Preamp - On a Power Trip

Where were we?? Oh yeah, preparing for power up.

Since most of the parts replacement in the Preamp has been in the power supply sections, I wanted to make sure that these sections were powered up first while disconnected from the main circuitry to ensure any voltage issues would not burn out the audio sections.

Power Supply
I was expecting the power voltages to be higher than the specs, mainly due to the fact that I replaced the Selenium rectifiers with silicon diodes (M1, M2, and M3)

Preamp Power Supply Circuit

Tube Filaments (12.5V - To FILS)
All tube based equipment I have worked on in the past used AC voltage to supply the Tube Filaments (called "Heaters"), so I was somewhat surprised to see rectifiers in the Heater supplies (M3). After some research, I learned that this was done in some older audio equipment to reduce noise. Of course, almost all tube equipment I have worked on in the past was for Ham Radio and those levels of noise were never an issue. Anyway, noise levels in the Preamp will be a project for later, once I am ready to put sound through it and have broken out the Oscilloscope.

The two Electrolytics in the heater supply (C5A, C5B) were not replaced so I will keep a close eye on these (see the earlier post).

High Voltage Output
All of the Electrolytic capacitors in the High Voltage sections were replaced and I was pretty confident that there would be no issues with this section, short of the higher voltages. The new Electrolytics were rated at 450 VDC as opposed to the originals which were rated at 300 VDC.


And then there was light!
After I was convinced that there would be no fire, I plugged the Preamp in to the AC and got the front lamp to light, always a good sign. I made some cursory voltage measurements with my trusty Micronta Multimeter (I probably bought this sometime in the 1990's) then pulled the power and connected the power supply output to the rest of the unit for the real acid test.
 

  Preamp powered up with Lamp glowing and the Multimeter showing voltage

A close look at the meter, for the sticklers...

Burn Rate
Once I was convinced the voltages were reasonable, I left the preamp powered up for about 3 hours for a good burn-in test. Next will be to introduce some waveforms into the Preamp input and check the output of the different stages on the scope.

If this checks out, I might actually plug the Preamp output into one channel of my Denon DRA-345R Stereo Receiver...the Receiver is only about 30 years old, unlike the Preamp which is closer to 60 years old!

Until then, stay cool!