Hi Frank -
First of all, is the circuit stock/original? You didn't say anything about this set's level of revision, or upgrade, or whether the output tubes are old NOS, or possibly worn out/depleted tubes from the 1960's, or modern day East European or Russian "
equivalents". ALL of these things have a vast affect on the sound, the reliability, and the potential safety of the Fisher 500c unit in operation ((you don't want to burn down the house, right!))
There is a tendency for the original siemens bridge rectifier present in the bias circuit to fail gradually over time. This Bias circuit also directly affects the filament voltages on several of the small 12AX7 audio tubes in the input and tone control circuits, something that will greatly affect the overall sound. So that bridge and a few other parts in that circuit must be replaced/adjusted in value in order to protect from over-voltages / under-voltages in the circuit, or the transformers and tubes are at risk for sudden catastrophic failure.
If parts have been revised and upgraded, one can measure the working bias voltage across the output tube grids (nominally -17 Volts on original 1960's 7591A's) since there are no cathode resistors and no pots for adjusting the bias (the circuit expects really matched pairs of output tubes) in the circuit. Also, if the four 330K resistors in the output circuit are quite close to their proper values, and the bias is close to the -17 Volts on each of the output tubes pin #6, and if the Plate voltages are also close to 430 Vdc., then you will have the expected bias levels present in the output circuit - if you are using NOS matched 7591/7591A tubes.
However, if using original 7591's that have gone "
soft" or become depleted somewhat, or modern day equivalent tubes, all bets are off, due to the lack of adjustment controls, and the fact that since 1965 the AC Line voltages have risen from about 110 VAC to more like 124 VAC about a 12-15% increase since the set was manufactured. As a result, the 430 Volts expected on the Plates of the 7591 output tube might even be near 490 Volts now, and the Bias voltage could be just about anywhere at this point.. The result could be that the circuit is possibly running the output tubes way too hard, resulting in gain depletion, which would sound mushy, with the output tubes running at a whole other Operational Point from the one intended. Also, as stated before, if the likely situation is that the bias voltage is too low, then the filament voltage of the 12AX7's is too low, and that can also affect the sound adversely, while also running the output tubes way too hot, the worst possible combination!
One might be quite tempted to replace various parts too moderate all of the listed voltages to those seen on the original schematic or maybe even put in some small value individual cathode resistors (say 20 ohms 1/2 Watt on each tube) using modern close tolerance parts, to make sure the bias currents are equally adjusted across each push-pull pair, and that perhaps it should even be made more negative than seen in the original schematic. I'd say -21 or -22 Volts might even be a better operational point to use with a nominal 450 Volts on the 7591's Plates (especially when using modern day tubes as opposed to NOS) or even with NOS - to avoid running the output tubes to depletion in just a few months.
Also testing the output tubes; the power supply caps; and the high voltage rectifiers might be a good idea, if the set is still all original... While I usually like to keep a vintage set as much "
All Original" as I can, replacing the coupling caps in there with modern day Polypropylene or Metalized Polypropylene of the same, or perhaps double the original values, might improve the overall sound satisfaction well away from the "Mushy" - all other things being equal.
Lastly, when measuring voltages near 500 Volts DC, remember these can be
LETHAL!... Sounds like this 500c needs
PROFESSIONAL HELP!-Steven L. Bender, Designer of Vintage Audio Equipment
The sound is getting a little mushy. How do I bias the tubes? Any help will be much appreciated!
Frank
