What is the optimal line voltage for a 500c?

Lo-Fidelity

Super Member
I understand that today’s line voltage are higher by say 5 to 15 volts than back when this gear was new. Voltage at my house is 122 tonight, but I have seen 124.

What is the ideal voltage to run a Fisher 400, 500, 800, etc? Currently I am using my Variac to hold 115, but might build a bucking transformer to knock the voltage down some.
 
I understand that today’s line voltage are higher by say 5 to 15 volts than back when this gear was new. Voltage at my house is 122 tonight, but I have seen 124.

What is the ideal voltage to run a Fisher 400, 500, 800, etc? Currently I am using my Variac to hold 115, but might build a bucking transformer to knock the voltage down some.
Same voltages at my house, I use a bucking transformer which takes the wall voltage down to around 117acv...
 
I took the following information from my 500C receiver (serial# 74907X):

Assuming that a single CL-80 type current limiting device has been installed in one primary lead of the power transformer, then 120.0 vac as received at the unit (i.e., before the CL-80, and therefore as measured at the unused auxiliary AC power receptacles) will produce 6.30 vac at signal strength meter's illumination lamp socket terminals after 1 minute of operation from a cold start (allowing time for the CL-80 device to stabilize). This lamp socket represents the "end of the line" for the audio related 6.3 vac heater circuit, and is therefore the best place to measure the voltage of this circuit. With 6.3 vac appearing at this point, it ensures that the other devices connected to this circuit will be receiving at least 6.3 vac, and/or minimally error slightly to the high side of it, which is the way any error should lean from optimum, with all voltages on the circuit then being well within a -0/+5% ideal range.

When the power transformer heats from extended operation, all voltages will drop slightly due to the properties of copper used in the windings of the transformer. As a result, I operate my 500C directly from my AC line (typically 122-123 vac), with no other voltage adjusting devices used other than the drop produced by the CL-80 current limiter. This typically results in no more than 6.45 vac at the meter lamp's terminals after a stabilized cold start, with it hovering just above 6.30 vac when the power transformer reaches its stabilized operating temperature, and 122 vac is applied as measured at the auxiliary outlets.

I hope this helps!

Dave
 
I took the following information from my 500C receiver (serial# 74907X):

Assuming that a single CL-80 type current limiting device has been installed in one primary lead of the power transformer, then 120.0 vac as received at the unit (i.e., before the CL-80, and therefore as measured at the unused auxiliary AC power receptacles) will produce 6.30 vac at signal strength meter's illumination lamp socket terminals after 1 minute of operation from a cold start (allowing time for the CL-80 device to stabilize). This lamp socket represents the "end of the line" for the audio related 6.3 vac heater circuit, and is therefore the best place to measure the voltage of this circuit. With 6.3 vac appearing at this point, it ensures that the other devices connected to this circuit will be receiving at least 6.3 vac, and/or minimally error slightly to the high side of it, which is the way any error should lean from optimum, with all voltages on the circuit then being well within a -0/+5% ideal range.

When the power transformer heats from extended operation, all voltages will drop slightly due to the properties of copper used in the windings of the transformer. As a result, I operate my 500C directly from my AC line (typically 122-123 vac), with no other voltage adjusting devices used other than the drop produced by the CL-80 current limiter. This typically results in no more than 6.45 vac at the meter lamp's terminals after a stabilized cold start, with it hovering just above 6.30 vac when the power transformer reaches its stabilized operating temperature, and 122 vac is applied as measured at the auxiliary outlets.

I hope this helps!

Dave
Hmmm , very interesting! I will have to check what i read on my 800c..... Thank you
 
Dave,

At 121vac input line I get 6.43vac on the illumination light pins.

Only potential issue is at 121vac input I am getting 17.8v on V 8-11 pin 6 instead of 17.0v. Guess I need a little larger R-132? Current R-132 is 5.9k with -25.25v at capacitors and 18.23v going out and 17.8v at the pin 6s.

Thoughts?

Forgot to add that the current limiter is a CL-60.

Rick
 
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Lo -- The proper current limiter would be a CL-80 for this unit -- a CL-60 would be good for a 5A circuit and is much too large. Changing from a CL-60 to a CL-80 won't change your voltage readings to any significant amount. Otherwise, it sounds like your standard line voltage is just fine for your unit.

The important measurement regarding your output tubes is not the pin 6 voltage, but the pin 5 voltage after you have removed the original pin 5 wiring (which grounded these pins), and installed 10Ω .25W 1% resistor from these pins to ground at each tube socket. The pin 6 voltage is based on the characteristics of a new bogey 7591 tube, as was made in America some 50+ years ago. However, between the somewhat different characteristics of the tubes manufactured today, normal manufacturing tolerances, and the still good and in use but well used American tubes of yesterday, the bogey value pin 6 voltage could be well off from what is really needed for the tubes to draw the proper recommended quiescent (no signal) current draw -- which is the end game of what you are really trying to achieve with the proper pin 6 voltage for the tubes you are using. If you have not installed cathode sampling resistors, it should be tops on your to-do list for the unit, which will then let you measure and know how much current the tubes are drawing in the stock configuration. From that information, you can then determine if the tubes are biased fine as is, are all over the map, and whether you need to do nothing, add a single bias control, or add an individual bias adjustment modification, so that you can bring each tube in line with the target value.

I hope this helps!

Dave
 
Dave,

Went to Fry’s today for the CL-80, but they didn’t have one (we’ll thats what they said anyway), so that will have to wait until I need to order bits.

The voltage over the pin 5 to ground resistors as follows.

V8. 288
V9. 345
V10. 298
V11. 369

The voltage after R-143 is 402v versus 375v. Any concern over this?

Still have 6.45vac at illumination bulb pins.

Thanks,
Rick
 
The voltage after R-143 is fine. V-9 is on the money, V11 a little hot, and V8 and V10 a little cool. The target is 35 mA, or 350 mV. Pair V8 and V10 together in one channel, and V9 and V11 together in the other channel. The unit is certainly safe to run as is, but with these tubes, you could at least benefit from the installation of dual bias controls -- one for each pair of tubes (i.e., each channel). When you get a CL-80 installed, it will drop the heater and B+ voltages slightly, bringing them even closer to target.

Dave
 
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