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First impression of Kenwood KW-55 receiver upgrade

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I always wanted to try updating one of those kind of cheap 1960s Asian tube receivers to find out if it could sound good. The receiver needed twenty NOS tubes, twenty-two Russian K40Y-9 capacitors, four BlackGate capacitors, one choke, seven power supply capacitors, four potentiometers and eight hours labor.

I have a fair amount of tubes to play with. The tuner wanted two expensive Telefunken ECC85 and that was that- period. Mullard ECC85 did not sound good & the Mitsushita was somewhat of a close second. I went with all new RCA tubes everywhere else in the tuner section. Hard to beat the common RCA 6BA6 & 6AU6 plus a few others like 6BL8. Of course, a tuner alignment was sorely needed. The power amp wanted GE 7189s and that was that. No Sylvania or RCA 6BQ5s in that position. The amp section places 375 volts on the 7189 plates so 7189 are likely a better tube at that voltage. I rewired the lousy cathode bias that shared with all tubes using one 100uF electrolytic for all plus the design 'Cheap Charlied' me with using the preamp 12AX7s for some of the cathode resistance for DC- errrr. I built a seperate PS with a 7812 regulator for the preamp tube filaments. Anyways, seperate bias pots and BlackGates at 100uF for each 7189 tube.

I tried a few various 12AX7s. I am out of Mullard 12AX7 long plate that seems to win every contest. I have GE mid 60s stock, RCA Command series black plate, Amperex, Matsushita and a few newer RCA. Surprisingly, the cathodyne phase splitter with pre-driver sounded best with Amperex 12AX7. BTW- most Asain amps use a 6AN8, not this Kenwood. A Matsushita 12AX7 won the first preamp section and the RCA Command series 12AX7 black plate was best in the other preamp position. Those RCA Command were close to the Amperex, but not the best in that position. The newer GE 12AX7s were lousy.

Most of the old oil/paper coupling capacitors were shot and leaky measuring from 7K to a high of 1.2 meg ohm. I used a whopping 18 various small K40Y-9 and four .22uF K40Y-9 caps on the PP 7189s. The power supply uses a doubler with only two 100uF in series. I installed new Sprague 330uF @ 250 volt caps in the doubler. Wanting to get rid of power supply noise, I installed two UF4007 diodes, an extra 50 ohm 1.5HY choke and placed a 50uF cap on the choke output. Other three 50uF sections were installed using new JJ can type capacitors. Three other Sprague 47uF rounded out the PS section down stream. Now, everyone knows oil or polyproplyene caps are superior, but no room in this receiver.

With only a few hours play time, I can say the receiver is not bad at all. With this receiver, one has to appreciate the pleasing musicial aspects of tubes, not ultimate refinement that a tube amp can achieve. Compared to my DIY amps, it is not even close. But, it is like a comparison to a Porsche whereas a GM Camero is not going to out run that Porsche. No surprise here. I would compare the modified Kenwood to a stock Dynaco SCA-35 and their matching tuner. With upgraded parts & tubes, the Dynaco is better.

I am going to replace those tricky 12AX7 cathode bypass capacitors with some new low ESR Panasonic parts. Plus the other parts I installed do require some break-in time. I will also try a CD player & my modded Eico 2200 tuner on the Kenwood amp section this week.

So, is this receiver worth all the work and a good investment? No, just a fun experiment. Fun does cost money at times.


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Topic - First impression of Kenwood KW-55 receiver upgrade - Neff 19:20:55 04/13/08 (7)

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