Don Cameron had me look at his 1962 Fender Princeton. I did a thorough assay of the amplifier, and discovered quite a few modifications to the circuit that contribute to making this brown Princeton have the best voice and tone of any amp I have heard in a long time.
RESEARCH
The amp chassis number is PO 1392, which dates it to 1962. The tube chart date stamp is “K L” (it’s hard to read), which would make the production date December of 1961. This makes it one of the first amps of 1962.
The speaker is not original to the amp, but has an interesting story:
The EIA Code marks the speaker as a CTS speaker made in the 38th week of 1980. Very few people have seen or heard of a “made in Mexico” CTS speaker – CTS moved production to Mexico sometime around 1978, and then went out of business sometime in the 1980s. The 10″ CTS speaker is known for having great tone. Fender made the 1962 Princeton with the Oxford 10J4 speaker, although Princetons with original Jensen C10Rs have also been found.
The amp is the 6G2 circuit design. It has a fixed-bias output stage, very different from the 5E3 Deluxe, to which it is sometimes compared, which is cathode-biased. The 6G2 uses a split-phase inverter (cathodyne); the circuit design gives the amp a certain level of pleasing distortion even at low levels.
The 6G2 uses a 5Y3GT rectifier, which lends sag and “brown sound” to the Princeton.
The tremolo is bias-modulating. The 6G2 Princeton is renowned for its lush tremolo; it is described as “smooth”, “organic”, “utterly dreamy” and “evocative”. One of the desirable characteristics of the bias-modulating tremolo is described in Vintage Guitar magazine, February 2011:
“…it’s a very playable effect, too; hit the strings hard and it steps aside so your note attack pops out proudly – allowing you to solo without switching out the effect – then makes itself known again as it throbs back into action on the decay of the note.”
PERFORMANCE
I brought the amp up slowly on the Variac. No surprises, except a blown indicator lamp, which was replaced. At proper voltage, the amp was so quiet I thought there was something wrong, but then I plugged Carmen (1954 Stratocaster reproduction) into it.
The sound out of this amp is glorious. I don’t know how to adequately express how wonderful this amp sounds. At moderate volumes it is growly and chimy, a splendid and just right tone that I have seldom heard from any other amp. Even at low volume it is commanding and “present”; the kind of amp that makes me play better than usual, because I am really paying attention to the incredible sound.
The master volume works as expected; the channel volume can be used to overdrive the preamp, and the master volume can be used to reduce the volume to the desired level while retaining the overdriven sound.
The reverb is excellent. The dwell is big and deep, without being too “springy” or bright.
I left the amp on for an hour or so. There was no evident overheating, and there was no heat-related capacitor or resistor noise evident. I would judge the circuit to be stable.
Circuit Evaluation
The amp circuit differs significantly from the original 6G2 design.
The added reverb circuitry is very busy – not surprising given the limited space. There is a mix of “orange drops”, film and carbon composition resistors, Sprague electrolytics, and other passive components. A lot of bare leads in close proximity to each other, and lots of questionable lead dress. Not the way I would have done it. (But of course I say that about every amp I work on). Regardless, the modifications work very well and don’t seem to have any unpleasant side effects, like noise.
Compare Don Cameron’s tone machine to a pristine original circuit, below.
I blueprinted the circuit as modified. The values of all resistors and capacitors were measured. All of the components were within 15% of their rated values. This includes those parts which have different values than those of the original 6G2 circuit and were part of the modifications.
Circuit Modifications
(I will refer to the preamp tubes as V1 through V4. V2 and V3 are part of the reverb modification, while V1 is the original preamp tube and V4 is the original phase inverter and tremolo tube.)
The input jacks have the 1 Meg resistor tied directly to ground – the original circuit uses the NC switch on the jack to tie the resistor to ground. This would have no noticeable effect on the performance of the amp.
The V1 cathode resistor is stock. The bypass cap is 8.2 μF, changed from the original 25 μF. This changes the tone of the first preamp stage and makes it brighter. This is often modified on the 6G2 amps because they are known for sounding “dark”.
The V1 plate capacitor is .047 μF – changed from the original 0.1 μF; again, changing the tone of the amp.
The V1 second plate capacitor is 0.01 μF, half the value of the original 0.02 μF.
The V2 cathode bypass capacitor is changed to 100 μF from the original 25 μF.
The V4 tremolo and phase inverter capacitors (3) have been changed to .047 μF from 0.1 μF.
The third filter capacitor section (“A”) has an added 40 μF/450 V electrolytic capacitor in parallel with the original 30 μF/450 V can capacitor. This raises the capacitance of the section to 70 μF. This was done presumably to add additional filtering for the added reverb section. The 5Y3GT rectifier tube has a rated maximum first capacitance of about 40 μF, and this capacitance should not be exceeded in the first filter stage. However, later filter stages have no restrictions on the level of capacitance, so this 70 μF of the third stage is not an issue.
The bias (tremolo) circuit originally had a 30K resistor across the 25/50 filter capacitor which set the bias of the output tubes. This 30K resistor has been paralleled with a 10K resistor, resulting in an actual resistance of 7.5K. This dramatically changes the bias of the amp. The 6V6 output tubes are running with much higher plate current than a stock 6G2 would. If there is an area of concern with this amp, this would be it.
The static dissipation of the amp is about 23 Watts – the amp is rated at 12 Watts. I did not notice any “redplating”, or overheating of the 6V6 tubes in prolonged testing. The bias does affect the tone of the amp considerably, so I would suggest leaving the bias where it is, since there is no immediate indication of failure of the tubes or the output transformer.
The reverb section consists of two tubes, a small transformer, various components, and a 100K Ω potentiometer. The reverb modification is basically a copy of the reverb section in an AB763 Fender Twin Reverb amp. The reverb modification to the amp only deviates from the standard Fender reverb in that it uses a 12AX7 tube instead of the standard 12AT7 for V3. This change is commonly seen in amps where the user wants to boost the power of the reverb.
The Master Volume modification is also an exact copy of the Fender master volume circuit from Fender’s early Silverface era amps (~1972). It uses a 500K audio taper potentiometer.
I have drawn a layout of this amp which includes all of the modifications and voltage notations. It is not a particularly neat drawing, but it is accurate. I have also included several photographs of the amplifier and the chassis.
