Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
There's good news and bad news for people who want to convert their silverface amps. The good news is that if you have an early silverface (68 to 76), you'll be able to regain much of the Blackface tone. It won't be exactly like a blackface due to things that can't be changed like wiring composition and lead dress. But it will greatly improve the tone of your amp.
The bad news is that, (generally speaking), if your amp was made after 76, the blackface conversion will do little to help your amp. This is when Fender started using more powerful transformers to increase their amp's volumes. To even begin to change this type of amp you'd need to replace the transformers which really isn't worth the expense.
The Parasitic Supressor Caps:
1. Remove the .002uf (or sometimes 1200pf) capacitors from the grids of the power tubes (pin 5). This gives you back the high end sparkle of the blackface era amps.
The Phase Inverter Differences:
2. Change the .001 Coupling Cap increased from back to the blackface era value .01 cap. This "redefines" the bass response that became overbearing during the silverface era.
3. Change the 330k Cathode Resistors on the grids of the Phase Inverter back to 1 meg. This lowers gain to the blackface era spec.
4. Change the 47k Plate Load Resistors back to either 82k or 100k. This also lowers gain to the blackface era spec.
The Reverb Differences: (*Note - Many "Prefer" Silverface Era Reverb)
5. Change the Reverb Driver (1.5k, 820, or 680) ohm unbypassed resistors back to the blackface era spec 2.2k resistors bypassed with a 25uf-25v electrolytic cap.
6. Remove the Reverb Driver Plate to Cathode Caps.
7. Remove the Cap to ground after the Reverb Tank.
The Bias Supply Differences:
8. Change the two 70uf Bias Supply Filter Caps back to a single 100uf 100v. This gives you back the ability to set the bias level.