• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Junk Pile Linestage

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jaymanaa

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Joined 2006
#1
Hey everyone, Thought I might start a thread on a little project I'm doing to try to learn about some of the pitfalls of preamp construction. When finished, I hope it will give me a place to try some different stuff. The PS is on a seperate chassis and consists of a CT TX, 6CA4, 10hy choke/25uf motor run cap, 5hychoke/motor run. It's putting out 211v into a 90ma test load. The filament supply is an old CDP PS which after a couple resistor changes is putting out ~ 25v of heavily filtered DC. I plan on running the 12B4 filaments in series, at least for now. The amplifier section will be a 12B4 per channel, with a 5k plate resistor. I'm thinking 100ohm resistor and 1000uf cap on the cathode? I have an assortment of output caps to try ranging from 3uf on up. Anyone ever use tantalium caps or motor run oilers for coupling? Anyhow, I hope to be able to design something someday, but for now I just have a lot of fun "slingin solder". If this gets to sounding like something I want to build for keeps, an RIAA stage will be the next project. Any comments, suggestions, or general poking fun is allways welcome and thanks in advance, Jay
 
#6
Very good reading there Geek, thanks for the link. I checked out your sites as well............very cool. Most of the new motor runs that I'm finding on the cheap at AC supply houses (2 or 3 bux) are made in China. I think I'll cut one open and see what they are made of. Jay
 
#7
Just thought I'd post a follow up. I finished this thing today, and am quite impressed with the 12B4A. I used some Japanese IECs which are apparently a good variety (I've heard you have to find good 12B4s). The sound is very good from top to bottom after a couple hours "burn in". I ended up with 220v B+, which gave me ~60v on the plates via a 4.7K resistor. I had a bit of trouble on one channel at first (High pitched squeal) but after changing the grid stoppers to 500 ohm, and replacing a suspect interconnect, all is well. I've allways been an octal fan, but this little 9pin has made a beleiver out of me. It has plenty of gain to make my 2A3s happy. I think I will build it for real, and add a RIAA circuit. I've never done an RIAA before, does anyone have a favorite? Oh yeh, I ended up using a .50uf output cap, and 1000uf cap with 100ohm R on the cathode. Jay
 
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