Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: waldner on January 22, 2024, 01:48:07 pm
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I've completed my Weber / Orange 6O100, kind of an OR120/Matamp build. It works and sounds good, like a raw vintage Orange type circuit should. The only issue I have is when I crank the MV up nearly all the way I get this low frequency feedback/oscillation.
I tried chopsticking the hell out of everything, especially the 4 wires going to and from the dual MV pot but nothing mattered. I was thinking maybe some or all of those wires need to be shielded? Any thoughts? I've attached a link to a video demonstrating the problem along with the schematic attached.
i=6bqOUcsiL-GgRvJx
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I would use shielded wire for all wires to the MV. What happens if you disconnect the NFB wire from the impedance selector or board?
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> when I crank the MV up nearly all the way I get this low frequency feedback/oscillation.
And what about the Presence and Boost knobs?
Have you tried reversing the OT Primary wires?
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Thanks guys. I'm just wrapping up work at 6pm and then will try all of the things mentioned. I didn't think to reverse OT primary wires because I didn't get any problem until way up on the MV and in previous experience amps would squeal like a banshee. Still, worth trying that and/or disconnect NFB and see what happens. I did try presence and boost knobs along with every other pot and none made any difference- just the MV. All good recommendations. Thanks!
Steve, please let me know what is dangerous with the bias circuit?
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That weber circuit will allow you to adjust the bias down to ZERO volts. Output tubes will die.
Not the first time I've harped on that circuit. I can't believe weber has not corrected that schematic.
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It turns out I did need to reverse the OT wires. I tried disconnecting the NFB and sure enough problem was gone. Then went and reversed OT wire and reconnected NFB, crossed fingers and sure enough- problem solved! :worthy1:
I also replaced the 4 wires to the MV with shielded cables too just in case.
Steve, can I fix that bias circuit without major surgery?
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Steve, can I fix that bias circuit without major surgery?[/size]
Yes. With the circuit as is, adjust R7 until the tubes are biased a bit hotter than you ever expect to run them. Switch the amp off and measure the resistance of the pot. Now get a fixed resistor (Rx) with that same measured value (or as close as you can get) and put it in series with R7 (between R7 and ground). See pic...
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Makes sense. Thanks Steve!