Tube DIY Asylum

RE: First impression of Kenwood KW-55 receiver upgrade

64.12.116.141

In Reply to: RE: First impression of Kenwood KW-55 receiver upgrade posted by Neff on April 13, 2008 at 19:20:55

Final impression & rebuild. I replaced the driver tubes and preamp tube with a polyproplyene type cathode capacitors available from Madisound. Cheap at 99 cents, 100 volt rating makes for a fairly compact installation. The amp section simply needed a boost here from Panasonic electrolytics I used at other 12AX7s.

The tuner is a 'bitch'. Align for max signal and tweak for best sound. Slightly tuning in one direction vs the other makes a difference when adjusted one or two degrees off maximum output (center) for best sound and consideration of long term plus warm-up drift. Tune RF/IF sections after a few hours operating, of course. The tuner is sensitive and selective for tube type. It does sound good. Remove that stereo dimension pot at top of chassis by the multiplexer, it trashes the clean stereo sound a rebuilt & upgraded Kenwood receiver. Use RCA tubes throughout the tuner with a pair of Telefunken ECC85 for the front-end. Some units use a nuvistor first RF amp tube. RCA brand should perform well.

Be aware about 21 new K40Y-9 caps are needed for a rebuild. This includes the multiplexer out and use .22uF there for good bass response. Rest use .033uf with 4x .22uf at EL84 grids. Careful at preamp tube 12AX7 in front center. Use a ground wire and connect to capacitor case with heatshrink applying contact pressure. Soldering a wire is not an option here. Terminate other end of wire to a ground. The steel case picks up hum- go figure. The inputs have a fair amount of crosstalk, not much can be done here or say I was too lazy to address. All 12AX7s perform well with Sovtek 12AX7LPS except the EL84 driver/splitter tubes. Amperex 1960s short plate are best there. The Amperex harmonic output seem to compliment the circuit driving the GE 6BQ5s that sound best. EL84 cathode bias is a mess. Seperate with a common mode cathode resistor for each tube pair and bypass with a BlackGate 100uF @ 25 volt capacitor for each EL84 pair. Option is a variable resistor of 100 ohms and a 100 ohm cathode resistor. The receiver applied 370 VDC and cathode bias should be about -13.5 to -14 volts. Not finished yet- you must install a new regulated PS for phono input tube filaments as as they originally derived the filament voltage from the EL84 cathode bias when used with one common resistor for all EL84s- double sick!

Use a 1.5 HY choke from B+ voltage doubler with at least 330uF pair of doubler caps. Connect choke to B_ out and use a new 50uF section on choke output to the EL84 audio transformers B+ leads. The receiver & amp is very quiet, I hear no hum from 90dB speakers. I used a pair of replacement JJ 50uf/50uf @ 500 volt electroyltics. Three new Sprague 50uF @ 450 vdc replaced the three existing 33uF @ 150 volts caps remaining in the circuits. A few other electrolytic caps should be replaced. One at an I.F can, a few in other places.

Is this receiver worth it? No, unless one enjoys the work. It does sound good though after about 250 hours break-in time. I prefer US vintage gear and seperates will be easier to upgrade due to more space available. I would approached stock vintage Asian based gear with caution. Most of the small value caps are just shot. Even an old US manufactured Sprague Black Beauty tests much better over the years. One good plus is the Asian gear uses quality resistors in many locations & resistors out number capacitors by a large amount. Most US manufactured gear and especially so in a tuner can always use an upgrade to Vishey resistors at the Mixer/IF cans. Use extreme caution at the first RF section. Those are tricky tuned circuits. Changing parts require compression or expansion on the length of an open air inductor plus tweaking of the capacitor used for linear frequency dial readout. That requires unusual skills to get in the right direction for accuracy.

Anyways, I am done here with this post.

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