2020 UPDATE:
It's been 5+ years since I started this thread, but I now finally have two of the EL34 versions of this amplifier here for service and restoration, and have determined some new information that I feel important enough to pass along to the Fisher community for those owning these amplifiers.
These amplifiers represent the last version of the 55A amplifiers produced, in that they employ EL34s -- and -- they both have a 4Ω output tap. Earlier versions of the 55A -- even those employing EL34 output tubes -- did not have a 4Ω tap like these do, so they do indeed represent the final production form of these amplifiers.
One piece of basic new information concerns the output transformers. It was unknown if Fisher changed these from the standpoint of reflected primary impedance when they changed from the 6550 to the EL34 in these amplifiers. Now that I've measured the transformers on both the 6550 and EL34 versions, it is clear that there were no changes made to them when the change was made between these tubes. And of course, there would be no reason for this specification to change when the 4Ω output tap was added. By all performance indications then, these transformer are identical to the transformers on the original 55A amplifier with 6550 output tubes, except they include a 4Ω output tap.
METER: The meters did change appearance wise from the original to final version of the 55A, with most such meters being very inaccurate by now. The original meter had an expanded meter face section for setting the bias, and a more compressed section to indicate power output. In later versions, this was changed to allow for a more expanded power output section to aid in its usefulness, As long as the field winding is still good within the meter, these can often be restored to a more usable level of service than they typically represent by now.
These meters are quite unique, with the power output indicating section of the scale having non-linear graduations, and a dual sensitivity design built into the meter movement itself. The non-linear graduations acoss the power output scale are necessary because the relationship between output stage current draw and power output produced is non-linear itself. The dual sensitivity is necessary to allow the meter to properly respond to both the static current conditions (only) that exist when setting the bias, and the static plus dynamic current conditions that exist when producing power output. The split in the sensitivity of the movement occurs at the 0 indicator on the scale, with the movement being much more sensitive to current flow above the 0 point, than it is below it. That is, with a properly operating meter, it takes 125 mA of current flow through the movement's field coil to produce an indication with the pointer centered inside the black 0 section at the top of the red scale. This level of current flow represents the nominal quiescent bias current for the output tubes. Yet the meter will also produce a full scale indication with a total current of 300 mA flowing through it as well. In other words, with just 140% more current flowing through the meter than it takes to produce a 0 mark indication, the meter will produce a full scale indication, which represents a 700% increase in response above the 0 mark, versus the current flow required to move the needle from its resting position to the 0 mark. That is quite a difference in sensitivity! Today, most of these meters are typically non-functional, or obviously reading incorrectly. With a little luck however, you can usually restore the most important part of their functionality, and with a lot of luck, all of it.
A potentially significant issue are two enemies of any permanent magnet, which is a principle part of the movement within the meter -- time, and heat. Time always tends to diminish the strength of a permanent magnet, and heat does too. With enough heat, a permanent magnet can have it's magnetic properties permanently diminished. It's been 60+ years since these meters were manufactured which is the time element, and the combination of amplifier heat and heat from the illumination lamp that was installed right under the movement inside these meters is the heat factor, that just piles on to the problem. Heat from the bulb alone has discolored the meter bezel in some examples of these meters I've seen. Additionally, it's also possible that time and heat acts to harden the characteristics of the return spring tension as well. The result is that over time, the meters then begin to read low, or hardly at all. As long as the internal field winding is good however, then with a DVM, a single resistor, and a variable low voltage power supply, it's not too hard for those so inclined to re-calibrate these meters to compensate for the losses of time and heat.
First, a look at how many examples of these meters operate today:
BELOW: A 15 volt 1A variable DC power supply along with a 50Ω 10W resistor, a good DVM (set for DC mA operation), and the Fisher meter are all connected in series. With the power supply adjusted for 125 mA of current flowing through this circuit, it produced the indication as shown on the meter when the amplifier arrived here. The nominal bias setting is achieved when the pointer is centered on the 0 mark with no signal passing through the amplifier. If this meter were used to bias the output tubes in the amplifier as is, then the output tubes would surely be red plating with such an adjustment:
BELOW: To correct this, carefully disassemble the meter, and you will find the adjustment that tensions the return pointer spring. It is the balance of return spring tension versus magnetic pull created by the field coil/magnet assembly that determines the calibration of the meter. With the circuit adjusted for 125 mA of current flow, careful movement of this adjustment will allow the pointer to again rest over the center of the 0 mark as it should. You will want to use a non-magnetic tool to make the adjustment, and preferably even a non-metallic tool if possible. Also, it will take some trial and error to achieve the best accuracy, since even the installation of the rear cover will change the calibration slightly. Patience then is the order of the day:
BELOW: After calibration, the meter is reassembled now, with the same 125 mA of current flow causing the pointer to now rest in the center of the 0 mark. Camera parallax error shows the pointer as slightly off center within the 0 mark, but in reality, it is properly centered:
Now, if you're really lucky, advance the current flow to 300 mA, which ideally produces a full scale indication, but this will not always be the case. Of the two meters from the amplifiers here, one will in fact produce a full scale indication with 300 mA total current flow through the meter, while also producing a centered 0 mark indication with 125 mA of current flow. When installed, this meter is also remarkably accurate in indicating the correct average power output of the amplifier as well.
BELOW: For the other meter pictured above however, I wasn't so lucky, with a 300 mA current flow indicating about 57 watts on the meter face as opposed to a full scale 65 watt indication. However, this is still way, way better than it was, and of the dual function features of the meter, the accuracy in setting and monitoring output tube current flow is by far much more important than the accuracy in monitoring power output, with the usefulness of that function being quite limited in practical use anyway. That's why of the two calibration points of the meter, adjusting it for an accurate 0 point indication at 125 mA current flow will produce the most useful results, and return the meter to the best overall operation for which it is capable today.
Up next, the strange case of instability that both of these amplifiers arrived with which would not be unique to just these two particular amplifiers, and an age old problem of the original 50A series of amplifiers rises again: poor HF transient response. No wonder my client was very disappointed in the sound these amplifiers produced when he received them! Next time.
Dave