You really need one less stage.
... what PRR says crossed my mind too. The re-amplification after the reverb recovery stage is present at any Fender BF reverb circuit though. There is somehow a lot of gain in this amp. ...
PRR 's route appears to be the way to go. Disconnecting thegain stage after the reverb recovery has cured both the excess reverb and the huge white noise problems. ...
Review the
AB763 Deluxe Reverb.
Your 150kΩ resistor between R and R2 performs the function of the AB763's 3.3MΩ. You have the Reverb level (100kΩ linear) and the 470kΩ mixing resistor, but you have added V3B to the reverb side of the ledger before the signal mixes with the Dry path again. The "reamplification gain stage after reverb recovery" that you noted amplifies the Dry signal, not just the Reverb signal.
The 3.3MΩ resistor in the Dry path and the 470kΩ resistor of the Reverb output are "Mixing Resistors" but also a Voltage Divider.
The AB763 also included a 220kΩ resistor to ground at the point the 3.3MΩ and 470kΩ join (further reduces signal level).
These 3 resistor (plus the Reverb pot) set the relative balance of Dry/Reverb. In the AB763, they also created the need for the gain stage
after this mixing network.
Assume a typical setting: Reverb control on "3."
- The reverb recovery stage before the Reverb pot looks like "38.5kΩ" output impedance.
- The Reverb pot looks like 70kΩ plus the recovery stage's 38.5kΩ from wiper to one end: 108.5kΩ
- The Reverb pot looks like "30kΩ to ground" from wiper to other end.
- These in parallel are 23.5kΩ to ground.
- The above is in-series with the 470kΩ resistor: 493.5kΩ
- The 220kΩ is a parallel path to ground for this 493.5kΩ: 152kΩ to ground
The Dry Signal sees a divider made of 3.3MΩ resistor and this "152kΩ resistor" composed of all the Reverb-path stuff:
152kΩ / (152kΩ + 3.3MΩ) = 0.044 = level reduction to 1/22.7 original size.
Now rinse & repeat for the Reverb signal:
- The Dry stage before the mixing network looks like "38.5kΩ" output impedance.
- The Dry stage's 3.3MΩ is in-series with that 38.5kΩ output impedance: 3.339MΩ
- The Reverb signal sees the 220kΩ resistor to ground as "in-parallel with the 3.3MΩ path": 206kΩ to ground
The Reverb Signal sees a divider made of the 470kΩ resistor and this "206kΩ resistor":
206kΩ / (206kΩ + 470kΩ) = 0.305 = level reduction to 1/3.3 original size.
- If the original AB763 provides Gain of 60 at the Mix stage between Reverb & phase inverter, that's only 60/22.7 = 2.6x for the Dry path. "Almost no extra gain."
- If the original AB763 provides Gain of 60 at the Mix stage between Reverb & phase inverter, that's 60/3.3 = 18x for the Dry path. "12AU7-worth of extra gain."
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Hopefully with this walk-through, you've noticed your circuit's V3B should really go between "R2" and the phase inverter.
- As drawn, it adds a gain of 60x to the Reverb path that doesn't exist in the AB763.
- The 470kΩ "mixing resistor" doesn't interact as-intended with the 150kΩ between "R" and "R2"
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Owning a 1962 brown Deluxe, I would not recommend trying to add reverb to it. The 6V6s will be the first thing to distort, so your reverb will get distorted. And the existing circuit already has lots of gain, not much loss. I'm not surprised you found a bunch of extra noise with your original setup.
Adding reverb to this amp is best done with something like a Fryette Power Station, where the "effects loop" is between the amp & speaker.
Topic: 6G3 ish build with reverb.. too much of it (Read 3075 times)