Tube Amps / Music Electronics
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|8/3/2005 6:30 AM|
|Matt||Re: Preliminary Results...|
lol, just be sure to use a bunch of ambiguous/vague terms so we're all on the same page.
Maybe this example will help me better understand this topic: I recently was messing with a homebrew 50W non-MV Marshall (but I added a KF MV). I swapped the 470R PI cathode resistor to 820R and the distortion got worse (it wasn't very good to begin with). So I put the 470R back in and then changed the 20K power supply dropping resistor to 6.6K to raise the plate voltage of the PI (for even more PI plate current). I then increased the next 10K PS dropping resistor to 20K to get the preamp voltages close to where they were before. Now the distortion sounds great, crunchy as hell but no 'fuzz-tone'. I assume that changing the operating point of the PI improved the distortion but which of the following applies:
1) I changed the distortion from cutoff to saturation.
2) The distortion is still cutoff but the change somehow 'improved' the tone of the cutoff distortion.
3) None of the above.
Feel free to elaborate on an answer.
I was actually shocked at the difference. With the 820R cathode R and 'stock' power supply resistors, the distortion was terrible and after the change it's awesome. Night and day.
|8/3/2005 7:31 AM|
This allowed you to drive the output stage grids a bit harder, and increased the headroom of the PI's positive waveform swing (the important one) a bit as well - see below:
If you hadn't done this you would not have benefitted from the increased PI headroom as much, since the PI drive signal would have increased as well - but since you kept the preamp B+ the same, the PI will be in cutoff for a smaller portion of the signal waveform.
As far as the cutoff/saturation questions... I think the changes you made allow you to drive the output tubes a little closer to "saturation", while keeping the PI out of cutoff for longer periods of time. You might want to continue down this road a bit further; pre-PI MV (maybe a ganged pot with the post-PI?), lower preamp B+, hotter PI and/or output stage bias, etc. Replacing the 6.6k with a small choke would allow you to run full screen voltage on the PI as well. Each one of these changes made separately (which they should be, to properly gauge their effects) may not be exactly earth-shattering, but cumulatively they may substantially improve your tone - and if not, the knowledge gained will be invaluable the next time 'round.
|8/3/2005 7:49 AM|
[QUOTE]Matt: I then increased the next 10K PS dropping resistor to 20K to get the preamp voltages close to where they were before.
Ray: If you hadn't done this you would not have benefitted from the increased PI headroom as much, since the PI drive signal would have increased as well - but since you kept the preamp B+ the same, the PI will be in cutoff for a smaller portion of the signal waveform.[/QUOTE]
I actually didn't do this at first and even though the distortion had improved (from lowering the 20K resistor), it still had some fuzz quality to it (especially the attack). But after I increased that 10K resistor, the attack was crispity-crunchity like I want it.
I was using the KF MV (dual-pot, post-PI, wiper to power tube grids) so the output tubes weren't seeing much signal and all of the distortion I was hearing was PI and preamp. But the part about "keeping the PI out of cutoff for longer periods of time" makes sense. When it was in cutoff for too long, it actually sounded less distorted and the distortion that was there, was bad. Now the distortion sounds better and there's more of it.
It's not buttoned up yet, so maybe this weekend I'll piddle with it some more. I do have a tiny choke lying around somewhere.
|8/3/2005 3:26 PM|
You juiced up the plates on the PI, so you are getting more swing out to the EL34 grids and so are getting more power stage crunch and less pre-amp crunch (relatively speaking) One of the neat things about the Marshall circuit is they took a circuit designed to deliver 52Vpk (ish) to 6L6's and while doubling the front end gain (going from 12AY7 to 12AX7) also dropped the drive requirements to 35Vpk (ish). So you get more distortion at each end.. but then they tweaked the PSU to try and adjust the plate swings to balance who was going to clip and when etc.. you just moved that balance to somewhere that works better for you in that amp.
jm2c and worth precisely what you paid for it.
|8/4/2005 1:32 AM|
Just some pondering.
You might have changed the balance of the PI triodes when rising the voltage. This might affect to cross-over charasteristics etc.
|8/4/2005 6:14 AM|
This might be saying the same thing but it seemed that the input to the PI was what was getting slammed too much. It didn't seem to matter where the post-PI MV was set (or even when I didn't have a post-PI MV), the distortion didn't sound right. The amp was extremely loud and sounded great clean so that made me think the power tubes were 'dialed in' OK. I had a pre-PI MV in the amp for years which allowed me to get familiar with how much distortion is generated before the PI (the good 'thick' stuff seems to be generated in the PI). If you cranked the preamp volume and kept the pre-PI MV low; the amp had the typical thin, crappy distortion associated with that setup. Then when I would crank the pre-PI MV for more crunch, the 'fuzz-tone' would start creeping in. So all that seemed to point to the PI. Since the PI had so much less current flowing at that time, the "PI spending too much time in cutoff" seemed to make sense.
I played it again last night and it's amazing how much that cutoff chokes the good crunch (and ruins the attack of the note). Not only did the quality of the distortion improve but the quantity of it seems increased too. I've seen this in an XTC type preamp I built as well. In that case, I kept reducing gain/bass until the cutoff stopped (actually, until the 'fuzz-tone' stopped) and then all this unheard crunch just popped up (less gain yet more crunch).
BTW, it is an OEI OT bought around 2001. My bad PI distortion was preventing from hearing it's full potential. This is one mean smallbox now.
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