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I took some simple measurements and scopes of my new build and jotted them down on the attachment...take a look and mention/discuss anything you see fit. This is an ODS prototype with Skyliner EQ, HRM, and just a regular Marshall (aka BluesMaster) PI and power amp. But this is only regarding the preamp signal. PAB = Preamp Boost.
And the following is the 'text' version of what I saw. Any comments appreciated. Clean sounds really good...overdrive is OK but has a little fizz on the notes.
When I put in a 100mV 880 Hz signal with PAB on and mid-boost off, at a Volume setting of about 6 or so, the waveform at V2a's grid was just starting to pull grid current and clip the top of the waveform. Measuring the AC voltage on that grid it measured 1.7vac. The idle DC cathode voltage of V2a is 1.7vdc. This is right where the Volume control hits its 'sensitive' zone which I assume is normal. At Volume setting of say 5.5 or so...V1b's plate is totally clean and the signal at V2a's grid is totally clean. At say 6.5 or so, V1b's plate is ever so slightly 'slanted' at the bottom and V2a's grid is slightly clipped on top. I'm guessing that the slight 'slant' I see on the bottom of V1b's plate is due to V2a drawing that grid current? In other words, it's not really V1b that's distorting?
I measured the DC voltage of the grids and the 'other' side of the grid stoppers with Volume on 6.5 and Drive on 8: V2b is at -8.8vdc with that 100mV signal at the input and the other side of the 150k grid stopper is at -3.7vdc so it appears that some major grid current is happening there.
And FWIW, it seems I liked the tone best when I backed off the Volume to just before that clipping on V2a's grid occured...or maybe right at it. VERY easy to go just past the sweet spot.
And the following is the 'text' version of what I saw. Any comments appreciated. Clean sounds really good...overdrive is OK but has a little fizz on the notes.
When I put in a 100mV 880 Hz signal with PAB on and mid-boost off, at a Volume setting of about 6 or so, the waveform at V2a's grid was just starting to pull grid current and clip the top of the waveform. Measuring the AC voltage on that grid it measured 1.7vac. The idle DC cathode voltage of V2a is 1.7vdc. This is right where the Volume control hits its 'sensitive' zone which I assume is normal. At Volume setting of say 5.5 or so...V1b's plate is totally clean and the signal at V2a's grid is totally clean. At say 6.5 or so, V1b's plate is ever so slightly 'slanted' at the bottom and V2a's grid is slightly clipped on top. I'm guessing that the slight 'slant' I see on the bottom of V1b's plate is due to V2a drawing that grid current? In other words, it's not really V1b that's distorting?
I measured the DC voltage of the grids and the 'other' side of the grid stoppers with Volume on 6.5 and Drive on 8: V2b is at -8.8vdc with that 100mV signal at the input and the other side of the 150k grid stopper is at -3.7vdc so it appears that some major grid current is happening there.
And FWIW, it seems I liked the tone best when I backed off the Volume to just before that clipping on V2a's grid occured...or maybe right at it. VERY easy to go just past the sweet spot.
). I actually don't like them because that tab is live, I'd like to put some sort of insulated cover on them. This amp was built using as much 'surplus' stuff as I could. I already had the PT/OT and much of the other stuff too...that's why the switches are non-standard for a Fender chassis and the pots are all over the place regarding CTS/Alpha/etc.
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