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previous: Gil Ayan Dominik, obviously you chose to do ... -- 4/25/2000 6:23 PM View Thread

Re: Dumb-Bell Preamp Clone almost finished

4/26/2000 10:41 AM
Re: Dumb-Bell Preamp Clone almost finished
Hi Gil and Andy,  
thanks for your replies. I should have been a bit more precise with my sayings:  
Indeed I chose a different layout. I drew two different versions and tried to realize the second one. I used two long terminal strip boards (facing each other) and the parts are solder across the strips:  
The tube sockets are mounted on a single brass raket in front of the terminal strips  
(CL1/2) (OD1/2) (Cl1/2) (OD1/2)(Dumbbellator)  
a b c d e f g h i j k  
front plate  
The tranny is located to the far right end side of the chassis. The powe supply is to the far right side in the middle (facing the amp).  
Maybe one reason for the bleeding of the channels is that I take the voltage for the FX-loop (350 V) after the first resistor of the PS. The OD1/2 of the first channel gets itīs voltage here as well, so no isolation between these two. Oddly enough, the other channel derives itīs voltage (306 V/ 303 V) further downstream (after three filter caps, each 22 uF and three resistors, like in a normal PS), but still the bleeding is there. As said the OD of channel one was very close to a plate cap of the FX-loop. I changed that. (Damn I wish I had some pics to show you). The clean channels sounded really nice upon first testing, altho I found that the tone-stack of the 70s generic needs some fine tuning yet. The thing about the drive pot:  
I swapped the 100 k log (new style, 10 %) with a 1 M lin pot I had at hand. I played around with the trimmers as you said, but found that the overall effect of the 100k log pot was not what I wanted.  
I think the 1 M pot will make the signal more compressed but that is not really a concern for me..  
I used one common ground for each tube, where the grid resistor and 1 M input resistor are connected to. This ground is very close to the tube (terminal strip). I then ran every local ground back to the ground of all the filter caps and from there one wire to the common ground of the AC-line bolted to the chassis.  
BTW: the fx-loop really fatens up the signal. One can even dial in some different tones by setting the return level and send level and using the shift switches (cap across pot).  
I still have to get the 6,3 V DC for the filaments right (maybe thats were some of the hum is coming from)and the OD channels and Clean channels need some fine tuning. If I would do the porject again, I would only build one channel and the Fx-loop.  
Thanks for any input,  

Gil Ayan Regarding your layout, yes, it coul... -- 4/26/2000 5:19 PM