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Dumb-Bell Preamp Clone almost finished

4/25/2000 7:49 AM
Dominik
Dumb-Bell Preamp Clone almost finished
Hi there,  
the last two weeks saw me assembling my newest project: a preamp with two seperate channels, one modeled after the Dumb-Bell Skyliner and the other after the 70s Dumb-Bell OD. I built the Dumb_bell-ator as well. I used a 19", 2 HE chassis , and had somebody make me an L-shaped brass thingy that the ceramic tube sockets are bolted on. I hopefully will be able to take some pics and post them on my homepage soon. I used a 80 mA tranny with 250 - 0 - 250 secondary and 6,3 - 0 V 2,7 A. I used two 1N4007 to rectify the B+ and three (two in series, one to ground) for the filament DC supply. I wanted to copy the DC-filament supply as posted by Randall Aiken, but my tranny did not have 2 3.15 V taps (with CT), but only one 6,3 V tap and 0 tap. I still used the 1000 uF cap to smooth the DC, but I guess it still has some ripple (don´t have a scope unfortunately). This DC supply also feeds the three relays (6 V). As in Mr Aiken´s plan, I tried to seperate the ground of the 6,3 V DC supply from the rest of the grounds (lifted ground so to say) but found that that leads to a bad hum. No wonder actually. I then assembled the Dumb-Bell-ator and tested it. Next were the clean sections of both channels. They sound really nice, and I have to admit, that the tone stack surely is very active in this design. The OD section came next, but after installing the channel switching and putting every thing together I witnessed a loud and nasty hum in the OD sections and some serious "bleeding" between channels. The power supply is pretty much standard with diodes- cap (33 uF) to ground - 1,25 kOhms (4 W)- 4,5 kOhms (1 W)-cap to ground (22 uF) - 4,25 kOhms (1 Watt) cap to ground (22uF) - 1,5 kOhms - cap to ground ( 22uF) and 1,0 kOHMs - cap to ground (22 uF). I witnessed bleeding into the Dumb-Bell-ator even so all signal wires of the two channels were unsoldered. I could still hear the OD signal. Channel II (70s OD) is very close to the Dumb-Bell-ator parts, so I changed the layout a bit. I have yet to see wether or not that did anything good. BTW I ran all the anode wires (wires to anodes) as one bundle back to the pS. I changed that as well, could that have been the problem?? BTW the schematics state a 100 log pot for the Drive control in the 70s OD section. I found that this is incorrect, since there is almost no effect on the content only in the very beginning of the sweeping of the wiper.  
 
That is all for now, I hope I did not bore yáll to death,  
Regards,  
Dominik

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