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Old 15th February 2004, 07:11 PM   #1
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Default some questions...

Hi,

Since electron tube is a voltage controlled device, what is the purpose of grid resistor if grid voltage is always more negative than cathode ? (to employ a path for grid leak ?)

Use of self biasing requires a cathode resistor and a by-pass capacitor in parallel to the resistor to prevent DC operating point drift when a signal is applied. This resistor/capacitor pair is bad for frequency response. Then, why is fixed biasing seldom used in amplifiers ? What is the disadvantages of fixed biasing ?

To control input signal level in front of the first stage with a potentiometer causes a variable input impedance. What is the proper way to control signal level ?
- Gain control ?
- Full passive network with fixed input/output impedance but variable voltage divider.
- Other ?

What is the advantages of tube rectification to solid-state rectification ?

What is the main parameters affecting tube life ?

Does a transformer coupled output stage require a protection circuit when there is an infinite or zero impedance (opened/shorted) at the speaker terminals ?

Thanks so much in advance...

MB
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Old 15th February 2004, 09:55 PM   #2
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Okay... the differences between fixed bias and bias using cathode resistor. If you want to design a tube amp you have to see the differences between both modes of operation. You can also design with a grounded grid configuration.

An amp using fixed bias; when the input signal is increased to a certain level there becomes a point where distortion becomes obvious. Distortion occurs in output stages when the grid current starts to flow, that a volts drop occurs across the grid resistor. If the working point of an output stage is AB, the increase in grid bias (due to grid current flow) may just clip cut-off point in class B, where serious distortion occurs. The problem gets worse by a higher grid bias causing an unregulated B+ to rise and distortion gets worse. The current swings with this mode of operation is much larger and an inductive input power supply is often used with perhaps regulation on certain parts. A small disadvantage is the need for bias adjustment pot(s).

An amp using cathode bias, (or auto bias) the transition from distortion free reproduction to audible distortion is far more gradual. If two amplifiers (one having fixed bias and the other with cathode bias with same quiescent currents) have identical output power ratings, the one with cathode bias would be capable of a larger distortion free output. Why ?
This bias method produces different results by creating a + cathode;... an output stage drive creates a drop in the anode & screen currents by automatically dropping the cathode voltage, so with the high time constant this self regulates to a point below cut off. This effect is opposite to the fixed bias mode. The current with cathode bias is more constant, so a capacitive input type power supply can be used. Tube matching isntí so important with this operation.

With fixed bias, sudden signals are followed by distortion with the signal. If the amp is designed with sufficient overhead, i.e no chance of grid current occurs for a given power, it makes no difference how the bias is obtained.

Fixed bias is very popular in MI applications and some Hi Fi designs. For a given B+ voltage, higher output power can be obtained .....Many well designed fixed bias amps can provide just as good sound quality as automatic bias types, for example I blow trumpet and I prefer fixed bias operation tube amp because it sounds sharper...same applies for my Hi Fi. The cathode bypass cap in the output stage is usually an electrolytic type that can effect the quality of the sound. The cathode bypass cap adds a LF 6dB pole and reduces the output power below the output transformer cut off frequency.

Advantages of tube rectums v.s SS ? indirectly heated rects warm up same time as amp tubes so B+ comes up gradually. This prolongs tube life. Supply electrolytic cap specs can be relaxed to lower voltages than if indirect tubes were used.. A drawback is the volts drop in a tube rectifier compared to a SS diode.... but using SS diode and direct heated rect creates a peak DC voltage of ACV x 1.414 at switch on and higher with the combination of an HV transformer with low regulation factor.

The only way to get a proper life out of tubes is to operate them at their specified ratings and that includes the specified heater voltage...

Your question.....Does a transformer coupled output stage require a protection circuit when there is an infinite or zero impedance (opened/shorted) at the speaker terminals ? No, for a tube amp..... A well designed amp should under BS3860 spec be stable with all loads and work in an open or short circuit. This is tough requirement and not many amps can tolerate this, even modern SS mosfet designs give up. The Radford PA tube amps of the mid 60ís were designed to accept this rough handling. Tube amps like the correct output Z for max power and lowest THD....guitar users often connect 8ohm speakers into 4 ohm taps to get more distortion for a lower power.

The purpose of the cathode bypass capacitor is to provide neg feedback (NFB) current by reflecting the audio freq component of the anode current back to the grid in antiphase. So what happens ? This lowers damping effect on the speaker by increasing the anode impedance... often a disadvantage.....although for a higher output power.... The cathode resistor may not be bypassed, and NFB can be applied as in the Quad amp designs by using feedback via a multi winding on the output tranny.

An output stage working in class A, biassed mid-point up the curve works more efficently at full vol by reducing anode heating power while B+ is pretty constant.

Itís getting late......Any mistakes......pse rectify !

rich
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Old 16th February 2004, 12:28 AM   #3
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Hi,

Quote:
To control input signal level in front of the first stage with a potentiometer causes a variable input impedance.
That can be a problem in some cases, basically it alters the frequency response at low level settings.

One way to isolate it is to use a buffer in front of it.

Quote:
- Full passive network with fixed input/output impedance but variable voltage divider.
That's another way.

Quote:
- Other ?
TVC.

Quote:
What is the advantages of tube rectification to solid-state rectification ?
No minority carriers with tube rectification so no spikes at turn off.

Quote:
Does a transformer coupled output stage require a protection circuit when there is an infinite or zero impedance (opened/shorted) at the speaker terminals ?
With no signal on the primary side this shouldn't kill it, however with signal present an open circuit ( infinite impedance) could kill the OPT.
To protect it from no load conditions I usually solder a 1K resistor accross the secondary winding.

Cheers,
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