Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lambertini on April 28, 2025, 06:51:31 am
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Hello,
Spring put me in the mood to improve the spring reverb of my Twin AB763 clone.
This reverb has always been too powerful, dispite having changed the 100K lin pot to log. It's usable from 2 to 3, intrusive from 4 to 6, crushing the dry signal from 8 to 10 - the wet signal is so loud that you can bearly hear the dry. I've inspected the circuit 10 times, without noticing any error.
I'm aiming to have the reverb pot usable from 1 to 10.
I've tried combinations of this 3 mods:
1. Swap V3 from 12AT7 to 12AU7.
2. Increase the reverb attenuation resistor value from 470K to 1M to reduce wet signal.
3. Decrease the reverb mix resistor value from 3.3K to 2.2M to increase dry signal.
All together is reducing too much, but either resistor change is fine and achieves the desired result, even with a 12AT7.
However I'm surprised to ear some AB763 amps keeping a proper dry signal with reverb on 10, surfy but audible, in many YouTube videos.
Why my beloved 99% AB763 stock circuit behave like this?
What could be the cause of this excessive wet signal? A bad reverb transformer sending too much signal? A bad tank? Something else?
Should I stop overthinking it, just change one of the resistor and leave the forum alone? ;)
Thank you!
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I think the overwhelming reverb is inherent to the AB763 design, so I don't think you are experiencing anything unusual. However, you might try a different reverb pan, because variations between them can have a big impact on how good it sounds - some pans are very washy and don't sound so good. And you might like one with a shorter decay.
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This reverb has always been too powerful, dispite having changed the 100K lin pot to log.
The diagram you included shows a 1MΩ resistor labeled "V3 Grid Leak." Replace that resistor with a 1MΩ pot for testing:
- Remove the 1MΩ resistor, and attach the 1MΩ pot so the "input lug" goes to the 500pF cap, and the "ground lug" goes to the ground eyelet.
- Attach the wire that goes to V3 Pin 7 to the wiper ("center lug") of the pot.
- Adjust for reduced drive to the Reverb tank.
Once you determine your ideal pot-setting, you can measure the resistance from wiper to either end, and install 2 fixed resistors from the 500pF to Ground to act as a control with a single setting.
Or skip all that & get a Medium decay tank (4AB2C1B) instead of the stock Long decay tank that gives too much reverb.
... However I'm surprised to ear some AB763 amps ... in many YouTube videos. ...
Careful: you have no idea how the sound you hear was developed, or if the audio has been doctored afterwards (or reverb added to the audio in a post-production tool).
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- Remove the 1MΩ resistor, and attach the 1MΩ pot so the "input lug" goes to the 500pF cap, and the "ground lug" goes to the ground eyelet.
- Attach the wire that goes to V3 Pin 7 to the wiper ("center lug") of the pot.
- Adjust for reduced drive to the Reverb tank.
Once you determine your ideal pot-setting, you can measure the resistance from wiper to either end, and install 2 fixed resistors from the 500pF to Ground to act as a control with a single setting.
I just received the 1MΩ pot. If I want to keep it as a permanent Dwell pot, located at the rear of the amp where you can find an humdigger pot in some silverface amps, I'll need to have a quite long wire to reach the 500pF cap. Should I wire it with simple wire or should I use shielded?
Would that layout be correct (mods in pinkish)?
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Should I wire it with simple wire or should I use shielded?
As a general rule, any signal wire going to a tube's control grid in the preamp should be shielded to reduce unwanted noise. Another way of achieving a similar result (with single stranded solid wire), is to keep the wire right laid right up against the chassis.
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In my AB763 single channel Deluxe Reverb, I didn't add a dwell pot but added a switch to go between 500k and 1M, and found the difference in the reverb to be hardly noticeable. So you may find with a 1M dwell pot that you end up turning it way down. I wonder if a 500k or even 250k pot would do better at producing a useful range for making the reverb sound better.
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... I didn't add a dwell pot but added a switch to go between 500k and 1M, and found the difference in the reverb to be hardly noticeable. ...
The change you describe does nothing to change the drive into the reverb tank.
Instead, replacing the 1MΩ with Input --> 500kΩ + 500kΩ --> Ground (with the 12AT7 grid fed from the junction of these resistors) would turn down the dwell "some."
Replacing the 1MΩ with Input --> 820kΩ + 100kΩ --> Ground would turn down the dwell very noticeably.
Yes, the 6G15 Reverb uses a 250kΩ linear pot, but using an audio taper pot would make the dwell reduction more apparent. Ultimately the pot-value (or total of the pair of resistors) just controls the bass roll-off of the feed into the reverb tank.
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In my AB763 single channel Deluxe Reverb, I didn't add a dwell pot but added a switch to go between 500k and 1M, and found the difference in the reverb to be hardly noticeable. So you may find with a 1M dwell pot that you end up turning it way down. I wonder if a 500k or even 250k pot would do better at producing a useful range for making the reverb sound better.
Your switch did little to affect the overall signal and actually shifted the high pass filter into the driver up significantly. You should have heard a difference in tonality, but not necessarily a difference in strength.
The pot suggested above creates a voltage divider which directly affects the amplitude of the signal into the driver tube while leaving the frequency response largely unaffected.
Do not change the pot value as this will shift the filter cutoff up a lot.
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Would that layout be correct (mods in pinkish)?
More like this:
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Thanks for the feedback on my attempt to tame the reverb in my AB763. I don't remember where I got the idea for the mod I did, but now I understand the circuit a bit better and how I can tweak it.