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Reverb success


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11/24/1999 2:14 AM
Mike B
Reverb success
I finally got around to adding reverb to one of my Spitfire clones. I used the standard Fender tank and transformer, drove the tank with a 12AT7, and used half a 12AX7 for recovery. I mixed the reverb signal back in at the unused phase inverter input. Sounds pretty good, I must say...  
 
Of course I can't resist the temptation to 'improve' the circuit. I played around and reduced the circuit to one 12AT7 - half for reverb drive and the other half for recovery. The drive half is running at about 1W dissipation so I'm no where near the 2.5W max for the AT7. It still sounds good to my ears... Anyone else fooled around with minimalist reverbs?  
 
Mike B
 
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11/24/1999 4:20 PM
Doc

Thanks for the report on adding reverb to this simple circuit. It makes for a more versatile amp.  
 
I understand how you returned the reverb signal to be mixed in the phase inverter (like some Vox & Matchless products did), but was wondering how you picked off the dry signal and sent it to the fender style reverb driver. Did you use any voltage divider resistors, or a drive control pot, or just come off the main signal path with a 500pf coupling capacitor?  
 
I'm fairly certain that earlier Mesa Boogie amps used a single section 12AX7 for driving the reverb transformer. Some of the Crate vintage club stuff used 12AX7. It seemed to me that the reverb driver in the boogies had higher voltage gain, but the general sound was thinner, like the 12AX7 didn't have current drive capability if you know what I mean.  
 
Most reverb circuits take their input drive signal somewhere after the volume control (I know Fender does). This has the effect of driving the reverb harder as the volume control is turned up, whether you like it or not, sometimes requiring the Reverb level knob to be turned down for a more pleasing overall sound. I was contemplating tapping off for the reverb drive after the last fixed gain preamp stage, before any gain pots, in order to drive the reverb at a constant proportion to the guitar signal. For a home brew amp, a voltage divider pot (Drive control) could be added between at the driver input grid, allowing for greater overall reverb tuning, such as with the 6G15 stand-alone reverb box. The Reverb (return signal level) knob could be set for a desired mix, and be left alone for a while. Has anybody experimented with this method in their amp?  
 
Doc
 
11/24/1999 4:51 PM
J Fletcher
I've done something similar to what you describe. I took the rev drive signal after the 1st preamp stage, with the dry signal going through a 100k or so resistor to the vol pot, fed it ( the rev signal) to a 1 meg pot ( dwell) , with the wiper going to a 12AT7 rev driver tube. The return signal went to a 12AX7 recovery stage , then the rev level pot, then to another 12AX7 gain stage, and then to the volume control, where it was mixed with the dry signal, via a mix resistor, about 470k I think. This would be in a 5F6 Bassman type circuit. Still uses 2 tubes though....Jerry
 
11/24/1999 8:47 PM
Mike B

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
 
11/24/1999 4:56 PM
Dave Stork

Mike wrote:  
Of course I can't resist the temptation to 'improve' the circuit. I played around and reduced the circuit to one 12AT7 - half for reverb drive and the other half for recovery. The drive half is running at about 1W dissipation so I'm no where near the 2.5W max for the AT7. It still sounds good to my ears... Anyone else fooled around with minimalist reverbs?  
 
I've done similar things... using one half of an AT7 as a driver, and the other half as a recovery amp or mixer. With careful selection of the operating points of the driver, it works just fine and gives plenty of reverb.  
 
I first attempted this when facing the challenge of installing an FX loop in a friend's silverface Deluxe without adding tubes or disabling the tremolo. I "stole" one half of the reverb driver tube to act as the return mixer in the loop. After adjusting the Q point of the reverb driver, the reverb sounded just as good as it had before.  
 
It goes without saying (but I'll say it anyway) that when using a single dual triode for both reverb drive and recovery, careful layout is essential to avoid oscillation in the reverb circuit.
 
11/24/1999 8:53 PM
Mike B

Dave,  
 
For the 1/2 12AT7 driver, how did you vary the Q-point from the original Fender design. The Deluxe Reverb used a paralled 12AT7 with Rk=2.2K and Ck = 22uF, if I'm not mistaken. What did you change these values to?  
 
Mike
 
11/25/1999 4:54 PM
Daver

Did you have to change the reverb drive transformer when using half a 12A*7? Does the primary impedance need to change when using half a tube or doesn't having an "ideal" match matter for the reverb?  
Daver
 

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