Fisher 500-C Bias
The Story: Recently, I pulled my Fisher 500-C out of the barn, where it had been stored since 1985. Cosmetically, it still looked to be in pretty good condition. I knew the challenge would be in getting it operating again, after 33 years sitting in the barn.
The first task was to test the B+ electrolytic capacitors. This was accomplished by disconnecting them from the circuit and slowly applying voltage while monitoring leakage current, using my trusty Sprague TO-6. As it turned out, I was able to reform all the original capacitors to where they showed low leakage up to their full rated voltage. Seeing has how they were still all good, I left them in place.
The second step was to carry out some modifications based on posts I had read here on AudioKarma. I added screen grid-stopper resistors and 10Ω cathode resistors for the output tubes. I also modified the bias supply, changing out the original capacitor and selenium rectifier and adding a bias adjust potentiometer, reducing the output tube control grid resistors from 330kΩ to 220kΩ and changing the four coupling capacitors feeding the output tube grids from 0.047µF to 0.068µF. A CL-80, current limit resistor, was added to the line input.
The third step was to slowly power up the receiver using my trusty Variac™. The good news is that as more voltage was applied, the Fisher awoke from its 33 year slumber and produced sound. Although, the FM tuner was a bit off, I was able to pull in radio stations in stereo. The less good news is that I discovered the output tube bias voltages and currents were not particularly well matched. The four output tubes were GE brand 7591A purchased sometime in the early eighties. The tubes were not a matched set when purchased and they were not well matched in terms of bias current for a given grid bias voltage. If I set the bias voltage to correctly bias the highest current tube, the lowest current tubes would be running about half current.
The fourth step was to modify my bias circuit to add individual bias voltage adjustments for each of the four output tubes. Once done, I now had a very good ability to adjust the individual bias current on each of the output tubes. Of course, once one can measure the bias current, one finds that it is not necessarily a set once and forget proposition. Over time there is some drift as the tube warms up and settles in and as the input AC line fluctuates the bias voltage and currents will also vary. At first, I recorded some readings manually, however; it was time consuming and a bit awkward.
Four Channel Bias Board:
The fifth step was to automate the recording of the bias current VS time. For this task, I used a DATAQ DL-145 data logger. For useful readings I had to build a buffer / gain interface between the 10Ω current sense resistors and the DL-145 data logger. The end result is that I now have the ability to monitor the bias current over time.
Here is an example of a Bias VS Time chart:
The Story: Recently, I pulled my Fisher 500-C out of the barn, where it had been stored since 1985. Cosmetically, it still looked to be in pretty good condition. I knew the challenge would be in getting it operating again, after 33 years sitting in the barn.
The first task was to test the B+ electrolytic capacitors. This was accomplished by disconnecting them from the circuit and slowly applying voltage while monitoring leakage current, using my trusty Sprague TO-6. As it turned out, I was able to reform all the original capacitors to where they showed low leakage up to their full rated voltage. Seeing has how they were still all good, I left them in place.
The second step was to carry out some modifications based on posts I had read here on AudioKarma. I added screen grid-stopper resistors and 10Ω cathode resistors for the output tubes. I also modified the bias supply, changing out the original capacitor and selenium rectifier and adding a bias adjust potentiometer, reducing the output tube control grid resistors from 330kΩ to 220kΩ and changing the four coupling capacitors feeding the output tube grids from 0.047µF to 0.068µF. A CL-80, current limit resistor, was added to the line input.
The third step was to slowly power up the receiver using my trusty Variac™. The good news is that as more voltage was applied, the Fisher awoke from its 33 year slumber and produced sound. Although, the FM tuner was a bit off, I was able to pull in radio stations in stereo. The less good news is that I discovered the output tube bias voltages and currents were not particularly well matched. The four output tubes were GE brand 7591A purchased sometime in the early eighties. The tubes were not a matched set when purchased and they were not well matched in terms of bias current for a given grid bias voltage. If I set the bias voltage to correctly bias the highest current tube, the lowest current tubes would be running about half current.
The fourth step was to modify my bias circuit to add individual bias voltage adjustments for each of the four output tubes. Once done, I now had a very good ability to adjust the individual bias current on each of the output tubes. Of course, once one can measure the bias current, one finds that it is not necessarily a set once and forget proposition. Over time there is some drift as the tube warms up and settles in and as the input AC line fluctuates the bias voltage and currents will also vary. At first, I recorded some readings manually, however; it was time consuming and a bit awkward.
Four Channel Bias Board:
The fifth step was to automate the recording of the bias current VS time. For this task, I used a DATAQ DL-145 data logger. For useful readings I had to build a buffer / gain interface between the 10Ω current sense resistors and the DL-145 data logger. The end result is that I now have the ability to monitor the bias current over time.
Here is an example of a Bias VS Time chart:
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