Parasitic oscillation problem untraceable

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geoff 1965
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Re: Parasitic oscillation problem untraceable

Post by geoff 1965 »

what's the procedure for the outer foil test? bear in mind some of us are neanderthal with analogue scopes!
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Sun 01/17/21 5:11 am
Did the testing. Great feature!!! It's really obvious.. and you were right about those striped vishays.. some are correct, while others are not.
Yeah, that stripe is basically just a label design now, and has nothing to do with the foils. But at least it's fun to test. Did you try doing it with some measurements on the screen?
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

geoff 1965 wrote:
Sun 01/17/21 7:43 am
what's the procedure for the outer foil test? bear in mind some of us are neanderthal with analogue scopes!
Read Aiken's post about it: https://www.aikenamps.com/index.php/whe ... capacitors

If you're using a digital scope, having measurements on screen can help when the difference isn't obvious.
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Bieworm
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Re: Parasitic oscillation problem untraceable

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JMPGuitars wrote:
Sun 01/17/21 8:44 am
Bieworm wrote:
Sun 01/17/21 5:11 am
Did the testing. Great feature!!! It's really obvious.. and you were right about those striped vishays.. some are correct, while others are not.
Yeah, that stripe is basically just a label design now, and has nothing to do with the foils. But at least it's fun to test. Did you try doing it with some measurements on the screen?
Ofcourse. I should have made screen shots.. but it was really obvious which side was the outside foil. So I marked all caps in my stash. The bigger caps were somewhat difficult, but the small ones were really obvious
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Bieworm
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Re: Parasitic oscillation problem untraceable

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Here I am again with some questions...sorry but I'd like the opinions of the good ones 😉

After converting the (no longer) imperial to the kinda british preamp ...or how should I call this?.. I found out the OD was not enough. So I replaced the voltage divider 470k-250k pot-47k to ground with a 1M pot to 47k to ground.. in order to release more signal to the grid of the 2nd gain stage. While that works just great I noticed that the clean channel looses clarity when the gain knob on the OD channel is set too low. I figure that part of the signal escapes to ground via the 1M pot for it probably chooses the least resistent path? I schematically added a 220k resistor between the plate and the coupling cap between stage 1 and 2 of thr OD channel. Is that something I shoild try? Or are there better options? What say yee?
Schematic attached...
schematic - problem interaction CH1 and 2.pdf
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Bieworm
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Re: Parasitic oscillation problem untraceable

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Ok I took a shot at it.. but started with 180k. Took too much signal from the OD channel, but at least the clean channel maintained its clarity. Sot I tried some more values and settled for 100k. Now it's a rockin'bird!!!
I might call it that way.. because there is not much left of the imperial at this stage of tweaking and altering. Thanks to y'all there is now a seriously good amp added to my collection!!!

Listen.. clean and moderate drive demo:

https://www.dropbox.com/s/6sz9lv2vi1ylu ... b.m4a?dl=0
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Bieworm
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Re: Parasitic oscillation problem untraceable

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Playing the amp makes it grow on me more and more. The OD is really nice since the last tweaks. Really raunchy and fattt. A cross-breed of a 5E3 and an 18W. Less tight but a real blooming buzzy mush in a very nice way. A nice addition to my collection..
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TriodeLuvr
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Excellent, love the sound! Congrats!
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

TriodeLuvr wrote:
Wed 02/03/21 2:14 am
Excellent, love the sound! Congrats!
I'll make a recording with more OD. This was only half of it..
It only gets fatter and more saturated
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