Filter Capacitor Sets
Capacitor Kit Installation Guidelines
These guidelines are deisgned for technicians - We do not recommed customers without an electrical background complete any filter cap replacements - Read completely before starting work on your amplifier.
Tube amplifiers store lethal high voltage in the filter capacitors even when unplugged. Always disconnect the amplifier from commercial power and thoroughly discharge all filter capacitors before touching any internal circuitry. If you are unfamiliar with safe discharge procedures, refer your service to a qualified professional technician.
1. Component Differences
Physical Size: Modern manufacturing allows new capacitors to be 50%–75% smaller than original vintage parts.
Value Substitution Standard: Values have adjusted over time due to industry standard shifts:
| Original Value | Kit Replacement |
|---|---|
| 70µF | 80µF |
| 20µF / 25µF | 22µF (or 25µF) |
| 8µF | 10µF |
| 5µF | 4.7µF |
| Bias Caps | 100µF/100V (or 25µF/70V for early '60s) |
2. Polarity Rules
Electrolytic capacitors are polarized. Incorrect installation causes catastrophic failure.
- Original Caps: Marked with
+and-indicators. - Tube Amp Doctor (TAD) Caps: Marked with an arrow pointing toward the negative (-) end.
- CRITICAL Bias Capacitors: The positive (+) lead connects to Ground; the negative (-) lead connects to the circuitry.
- Doghouse Warning: Caps inside the capacitor housing are not always oriented the same way (e.g., dual 70µF caps are installed in opposite directions).
Always photograph orientation and mark polarity inside the amp (using a silver Sharpie or tape) before removing old caps.
3. Installation Step-by-Step
- Replace One at a Time: Work on a single capacitor start-to-finish before moving to the next. Never gut all capacitors at once.
- Clean the Eyelet: Snipping leads close to the eyelet. Clear the board hole using solder wick or a solder sucker. Leave underlying factory leads intact under the fiberboard.
- Fit Leads Properly: Cut new leads to leave ~½" extra length for insertion. Never twist new leads onto old clipped leads—this compromises reliability and destroys resale value.
- Secure Large Capacitors: Due to smaller modern sizes, place a bead of RTV silicone caulk or hot glue under large filter caps in the doghouse to prevent movement. Unsecured caps will crack their leads from vibration over time. (Small caps do not need glue).
- Swap Resistors (Do Not Skip): If your kit contains balancing/dropping resistors, replace them one-at-a-time matching color codes. Ignore the final tolerance stripe (gold/silver) when matching old to new. Some kits may have extra resistors which can be disguarded
Note: Complete the full installation kit before connecting power to the amplifier!
4. Special Model Guidelines
Later Fender® Silverface Amps (Post-1970):
- Locate the vibrato cathode bypass capacitor (changed from 25µF to 5µF/4.7µF). Replace this first in the cathode section to avoid placement mix-ups.
- Reference location: Paralleled across a 100K resistor, with one end tied to Pin 8 of the vibrato tube (5th 12AX7 on 6-preamp tube models).
- Some cathode bypass caps may be grounded directly to rear RCA jacks rather than the main fiberboard.
Early Fender® Blackface Amps:
- Early models used 3-leg dual 25µF caps (common ground, two
+terminals) in the preamp. - Replace each dual cap with two individual 22µF or 25µF capacitors. Connect both negative leads to the single original ground eyelet, and route the two positive leads to their respective circuitry points.
