A worthy project that really paid off, from the golden age of hifi - a restored Stromberg Carlson 433 stereo 24 amplifier. I started the restoration by recapping the power supply with good fresh Japan Nichicon under the chassis, importantly completely disconnecting the old multi FP can cap.Hooked to a cd player and the resulting sound was low and distorted, dirty, but the B+ and other voltages were nearly good. Checked the original 6BQ5s on my Hickok 547A and they were junk, testing at just 50-60% - shot. Replaced with a Hickok matched quad of big bottle Soviet military 6BQ5 with light use that test very near new. Kept replacing electrolytic caps until all were replaced, 10 in all. That brought back volume level but still a bit mushy. Checked the 6U8 dual triode pre-tubes, they were also both very weak causing the distortion. I installed a Hickok tested matched and balanced pair of NOS blue ink Philips 6U8. Checked all the resistors around these pre-tubes and the power tubes, all were within normal range.
Now I was getting somewhere, sound quality was restored and very good but one output plate started glowing red. I thought this might happen as a few volts were leaking through an old coupling film cap allowing positive voltage. On the grid of the overheating output tube. Grids should be at 0 Vdc for this self-biased amp.Had a set of incredible NOS Soviet K40Y-9 paper in oil, metal cased, glass end sealed coupling film caps at the correct. 047uF at 400dc max voltage. These are just like the famous USA Sprague Vitamin Q caps of old, from the 60s at the height of the tube era - so in demand, extremely popular, the best sounding ever made many feel. That solved the red tube run away condition of course, and further improved sound quality at the same time.
These paper in oil caps are creating an amazing sound stage with intensity, impact, and smoothness that will actually continue to improve as they break in. Sound now though is excellent I'de say, smooth, powerful and clean, even with the vol knob up past 2 o'clock position and beyond! Most impressive for a pair of small output tubes. Makes my Klipsch KG4 sing cleanly with plenty of volume to spare. Makes you wonder why people thought they needed more power than a pair of 6BQ5 can provide.Granted my Klipsch are pretty efficient, but so are many other brands. Sounds superior as is and is of course dead quiet at idle. No hum, no buzz, no crackle. I worked the pots and switches with De-oxit spray for smoothness, quietness, and the best electric contact. If you get out a magnifier you'd probably find something, but this one sparkles with next to no spotting of the gold colored trim.
I didn't even clean the face or the knobs so after detailing it will look amazing. I see no flaws, no marks on the face and no lettering flaws. Nice clean chassis, no rust, excellent lettering there too.A real keeper here and with it's nice looks will certainly appreciate nicely in value. Meanwhile, it is plug and play for your enjoyment. Ready for more years of fine service. And as with all vintage high voltage tube type equipment, this amp may need electrical attention at any time. I'm always available to fix tubed gear if you cann't find anyone locally, no worries. Will pack well, to arrive the way it left. Pictured tube tester not included. The item "RESTORED STROMBERG CARLSON Stereo 24 TUBE AMPLIFIER 433 PLUG-N-PLAY + TUNER" is in sale since Thursday, April 4, 2019. This item is in the category "Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps". The seller is "rwt34221" and is located in Bradenton, Florida. This item can be shipped to United States.