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I have a 1x10" Gibson GA-15 (Trace Elliot Velocette in different clothing) that has a phasor-like distortion to it when cranked (sounds fine until it starts to distort). I've tried the zener-mod on the grids of the EL84s using both 10v and 15v zeners (idle cathode voltage is 13.8v, cathode R is bypassed with a 220u cap) and they didn't seem to address it. I've also tried the zener mod on the grid of the second stage using both 5.1v and 6.8v zeners (idle cathode voltage on V1b is 1.6v, cathode R is bypassed with 2u2 cap), no help. I've also just tried a 10K/10W and .0022uF cap across the OT primaries as a conjunctive filter and that didn't seem to address this problem either.
Any idea where this phasor-like distortion is coming from? The PI? This amp has a long-tail PI using the 'Vox' values of 47K/1K2 with 1M grid load resistors, 100K plates. PI plates are 280v (355v supply) and the cathode voltage is 70v. Why do many amps using EL84s use these values where many EL34 amps seem to copy Marshall and use the 10K/470R values? I'm used to Marshalls where I would expect more current, more voltage drop (say, around 100v) across the plate resistors. Does that tend to make it overdrive better?
Lots of questions...some of them I'm just thinking out loud lol. Any help appreciated.
BTW - I have written all the ID numbers (i.e. R1, R2, C1, C2, etc) on a schematic if anyone wants it. I still have to 'redo' it so other people can read it lol but it sure makes modding much easier when you know which component is which. I'll probably send a copy to Steve A. Also, my amp was missing two components (220K across first filter cap and the snubber cap across the SB switch) and three of the caps that were supposed to be .1uF (already large) were .22uF (not .022uF, .22uF...I measured them...220nF). No wonder it was so bassy.
Edit: Two of those .22 caps were swapped for .022uF caps, the third is the 'blocking' cap for the second (bottom) grid of the PI so I left it (it only needs to be 'sufficiently large'...correct?).
Any idea where this phasor-like distortion is coming from? The PI? This amp has a long-tail PI using the 'Vox' values of 47K/1K2 with 1M grid load resistors, 100K plates. PI plates are 280v (355v supply) and the cathode voltage is 70v. Why do many amps using EL84s use these values where many EL34 amps seem to copy Marshall and use the 10K/470R values? I'm used to Marshalls where I would expect more current, more voltage drop (say, around 100v) across the plate resistors. Does that tend to make it overdrive better?
Lots of questions...some of them I'm just thinking out loud lol. Any help appreciated.
BTW - I have written all the ID numbers (i.e. R1, R2, C1, C2, etc) on a schematic if anyone wants it. I still have to 'redo' it so other people can read it lol but it sure makes modding much easier when you know which component is which. I'll probably send a copy to Steve A. Also, my amp was missing two components (220K across first filter cap and the snubber cap across the SB switch) and three of the caps that were supposed to be .1uF (already large) were .22uF (not .022uF, .22uF...I measured them...220nF). No wonder it was so bassy.
Edit: Two of those .22 caps were swapped for .022uF caps, the third is the 'blocking' cap for the second (bottom) grid of the PI so I left it (it only needs to be 'sufficiently large'...correct?).