Fender 5F2-A Princeton


The story about this one began when I found out that my grandmother has ancient radio in her basement. It turned out to be 1959 Tesla radio manufactured here in Serbia. Next moment I rushed to get it and took it apart within minutes without even trying to turn it on. Afterwards I heard that it could have been sold very well, especially if it was working. So now I kinda feel sorry for it, partially because it’s very old radio, nice piece of history and partially because I could earn some cash 🙂 .

Anyway, it’s done and can’t be turned back. So I salvaged very nice steel chassis, great 285-0-285V power transformer, 6500/8ohm output transformer and some cool braided shielded wire which all can be used for Princeton. Interesting coincidence is that both radio and Princeton 5F2-A come roughly from the same year. As with all my other builds I decided to use low noise metal film resistor in signal path. Also, to further reduce noise I decided to use shielded wire to pots and input jack and to place input resistor (68K) directly to tube socket. I got a 4 ohm speaker so salvaged OT wasn’t good for it. Fortunately, I got another salvaged transformer that’s just rigth for this with 7500/4 ratio.

Original Fender Princeton tweed

Now some tube talk. For power tube I got some NOS Reflektor 6P6S (or cyrillic 6Π6С) very cheap from eBay (only 2.5$ for pair). It’s Russian equivalent for 6V6GT only with slightly different voltage ratings. But in this case it doesn’t matter because voltages are below 350V anyway. As for preamp tube, I choose JJ/Tesla ECC83S that was left from my Tube Reactor project.

Reflektor 6P6S tube and JJ/Tesla ECC83S

As for speaker, I decided to try Weber because all comments I’ve heard about them were great. Being tight with money I got a Ceramic Signature 8″. I don’t believe in AlNiCo mojo anyway 🙂 .

  • Switchable cathode bypass caps on first gain stage. I placed two caps on cathode: 22uF 25V electrolyte and 1uF poly film cap which give 4 different combinations: without cap, only 1uF, only 22uF, both 1uF and 22uF in parallel. Two small SPST toggle switches are soldered directly to circuit board, each assigned to one cap.
  • Switchable negative feedback loop. I added SPST switch between OT secondary and 22K resistor which breaks that connection.
  • Improved heater wiring . To reduce noise caused by AC powered heaters I employed two techniques: virtual center tap and DC heater reference. Below is schematic which shows that circuit. 100ohm resistors connected from heater taps to elevated ground should be selected to be as close as possible. If power transformer has heater center tap those resistors are not needed and elevated ground should be connected to center tap. The ones I used are 100.3 and 100.2 ohms. Elevated ground is accomplished with simple voltage divider combined with 22uF filter cap which should make nice smooth reference. With B+3 at 260VDC I should get 260 / 4.7 = 55V as a reference. This mod costs only about 0.2€ but it can significantly reduce hum in amp so it’s highly recommended. Thanks to great guys from AX84.com for help with that!

  • Switchable volume reduction. In situations when I need really low volume I added SPDT switch which reduces volume for 7dB when activated. Using my L-Pad calculator I got values for series and parallel resistors: Rs=2.2ohm, Rp=3.3ohm. I choosed that attenuation level because 2.2ohm and 3.3ohm resistors are easy to find. Also, I added treble bypass cap which restores some treble lost by attenuator. It’s a 4uF non-polarized capacitor wired in parallel with Rs. (actually, four 1uF WIMA poly film caps). Wiring diagram is shown below. I decided to place this circuit inside separate box that can be used with other similar amps.

  • Switchable tone control. Passive tone controls will always suck tone and gain. After I completed my Firefly build I noticed that I always use tone pot maxed. Sometimes I wish I made it switchable. Toggle switch is cheap enough and offers more flexibility. DPDT ON/OFF/ON switch offers three different sounds: stock (tone stack is on), bright (only 500pF cap is left across gain pot) and completely bypassed tone stack in middle position. The next diagram shows how it’s done.

  • Solid state rectifier. Instead of 5Y3 rectifier I used two 1N4007 rectifier diodes. It’s cheaper and easier to do. I’ve read that tube rectifier sag does not affects class A amps, so it’s a waste of a tube and current to supply it. To accomplish voltage drop of tube rectifier I added one more RC filter stage with 47uF cap and 100ohm 2W resistor.
  • Improved power supply filtering. Solid state rectifier doesn’t have voltage sag so I added one more RC filter stage with 47uF cap and 100ohm resistor. In addition to this I increased first filter cap to 47uF and added 0.22uF 630V poly film cap in parallel with last filter cap. Also, instead of 8uF caps I got 10uF because those were available in local store.
  • Improved power supply filtering. Solid state rectifier doesn’t have voltage sag so I added one more RC filter stage with 47uF cap and 100ohm resistor. In addition to this I increased first filter cap to 47uF and added 0.22uF 630V poly film cap in parallel with last filter cap. Also, instead of 8uF caps I got 10uF because those were available in local store.

  • Standby switch. Original Princeton doesn’t have it, but it can’t hurt to have one 🙂 .
  • Only one input. I don’t need second “microphone” input so I didn’t include it.

Click on an image to see more details.


And finally sound: I’m very surprised by volume of this thing. It’s really loud for 5W, much louder than my solid state 15W Marshall MG. Highly efficient speaker must have contributed to this. Clean is great. I couldn’t make it overdrive with my Squier Strat, but it’s weak pickups to blame. I’m glad I added tone switch, especially “bright” mode. It’s really nice and bright. NFB switch also changes tone, when off it adds a bit more overdrive. Noise wise, it’s very quiet. Shielded wire and DC elevated heaters have done the magic. The only noise problem comes from power transformer noise. It’s quiet, but it’s there even when amp is on standby. I’ll have to find the way to shield the transformer.

Voltage reference

Voltage reference
6P6S plate
6P6S cathode
6P6S plate current
Heater reference

All voltages measured in respect to ground

12 Responses to “Fender 5F2-A Princeton”
  1. Old Crow says:

    Really nice effort and yes, they really are loud enough for small gigs. That board you have built it on is good stuff, it simply won’t burn and it’s easy to bend the leads. I’ve used it for small builds. Thanks for the mods, I must try the switch on the tone stack.
    My version has a lower voltage and sounds darker but I do have an old 275v transformer so I must try it.
    Good job on the mounting and cabinet build, very neat.
    Thanks for sharing.

  2. Amp Geek says:

    I think you’ve been reading the wrong material – rectifier sag is a universal effect – it lowers the high voltage rail and adds a compression to the signal. Doesn’t matter if it’s SE or Push-Pull. Your Princeton could sound much better with it, and break up sooner.

    • Bancika says:

      I think it’s more related to the class of the amp than topology. This is class A, so it will draw full current all the time. It will not sag under heavy load because it’s already under load and voltage is already as low as it’s going to get. Maybe I’m missing something, but I think it should work like that?

      I have updated the text.


  3. hubert says:

    the project is very good.
    can you send me the modified schematic.?

  4. fox says:

    why not elevate heater dc ref by connecting to 6v6’s cathod? That’s easier and is a typical ax84 way.

  5. TeoZh says:

    Hi Bane.
    Could you give us some schematics or something? There are a few on the internet, i’ve googled after some good schematics, and they where very slappy drawn, and unreadeable. mail me, or publish some right here.
    Great respect for what are you doing here. greetings from Romania.

  6. John Ott says:

    I came upon your nice website thru the 5f2-a article. I built a 5f2-a last year, my first guitar amp project.
    Thanks for sharing your experiences with DIY.

  7. GodsaveMetal says:

    The plans layout Bill material and whetever it fits, please !!!!!!!!!!!!!!!!!!!!!

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.