The Fisher 500C

The Fisher 500C is probably one of the best known pieces of Fisher's vacuum tube hi-fi equipment

It is a FM stereo receiver. The front end of the tuner uses Nuvistors which are very small

metal encased vacuum tubes. They were an attempt by tube designers at miniaturization.

They would most probably have been successful if it was not for the introduction of

the transistor. The two used in the 500C are 6CW4's. They are single triodes and sit

on the small subchassis that makes up the front end of the FM tuner. If you own a 500C,

it would be a good idea to try and pick up a couple spare 6CW4's since they are a bit hard to find

and no one makes them now.

Schematic for 500C

The 500C uses a fairly straight forward power amp section consisting of a 12AX7 phase splitter and

a pair of 7591 triodes in push pull for the output stage of each channel. The Fisher 500C is capable of

producing 35 watts per channel which is comparable to the Dyna ST70 and the Scott 299 C and D integrated amps..

The 500C's competition in the FM receiver market comes in the form of the H.H. Scott 340B. I own both the 500C and

340B and can say that the Fisher is a bit louder, but to me, the Scott sounds a bit better. Don't get me wrong, I really like my 500C and was using it on almost a daily basis until I decided to set it aside for a complete restoration both electronically

and cosmetically. Both the Scott 340B and the Fisher 500C use a solid state power supply as opposed to the

traditional vacuum tube power supply found in the Dyna ST70 and Scott 299 series of amps. To my ears, The 340B and

500C have a little bit more punch then the ST70 or 299, but the receivers lack a bit of that classic tube sound that

only an all tube amp can deliver. Because of the high current demands of these receivers, the manufacturers had

two choices. Use a two rectifier tubes connected in parallel like the 50 watt Operadio power amps of the 40's or

go with a solid state rectifier. I'm sure that the cost factor dramatically influenced both Scott and Fisher's decisions

to abandon the 5AR4 in favor of the silicon rectifiers. What is different about the 500C's power supply is that

it employs a voltage doubler circuit as opposed to the bridge rectifier found in the Scott 340B. Although

both types of power supplies will get the job done, the voltage doubler will require substantially larger filter capacitors

to keep the power supply ripple under control and eliminate any objectionable hum.

The 500C is prone to power supply capacitor failure like any other 35 year old piece of hi-fi equipment.

The pain with this receiver is that the values for these capacitors are hard to find and you usually end up having to

be creative with respect to what you use. I have actually drilled out the guts of the capacitor cans and slipped

a new axial lead capacitor in the can to replace at least one part of the capacitor. Then I add what ever other capacitors

I need to some solder lug strips that I attach to the chassis. By doing this, the amp looks cosmetically correct

from the top. The only way you could tell that it was worked on would be to pull the chassis out of it's wood case,

turn it over and remove the bottom plate.

Here is the list of things I have found advantageous to replace in the 500C:

1:The 4 grid coupling capacitors from the 12AX7 to the 7591's. (.047uf @ 400v preferably metalized polypropylene)

2: The power supply filter capacitors.

3: The bias rectifier (5A 250 piv this is readily available at Radio Shack)

4: The filter capacitors for the bias circuit

5: All the electrolytic cathode bypass capacitors (there are two for the 12AX7 driver/phasesplitter, and a couple in the tuner)

6: The 4 resistors that connect the negative grid bias to the grids of the 7591's. (If they are over 1% different in value.

They should be 330K ohm.)

7: After replacing the bias rectifier, check to see that the 7591's do NOT exceed 70 ma of plate current for both tubes.

Because the 7591 is so hard to find and so expensive, I prefer to bias them at no more then 65ma per pair.

The amp sounds no different, but the tubes last longer and the transformers run notably cooler. There are

several mods that can be made to the tuner, and if you do a deja-news search, you should be able to locate them.

Personally, I like my 500C just the way it is. One more oddity of the 500C is the fact that it uses a relay to

switch on the stereo circuit when the tuner receives a FM stereo broadcast. This relay is what causes the

distinct click you hear when you tune in a stereo station. The 500C has a simple but nice sounding preamp

section that is capable of processing signals from CD players as well as magnetic phono cartridges. It does

not, how ever, have sufficient gain to properly reproduce signals coming out of low level moving coil phono

cartridges. But, most vintage amps do not have the ability to properly reproduce signals coming from

moving coil cartridges anyway. So, the Fisher is not alone in this.

It has a tape send and receive, and also has the added bonus of a pair of connectors that allow you to

inject a reverb or other processor in the tape loop. You could use this for an equalizer or reverb.

The amplifier section of the 500C is extremely versatile, simple and great sounding.

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