Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tiny_Daddy on January 01, 2012, 06:36:04 pm
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Someone brought a Sovtek Mig 100 and a box of 6AV5 and asked for cathode bias. I used 390 ohm / 47uF cathode resistors on each tube and connected the plate voltage to the 270V with 120V zeners following a 1.2K 10W resistor and 22uF filter cap dropping the screen voltage to 150V. After getting a bad oscillation I added 100 ohm resistors in series with each plate. Tubes are various and run 10-14 watts with no input. Measures 20 watts out before clipping.
Fortunately the power supply with bridge and center-tap allows this mod, and the double-pole standby switch disconnects both AC leads to the bridge.
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I've used 12AV5's, and had no problems with unwanted oscillation. I did use grid stoppers and screen stoppers as a routine precaution, but did not require plate stoppers for stability. The power supply was a voltage doubler, supplying 310VDC B+ and 155VDC for the screens, and the amp had fixed bias.
(http://i497.photobucket.com/albums/rr333/valvetone/TeeVee%2015/front.jpg)
The construction details are here: http://www.guitargear.net.au/discussion/index.php?topic=29864.0 (http://www.guitargear.net.au/discussion/index.php?topic=29864.0)
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The amp works fine with 2 tubes. The oscillation was with 4 output tubes.
NICE LOOKING AMP.
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The amp works fine with 2 tubes. The oscillation was with 4 output tubes.
NICE LOOKING AMP.
air gap inductor: some 20AWG enamel wire wound on a pencil form; about 20-25 turns then connected from plate to plate would help tame oscillation as well.
agreed - very nice looking amp.
--DL
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The oscillation was with 4 output tubes.
Sorry, I should have taken more notice of "Mig 100". (http://serve.mysmiley.net/ashamed/ashamed0001.gif)