GAA's Tweed Princeton Clone

This amp is based on a Fender 5F2-A Princeton. The 5F2-A was a guitar amp from the late 50's. It was a 5 watt Class A amp with a 5Y3 rectifier, a 12AX7 and a single 6V6.

GAA's 5F2A Princeton

 

Modifications

I simplified the original 5F2-A circuit but I also added some extra controls. I was interested in hearing the effect of negative feedback and preamp/power tube distortion. I made the following changes to the original circuit:

  • The Mic input was eliminated.
  • A beefier Hammond 125E universal output transformer is substituted for the Fender OT
  • The power supply is different. A pair of 1N4007 solid state diodes are used instead of a 5Y3. Because SS diodes are used, a lower voltage Hammond 270DX power transformer (275-0-275) is used in place of the (300-0-300) Fender PT, a 33uF cap and a 120 ohm/5W resistor were added between the diodes and the first filter cap. Even with the lower voltage PT the plate voltage was 373 V. I added a 15V zener on the center tap to ground to knock it down by 20V
  • A Variable Negative Feedback control was added. A 100K pot was put in series with the stock 22K negative feedback resistor.
  • A Master Volume was added. The 220K resistor at the grid of the 6V6 was replaced with a 250K pot.
  • A 1K grid stopper resistor was added to the 6V6 grid to eliminate a problem with oscillations.
  • Star grounding was used and the filament was referenced to ground with a pair of 100 ohm resistors to reduce hum.
  • I built it as a head and made a compact cabinet from 1/4" birch plywood covered with maple veneer. The wood finish and size give it an early-60's tabletop radio vibe.

    • Schematic

    Below is the modified schematic. Click on schematic for a larger image.

    GAA's 5F2-A Schematic

    Plate voltage is 352V. Cathode voltage is 26V, so current is (26/470) = 55mA. There is about 8.5mA on the 2K/2W screen resistor. After subtracting the 12AX7 plate current the 6V6 screen current is about 6mA. The 6V6 is dissipating about (352-26)*(26/470 - .006) = 16.1 Watts. That's high based on the 6V6 datasheet, but pretty common for modern single-ended 6V6 guitar amps.
     

    Views

    Below is a front view. The chassis is a Hammond 10x6x2 steel chassis spray painted with a hammertone finish. From left to right on the front panel are the input jack, volume, tone, master volume, Negative Feedback control, power switch and indicator light. On top are the OT, the 12AX7, the 6V6 and the PT. The chassis has a few extra hole because I've used it in other projects.

    Front view.

    The amp originally had a standby switch on the front (I was worried about the SS diodes bringing up the B+ too quickly) and the Negative Feedback control on the back. Prevailing internet wisdom says that a standby switch is unnecessary so I scrapped the standby switch and moved the Negative Feedback contol to the front panel in it's place. It makes more sense to have all the controls where you can get to them even if you don't use them often.

    The Master Volume and Negative Feedback have mini-knobs since I don't use them much and tend to keep them dimed (i.e. normal volume and normal feedback).

    Below is the back side. From left to right you see the fuse, AC jack and speaker jack along with more extra holes.

    Rear View.

     

    Here's the underside. Eliminating the rectifier tube and building the amp as a head made the layout very compact. As an experiment I used Vectorboard to mount the components. It made assembly quick and easy. Vectorboard is okay for a light duty amp. I wouldn't recommend it if you plan to do lots of mods.

    Bottom View.


     

    Variable Negative Feedback

    The variable negative feedback was an interesting experiment. Decrease the feedback and the sound "loosens up" - it gets louder and brighter, the bass is reduced and it distorts more easily. With the feedback enabled you get a cleaner, more hi-fi sound.

    Originally I had a 4.7K resistor in series with the 100K pot. That allowed me to dial in more than normal feedback. With that setup the feedback effect is much more pronounced. With 4.7K of feedback the sound is clean, but not very loud which is not very useful for a guitar amp.

    The amp sounds good with the stock 22K feedback resistor. I ended up putting a 22K in series with the 100K pot so when the feedback control is dimed at 0 ohms I get the stock feedback. If you really like tweaking the feedback use a 12K in series. In my opinion using a potentiometer for the feedback control is overkill. A simple on/off switch with 22K of feedback is sufficient.

    You really need a tone control if you open up the feedback loop otherwise the amp is way too bright.
     

    Master Volume

    The Master Volume was also an interesting experiment. If you set it to 10 it's equivalent to having the original fixed resistor in the circuit. Turn the master volume down and crank the volume and you'll get preamp distortion at low volume. If I were to use it more often I'd replace the linear pot with an audio taper pot since the useful settings are near the end of the range.
     

    6L6 Substitution

    I tried a couple of very old 6L6s in place of the new 6V6.

    The 6L6 requires 0.9 amp of heater current versus 0.45 for the 6V6. The PT was rated for 3A at 6.3V so it could easily handle the 6L6, 12AX7 (300mA) and pilot light (~300mA). I changed the OT impedance from about 6K for the 6V6 to 3K for the 6L6.

    With the existing 470 ohm cathode resistor the cathode voltage was 25.5V. Plate was 354V. Dissipation was about 17W. That's probably a little low for a 6L6. I could have used a smaller cathode resistor.

    It sounded pretty similar to the 6V6 - maybe a little louder and one tube, an RCA, seemed like it had slightly smoother distortion.

    Unfortunately I didn't consider 6L6s when I built the cabinet. The cabinet has enough clearance for a 6V6 but it is too small for the taller 6L6.
     

    Do It Yourself

    Here is the Bill of Materials. The BOM includes Mouser Part numbers and prices circa 1999. The BOM is also available in the form of a spreadsheet. Click here for an Excel spreadsheet compressed into a ZIP file.

    Links

  • The Angela Instruments website had a 6V6 amp project that is similar to my amp. It's also a Tweed Princeton clone, but it's truer to the the original design.
  • The Fender Amp Field Guide has descriptions, pictures, schematics and layouts for Princetons and other older Fender amps.
  • For something slightly different, Runoffgroove.com has a Tweed Princeton Pedal. It's nearly the same circuit but with JFETs in place of the 12AX7 and 6V6.

    • Feel free to contact me if you have any questions or comments. My other DIY amp page can be found here

    Updates:
    5/16/01 - added schematic
    2/06/02 - added BOM
    4/10/06 - moved to new server
    2/22/07 - added Runoffgroove link
    11/07/09 - fixed bad links
    06/25/11 - Put new pictures and schematic on main page