at music-electronics-forum, in particular, Steve Ahola have come up with some ideas so I thought let’s give them a try.
In stock form this unassuming amp is a great clean fender amp and most people might be surprised to learn that it is a light weight solid state amp. In fact second hand these amps are pretty cheap second hand at about £70-120 in the UK.
The mods below just help to improve usability and versatility.
Clean Volume Pot Taper Improvement Mod
First mod is to replace the clean volume pot with something more gradual. The stock pot is an anti-log pot 50KC and the suggested solution is to use a 50K Log (audio) taper pot. I was a bit sceptical about this since an anti log pot is usually a bit more expensive compared to a log pot so it seems like Fender consciously thought that this was for the best. The results are quite dramatic with volume contol about half as sensitive. On the stock amp full power is achieved at about volume of 4 or 5 but with the mod this is moved way over towards 9-10. Definitely a keeper.
| With anti-log pot stock | With 50KA pot mod | |||
| Volume Control Level | Speaker Signal (volts RMS) | Power (watts) | Speaker Signal (volts RMS) | Power (watts) |
| 0 | 0.022 | 0 | ||
| 1 | 3.6 | 1.2 | 1.8 | 0.33w |
| 2 | 9 | 8.1 | 4.5 | 2.0w |
| 3 | 15 | 22.5 | 8 | 6.4 |
| 4 | 19 | 36 | ||
| 5 | 27 | 73 |
Input conditions: 100mV sine and EQ flat
Drive Channel Gain Reduction Mod
This next mod is just to tame the drive channel a little bit. Steve suggests adding a 68K resistor across C14 which turns the metal more into rock. I went with a 56K which added a little more control but I think this is worth experimenting further in the future.
Master Volume Mod
This simple mod just adds a master volume control like we see on many amps and allows us a little more control such that we can operate at night-time practice levels without waking the baby.
The MV control is simply an extra potentiometer effectively controlling the gain beteen the power amp and the pre amp. At full volume the amp behaves as stock and as the pot sweeps down the level is adjusted progressively. For home use I have been keeping at about half level. The pot is most easily added to the rear panel due the limited space on the front and also keeps the front panel un-modified. It is actually quite usable on the rear and also discourages the player from constant tweeking. If you really want to have it one the front panel then the only option practically speaking is to fit the pot right over on the right near the power switch which would mean cutting a hole in the chassis wherer the “princeton 112 plus” legend is displayed.