• Tweaking a SF Deluxe Reverb
When I got this amp it showed signs of abuse not only by post-CBS construction, but by one of its owners. The baffle board had 1/4" holes drilled in a circular pattern about 3" in diameter. One screen resistor had blown and cracked in half, and I noticed the other had been replaced. From this I gathered that the owner had cobbled in a tweeter, inadvertantly lowering the speaker impedance, and played the amp loud.The tolex on the top had blistered in in a 4" x 4" area near the handle. I figured this was the result of some owner's actions, but when I pulled off the tolex, it appeared that that area never had any adhesive on it. It leads me to believe that it might have left the factory this way, and didn't become noticeable until the tolex had a chance to stretch out a bit.
Inside the wiring was sloppy when compared to a blackface amp. This caused the tremolo to tick. Re-routing some wires cured that.
I planned on replacing the filter capacitors, the ceramic caps used in the tone stacks, and a ceramic cap that was used in the phase inverter (!). When I called Doug Hoffman to order these parts, he advised me to also replace the cathode bypass caps. Out of everything I did, this had the greatest effect on bringing the amp back to life.
The ceramic caps in the tone stacks were replaced with silver mica, but I don't hear much difference between the two.
I converted the phase inverter to blackface specs. More on this in the phase inverter section.
This model DR had the pull-boost switch on the volume control. Talk about ice-pick in the ear tone. This boost is achieved by stealing some of the signal off the transformer feeding the reverb springs and adding it back in after the reverb recovery tube. Unfortunately, the signal passes through a 500pf cap before hitting the tube driving the reverb springs, so it's extremely bright. This is OK going into the reverb springs where bass would muddy the reverb sound and the springs attenuate the signal, but it's not suitable to be put back in the audio path. Fortunately it's easy to unhook this.
The now unused boost switch on the volume pot is a great way to disable the tremolo when it's not in use. Even when the tremolo is off, it's stealing a little of the audio signal because the 50K pot is always in the circuit. You can use the boost switch to open the line to this pot, and get a slight boost in volume. When the knob is in the tremolo is disabled, and when it's pulled, it's the stock signal path.
Two other things bothered me about the stock Fender design. Two tubes stages share a common cathode resistor and bypass capacitor. Fender may need to be concerned about the economics of adding 2 resistors and 2 capacitors with the thousands of amps they made, but I can afford the $2 to separate them. The 820 ohm shared resistor was replaced by a 1K5 resistor on each cathode, and each cathode got it's own 22µf cap.
The second thing that I'd read is that the newer Fender design, using a 680 ohm resistor on the tube stages to drive the reverb springs, runs those tube stages too hard. Older designs used 2K2 bypassed with a 22µf cap. I replaced the 680 ohm resistor with the 2K2 and cap.
Neither of these changes made a noticeable change in sound to my ears, but I felt better about it.
Another tweak I played with was the voltage on the preamp tubes. In the AB868 circuit, 10K resistors separate the filter capacitors. On this amp, a 2K was substituted for the first resistor. This raises the voltage on the phase inverter and preamp tube plates about 40 volts. The lower voltages seemed to have a crisper, twangy-er sound, the higher voltage seemed fuller sounding.
I was intrigued by the differences of the voltages and the phase inverter values, especially since different speakers emphasized the differences. So I wired up a DPDT switch to select either BF or SF sound. The switch was mounted in the hole left by the jack of the foot-pedal boost. I installed a 10K resistor in place of the 2K, and used one side of this switch to add a 3.3K resistor in parallel to the 10K. When switched in, this drops the resistance to 2K5, and the voltage rises. Similarly, I installed a 0.001µf cap in the phase inverter, and let the switch add a 0.002µf cap in parallel. This raises the total capacitance to 0.003µf, which combined with the 1M resistor gives a frequency of 53hz--close enough to the 48hz of the SF phase inverter.
The last tweak was to give each tube it's own bias pot.At the time, NOS 6V6 tubes were hard to come by, especially in matched sets, and this lets you set the bias individually. I changed the 10K pot with its 10K resistor to two 25K pots with 15K resistors in parallel. Now current production 6V6 tubes are pretty good.