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Hey fellow techs,
The symptom is not just a constant oscillation but when you hit a note and them mute it, that's when you get a short spurt of the oscillation. Just sounds like a very short buzz. Easy to see on the scope. Almost doesn't occur when you drive the amp with an oscillator or consistent source.
I looked through all the Sunn Concert Bass or Lead entries for this symptom & did not find it, however I did find the schematic that a tech posted on this forum that included the parts list as well as a output stability mod that Sunn put out in '73. I was about to just bridge some disc caps across the B-C of the driver xistors when I stumbled on these mods. Thanx to whomever posted that. Can't find it now.
I guess even in '73 there must have been some higher frequency response T03 power transistors that had a higher freq response then the orig 2N3055's which is usually the cause of these type of issues.
I guess the point of this post is to enlighten some of the existence of the mod as well as the fact that it actually works. The mod had you also removing the insulating washers from the speaker jacks but on this version that actually shorts the output to ground, so just be aware there are different versions of this amp out there. I didn't swap the + and - of the speakers in order to be able to ground the jacks as that would have also changed the input to output phase relation which I didn't think would be wise in the long run.
They also suggested removing C305 in a 3 state pi filter in the power supply which I did not do.
glen
The symptom is not just a constant oscillation but when you hit a note and them mute it, that's when you get a short spurt of the oscillation. Just sounds like a very short buzz. Easy to see on the scope. Almost doesn't occur when you drive the amp with an oscillator or consistent source.
I looked through all the Sunn Concert Bass or Lead entries for this symptom & did not find it, however I did find the schematic that a tech posted on this forum that included the parts list as well as a output stability mod that Sunn put out in '73. I was about to just bridge some disc caps across the B-C of the driver xistors when I stumbled on these mods. Thanx to whomever posted that. Can't find it now.
I guess even in '73 there must have been some higher frequency response T03 power transistors that had a higher freq response then the orig 2N3055's which is usually the cause of these type of issues.
I guess the point of this post is to enlighten some of the existence of the mod as well as the fact that it actually works. The mod had you also removing the insulating washers from the speaker jacks but on this version that actually shorts the output to ground, so just be aware there are different versions of this amp out there. I didn't swap the + and - of the speakers in order to be able to ground the jacks as that would have also changed the input to output phase relation which I didn't think would be wise in the long run.
They also suggested removing C305 in a 3 state pi filter in the power supply which I did not do.
glen
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