metaruss

♦ Back ♦ History ♦ Description ♦ Schematics ♦ Performance ♦ Pictures ♦

This was the very first electron tube related piece of hardware I ever owned.  It was the stereo power amplifier from an electronic organ made by Thomas (don’t know the model) that I received as a child around 1980.  Though I never learned to play it, it did come with a built-in ceramic turntable and a pair of RCA jacks on the back for a line level input.  I grew up listening to LPs, cassettes, and the radio through this admittedly low-fi amp and speakers, which meant that it was on for at least a few hours every day for about 12 years.  It had never been retubed in all that time, even though I went through several styli in the turntable and one tape head in my tape deck.

Eventually the organ was thrown out, but for some reason I decided to scavenge the power amplifier and the preamp.  I also pulled all of the oscillator tubes from the organ portion (all 12AU7s).  For a long time, the amp was forgotten as I listened to all of my audio through a Heathkit 60W transistor amp (built by my father) and eventually a “100W” Realistic transistor amp of reasonable quality.

At some point I either came across the amp or felt the urge to fire it up again (this was while I was in high school).  Only knowing the basics of electronics at the time, I managed to figure out how to hook it up to my system without killing myself and was rewarded with the sweet sound of tubes again.  The brash, gritty sound of the transistor, which I couldn’t put my finger on until I was listening to the Thomas amp again, was gone.  I then started to build a sort of wooden chassis for it that would have allowed me to stand it upright to save space.  In the process, I broke the rectifier tube (5U4GB) and had to rig it with a pair of silicon diodes, thinking that there was no way I’d be able to get my hands on another tube.  Knowing nothing about proper tube biasing, I was lucky that the amp was biased relatively cold (305V plate voltage, fixed bias at about -27.5V) and that the tubes were quite worn (they were all original).  I was also lucky that I never finished the chassis, as it would probably have started a fire. 🙂

Later I went on to college and the amp was again forgotten and eventually lost after I moved away from home.  It was my fairly recent delve back into tube electronics that rekindled my desire to find this amp, but it did not turn up anywhere.  Eventually my parents found it while cleaning out my sister’s closet (!) and I was again reunited with the little beast.  At that point, I actually knew what I was doing and quickly figured out what it was that I had.  You can continue on to the description to find out more about it and the modifications I have made.