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. 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)

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 , 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.

b. Reverb Recovery Path

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 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.

c. Dry Signal Path

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!

C11 is a non-polarized, metalized polyester film capacitor, which is located across the high voltage transformer

secondary, suppresses EMI frequencies from 20kHz to 30mHz. This is necessary to conform to modern day NEMKO

safety requirements.

The pot mounting brackets are all connected together via traces on the circuit board. The trace connects to the star

audio ground through R22, R22 places 15 ohms of resistance between audio ground and the pot mounting brackets.

This is done for two reasons. Fist, with the circuit board installed, the pot brackets are physically connected to

chassis/earth ground. R22 isolates audio ground from earth ground through the pot brackets. This eliminates internal

ground loops. Secondly, when the circuit board is removed for repair and testing, R22 provides a ground reference

(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

SG101