Tone Analysis III

I've done a variety of testing: So interesting, yet so tedious to produce a comprehensive enough collection of data to make definitive pronouncements. But here's what I think I can say. 

The TINA modeler does a great job showing what is likely to happen with my tube-driven circuit, especially when it's operating in linear territory, and it's even pretty good at showing saturation or cut-off conditions, when the inputs to tubes are driving them beyond their design limits. 

For example, here's a comparison of a modeled vs. actual 1kHz/200mv signal after is passes through channel I (full wide-open gain setting), cascaded through channel II set up as a "cold clipper" (very cool bias to force the clipping off of one lobe of the waveform, set to 25% gain), and measured at the grid of the second pre-amp tube:

Modeled using TINA:

Measured using a tone generator and capturing the signal in Visual Analyzer (VA):

Phase differences aside, they're pretty close in shape and proportion, and I think in actual measured signal voltage compared to modeled (I did some testing with volt meter and guitar generated signals, which conformed to expectations of signal voltage levels - no pics - but I haven't followed up using the tone generator). VA also shows the actual higher order harmonics accentuated by the circuit up to this point at these settings, in the lower panel. 

I also set up a suite of tone testing, but all the variations are too involved to capture and explain in detail, plus I changed some things as I went, and didn't want to go back and regenerate prior results to match up... Never the less, here are some examples of what I did and some key results. 

Using channel I and the stock TMB tone stack, I plugged in my MC500, set to neck humbucker PU only, and dialed in a sound that like - a full, round, straight-ahead jazz comping and lead tone. Then I replaced the guitar input with the white noise generator, and captured the plot in VA (the left side of figure below, green on black). Then I recreated the very same amplifier settings in TINA and plotted the modeled result (the right side of figure below), where the green curve is the signal prior to entering the tone control part of the circuit and the dark blue curve is the signal after being shaped by the tone controls. Here they are side by side:

Comparing the green curve on the left to the dark blue curve the right, you can see the similarities in overall shape, but the actual output appears to amplify the higher frequencies a bit more and extends them another 1k or so before beginning to fall off. There are also some spikes around 100 Hz which I'm not sure I actually hear.

This set shows use of channel II with the TMB tone stack, single coil on the neck PU, with a much different, very nice acoustic tone for comping or finger style playing - different visual orientation to widen the scale a bit. 

Here, the modeled vs. actual plots appear quite similar. with a pronounced notch at 600Hz. 

And, as below with the James tone stack set up with maximum mid frequency notch (close to 300Hz, nearly as predicted), we can see it's similar with regard to frequency response but doesn't represent the levels quite right.

Also (and this is important) I'm just getting my feet wet with this. I'm a complete novice and I just pick up what I can from reading and asking questions of people who actually know things. So, I'm sure I have based some of my findings and conclusions on inaccurate assumptions, misuse of complex software tools I have just learned for this application, and just plain incorrect interpretation of facts, all due to my limited knowledge and experience. 

Results 

Overall, TINA the modeler did a GREAT job guiding choices with regard to both tone and gain management from one stage to the next, and predicting actual outcomes. Bravo Texas Instruments! Now that I'm playing around with the amp in high gain territory, listening to the tonal qualities and overtones of just-breaking-up and highly distorted signals, I think all bets are off in terms of accurate modeling. But TINA got me 95% of the way there, and that's pretty impressive. 

The real result, the range of "clean" and "on the verge of breakup" tones that the amp creates with my guitars, is really quite impressive and everything I hoped for - and it's damn loud, which is just so satisfying in ways that are hard to convey in words (although neighbors have come up with a few!). It's sonically vast, and while I've got a handful of distinct and delicious tones I can use right now, it also provides so many avenues to explore and a lot more potential, as I swap out tubes (I have to try 5881 power tubes again and also some 6L6's, and a GZ34 rectifier I haven't tested out yet), and try new combinations of built-in settings.

I'd say I'm nearly done at this point. I will probably modify one last resistor value in the V2GL switch, to help manage signal level going into V2 in Cascade mode. The cabinet for it is basically done and in use, but I need to do the related upholstery tasks. And the art and layout for the faceplate is being finalized.