Fixed Bias is a confusing tube-industry term.
Only a handful of vintage H.H. Scott
amplifiers employ Fixed Bias that is truly fixed. By that we mean there's not a
Bias Control potentiometer to be found! Such models include: Types 208, 222-C,
299-C, 296, and 340 receiver. Assuming everything else is healthy, you're good
to go. (No need for you to continue reading this page.)
However, the
majority of remaining vintage H. H. Scott amplifiers employ Fixed Bias
featuring a variable Bias Control adjustment (potentiometer) for each
output channel. (Also see Self-Bias).
Setting
the (DC) Bias is fairly easy if you follow these steps.
Disconnect the speakers and connect a 16-ohm, 2-watt, (non-inductive)
load resistor. Carbon-compositions will work fine as no signal load will be
passing through the amplifier on this test. (Do not use partial, 2-watt loads on the AC
Balance adjustment!)
Use a low-resistance, mA (milliammeter) and connect it across the current
sensing resistor in the output tubes' cathode circuit of the channel under
measurement. (Some Scott amps provide convenient test points, while others
require you to measure directly from the underside of the chassis. (CAUTION:
If you are unsure of what you are doing, seek qualified assistance, tangling
with 400+ volts can ruin your day!)
The original factory current
specification is listed. But with modern AC line voltages being higher, many
vintage tube hi-fi collector's choose to run their bias current roughly 10
per cent "cooler" to extend component life. While this
"compromise" does yield slightly lower output power, it typically
cannot be "heard" by most critical listeners.
| H.H. Scott Type |
Total Bias Current
per Channel
(not per tube) |
Output Tubes |
| 200-B/LK-30 |
35 mA |
6GW8 |
| 208 |
70 mA |
7591 |
| 299-B |
55 mA |
7189 |
| 222-C/D |
44 mA |
7189 |
| LK-48-A/B |
44 mA |
7189 |
| 233 |
65 mA |
7591 |
| 299-C/D |
70 mA |
7591 |
| LK-72-A/B |
70 mA |
7591 |
| 340-B |
70 mA |
7591 |
After allowing the amplifier to reach stable operating temperature, (and
at a calibrated AC line voltage); adjust the appropriate channel's (DC) Bias Adjustment control to match
your amplifier's (see above chart) total cathode current for the push-pull
outputs in that channel. Repeat for remaining channel. The channels can
interact, so recheck and readjust if necessary. The 345 requires a bias
adjustment on each tube and the values for each tube are half the channel
total.
