Fixed Bias

Alternate Fixed Bias Method

 


Archives Schematics
Site Map InstaBase

Up
Next


 

Reading Room
Thermionic
Audio Books


Last Edited:
12-Sep-2004


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.

 

Home Archives Join Scott Forum Join Scott E-List Link Guide

Search this site
powered by FreeFind

Copyright 1998-2003 HHSCOTT.COM. All rights reserved.
Terms of Use

Web Designed & Managed by Lee K. Shuster