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Hey, I recently converted a Bogen K15 into what I thought would be a fairly nice guitar amp. I redid the preamp but the PI was a paraphase type, I'm not too familiar with them and there's not a lot of info out there about them so I just kind of left it the ay it was.
Anyway, it didn't turn out as good as I thought, it sounds great clean but when you play "on the edge" of breakup, you can hear some nasty distortion mixed in. I ran through it with my scope and I saw that the source of the nastiness was indeed the phase splitter stage. There are two preamp stages before it, after the first one there's no breakup at all (as expected) and after the second there's a little smooth, asymmetrical clipping, also as expected. However, when I checked the signal coming from the "first" half of the PI, I noticed that it would reach a certain pont and clip hard, very SS like, and only one side of the wave. This signal goes to one output tube and also the grid of the other half of the PI. When it gets here it "flips" it (as it should) and does the same thing to it, so now both sides of the wave are clipping hard, and this goes to the other output tube. This, as you can imagine, sounds like a mess.
This is the original K15 schematic by the way. Don't bother looking at the preamp, just the two halves of V3 onward.
http://www.flickerdown.com/forsale/k-15.pdf
So waht I want to know is, is this "SS clipping" inherent with this style of phase splitter? I tried a few different tubes (a couple Sylvanias and a Mullard) and they all did the same thing. Can I tweak some values to get it to sound better or am I going to have to switch to a LTP design? I really don't want to do that because this amp is a horribly laid-out mess inside (not my fault, it was a mess from the factory) and I'd rather not go to that extent with this amp unless it's 100% neccesary. I suppose there's a reason why everyone stopped using this design in the 50's though
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Edit: Forgot to mention that the original components are still in the PI stage. What exactly constitutes a "leaky" cap? I checked the coupling caps and they were only letting a few mV get through, is that enough to cause a problem?
Thanks in advance.
-Darren
Anyway, it didn't turn out as good as I thought, it sounds great clean but when you play "on the edge" of breakup, you can hear some nasty distortion mixed in. I ran through it with my scope and I saw that the source of the nastiness was indeed the phase splitter stage. There are two preamp stages before it, after the first one there's no breakup at all (as expected) and after the second there's a little smooth, asymmetrical clipping, also as expected. However, when I checked the signal coming from the "first" half of the PI, I noticed that it would reach a certain pont and clip hard, very SS like, and only one side of the wave. This signal goes to one output tube and also the grid of the other half of the PI. When it gets here it "flips" it (as it should) and does the same thing to it, so now both sides of the wave are clipping hard, and this goes to the other output tube. This, as you can imagine, sounds like a mess.
This is the original K15 schematic by the way. Don't bother looking at the preamp, just the two halves of V3 onward.
http://www.flickerdown.com/forsale/k-15.pdf
So waht I want to know is, is this "SS clipping" inherent with this style of phase splitter? I tried a few different tubes (a couple Sylvanias and a Mullard) and they all did the same thing. Can I tweak some values to get it to sound better or am I going to have to switch to a LTP design? I really don't want to do that because this amp is a horribly laid-out mess inside (not my fault, it was a mess from the factory) and I'd rather not go to that extent with this amp unless it's 100% neccesary. I suppose there's a reason why everyone stopped using this design in the 50's though
.Edit: Forgot to mention that the original components are still in the PI stage. What exactly constitutes a "leaky" cap? I checked the coupling caps and they were only letting a few mV get through, is that enough to cause a problem?
Thanks in advance.
-Darren
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I've been toying with this idea for a while. And last but not least, if you realy want power tube distortion,
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