THE SECRET 6G2 CIRCUIT CHANGES FOR GOD-LIKE TONE
Here is a layout I made of the 1962 Princeton with Reverb and Master Volume. Of most importance are the circuit changes which give it such glorious tone. Numbers in red are voltage readings when the wall voltage was 123 VAC.
The very disturbing 10K bias resistor in parallel with the original 30K resistor results in - 19.05 V bias. The 6V6 tubes are well matched, but running at ~ 100% dissipation because of this bias - and this is not a class A amp. You would expect the tubes to be red-plating, but they are not. Not even a hint of red-plating. Idle dissipation of this little Princeton is 23 Watts!
You have to peer at tone cap values and compare them to the stock 6G2 schematic or layout to see the magic. All of the "tone" caps have their values changed from 0.1 to 0.047 µF. Although the bypass caps changed, the resistors more or less stay the same. This changes the preamp tubes bias from the way the amp was designed, but it works, from a tone standpoint.
The added reverb section is a copy of a Fender AB763 reverb, and the master volume is a copy of the early Silverface master volume circuit (without the push-pull nonsense).
Of particular interest is the PI cathode cap, which is 100 µF. The stock cap is 25 µF. This basically doesn't roll of any bass from the guitar at the PI; but in the first preamp stage the cathode bypass cap is 8.2 µF, instead of 25 µF - so the first stage rolls off quite a bit of bass. I suspect this was to reduce the "flabbiness" of the bass often associated with the brown Princeton.
The 3rd filter section was changed by adding a 40/450 electrolytic cap in parallel with the existing 30/450 can cap, giving 70 µF to that section. That would be way to much for the rectifier if it were the first filter section, but since it is the third, the rectifier doesn't care. This extra filtration was done for the added reverb section.
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, that were part of the modifications.
A customer brought me this lovely brown Princeton to look over.
( I have since purchased this amp from the owner.)
It had not been turned on for years, so I brought it up slowly on the Variac. No surprises, except a blown indicator lamp.
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.
Fender only made the 6G2 Princeton for two years.
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 is the basis for Mesa Boogie's Mark I.
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."
FOR SALE: $ 1,100
The date stamp "KL" is December, 1961. This is one of the first amps of 1962.
The added reverb circuitry is very busy - not surprising given the limited space. There is a mix of orange drops, film resistors, Sprague electrolytics, and whatnot. 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.
There is something funny going on in the first preamp stage, starting at the input jack. I'm very interested in this because it almost certainly affects the tone of the amp, may hold a clue to this amp's amazing sound.
Also note that almost every capacitor on the board has been changed - they were originally blue tubular "Ajax" caps. Just the original tremolo section remains. I need to check the values of the replaced caps.
Frederic from France pointed out a couple of errors in the above layout; Here are what the inputs and outputs should look like.
This Princeton has been modified over the years. The speaker, which originally would have been an Oxford 10J4, or a Jensen C10R, has been replaced with a rare CTS 10" ceramic speaker, made in Mexico. Few people have seen or heard of this speaker, because CTS only moved production to Mexico in 1978, and then went out of business a few years later.
A reverb section has been added to the amp - a full size tank from a Twin (which just barely fits), two 12AX7 tubes for the send & receive, a reverb transformer, and a dwell knob (the top knob on the bracket).
A master volume has been added (the knob at the bottom).
Some people, I guess myself included, would be aghast at these modifications to a rare brown Fender, but the fact is that the reverb sounds really excellent, and the master volume works precisely as it should, not at all like the master volume on later Fender amps.
I've thought about it quite a bit, and I just can't advise the customer to un-do the modifications. Without the modifications the amp might be worth more to a collector, but the sound of this amp is truly outstanding and any player interested in tone would want to own it.
Over on the power side, a 10 µF electrolytic capacitor has been added to the filter can. The bias board has a resistor added in parallel, seems untouched otherwise.
A ground wire on the power transformer primary has been cut off - this is the center tap, and wasn't used, so the factory cut it off.
(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 change 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 your 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.
The bias calculations of the amp are as follows:
6V6 outboard: 324 V Plate, 38.9 mA = ~12 W dissipation (100%)
6V6 inboard: 324 V Plate, 37.3 mA = ~11 W dissipation (100%)
The input voltage at the time was ~ 123.3 VAC. The tubes are well matched.
The plate current is very high. The recommended dissipation is 70% and here we are running at 100%.
Normally with these conditions I would expect to see some evidence of "red-plating" from the tubes, but there is no hint of redplating here, even in a dark room. As mentioned earlier, this bias was modified from stock by changing the bias resistor from 30K to 7.5K. It is a simple matter to remove the 10K resistor which is in parallel to the stock 30K and take the bias back to a more stock level. I did not do this, because the bias is so critical to the tone of an amp, and this amp does sound very, very good as is.
TUBES, TRANSFORMERS, FILTER CAPACITORS
V1, the first preamp tube, is a Peavey 12AX7A. These tubes were made in China and branded by Peavey. They are not known for good performance. This amp would benefit from a high quality V1 tube; either a vintage RCA tube or similar, or a new production highly rated 12AX7, such as the Tung-Sol or Mullard reissues.
V2, the first reverb modification tube, is a Fender-branded 7025 Special. The 7025 is a high-quality version of the 12AX7A, made in the 50s and 60s when regular production 12AX7s were sometimes (not always) suspect for use in audio amplifiers due to microphonics.
V3 is the second reverb tube. It is a GT (Groove Tubes) 12AX7A, again, probably a rebranded Chinese tube.
V4 is the phase inverter tube (and tremolo, in the 6G2). In this amp it is an RCA 7025 greyplate. This is a highly rated USA made tube.
The two 6V6 power tubes are both Sylvania 6V6GTA. The tubes are from the same production lot and date.
The rectifier tube is a GE USA 5Y3GT. This is an excellent rectifier tube.
The output transformer appears to be original. #125A10B.
The reverb transformer is #EIA606-022 (022924)
The power transformer appears to be original. #22R02
The can-type filter capacitors are Astron, and appear to be original. Section values are 20/450, 20/450, 20/450.