LPSGME wrote:I was getting some undesirable fizz at one point.
Adding a 47PF cap to ground at the grid of the PI tube, right after the .02 input cap, vastly smoothed out the tone.
The 47PF cap also introduced a nice compressed attack and a brassy sustain to my Les Paul's bridge PU that it lacked before.
....and thereby getting rid of parasitic oscillation
Quite right. No doubt. I have wires running chaotically all over the place. The cap worked similarly at several places, but the PI point sounded best.
And as i said, it didn't just take out a high end edge, it radically improved the smoothness - not so much creamy as polished and shinny.
No, because the cutoff frequency is around 10 kHz (1Mohm/47pf)...way out of the guitar speakers range. I haven't tried a 47pf on PI input, but I usually put a 10pf across the OD input, which has the same effect for me.
jakehop wrote:Just for kicks, I placed two 300pF from the anodes to the gates of both halves of the OD-tubes, as I see on some of the amps Howard made - no difference. Shouldn't that kill the oscillation?
Only if the OD tube is the source of the PO. You need to locate the source.
Post some pictures of the build, maybe we can see where the lead dress can be improved.
bluesfendermanblues wrote:
....and thereby getting rid of parasitic oscillation
Quite right. No doubt. I have wires running chaotically all over the place. The cap worked similarly at several places, but the PI point sounded best.
And as i said, it didn't just take out a high end edge, it radically improved the smoothness - not so much creamy as polished and shinny.
No, because the cutoff frequency is around 10 kHz (1Mohm/47pf)...way out of the guitar speakers range. I haven't tried a 47pf on PI input, but I usually put a 10pf across the OD input, which has the same effect for me.
I didn't try any other cap values. I just grabbed what was handy and it seemed to work. The change was similar when inserted at various points along the signal path - except when closing my eyes and concentrating. Then the PI point sounded slightly better to me.
In any case it took the sound to a superior level that I hadn't heard before. I was also amazed that now my d'lator further compresses and improves whereas it didn't before.
