Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: WillieBoy on November 26, 2010, 02:13:19 pm
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On a DRRI, you can add reverb/trem to the normal channel by moving a wire from the plate resistor (pin 6 V1B) to the node right after the coupling cap on the plate resistor of V2B.
Looking at Hoffman AB763, it would be connecting a wire from the other side of the .047uF cap on the plate of V1A to the node between the .022uF coupling cap and the 10pF/3.3 meg resistor located between Pin 6 of V2B and Pin 2 of V3. You also would lift the wire coming off of the .047uF (Pin 6 V1B) cap going to the 220K resistor on Pin 2 of V6.
Is this the correct approach? I have read that the 220K resistors have somehow got to move when doing this mod?
I'm sorry I keep asking questions. Thanks in advance for not losing patience with me folks!
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This works:
(http://i222.photobucket.com/albums/dd298/Chipster457/Super%20Reverb/UberVerbSchematic-Preamp.gif)
No need for 220K mixing resistors. They will create voltage dividers with unintended consequences. Trust me - I went down that road.
It is important that you keep the 220K resistor shown right before going to the PI.
Cheers,
Chip
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Is this the best way to go to add reverb to the normal channel and put the normal and vib channels in phase?
Also what are the mods to the tone stack doing?
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Is this the best way to go to add reverb to the normal channel and put the normal and vib channels in phase?
Also what are the mods to the tone stack doing?
Best way? You can just tie the two plates together (V1-B and V2-B). That's the "classic" way of doing it, but if you look closely you'll notice that I used different value coupling caps after each of those triodes. The Normal channel uses a .1 uf coupling cap (stock .047uf), adding a bit of bass response. Also, I have separate power rail nodes for each channel. The coupling caps block DC voltage and keep the two power rail nodes isolated. Otherwise they'd be connected through the two plate resistors. FWIW I figured that last part out AFTER I built the amp.
The Normal channel tone stack uses an unusual pot with a tap inserted part way, like the Brown Vibroverb and some other transition amps between classic Tweed and classic Blackface. Doug was nice enough to add those 350K pots to his inventory at my request, so PLEASE buy some and experiment with them! The 82K slope resistor just was what sounded best to me after experimenting. Same with the .033uf Mid cap and 9K Mid resistor (like Mids at 10 all the time - more mids and more gain).
The .047uf Mid cap in the Reverb channel actually is a "mistake" I only picked up on a few months ago. That channel was supposed to be dead stock and should be .022uf in a Super Reverb, raising the cutoff frequency of the Mid control. The owner loves the amp, so I'm not changing it.
Hope that helps.
Cheers,
Chip