Yeeeup, you've got a two part problem there. Based on the design, and in my best used car salesman's voice, I can most assuredly say, " They all do it!"
It is a problem of light filtering, coupled with Fisher's absolute devotion to operating the heaters of the audio tubes in the most sensitive portion of the circuit on DC voltage. In this case, it's the old power the phono preamp and line amplifier tube heaters from the cathode current from the output stage trick. In this case, it's the 12AT7 phono preamp, and 12AV6 line stage tube heaters that are connected in series, with the series string then powered from the quad of 6V6 output tubes. However, while the series string wants 25.2 volts to properly power both heaters, the cathode voltage a the output tubes is only 21 volts. Therefore, the warmup time would be veeerry long indeed -- and on all functions, too.
But during the extended warmup time, the unit would go through a series of different hum tones, based on what was happening in the unit at the time. At turn on:
1. There should be nothing other than any hum induced from magnetic coupling between the power and output transformers -- but as I recall, they are far enough apart that this should not occur. B+ voltage should appear within about 3 seconds of turn on, due to use of the quick warming 5U4 rectifier tube, but this is not a cause for any noise. Then,
2. The output tubes warm up (at about the 15 second mark), but as they do, the unequal plate currents through each half of the OPT will cause incomplete cancellation of B+ ripple, so some 120 Hz hum will appear, but should fade rather quickly with good tubes as they heat (assuming they are well balanced as well). Then,
3. The 12AU7 phase inverter tube is also heated by now, but two conditions cause it to pick up and amplify the hum signal that endures until sound is produced:
A. The AC heater circuit of this unit utilizes one winding for all heaters. Since one side must be directly grounded to satisfy the proper decoupling needs for the RF/IF circuits, this prevents the use of balancing resistors to minimize AC hum in the audio circuits. Note that Fisher wisely went to dual heater windings in their stereo receivers, so that the tuner winding could be directly grounded, while the audio winding could be balanced to ground.
B. Until the 12AV6 warms up -- which could take a year or two -- it does not draw any current. But the 12AU7 phase inverter tube immediately after this tube is hot, primed, and ready to go. With the minimal filtering in place supplying the 12AV6, and no voltage drop occurring through the B+ dropping resistor or the plate resistor for this stage, it means that the 12AU7 is effectively amplifying the noise on the MAIN B+ line until the 12AV6 warms up. When it does, voltage drop occurs across the B+ dropping and plate resistors, which improves the noise rejection at the plate of the 12AV6, and the hum goes away. But it takes a year and a half for it too!!!
A couple of points for resolve of both issues:
1. Consider dropping in a small 12.6 vac 1.2 amp R/S transformer, with bridge rectifier, appropriate dropping resistor, and a couple of 4700 uF filter caps to operate the 12AT7 phono preamp and 12AV6 line stage amp heaters from (rewiring the heaters to be a 12.6 volt circuit). You would also have to drop in an appropriate cathode bias resistor for the output stage to replace the heater string no longer connected there.
You could even operate the 12AU7 phase inverter tube from this supply, which would not only maintain DC on the original heaters that operated from this type of voltage, but also extend DC heater operation to the phase inverter stage, AND allow for all of these heaters to operate in a balanced fashion to ground if necessary, because they would be operating from a separate supply. That would kill two birds with one stone, eliminating the extended warmup time, and therefore the long period where hum -- albeit now at a likely lower level -- would be audible.
Short of that exercise, you might also try this (or in tandem with the previous suggestion):
I can't tell what size the filter cap is that is supporting the 12AV6 stage, but up it as necessary to ensure that it is at least 30 uf. Then, insert a common 1N4007 diode between that cap, and the B+ dropping resistor supplying that cap with B+ voltage (NOT the plate resistor). The banded end of the diode should connect to the bypass cap and plate load resistor, while the other end of the diode should connect to the end of the B+ dropping resistor that has been disconnected from the cap. This little trick just might help with the hum issue during the cold winter warm up period, if the heaters of the tubes in question are still powered from the output stage cathode current.
I hope this helps!
Dave