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Conrad Johnson Premier Two: restoration
It’s not often that genuine “high-end” audio equipment crosses my
workbench, so I’m sharing my experience of the special occasion. I’m not going to wax poetic about the “tube sound” but I will talk about the design and my efforts toward restoration. You’ll find photos at http://picasaweb.google.com/circuitsmith/ConradJohnson The subject at hand is a pre-amplifier that appears to have been made in the early ‘80s (1982 or later). It uses vacuum tubes in the signal path and high voltage rectifier, a hand full of transistors to regulate the B+ voltages and IC regulators to supply the tube filaments with DC voltage. The reason it crossed my path is a small child was playing with it; repeatedly turning it on and off. The resulting hot-switching transients opened up several resistors in the HV power supply and blew one of the two filament regulators. I also found the output mute circuit inoperative and a shaky mod job of replacing capacitors. Thankfully, the owner had a schematic. Clearly the designer gives high priority to supplying clean DC power to the amplifiers. Each cascode triode stage has its own R-C filter (8 total) after the voltage regulator. These filters take up a sizeable chunk of the main PC board. The 12X4 rectifier delivers a nominal output of 510VDC. Seems quaint in this age of MP3 players that run on one AA cell. Unfortunately the rectifier filament is not on a separate transformer winding, so the heater-cathode voltage is also 510V, far in excess of the 100V average 400V peak max rating listed in my RCA tube manual for the 12X4. This is one oversight in an otherwise conservative design. Line level, phono and filament grounds are loosely coupled with big Rs and small Cs. The smoothing capacitors were originally two 200uF 350V caps in series but whoever “re-capped” the unit replaced them with a “universal” 60-90uF 500V cap in parallel with one of several 20uF non-electrolytic and put a 2K resistor in series with the rectifier cathode to reduce the voltage (to 430V nominal) and keep from over stressing the cap. Unfortunately this nearly eliminates the regulators ability to tolerate low line voltage. I restored the original configuration with two Nichicon high temp (105C) 220uF 350V caps and bleeder resistors. I like the idea of some extra resistance to limit cathode current of the rectifier since it feeds such a large smoothing capacitor (10-20uF would be run-of-the-mill with a 12X4) so I replaced the 2K with a 649 ohm 3W resistor. Now the HV regulators deliver clean DC down to a line voltage of 104VAC. The first HV regulator uses a stack of zener diodes, fed a constant current, plus an emitter follower to deliver 410VDC. Four R-C filters provide extra smoothing to each high level amplifying stage. The Cs were originally 60uF electrolytics but have been replaced with non- electrolytic 20uF units. All the Rs were blown from the switching transients. A couple of the zeners were shorted and reduced the output voltage to ~350V. The second regulator starts with the 410V and uses more zeners and an emitter follower to deliver 380V to the low level phono amplifiers, through 4 more R-C filters. The mute circuit uses a relay and a unijunction transistor to short the output lines to ground for approx. one minute after the unit is turned on. There is also a mute button on the front that controls this relay. A wise choice, considering it was able to deliver a 200V peak- to-peak sine wave without clipping, driven from the phono input. This circuit had been bypassed however. A timing capacitor became leaky and prevented the timer from timing out. Evidently, someone mistakenly changed the relay and miswired the replacement. I restored the circuit to its original configuration. Another relay and timer to prevent hot switching would make the unit more robust and toddler resistant. Tim Brown circuitsmith * at * verizon * dot * net |
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