Making the fisher 49T a standalone preamp

[Carbsandcaps]

I had a fisher 49t/49a combo which are bulky and awkward as an amp/preamp outside of a console. I decided to use the amplifier separate which isn’t a big project (don’t forget to wire the 12ax7 heater!). This left me with the tuner/ preamp with no power. Instead of harvesting it for parts I decided to make a power supply for it, or at least try. With a lot of help from the people in this community I was able to do it.

I had a transformer with a 350/0/350 output that I figured I could use one leg of but was talked out of it by members on here and rightfully so. I had found 2 heathkit transformers on the eBay’s that powered only preamp tubes so I jumped on them. At the same time someone on here was had found a schematic for the same transformer I just bought! It had 117v and 6.3v outputs which is all that is needed. He had recommended a voltage doubler circuit like the original fisher.

Mounting the transformer was simple enough and for the most part fits on the chassis. Stuffing everything else underneath was another story.
Sorry for my terrible drawing abilities but here’s the diagram.




where it says to #1 that’s #1 on the harness that connects the power amp to the preamp. You can cut the wires and pull them back into the chassis and connect them right to the capacitors. It’s basically the same as the original fisher circuit but with 100uf caps (to make up for some lost capacitance) at the end and 220uf caps at the voltage doubler. You’ll see in my case there are a few more capacitors and that’s because I decided to replace the caps inside the can while I was in there. They’re basically shoved wherever I could fit them. The diodes are in the upper corner. The mains were wired to the original switch and a cheap inline fuse holder for safety.





You will also have to rewire the 2 phono 12ax7 and the single preamp 12ax7 heaters by connecting pins 4 and 5 and connecting those 2 pins to one side of the 6.3v feed and pin 9 of the tubes in parallel since they were ran in series with the 24vdc in the fisher design. The swore point might be the 10k ohm resistor. This was needed to get the b+ voltage correct. I used (2) 20k ohm 5 watt resistors in parallel to make the 10k because that’s what I had but it also helps to dissipate better. The resistors never got above 95 degrees so I’m happy with it. The voltage is actually a tad low but I’m ok with that too The transformer also never got hotter than warm to the touch. I also removed the speaker select switch and the auto on switch because they aren’t needed anymore and could use them for future projects.

This all makes for a stand alone stereo fisher preamp/ tuner for under $100 (depending on how much you get the preamp for). And let me tell you, this thing sounds amazing! Picks up stations inside the building without an antenna AND has built in multiplex! I’m currently using it at work with my cheapo dewald tube amp and it’s singing sweetly. I may build a case for it but for now its just going to be covered in the bottom with some wood and the face plate is glued on.

I’m sorry I don’t know how to add people’s names to the post otherwise I would because the people who helped with this are owed a great debt of gratitude. If anyone can tell me how to add names I’d appreciate it.

 

[dcgillespie]

Neat project! I did something very similar 10 years ago with an Electra 590-T tuner preamp head -- which is a slightly earlier model to your 49T unit, identical in most ways except my unit still uses the classic 3 tube MPX-65 multiplex section. Performance wise, in the tuner section these units are virtually identical to that of an 800B receiver, except that the AM and FM sections can only be operated one at a time with these units, and as a result, only employ one eye tube. Otherwise however, the actual performance in both the AM and FM sections is basically the same. The audio section is much simpler of course, but you are absolutely correct, these units can make for the basis of a very neat stand alone Fisher tuner/preamp unit -- particularly so since Fisher ceased producing any real self contained tuner/preamp units after the 100T/202T -- both of which were originally designed for the Crosby MPX system before the FCC announced its decision -- while these tuner/preamp heads were designed from the get-go for use with the chosen GE/Zenith system in place today. In that regard then they are very unique in the Fisher stable, which makes such a project so neat.

I took a little bit of a different approach with my project, building a specialized outboard power supply for the unit. It is of course much smaller than the original amplifier that powered it, so while it doesn't completely eliminate the external power source/unbilical cord arrangement of the original installation like your project does, because of the supply's compact size, it does all but virtually eliminate all of the cumbersome aspect of it. In the process, I was also able to maintain DC power to the audio tube heaters so as to maintain the lowest possible overall noise from the unit as well.

If I might offer a suggestion for your project, you might consider also adding at least one dropping/filter resistor in the B+ line to the tuner circuits, similar to that in the original design (which used two sections of filtering). You're showing 190 vdc being supplied to that section, with no dropping resistor to aid in filtering, or to help target the correct operating voltage. The tubes in the tuner section work very hard (getting very hot!) even at the operating voltages they are shown to operate at, so any elevated voltage above that just acts to shorten tube life, while the reduced filtering can add its own element of hum modulation to the tuner signals as well. Just a suggestion.

I offered my project long ago in this thread which you might enjoy:

Junkbox Fisher Project

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...

audiokarma.org

Congrats on the success of your project!

Dave

 

[Carbsandcaps]

Neat project! I did something very similar 10 years ago with an Electra 590-T tuner preamp head -- which is a slightly earlier model to your 49T unit, identical in most ways except my unit still uses the classic 3 tube MPX-65 multiplex section. Performance wise, in the tuner section these units are virtually identical to that of an 800B receiver, except that the AM and FM sections can only be operated one at a time with these units, and as a result, only employ one eye tube. Otherwise however, the actual performance in both the AM and FM sections is basically the same. The audio section is much simpler of course, but you are absolutely correct, these units can make for the basis of a very neat stand alone Fisher tuner/preamp unit -- particularly so since Fisher ceased producing any real self contained tuner/preamp units after the 100T/202T -- both of which were originally designed for the Crosby MPX system before the FCC announced its decision -- while these tuner/preamp heads were designed from the get-go for use with the chosen GE/Zenith system in place today. In that regard then they are very unique in the Fisher stable, which makes such a project so neat.

I took a little bit of a different approach with my project, building a specialized outboard power supply for the unit. It is of course much smaller than the original amplifier that powered it, so while it doesn't completely eliminate the external power source/unbilical cord arrangement of the original installation like your project does, because of the supply's compact size, it does all but virtually eliminate all of the cumbersome aspect of it. In the process, I was also able to maintain DC power to the audio tube heaters so as to maintain the lowest possible overall noise from the unit as well.

If I might offer a suggestion for your project, you might consider also adding at least one dropping/filter resistor in the B+ line to the tuner circuits, similar to that in the original design (which used two sections of filtering). You're showing 190 vdc being supplied to that section, with no dropping resistor to aid in filtering, or to help target the correct operating voltage. The tubes in the tuner section work very hard (getting very hot!) even at the operating voltages they are shown to operate at, so any elevated voltage above that just acts to shorten tube life, while the reduced filtering can add its own element of hum modulation to the tuner signals as well. Just a suggestion.

I offered my project long ago in this thread which you might enjoy:

Junkbox Fisher Project

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...

audiokarma.org

Congrats on the success of your project!

Dave

Thank you! I was originally going to do it like yours because I figured there’d be a ton of noise from the transformer but there but it’s surprisingly quiet. I’ll have to double check the 190vdc. The voltages feed right into where the umbilical did so everything is kept intact inside the preamp wiring. What did you make the box from for the power supply?

 

[dcgillespie]

Oh my gosh, that was an old little two piece aluminum box from who knows -- probably (literally) 50 years ago, that had already housed a half dozen different projects over its life. But it truly found a permanent home with this project. I continue to use my tuner/preamp regularly all these years later, and still enjoy it as much as I did after I first made it. If you want a "modern" Fisher tuner/preamp that's ready to go with builtin MPX, then this is as good as it gets, and the only game in town from Fisher.

Dave