|John Broskie's Guide to Tube Circuit Analysis & Design|
Beige Bag Software
29 Nov 2002
The new position I have is at Beige Bag Software [I no longer work for them, but I do still use the program daily, JRB], the people who make the excellent B2 Spice A/D program. And by the way, I thought their program was excellent long before I began working there. In fact, two years ago, I wanted to do a review of several inexpensive SPICE programs and I quickly discovered that inexpensive and SPICE seldom went together, that $30,000 to 75,000 per seat installations were common, and that finding a capable SPICE program for under $1,000 was not easy. At first I was wowed by a competitor’s $1,000 dollar program — the manual alone had to be worth a few hundred dollars, big and glossy, snuggly fitting in its shrouding box. Well, a month later, the only thing I still liked about the program was its manual. Why?
I couldn’t quickly draw complex schematics. I draw up to ten schematics a day and it was nerve racking to use that program. With the B2 Spice A/D program I was able to draw a complex hybrid amplifier in 15 minutes and I had a useful test running in the next 3 minutes.
So what you can expect see in upcoming articles (beyond the usual schematics) is B2 Spice A/D circuit files, which means you can download the circuit and run your simulations on it (and no, I will not be offering the circuits in P-SPICE or Protel or EWB or any other version of SPICE.) To see what this might be like, check out the two case studies I have posted on the Beige Bag website http://www.beigebag.com/resources.htm The first study covers RIAA preamps and the second covers headphone amplifiers. If you don’t see a tube for the first few pages, do not despair: they are there.
Once, in a moment of extreme stupidity, I signed a non-disclosure form, as the business owner who was so nervous over his highly original tube circuitry that he could not speak to me otherwise. For about six months afterwards, I was nervous about including a tube differential input stage in this journal, so as not to bring the wrath of voracious lawyers upon me.
Yes, that was his big trade secret, two triodes sharing a common cathode resistor. Yes, that circuit has appeared in this journal prior to his showing me it; it also appeared thousands of preexisting products, BW TVs, oscilloscopes, voltage meters, test equipment, and dozens of amplifiers, but would any of that necessarily hold off a hungry lawyer or persuade the jurors of the OJ trial?
I can imagine a dramatic scene in which my attorney would bring in a large wheel barrel overflowing with old electronic textbooks and user manuals. “Your honor, each and everyone of these old books holds a drawing of a tube-based differential amplifier and we have seven more wheel barrels in the backroom, should this evidence prove inconclusive.” Unfortunately, I can also easily imagine the prosecuting attorney’s reply (imagine John Edwards' drawling voice):
Beyond legal hassles, there is the general problem of constraining this journal’s scope, which is at odds with its mission to inform and reveal the art and science of tube circuit design, not to form an exclusive club or, even less, a confessional booth. For example, a perfectly good-natured reader writes showing and describing his new circuit, but requests secrecy. Now, even if his circuit is not as newborn as he imagines nor as not obvious as thinks, I do not want him to feel betrayed, when I publish the circuit or a similar circuit in this journal; yep, it’s the differential circuit all over again. And what is imagined to be wildly bold and startlingly creative can be as trite and tiresome as using a 100k grid resistor instead of a 1M or 47k resistor.
So rather than risk court time or hurt feelings or a constrained journal, please do not send me any schematics that you are not willing to share with your fellow readers.
Tube CAD does the hard math for you.
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