I had to re-route the turntable signal to the PC because for some reason, the Nuforce AVP did not work properly with analog inputs! Really?! I thought it would be simple matter to use any audio program to playback the input of the soundcard – not so! After trying a bunch of programs – foobar, Audacity, CoolEdit, etc. I discovered that I need to bypass the kmixer in Windows and use ASIO (for better fidelity and monitoring). Unfortunately, none of the program on hand did that by default, but after some Googling, the solution was simple, I just needed to get a version of Audacity complied to run with ASIO which standard version does not support due to licensing issue.
Here is a shot of the Audacity re-complied to use the ASIO I/O:
Since most soundcards have low input impedance, they load down the circuit being measured. The solution is a buffer/amplifier for the soundcard inputs. It’s a very simple circuit using a dual opamp. I built one this morning…
I spent a good part of the weekend trying to sort out the various worksheets, software, Spice models used for vacuum tubes. It was pretty messy… As there are numerous ways to do the same thing and I have already collect a lot of material over the past year or so. The goal was to distill the process down to a few simple steps and to nail down the simulation platform.
Let’s get the simple thing out of the way – picking the simulation platform. While most of the online Spice models and discussions on simulations tend to gravitate to the older PSpice or the newer LTSpice, neither were to my liking after spending time with SIMetrix, so there is just no going back to those platforms. Now the task before me, was to gather tube models specifically for SIMetrix, which wasn’t really that difficult, since it can already import various models written in several popular Spice languages. Not only that, I also discovered a complete SIMetrix-ready tube library at the great Ayumi’s Lab, thanks Nakabayashi-Son!
Of course, there are always going to be tubes that are not included in these all-in-one libraries, especially when I tend to use a lot of Chinese and Russian tubes, so the need to generate individual tube models based on datasheets is a dreaded but necessary task.
As discussed in an earlier blog post, I have developed the process for this task, but it is worth repeating here, as there have been some updates:
1) data capture – I use Andrei Frolov’s CurveCaptor to quickly capture the basic curve data from scanned datasheets, there are many other programs that can do the same thing, but this one is preferred because it can also generate triode models directly as we will see shortly;
2) modeling the triode – CurveCaptor is specifically designed for this task, so this is what I use to get the basic Koren or Rydel parameters, but these models do not include grid current parameters, so they will need to be added in the step below;
3) triode grid current parameters – since guitar amps often push the input signal of the tubes beyond typical “hi-fi” operating limits, the grid current needs to be modeled, as it affects the clipping and compression characteristics of the tube thus the amplifier. There are a couple of ways to do this, such as a simple diode model or using the more complex models proposed by Hutt and Sjursens, a good discussion is available at AX84.
4) modeling the pentode – this is a tough one, but I managed to get a copy of Robert McLean’s worksheet which makes the task easier, I think Robert’s model is a definite improvement over Koren’s version. An alternative is to use Eureqa Formulize to automatically generate the parameters based on the pentode characteristic curves.
During my research, I also discovered quite a few good sources of information that I have not seen before, these fine gentlemen have clearly gone through the modeling process in great detail as reflected in their write-ups, programs and worksheets. I also had some short email exchanges with them, they are both great and I want to give my thanks to them again here.
David Heiser’s site contains a treasure trove of information, he does a thorough review of the methodologies used by the past practitioners, and provides great insights to the short-comings as well as suggestions for improvement on those methodologies. This is a must read for anyone that’s interested in the subject.
Bill Elliott’s site contains a very detail discussion on the Koren’s models with a heavy mathematics bend. Bill also provides a novel methodology to extract key tube parameters from the datasheet with as little as 5 data points. For those that have the Derive 6 software, they can also use Bill’s program to solve the Koren parameters automatically.
As usual, AX84 proved to be a great source of information, as many experienced amp designers/DIYers frequently use Spice in their work, and searching the site always turns up some useful information on the subject.
I have been trying out different ways to generate pentode models, out of curiosity mostly… Given the wide tolerance of vacuum tube specs, I don’t think the differences between these models would mean much, but nevertheless, I am hoping that through actual bread-boarding, testing, I will eventually find a model that best matches real life performance data.
Here is a summary on how these models were derived:
1) The McLean Model – this model comes courtesy of Robert McLean’s BeamPentodeModel Excel worksheet, Robert was kind enough to share his proprietary worksheet with me. Since Robert is well known as the tube SPICE guru, I have good faith in this model, although Robert did mention that the model is still well short of his own expectation.
2) The Eureqa Model – this model comes from Eureqa Formulize, a curve fitting program designed by the smart folks at Cornell, which uses sophisticated algorithms to derive equations given a set of data. I scanned the IV curves from the datasheet with CurveCaptor (which is great for generating triode models BTW), and imported the data into Formulize. The optimization process took about 10 minutes, the longer the program runs – the better is the fit. But for my purpose, I don’t think the extra precision is really needed. Close enough for rock & roll, as they say.
3) The Koren Model – this is the standard Koren pentode model using the same data set from the above curve capture process. Robert McLean’s KorenModel worksheet was used to generate the model parameters – this one comes in handy for another reason – the tube parameters could also be used in Chuck Doose’s power amplifier worksheet, none of the other two models can do that since they do not use the same set of parameters as the Koren model.
As mentioned, actual testing of the tubes in real circuit designs will be needed to verify the usefulness of these models, but at least it is a starting point…
Don’t know if it was coincidental, but one of the disks of unRAID array failed after a water-leak in the electrical panel that forced a complete shut down of the main AC into the house! (that’s another sad story altogether)
I ordered another drive, the price of which seem to have shot up by more than 50% in the past year, WTF!? Hope that will solve the problem, hate to think there is something wrong with the RAID chassis… It turned out that one of the cooling fans in the disk array died, and I guess the system purposely disable one of the drives, so someone (me) would get it replaced, I did, and the array came right up, now I have a spare 1TB disk for future replacement…
Got a bunch of projects done that have been piling up…
1) Put in the Cabinet Sim/Boost & Speaker Mute switch for the Vox VT15, added the external speaker jack as well. What a difference! The 8″ in combo absolutely blows, now plugging in the Ragin’ Cajun actually make the amp sound half decent – still no match for true tube amp though…
2) Completed the wiring for grounded wall wart power supply that will be used by the SMPS inside the Octal Amp. The SMPS needs to be both referenced to chassis and earth ground, otherwise the amp generates very loud squelch when volume is turned pass 4-5. Cobbled together the wiring using old S-Video cable and 3-pin DIN plug/jack, so no one can plug in a regular wall wart by mistake. The amp is still a bit noisy for my liking, but when it is cranked up, you don’t hear the noise at all.
3) Back up the HTPC, I think for the first time in 3-4 years! The XP machine is rock stable, so it will be used til who knows when, if it ain’t broke, don’t mess with it! Who needs Vista or Win7?!
4) Received a bunch spreadsheets for plotting loadlines, biasing and Spice modeling from the kind souls at diyaudio and antique radio forums, we are a gang of geeky brothers ;p Now I just got to learn to use these wonderful tools.
A good day indeed!