Fisher 400C Tube Preamp

I just finished repairing one of these amazing sounding preamps. These were made in the late 1950's. This unit has a EZ80 tube rectifier , 4 tubes for the main line stage, 2 are 12ax7,the other 2 are 12at7.There are also 2 12ax7 for the phono stage.

Very clever design, one channel  uses 1/2 of one 12ax7 and then feeding to the cathode follower stage  stage using 1/2 of the 12at7. After the bass and treble filters the signal goes to the other half of the 12ax7 and the output to the second half of the 12at7. The other channel is reversed on the pins on both the 12ax7 and 12at7 so the shielded cable lines up from the 12ax7 to the 12at7. See the pix below.

The above unit has been fully recapped and a lot of out of spec resistors replaced. The preamp uses DC heaters to feed all 6 tubes. It originally used a selinium rectifier and a can cap with 2 1000uf/50v sections to do the filtering, plus a 10ohm resisitor between the caps. The unit had bad filter caps and the DC heaters were only producing about 15 volts, where it should be about 25vdc as per schematic. As you see on the right a new bridge rectifier and 2 1000uf/50v caps were replaced. I had to raise the 10 ohm resisitor to 15 ohms to get the  proper voltage for the heaters.

The other 2 can caps were made by Hayseed Hamfest. Very nice how the final stage of rectification, the 2 10k resistors and 100k resisitors fed each channel seperately. Some one before me did a cap replacement that was just horrible. They had a few wrong values in caps, the leads were just wrapped around terminals and some were barely soldered. See below for the original pix.

I was very surprised how good this preamp sounded. The line section and phono section both has really nice overall soundstage and nice highs and  bass. 

The 2 pix above is how the preamp was sent to me. Notice the blue selinium rectifier and the heat marks around the 10ohm resistor for the DC heaters.

The pix above shows the 2 filter caps form Hayseed Hamfest and I left the original bad centre cap for the DC heaters. Those caps were mounted on terminal strips next to the new bridge rectifier.

If you can find one of these units grab it...they sound amazing.