Specifications

The second point is obscure, but relates to a feature that Hafler always championed. The Dyna

Biaset resistors in his amplifiers serve as a current measuring device similar to the 5.0 ohm resis-

tors mentioned above. But they also act to significantly reduce IM distortion in fixed bias

amplifiers. Hafler went to some length describing this fact with the introduction of his Mk III

amplifier. It is a fact I have also confirmed for myself. The value Hafler chose produces readings

equal to that of a fresh D battery when the bias is properly set, but it also typically cuts IM dis-

tortion by 35%, as opposed to operation without the resistor. However, to gain all of the

improvements this resistor provides requires that the cathodes remain connected directly togeth-

er. As opposed to using separate resistors for each tube, maintaining a direct connection

minimizes the internal impedance of the amplifier, which maximizes speaker damping. With my

modification to the SCA-35, I have elected to retain this feature, and have chosen a value for a

common resistor that produces maximum reduction in IM distortion. Providing individual bias

controls for each channel will ease the tube matching requirements to two matched pair, and also

allow the direct cathode connection to remain permanently in place. However, if independent

biasing for each tube is desired, for best performance, it should not be done at the expense of the

direct connection.

The proper way to install individual adjustments then is to install individual 10 ohm cathode

resistors and appropriate bias controls without a permanent direct connection, but also include a

switch or jumpers to remake the cathode connection in each channel after the adjustments are

made. Of course, this requires further modification from the original design. In my amplifier, I

was interested in keeping the modification as simple as possible, so the direct connection was

made permanent in each channel, and a matched quad of output tubes installed to obtain the

results provided.

3. With the conversion to fixed bias, control

grid swamper resistors and screen stability

resistors are now required. These are shown on

the schematic of the complete modification,

and can be added quite easily at each output

tube socket as shown in Figure-7. The 1000

ohm control grid resistors are simply installed

in place of the wire that originally connected

pin #2 of each output tube to the appropriate

eyelet on the circuit board, while the 100 ohm

screen stability resistors are mounted directly to

the output tube sockets themselves.

To install them, first move all wiring that origi-

nally appears on pin #9 of each output tube to

pin #8. Then, connect the screen resistors between pins #8 & #9. The control grid resistors elimi-

nate any possibilities of parasitic oscillations, while the screen stability resistors eliminate the

possibility of output tube arcing in fixed bias amplifiers under certain conditions. For more infor-

mation on this, I refer you to my article entitled “Techniques To Maximize Power Tube Life”,

published in AudioXpress as a web exclusive article on their web site in 2005.

http://www.audioxpress.com/magsdirx/ax/addenda/index.htm

Figure-7. 1000 ohm grid resistors and 100 ohm screen

resistors are installed at the output tube sockets.