1973 Fender Super Reverb
AB763 Schematic - Blackface Fender Super Reverb
AB763 Layout - Blackface Fender Super Reverb
Silverface to Blackface Conversion
I wouldn't consider myself an expert, but I have been playing with Fenders for quite a while, and have modded quite a few to sound the way I like. Converting a silverface to blackface actually has quite a bit to it, but there are some basic mods that can get you pretty close pretty quick.
I think the single most significant mod to get closer to blackface tone is to replace the .01 coupling cap on the input of the PI with a good .001 cap. The .01 cap contributes significantly to the bass heavy tone of the Silverface circuitry
The next thing I would do is change all the brown coupling caps (known as sonic "turds") in the circuit to a better style of cap. I like orange drops, but Mallory 150's are good, too. The orange drops tend to give a very detailed sound, and provide a lot of dynamics. To some people this is seen as brite and clinical. To each their own, I guess.
The next mod I would do is to convert the bias circuitry to a standard adjustable bias. This can easily be done with the current Hum Balance pot that is installed in the amp. Clipping the suppression caps on the output tubes is also a good idea, as there really isn't any need for them in these amps, unless you have some poorly matched tubes, and they will leach some highs if left in.
Next I would ensure all the values in the PI are the same as a blackface circuit. Some SF circuits have 100k plate resistors on both sides of the PI. All the BF circuits had a 82K on the (+) side and a 100k on the (-) side.
At this point, I would probably have the customer come in and give it a listen. If he / she is happy, then I would stop, but there is still quite a bit more that you can do...
Next thing to tackle is the reverb circuit. Clip the suppression cap and then make sure all the values in the circuit match blackface specs. This would include the .033 coupling cap. Many SF used varying values.
Next, switch the rectifier tube to a GZ34 / 5AR4. This may require you to rebias, as the B+ will increase about 10-15 V. The GZ34 will provide a stiffer power supply grid and breaks up nicer in my opinion. The general consensus is that the best one out there right now is the Chinese one, unless you want to spring for NOS. If the amp doesn't have a rectifier tube, I have had pretty good luck with the Weber copper cap rectifier or the THD rectifier simulator.
At this point, I would again have the customer listen to it. If they want more, there are still a few more things...
Carefully check all the voltage values on the preamp tubes. I typically like to run the preamp tubes a little colder with the B+ voltage equal to spec to about 10% below the rated values given on the schematics. This tends to give a warmer tone and a more natural breakup when the amp is pushed. If the voltages are a little high, you increase the resistor values in the supply rail slightly to drop the voltage. Typically adding about 5K to the value that is currently there will do the trick. This is especially important for V1.
I would also recommend experimenting with different preamp tubes at this point. I find the BF Fenders like a medium to lo gain 12AX7 in the V1 slot. I have had pretty good luck with RCA grey and black plate tubes. The Tesla is a pretty good current production tube for this application. I would probably stay away from the Sovtek / EH tubes, as they have higher gain and tend to be brighter. Again, this is matter of personal preference.
At this point I would go back through the amp and begin replacing the blue coupling caps until something close to the desired tone is reached. Start with the .1 coupling caps in the PI and move toward the preamp. I would check the sound after nearly every cap. Again, use Orange Drops or Mallory 150's.
The next mod that I really like for Fenders is to install some KT66's. These really have a great tone in Fenders, IMHO.
The last step would be to put some good speakers in the box. You may want to get the customer to do this earlier, potentially after the first time they come in to check the progress. In my Super Reverb, I currently amp running a pair of Fender Blue Alnico's on top and a pair of Celestion Vintage 10's in the bottom. It really gives a nice balance. The stock speakers in a lot of silverface amps were downright crap, and tend to contribute to the bad rap of the silverface amps. A reissue Jensen or a Weber should do the trick. If you do change the speaker in the amp, try to encourage your customer to spend a week or two playing it before doing any more mods. This is because the speaker will take some time at volume to break in. This is doubly true of Jensens and Webers. Both of them take significant playing time at volume to reach their final sound.
Another mod I have done with some success is to reduce the value of the power supply caps slightly. I like to go to values in the 16-20 uf range. This will make the amp a little more responsive, but will also increase the noise and hum and will allow the amp to be a little more compressed at high volumes. This is really a matter of personal preference. Some people like really big power supply caps to get that really stiff sound that usually has less breakup.
GOOD LUCK!
