Fender Princeton 5F2 clone with a tone switch and mods
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- 2x 6SQ7 tubes instead of 12AX7,
- 6F6 power pentode instead of beam 6V6,
- Switchable tone control with three bright capacitor values,
- Switchable negative feedback,
- Second stage's three bias points: cold, normal, and warm.
The amp turns to have a good gain and a very Fender tone. It is breaking up somewhere in a mid position of a volume pot when on High input. 6F6 as a Single Ended output is capable for about 5W of output power.
Assemble wise, the amp is built in a proudly Canadian RCA A-23 Victor radio cabinet. 5Y3 tube is serving as a rectifier. "P-TGO-001 Champ Output" by Amplified Parts was chosen as an output transformer. Cabinet was covered with a couple coats of lacquer on top of original one. Speaker grid and corners were dyed with a dark stain.
Circuit overview
5F2 Princeton is a single ended amp with a negative feedback and bright channel, equipped with a tone control. Following is an original schematic.
Mod #1. "Tone control"
Tone section is based on the Princeton 5F2 circuit, quite unique among other Fender amps. In a donor radio I had three knobs which didn't give me a room for a full tone control. Also there was a bult in switch in the middle position, so I decided to re-use it along with volume\tone pots. The idea was to have three different values for a "bright" capacitor or completely switch off brightness and have a raw tone like Fender Champ.
Mod #2. "Negative feedback switch"
Some of the Fender amps like early Champ or Deluxe are not equipped with any global negative feedback. Canonical 5F1 Champ or Princeton has this feedback. Why not have this as an switchable option?
With a switch you can turn it ON and have a more linear and plain tone. Or switch it OFF and have an earlier breakup and more colorful tone like Fender Deluxe.
Mod #3. "Bias switch"
If we dig dipper into a tube circuit theory, bias is an idle operation point, placing a tube control grid somewhat between a cutoff where the tube is not passing any signal and saturation where the tube is passing its max current. Usually, bias is set in a middle of tube limits, giving the tube a symmetrical headroom for input signal. When the bias is set close to cut off, this is called a "cold" bias. Position close to a saturation point is as you can guess a "warm" one.
Again, in a donor radio I had a three position switch which I turned into a bias mode switch. The expectation is to have a different tone and different breakup point. The difference is audible, but I can't tell which one is better.
To avoid a difference in gain, it bypassed cathode resistors with a capacitor, leaving a small resistor in series un-bypassed and serve a NFB.
Misc. and guts
Finally, here are the guts of the amp. 6F6 was heating as hell, so I put a little heatsink made from an alumni profile just on tube's body. To lit the dial glass I put a short stripe of LED, driven by a doubling solid state rectifier, mounted on a little metal post. The two input jacks are "High" and "Low" as in any other Fender amps. 1/4 jack for an external 8 Ohm speaker was mounted on a metal brace as no room left of the chassis back.
P.S.: did not list the amp on the market yet. I'm building another one, the Push-pull in a old Yamaha amp cabinet. The intent is to have two different amps to give them to whoever is interested in trying and collect some feedback.