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previous: Doc Thanks for the report on adding rev... -- 11/24/1999 4:20 PM view thread

Re: Reverb success

11/24/1999 8:47 PM
Mike B
Re: Reverb success
Doc,  
 
I picked off the signal for the reverb drive directly from the wiper of the volume pot. The wiper connects directly to the 12AT7 grid. The traditional method uses a 500pf cap and 1Meg resistor to form a high pass filter at the input of the driver. Unfortunately, this method adds some loading to the output of the preamp. In a multi-stage preamp where you have tons of gain on tap, this is not a problem. In the spitfire circuit there is a limited amount of preamp gain and I did not want to affect the tone of the amp in any way. I know that I'm probably being obsessive, but Spice modeling and several hours of listening confirmed that this was the way to go.  
 
One thing - since the high pass at the input has been eliminated, I still had to find a way to roll off the lows before they hit the tank. Otherwise the reverb tends to sound muddy. I did this by selecting a Ck/Rk combination for the driver that provided some low-end roll off. Of course, as Randall Aiken pointed out in his excellent article on triode gain calculations, this method only provides 6dB of cut. This seems to be enough for my purposes.  
 
So, the driver consists of a 12AT7 with the grid connected directly to the volume pot wiper, plate connected to reverb tranny primary to B+ (about 300V), and 1.5K paralled with 1uF in the cathode circuit. I fooled around with different values of Rk, dropping down as low as 750 ohms. I couldn't hear much of a difference. I'm going to play with Ck a little more but I'm basically satisfied with the current configuration.  
 
In a multi-stage preamp, you could steal some signal prior to the volume control and set the reverb up with a dwell control, just as you suggest. The circuit that I'm experimenting with does suffer from reverb level variations with different volume settings. That doesn't bother me too much, so I'll probably leave it alone, for now....  
 
Hope this helps,  
 
Mike