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Size: 4263
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Size: 4515
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| Deletions are marked like this. | Additions are marked like this. |
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| 5. Mods | 5. Honorable Mention (Vibro-King, Dual Professional Reverb) 6. Mods |
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| 3. Theory of Operation (it's based on the Fender service manual A block diagram is probably a good idea) | 3. Theory of Operation (it's based on the Fender service manual A block diagram is probably a good idea. This is pretty detailed and would put most people to sleep, but the techs need to know this) |
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| The first stage uses one half of the 12AT7 and provides a voltage gain A voltage divider reduce the signal and then it is sent to the Dwell , which determines the amount of signal sent to the reverb drive circuit. | The first stage uses one half of the 12AT7 and provides a voltage gain A voltage divider reduce the signal and then it is sent to the Dwell control (250K pot), which determines the amount of signal sent to the reverb drive circuit. |
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| The 6K6GT power tube supplies power necessary to drive the reverb transformer. | The 6K6GT power tube supplies power necessary to drive the reverb transformer. Although a 6V6 driver tube is used in the 90's reissue, the power tube biasing resistor is the same value used by the 6K6 circuit. This larger value causes the 6V6 to be overbiased (cold), limiting output power and causing early distortion. |
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| The footswitch turns the reverb on and off by grounding the input to the reverb recovery circuit. The senses the reverb return signal and amplifies it with a gain of about 37. | The footswitch turns the reverb on and off by grounding the input to the reverb recovery circuit. One half of the 12AX7 tube amplifies the reverb return signal. |
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| The Tone control (R17) attenuates the high frequencies above 3kHz, through C9. C10 provides a slight bass roll off which is affected by the position of the Mix control in relation to the input impedance of the guitar amplifier. |
The Tone control attenuates the high frequencies ( I can run a PSpice simulation to illustrate the effect of the tone control settings) |
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| Cathode follower, unity gain buffer amplifier for the dry signal. The input to the dry signal path is directly in parallel with the reverb drive path! | 1/2 of the 12AX7 is used as a cathode follower buffer amplifier for the dry signal. The input to the dry signal path is directly in parallel with the reverb drive path! |
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| C11 is a non-polarized, metalized polyester film capacitor, which is located across the high voltage transformer | d. Mixer Circuit |
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| secondary, suppresses EMI frequencies from 20kHz to 30mHz. This is necessary to conform to modern day NEMKO | The Mixer circuit is a simple resistive mixer consisting of a 50K linear taper pot. One side of the pot is fed by the dry signal path and the other is fed by the reverb recovery circuit. The control is essentially a pan pot that favors the dry signal at 0 and the wet signal when set to 10. This is in contrast to the Reverb control used in the combo amps, which simply controls the amount of reverb recovery signal mixed into the dry signal path. |
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| safety requirements. | As the Mixer control is turned up from zero, additional resistance is placed in series with the output of the dry signal path (the cathode follower). This additional resistance forms a low-pass RC filter with the capacitance of the cable used to connect the reverb unit to the amplifier. The resultant filter removes high frequencies from the dry signal, and effect known as 'tone suck'. (I can run PSpice simulations to illustrate the amount of high-frequency loss in the dry signal caused by the Mixer control settings) |
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| The pot mounting brackets are all connected together via traces on the circuit board. The trace connects to the star | e. Output Buffer Circuit (70's reissue only) |
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| audio ground through R22, R22 places 15 ohms of resistance between audio ground and the pot mounting brackets. | A 4th preamp tube is added. A cathode follower is used to isolate the Mixer output from the guitar cable/amplifier load imedance. The goal is to prevent tonal variations (tone suck) as the Mixer control is turned up. The buffer prevents the patch cable capacitance from interacting with the Mixer control resistance, mitigating the RC filtering effect. |
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| This is done for two reasons. Fist, with the circuit board installed, the pot brackets are physically connected to | 4. Others |
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| chassis/earth ground. R22 isolates audio ground from earth ground through the pot brackets. This eliminates internal | TBD |
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| ground loops. Secondly, when the circuit board is removed for repair and testing, R22 provides a ground reference | 5. Honorable Mention |
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| (15 ohms) for the pot brackets. R23 places 15 ohms of resistance between audio ground and chassis/earth ground. This minimizes hum by eliminating ground loops internally and externally when connection with other units. R22 and R23 are Flame Proof/Fusible resistors. If excessive current flows through theses resistors, they will not burn, they will simply open. CR5 & 6 (across R23) provide an important safety feature. IF the guitar amp chassis becomes electrified, current will flow through the coax cable to the power supply ground of the Fender Reverb unit. The current will seek earth ground through R23. When R23 opens, the earth ground connection is broken. This will electrify the reverb unit’s ground and |
The Fender amps that have a 3-knob reverb circuit |
Fender 6G15 Reverb Unit
Contents 1. General description 2. Variants (original, 70's reissue, 90's reissue) 3. Theory of operation 4. Other (Kendrick, Texotica, G-Spring, etc.) 5. Honorable Mention (Vibro-King, Dual Professional Reverb) 6. Mods
1. General Description
The Fender Reverb Unit (6G15) was a tube, spring reverb-equipped effects unit made by Fender. The Reverb Unit was originally introduced in 1961. It was discontinued in 1966 and was replaced by a solid-state model, the FR1000.[[http://en.wikipedia.org/wiki/Fender_Reverb_Unit#cite_note-1|[1]]] The unit features three controls: Dwell, Mix and Tone and is run by two pre-amp tubes and a power tube. Physically the unit looks like a small ampilfier head and since the early '60s the unit has become synonymous with surf music.
OK, so I stole this from Wikipedia. Sue me.
2. Variants
1961-1966 - Handwired, 6K6 powertube, Hammond reverb pan (61-64)Gibbs reverb pan(64-66), appeared in every cosmetic variation in that era.
1976-1978 - Handwired, 6V6 power tube, Accutronics reverb pan, silverface cosmetics. Super rare, I think somebody here owns one. I've never seen one is person. I think they are just the same as 61-66...
1994- Present Reissue - PCB, 6V6 powertube, Accutronics pan, Blonde/oxblood, brown/wheat, black/silver, tweed.
3. Theory of Operation (it's based on the Fender service manual A block diagram is probably a good idea. This is pretty detailed and would put most people to sleep, but the techs need to know this)
a. Reverb Drive Path
The first stage uses one half of the 12AT7 and provides a voltage gain A voltage divider reduce the signal and then it is sent to the Dwell control (250K pot), which determines the amount of signal sent to the reverb drive circuit.
The reverb drive circuit consists of the second half of the 12AT7 preamp tube, a high-pass filter, a 6K6 power tube, and the reverb transformer.
The signal is amplified by the 12AT7 and sent through an RC high-pass filter which rolls off low frequencies below 300Hz.
The 6K6GT power tube supplies power necessary to drive the reverb transformer. Although a 6V6 driver tube is used in the 90's reissue, the power tube biasing resistor is the same value used by the 6K6 circuit. This larger value causes the 6V6 to be overbiased (cold), limiting output power and causing early distortion.
b. Reverb Recovery Path
The footswitch turns the reverb on and off by grounding the input to the reverb recovery circuit. One half of the 12AX7 tube amplifies the reverb return signal.
The Tone control attenuates the high frequencies ( I can run a PSpice simulation to illustrate the effect of the tone control settings)
c. Dry Signal Path
1/2 of the 12AX7 is used as a cathode follower buffer amplifier for the dry signal. The input to the dry signal path is directly in parallel with the reverb drive path!
d. Mixer Circuit
The Mixer circuit is a simple resistive mixer consisting of a 50K linear taper pot. One side of the pot is fed by the dry signal path and the other is fed by the reverb recovery circuit. The control is essentially a pan pot that favors the dry signal at 0 and the wet signal when set to 10. This is in contrast to the Reverb control used in the combo amps, which simply controls the amount of reverb recovery signal mixed into the dry signal path.
As the Mixer control is turned up from zero, additional resistance is placed in series with the output of the dry signal path (the cathode follower). This additional resistance forms a low-pass RC filter with the capacitance of the cable used to connect the reverb unit to the amplifier. The resultant filter removes high frequencies from the dry signal, and effect known as 'tone suck'. (I can run PSpice simulations to illustrate the amount of high-frequency loss in the dry signal caused by the Mixer control settings)
e. Output Buffer Circuit (70's reissue only)
A 4th preamp tube is added. A cathode follower is used to isolate the Mixer output from the guitar cable/amplifier load imedance. The goal is to prevent tonal variations (tone suck) as the Mixer control is turned up. The buffer prevents the patch cable capacitance from interacting with the Mixer control resistance, mitigating the RC filtering effect.
4. Others
TBD
5. Honorable Mention
The Fender amps that have a 3-knob reverb circuit
