Shop Talk: Silverface to Blackface Mod

July 10, 2008

It’s not difficult to convert a silverface Super Reverb to blackface spec, and the end result will usually sound as good as or even better than its blackface counterpart. That said, do not attempt this modification unless you are certain that you know what you are doing. Silverface Supers are still very affordable, so it will be well worth paying a qualified amp tech to do this work if you’re in doubt about your electronics skills. If you decide to tackle it on your own, always be sure to unplug the amp and drain the power capacitors before getting started to avoid electrical shock. All resistors used should be carbon composition w watt, 5 percent tolerance or better.

Step 1: Rewire bias supply and add two 220kΩ resistors

  • Look at Fig. 1 and locate these parts in the chassis.
  1. Remove both 100kΩ resistors (brown, black, yellow) between points B and E, and F and G. 
  2. Add two 220kΩ resistors—one between points “F” and “G”, and another between points “E” and “G”. 
  3. Move the existing resistor and cap lead on point “A” to point “B”. 
  4. Move resistor on point “C” to “D” and move the wire on point “D” to “C”. Now look at Fig. 2. This is how your chassis should look now.

Step 2: Replace four resistors in the phase inverter

  • Look at Fig.3 and locate these parts inside the chassis.
  • Replace the 47kΩ resistor (yellow, violet, orange) going from point “H” to “I” with a 100kΩ.
  • Replace 47kΩ resistor going from “H” to “J” with an 82kΩ.
  • Replace the two 330kΩ resistors (orange, orange, orange) going from point “K” to “O” and “M” to “O” with two 1 Megž resistors if needed. Some silverfaces already have these. Look at Fig. 4. This is how your component board should look now.

Step 3: Remove the parasitic suppression caps

  • Look at pin 1 on each power tube and notice the 2000 pf capacitor. These are usually grounded on pin 8. Cut these two caps out and discard them.

Step 4: Change the 5U4 rectifier tube to a 5AR4/GZ34 and you are ready to check bias range

Plug the AC line cord in and with the amp power switch “on” and the standby switch in the “standby” mode, put a DC voltmeter between point “G” (see Fig. 4) and the chassis ground. Adjust the bias pot until the meter reads -48 to -54 volts. If you cannot adjust the bias to get in this range, then the 15kž resistor that is soldered from the bias pot to ground (point “D” on figure 2) may have to be replaced with a 27kΩ resistor. There are several ways to bias an amp, but a nominal setting using the above method will be about -52 volts. Now go out and enjoy the
sound of your blackface-converted Super!
Gerald Weber, Kendrick Amplifiers

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