Reverb Tanks Explained
Author: Kevin
Reverb Tank Code Decoder
Learn how to read the 7-digit part numbers on MOD & Accutronics spring reverb tanks to find the exact replacement for your amplifier.
Reverb Tank Sizes & Springs
Type 4 & Type 9 (Large Tanks)
Standard full-sized reverb tanks found in classic combo amps.
L: 425mm W: 108mm H: 38mm
- Type 4: 4 counter-wound & coupled springs (Classic vintage sound).
- Type 9: 6 springs arranged in parallel (Luxurious, deep reverb for surf or pedal steel).
Type 8 (Small Tanks)
Compact reverb tanks designed for tight amplifier cabinets.
L: 235mm W: 86mm H: 38mm
- Type 8: 3 springs engineered for tight spaces without sacrificing rich, authentic spring reverb tone.
Note on Dimensions: Dimensions assume the tank is placed flat on a table. Actual mounting orientation varies depending on amp layout.
Understanding the 7-Digit Part Number
Digit 1
8
Type
Digit 2
A
Input Imp.
Digit 3
B
Output Imp.
Digit 4
2
Decay
Digit 5
A
Connectors
Digit 6
1
Locking
Digit 7
B
Mounting
1st Digit: Reverb Type
- 1 / 4: Large Tank (4 Springs)
- 8: Small Tank (3 Springs)
- 9: Large Tank (6 Springs)
2nd Digit: Input Impedance
- A: 8 Ω – 10 Ω
- B: 105 Ω – 190 Ω
- C: 200 Ω – 240 Ω
- D: 250 Ω – 310 Ω
- E: 600 Ω – 800 Ω
- F: 1,475 Ω – 1,925 Ω
3rd Digit: Output Impedance
- A: 500 Ω – 600 Ω
- B: 2,250 Ω – 2,575 Ω
- C: 10,000 Ω – 12,000 Ω
4th Digit: Decay Time
- 1: Short (1.2 to 2 sec)
- 2: Medium (1.75 to 3 sec)
- 3: Long (2.75 to 4 sec)
5th Digit: Connector Grounding
- A: Input Grounded / Output Grounded
- B: Input Grounded / Output Insulated
- C: Input Insulated / Output Grounded
- D: Input Insulated / Output Insulated
- E: No Outer Channel
6th & 7th Digits: Lock & Mounting
- 6th (Lock): 1 = No Lock
- 7th A: Horizontal (Open Side Up)
- 7th B: Horizontal (Open Side Down)
- 7th C/D: Vertical (Connectors Up / Down)
- 7th E/F: On End (Input Up / Output Up)
Example Code Breakdown: 8AB2A1B
| Digit Pos. | Code Value | Description & Specs |
|---|---|---|
| Digit 1 | 8 | Small Tank Type (3 Springs, Compact) |
| Digit 2 | A | 8 Ω – 10 Ω Input Impedance |
| Digit 3 | B | 2,250 Ω – 2,575 Ω Output Impedance |
| Digit 4 | 2 | Medium Decay (1.75 to 3 Seconds) |
| Digit 5 | A | Input Grounded / Output Grounded Connectors |
| Digit 6 | 1 | No Locking Device |
| Digit 7 | B | Horizontal Mounting (Open Side Down) |
Can't Find an Exact Match?
If your tank is missing a number or a direct match is unavailable, use this compatibility priority guide when selecting a replacement:
⚠️ Critical Matching Required
- Digit 2 (Input Impedance): Must match the circuit drive specs.
- Digit 3 (Output Impedance): Must match the recovery circuit specs.
- Digit 5 (Connectors): Grounding scheme must match to avoid hum/ground loops.
✓ Flexible / Optional Matching
- Digit 1 (Tank Size): Flexible if physical space permits.
- Digit 4 (Decay Time): Personal choice (Short vs Long delay).
- Digit 6 (Locking): Not required for standard amps.
- Digit 7 (Mounting): Ideal to match, but often ignored in practice.
