I have an amp on the bench right now that was doing something "I" found most annoying. The LTP PI would skew hard upon clipping. I know they do this, but the imbalance just happened to trigger at the same time as the onset of power tube grid conduction. The power tubes are clipping well ahead of the PI, with nice symmetrical flat lines top and bottom, but before getting to a crunchy square wave the PI impedance would tank, the PI balance would skew and the onset of grid conduction would only occur on one power tube. So the waveform looked like this:
I know you can look at a bunch of old Fenders and see a similar wave form, but I just didn't like the look of it. With the one tube biased all chilly and the other tanking bottom end. So I tried a bunch of stuff to reduce the affects. Or at least get the amp to NOT do all the bad things at the same damn time. Changing PI bias, tail resistor, plate load balance/imbalance, PI voltage, different PI tubes, and, of course I swapped the power tubes just to see if that would change the anomaly, etc. Even changing the NFB circuit had little affect. Adding a large grid stop to the PI grid had very limited affect. So I thought on it this week while I was at my day job and came up with some ideas. Most didn't work or at least not well enough. But one did! I measured the AC volts on the PI input at the onset of PI clipping at just over 3V. The signal going into the PI could be turned up to about 8.5V. I want the amp to grind so just reducing the PI input wasn't going to work. Instead I installed face to face 5.1V zeners across the PI grids. Bam! Nice square, symmetrical waveform that doesn't introduce any diode clipping because the power tubes are well into clipping before that, so, also no affect on clean tones. No more impedance tank or imbalance on the PI either with it's cursory LF roll off and the tiny 2V of grid conduction is somewhat even on each power tube so the push/pull is much more even. I connected a pot in series with the diodes and found that adding about 8k of resistance really dialed it in.
In fact the wave form may be TOO square and symmetrical now. I haven't heard it yet. I may need to increase the ZV and add more resistance to lessen the affect if the clipping sounds too hollow. BUT! This could be just the thing for those guys that like to play metal through their 18W Marshall clones and Tiny Terrors or if you just want a more hard rock/less "bluesy" overdrive from a BF type amp!
That's it on this one for now. I'll report on how it sounds when I can.
I know you can look at a bunch of old Fenders and see a similar wave form, but I just didn't like the look of it. With the one tube biased all chilly and the other tanking bottom end. So I tried a bunch of stuff to reduce the affects. Or at least get the amp to NOT do all the bad things at the same damn time. Changing PI bias, tail resistor, plate load balance/imbalance, PI voltage, different PI tubes, and, of course I swapped the power tubes just to see if that would change the anomaly, etc. Even changing the NFB circuit had little affect. Adding a large grid stop to the PI grid had very limited affect. So I thought on it this week while I was at my day job and came up with some ideas. Most didn't work or at least not well enough. But one did! I measured the AC volts on the PI input at the onset of PI clipping at just over 3V. The signal going into the PI could be turned up to about 8.5V. I want the amp to grind so just reducing the PI input wasn't going to work. Instead I installed face to face 5.1V zeners across the PI grids. Bam! Nice square, symmetrical waveform that doesn't introduce any diode clipping because the power tubes are well into clipping before that, so, also no affect on clean tones. No more impedance tank or imbalance on the PI either with it's cursory LF roll off and the tiny 2V of grid conduction is somewhat even on each power tube so the push/pull is much more even. I connected a pot in series with the diodes and found that adding about 8k of resistance really dialed it in.
In fact the wave form may be TOO square and symmetrical now. I haven't heard it yet. I may need to increase the ZV and add more resistance to lessen the affect if the clipping sounds too hollow. BUT! This could be just the thing for those guys that like to play metal through their 18W Marshall clones and Tiny Terrors or if you just want a more hard rock/less "bluesy" overdrive from a BF type amp!
That's it on this one for now. I'll report on how it sounds when I can.
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