Junkbox Fisher Project

dcgillespie

Fisher SA-100 Clone
Subscriber
Back when I was looking for an IF transformer to repair the AM section of my 800C, I discovered that the the typical Custom Electra tuner/preamp chassis also used the same IF transformer for its AM section. I found a nice enough example of one of these tuner/preamp chassis on the auction site (nice, primarily to help ensure that the transformer sought would be good), and bought it. Of course, while I was awaiting its arrival, a kind AKer stepped up and offered a known good IF transformer pull that I couldn't pass up. So now I had a good IF transformer that in fact made everything right with the AM section of my 800C, and an unmolested, good condition 590-T Custom Electra tuner/preamp chassis as well. What to do with it? As is, without a matching power amp chassis, it's pretty much useless operationally, becoming either a parts unit itself, or at the very least, starts taking up space on a storage shelf. Unless however, a suitable power supply could be built to operate it........

Based on their rather common enough availability, and the price they go for (versus that of the power amplifier section), these things are clearly thought of as a sort of Fisher stepchild when sold as a stand alone item. But they really have much to commend them for:

1. Except for the Broad/Sharp AM IF response feature, the AM section is (for all intents and purposes) identical to that of the 800C.

2. The FM RF front end is clearly low brow for Fisher, using a single (but common) dual triode 6AQ8 to perform RF amplifier, Oscillator, and Mixer functions, but performs these functions quite well enough, even for good stereo reception in my rather rural location.

3. The IF strip is a 3 tube design that is very similar to early version 400 receivers, but uses instead an easier to obtain, and even higher Gm 6HS6, rather than the near unobtanium 6HR6 as used in both the 800C and early 400 receivers.

4. It employs the excellent MX-65 multiplex unit as found in Fisher's best tuners and receivers.

5. Except for including (an easily defeatable) permanently wired "on" rumble feature, the on board magnetic phono preamps are the same basic design as employed in virtually all other stereo Fisher offerings. And, since their heaters were operated off of DC from the power amplifier, they are just as quiet as well.

6. The tone control design is identical to that used in the X-101C, and virtually all other stereo Fisher units that use passive tone control designs.

With flexibility that provides Tape Output and high level Aux and Tape input features, as well as the coveted Reverb In and Out jacks (in case you were wondering, that was a joke), what's not to like about this baby? Yeah, it's kind of physically cumbersome in a stand alone setting, but functionally and by appearance, it's all Fisher.

Building a power supply for such a unit to mimic its original operating conditions is not a casual throw together project, because the supply must provide four very different and significant supply sources. To produce the same caliber of original operating qualities, it must provide:

1. 6.3 vac at 4 amps(!) -- by the time a small rectifier tube is included -- to power the heaters in 10 of the 13 tubes this unit employs, along with its two dial lamps,

2. 35 volts at 150 ma of well filtered DC to power the heaters of the three audio tubes in this unit,

3. 180 volts of decently regulated DC to power the widely varying B+ current requirements of the AM and FM tuner sections (as well as the 19 and 38 kHz portions of the MPX section) when they are engaged, which is then drastically reduced when they are not selected. This can therefore vary from as little as about 15 ma, to over 50 ma, depending on the function chosen, and whether a station is tuned in or not.

4. 275 volts of well filtered DC at 15 ma to power the B+ requirements of the phono preamps, tone control, and audio portions of the MPX section.

Providing this wide array of power sources usually requires a specially made transformer as the original power transformers of the Fisher tuner/receivers represent. However, I found a junk box transformer that could handle the job, with a slight alteration to the 590-T.

The transformer could supply the necessary 6.3 vac 4A requirement, and with appropriate separate (dual) B+ filter paths (just like Fisher used in their receivers) and a (6X4) rectifier tube, the appropriate B+ requirements as well. One path employs an isolated cap input filter to ultimately provide the 275 volt requirement, while the other path is a "virtual" choke input design to supply the 180 volt needs of the tuner circuits. That left the DC power for the audio tube heaters. This was addressed by rewiring that heater circuit on the 590-T to require 12 vdc @ 450 ma, rather than the 35 vdc @ 150 ma of the original design. The transformer has a 5 volt 2 amp winding that was then sent to a voltage doubler and choke filter circuit to produce a well filtered 12 vdc under load.

An old defunct project of some two decades ago donated the chassis box that could be reconfigured to house the power supply, while the various other parts and pieces were all on-hand items as well.

Besides the rewiring of the heater circuit, the only other significant modification made to the 590-T was to ground the low side of the volume controls, which normally were grounded to the power amp chassis by way of the audio interconnects used between the two units. Therefore, this modification is required when operating the unit as a stand alone device. I also removed the speaker switch wiring, as well as the auto shut-off wiring in the power connector cable bundle, and used a more common 8 pin octal plug to connect the 590-T to its new power supply companion. Finally, I also corrected the connections to the can cap which were wired incorrectly during production, as discussed in a separate thread regarding this unit. Along the way, various schematic inaccuracies were corrected as well.

The pics show the final results:

1. The unit is small and compact, but does its job perfectly. The 6X4 is an indirectly heated rectifier tube, eliminating virtually all of the turn on voltage rise normally associated with SS B+ supplies. The 8 pin octal plug make for a much more conventional diy connector. The small red switch on top of the unit is to turn the power supply on, when it is not connected to the 590-T.

2. The underside is well populated, but neatly laid out in executing the build. Note the small choke used to filter the DC heater power, with very little voltage loss in the process. The small diode on the rectifier tube socket is used to isolate the two separate B+ filter paths from each other to prevent interaction.

3. With 13 tubes on this puppy, it actually draws some significant power to operate it all combined -- easily enough to power a typical mono 10-12 watt power amplifier.

4. Those familiar with these units can see the rewired power supply cap, grounded volume controls, and rewired audio heater circuit.

5. All connected and powered up. The power switch on the 590-T even triggers the companion power supply appropriately. This combo makes a great lab mule for general listening of multiple sources, and also provides wonderful test signal sources as needed.

All that remains then is to build some sort of housing for it to make it more presentable for general use. I do have the original front mounting board and face plate that came with it, so that should make that portion of the project rather straight forward by comparison. It's been a fun project though, which as the central piece of the Custom Electra console, shows just how great even the mid-range Fisher consoles are -- even just the tuner/preamp unit operating all by itself!

Dave
 

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OUTFRIGGINSTANDING!!! We'll make a console addict of you yet!
and the cool thing is you can plug it in to ANY AMP your little heart desires.
 
Just for clarification, the 1964 Custom Electra VII designation is E-49. The Futura V is a F59.
The 1965 C.E. VIII is E-490, and Futura VI is F-590.
Your's is a E-49.

Easy way to tell the difference between the e-46 to e-49, and E490 is a STEREO Indicator bulb (red) centered on the dial just above the word STEREOPHONIC. The E-46 to 49 has no bulb, the E-490 does.
 
Larry -- My chassis is the 590-T from the Custom Electra. Which version of the Electra is it from then?

Dave
 
Dave; my apologies.

590-T Tuner is from a Electra VII

The 590 thru me. When someone says Custom Electra I automatically see a 4 series model # in the tuner/preamp and the amp. When I saw 590 I immediately thought FUTURA, but That year Futura ('65 is is F-590) they used the 69-T/69A (which was the # for the ambassador which is a F-690). So I had to go look. You are correct in that your 590-T is from a 1964 Custom Electra VII. Should you want to find the corresponding amp, it would be a 481-A.

Sorry for the confusion.

Larry
 
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