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BIAS CONTROL SYSTEM (BCS) - AUTOBIAS FOR VACUUM TUBE POWER AMPLIFIER

All owners of vacuum tube amplifiers are fully aware of the benefits - a musical quality that approaches reality more closely than any other technology. This performance advantage has a counterbalancing downside - the constant worry that the output tube bias has drifted.

Output Tube Bias Current is the required idling current flowing through each output tube when there is no audio present (not unlike your car engine's idle speed). This idle current is required to keep your tube in its lowest distortion operating point. All power amplifiers have a circuit that generates a control voltage that is fed to the output tubes and this voltage in turn determines the current flowing through the output tube (in some designs, like Dynaco amplifiers, it is fed to two tubes at a time - causing even more imbalance). Many factors effect both the generated voltage including your power company and the flowing current including each tube individually. Unfortunately, all power amps today are designed to simply set it and hope that nothing changes and that it will stay constant forever - but as all tube amp owners know, it usually doesn't. Think of it as cruise control where you place a brick on your accelerator pedal hoping nothing in the road or your car changes. With tube amps, tube aging (individually and from tube to tube), grid temperature variations, power line voltage changes, and component changes all contribute to the departure from assumed ideal conditions. When the bias current is too low, distortions will rise quickly rise with audible consequences. When the current drifts too high, all the high cost power circuits are strained including the output tubes themselves as well as the power supply components (Power Transformer, Rectifier, filter caps) shortening their life (and impacting your checkbook). Consequently, tube owners have accepted the need to monitor and reset the bias periodically hoping that it has not departed too far from ideal.

And so it has been - until now.

Stereo 70 Owners Manual (original full)

BCS Assembly & Installation Manual

CAE ASM-BCS BIAS CONTROL SYSTEM

BCS OUTPUT TUBE BIAS CONTROL SYSTEM

Schematic Diagram

  • Constantly Monitors & Controls the Bias Current of Each Output Tube
  • Each Tube is Independently Monitored & Controlled
  • Compensates for Tube Aging, Power Line Variances
  • Completely Eliminates Costly Matched Output Tubes
  • Dramatically Extends Life of Output Tubes + All Power Supply Components (Power Transformer, Rectifier, Filter Caps)
  • Improves Audible Performance by Maintaining Lowest Distortion Operating Point
  • Instrumentation Grounding / Signal Transmission Layout Delivers Instrument Grade Stability & Performance
  • One Simple "Set & Forget" Adjustment - Tubes Will Remain at This Setting Over Their Lifetime
  • Comprehensive Assembly and Installation Documentation
  • Easy to Build & Install
Our Bias Control System (BCS) is a small intelligent module that constantly monitors the current flowing through each tube and constantly maintains the current at it's optimum level. Let me emphasize that there are four independent circuits - each tube is monitored and controlled individually and independently. The result is that no tube can be effected by the current draw of any of the others so your amplifier will always be at its optimum operating condition. Not too low to cause distortion nor too high resulting in life threatening amplifier strain.

Even over one listening session, it is not uncommon for the individual tubes to exhibit drift out of the desired bias current setting. If we can maintain this control, the audible improvements can be significant and even dramatic.

Over the longer term, should the bias drift into the high bias state, for one or more tubes, their life can be dramatically shortened. With control, tube failures drop significantly while tube life can be extended from months to years.

If you are a proponent of matched tubes, you will never have to pay the premium prices for matched tubes again. Since each tube is monitored and controlled individually and independently, you can be sure that they all will have the same bias current regardless of the tube's age or operating characteristics. This is even more important when you realize that the characteristics of matched tubes do not remain matched as they age - with our BCS system they remain matched all the time & over their lifetime.

Our BCS system has a very wide dynamic operating range allowing control over the full life of the tube - so much so that you can even mix brands (but not recommended). Our PCB has been designed using instrumentation grounding techniques to deliver consistent long term stable performance. It has been developed, designed and refined to be completely sonically transparent - but in fact, there are significant improvements in the audible performance of the amplifiers we have tested.

For you, the bottom line is that your amplifier is guaranteed to provide consistent peak performance.

Although the design can be used with virtually any "fixed bias" output stage topology, the PC Board and instructions specifically address installation in Dynaco Stereo-70 and Mark 2/3 power amplifiers. Assembly and installation is quite straightforward however you should be reasonably versed in reading a schematic diagram and the use of a DVM. Feel free to download the Assembly and Installation Manual to review the project.

Installation of our Bias Control System (BCS) is perfectly straightforward. The PC Board contains everything including a new bias power supply. In Dynaco Stereo-70 and Mark2/3 power amplifiers, it mounts in existing holes slightly below chassis.

Feel free to download our comprehensive instructions to see for yourself just how easy it is. You may purchase the board blank or fully assembled. You may also purchase the board and parts as a kit. 

·        Blank Board (P/N PWB-BCS) @ $55

·        Blank Board w/ parts (P/N KIT-BCS) @ $89

·        Assembled & Tested Board (P/N ASM-BCS) @ $121

Recommended Options

·        Teflon Rewire Kit (4X 10ft Red, Black, Wht, Grn) (P/N TWK) @ $24/ set

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