The bias voltage should be higher (more negative) than you're showing. Sounds like some work has already been done, as you have the 10 ohm cathode resistors installed to determine current draw on the output tubes.
Is your unit originally set up for EL37 tubes or 5881 (6L6 family) tubes? It will be labeled on the chassis. The 5881's came later and Fisher modified the bias circuit to provide a higher (more negative) voltage to properly bias the 5881's. What set up you have is part of the answer. You want yours set up for the 5881 type tubes to start.
Lift one end and check the values of the power resistor in the string with the 12AX7 heaters and the grid supply resistor (33K in some models) in the supply line to both output tube grids.
Realize that the Sams schematic is in error in listing EL34 or KT66 tubes. The Fisher 500 mono receiver never used either.
Also, when calculating plate current and dissipation, you need to subtract the screen current draw from the cathode current you calculate from the voltage across the 10 ohm cathode resistors. You can actually determine screen current draw by measuring the voltage drop across the 68 ohm screen resistors on the output tubes and using Ohm's Law.
You may find this thread helpful--it dealt with the same question and Dave Gillespie (in post #7) gives a great explanation of how the bias circuit in the 500 functions and how you can modify it to be able to adjust the negative bias voltage in the -36V supply:
This circuit is a variation of Fisher's tried and true formulation of having the output tube cathode current power the heaters of certain small signal tubes. In this case however, it is the current flow of the entire receiver drawing current through the small signal tubes -- and of course that 400Ω resistor. As such then the success of the circuit depends on the bias voltage needs of the output tubes (to produce the correct quiescent current) being basically the same voltage that it takes to light the heaters of three 12AX7 tubes wired in series using their 12 volt heater configuration...
This thread may also help:
EL34 and KT66 were never used in the 500 - only EL37 or 5881 - mistake on Sams part. Fisher had a service bulletin, I believe, to increase the bias voltage when 5881 were used, adding a 50 Ohm 2W resistor in series with the cathodes to ground. There's a K-100 manual (console version of 500, with an eye tube) on my website:
http://www.audiophool.com/Schem_A/Fisher_K100_SM.pdf. This shows -31V bias, probably right for EL37, but Sams shows -36V for 5881. The three preamp tube heaters are the negative B+ lead, giving around -36V for bias - if it's higher, then add a cathode resistor...
