Low frequency oscillation(?) when MV turned up Weber/Orange build

[waldner]

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

[sluckey]

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?

[PRR]

> 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?

[sluckey]

...

[waldner]

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? 

[sluckey]

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.

[waldner]

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!     


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?

[sluckey]

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...

[waldner]

Makes sense. Thanks Steve!