• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What is this tube amp

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Does anyone know what this amp is or what it was for?

It says "Quad/Eight" "TFB200" on the faceplate.

I can't find anything on it.

Here are some pics.


  • tfb200 #1.jpg
    tfb200 #1.jpg
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This post looks pretty old, but for the record the TFB200 was a tube amp made by Quad-Eight Electronics (the console company) as a drive in theater amplifier. I worked there at the time they were made. The '200' referred to the supposed output power of the amp, but it was typically actually less than 200W.

Because the amplifier output lines at drive ins were buried in the ground to distribute signal to each speaker car post, they were notoriously susceptible to picking up large voltage transients from nearby lightning strikes. Solid state amps were prone to blow out from this and killing the sound.

So the idea was that by using an amplifier with tube output, the reliability could be increased since tubes are not particularly sensitive to static discharge.

The TFB 200 used 4X television sweep tubes for the output in a parallel push pull configuration, although the voltage amplification stages were solid state. Sweep tubes were very rugged and expected to last a long time. Additional protection was provided for internal shorts in the output tubes that QR called 'Scorch Guard'.

Unfortunately to keep the rack mounting height reasonable the tubes were mounted horizontally against the tube manufacturer's recommendations. When the tubes heated the internal grids sagged under gravity and shorted out internally. The failure mode was such that the solid state protection circuitry was of little value. Additionally the output transformer was of low quality with high leakage inductance which led to a high amount of what effectively was crossover distortion when the conducting pair of tubes switched to the other set when the signal crossed through zero. The amp was notorious for producing HF oscillation in the high MHz range which was very difficult to eliminate. Dale Connaly (since passed on) was the service tech for them and he quite understandably loathed them. We all agreed that 'TFB' stood for 'Too F****** Bad'.

So although QE had a well deserved reputation for some excellent products, and consoles, the TFB-200 was unfortunately not among them. Every manufacturer has produced some turkey products and this was one of Q.E.s.
I know absolutely nothing about electronics peculiar to drive-in theaters but I vaguely recall Quad Eight as a company name - did they also do some broadcast studio equipment?
. . . . Every manufacturer has produced some turkey products and this was one of Q.E.s.
Brings to mind some of the writings from Dr Henry Petroski ("To Engineer is Human", et al). He often makes the point that design SUCCESSES can build on themselves, resulting in better products through incremental improvements; but design FAILURES create better engineers, who eventually drive large-scale performance improvements or even whole new classes of products.

Yes, looking back over my several incarnations I have made some technical decisions that turned out to be . . . . ummm . . . let's say, "sub-optimum". Fortunately none of them turned out to be catastrophic in either a business sense or a human sense. Those instances don't bother me nearly as much as the several times when my decisions were over-ruled, or even ridiculed, by an alternative that I felt was clearly inferior, and justified on the basis of short-term cost . . . . but turned out to have much higher costs over the product life-cycle.
At one time I was working at a company that made a major revision to their performance evaluation system for engineers and designers. One factor in the revised system was the amount of formal documentation - drawings, and change requests - produced by an engineer. I suggested that a better measure of an engineer's performance was not the documentation he created, but rather the number of changes and revisions that were submitted, over a few years, against the documentation he originated. The idea was soundly dismissed, with the comment that the company wasn't in the business of training engineers to design it's products, and they couldn't afford to wait that long to see if an engineer was capable.

About Quad-Eight

Quad-Eight was, at its day in the mid-70s, the premier manufacturer of large custom recording consoles for the film industry. 42 input channels and 8 output channels might be a typical one. They had clients like Burbank Studios, Capitol Records, Disney, etc. Their patchbays had literally miles of wire. The module panels in the console were aluminum, covered in Formica, with hand engraved lettering for all the setting positions. The console cabinets were so large they had to be lifted with a gantry, and were made of wood covered with wood grain Formica for a durable finish. Later on, they also made stock recording consoles directed more at the music industry. The modules in those were very good, but they reduced manufacturing cost by doing things like changing the lettering to epoxy ink instead of hand engraved, using sheet metal cabinetry, etc.

Since they designed and built the necessary modules in-house, some of these were repackaged and sold as stand alone products - things like compressors, equalizers, and line amps. Many of these found their way into broadcast studios.

They also tried entering the cinema theater market, and that was why the TFB-200 was designed.

The company's name, although well suited for a recording console company, actually came from their first product, a slitting machine that would produce 4 strips of 8mm film from a single 35 mm original roll.

Hope this clarifies the nature of the company. :film:
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