Fisher KX-200 Integrated - Blast from the Past

Discussion in 'Power Amps' started by purr1n, May 11, 2021.

  1. purr1n

    purr1n Desire for betterer is endless.

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    The timing of this conversation and the necro of the Leben CS300XS thread is rather very very interesting! It's super funny because I fortuitously happened upon another classic push-pull pentode amp just last night. This one is much better IMO! So instead of Leben, why not a reconditioned Fisher KX-200?

    PXL_20210511_063616661.jpg
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    In retrospect with today's gear prices, perhaps the Leben CS300XS isn't so bad if you like the sound. I still cannot get over the Leben's loose bass however. I also can't say that I'm a big fan of the EL84 / 6BQ5, unless one can find exotic EL84 pieces. Personally, I feel the EL84's harmonic characteristics sound better in overdriven guitar amplifiers. I like the 6L6 or 6V6 better. Ultimately, it depends upon implementation.

    The Leben is autobias, and I don't think I've heard a PP of these designs which wasn't loose. TBH, I'm not even sure if the Leben does the mids better than a cheap Chinese EL84PP. Now fixed bias is a different story. But fixed bias requires some effort and know-how from the user, and by doing it wrong, one can end up with distorted weaksauce sound or red-hot glowing plates in the power tube about to self-destruct. I don't know what it is with the Leben. Maybe on the two occasions I heard it, it was set up wrong. Heck, maybe it's the output transformers. The Fisher KX-200 I have in the house came out during the golden age of output transformers. It's just not quite the same today with respect to plentiful good quality transformers. The $3500 asking price for the Leben CS300XS isn't that high today in the context of ORFAS.org so I have serious doubts good transformers are used.

    And how much do vintage (real NOS is practically non-existent today) EL84's cost today? Let's keep in mind that for a two-channel PP, we need four of them. I don't know how hard the Leben runs the power tubes, but these things tend to be driven harder in a speaker than DHTs used in headphone amps. That is, they are wear items that will die. Finally, there's no way for Leben to offer vintage tubes because of cost, availability, and reliability. A good percentage of "strong" or "NOS" tubes bought on eBay aren't really so when I put them on the Amplitrex tube tester. I also eschew amps that need spheshal tubes to sound good (mids and highs will be better, but the lows will still be loose). Great amps IMO sound good with current production tubes, but sound better with exotic tubes. FWIW, the KX-200 uses the rather unknown 7591. Unknown is that it isn't a EL84 or EL34. I am beginning to think that the 7591 is darn good sounding tube overlooked by PP amp designers today. I'm using new production JJ 7591 in my KX-200 right now and the amp sounds darn good.

    As an aside, I'm pretty sure I've found the amp for my big vintage 80s JBLs (4-ohm woofers). I couldn't decide among a low power SET, Aegir, x2 Aegir, high-power class AB SS. The KX-200 takes the cake. Well, at least it does most things well without doing anything not well. The big JBLs are rated close to 100db sensitivity, but they need some power. The KX-200 isn't as resolving or spacious as my custom 45, but it most certainly has more grunt and is less rounded. Compared to the single Aegir, which I regularly run, the KX-200 has lusher midrange, tube microdynamics, and slightly deeper and more holographic staging. A single Aegir can at times sound too soft. (not enough power). Dual Aegir changes the sound to not my liking - not what I want to accomplish. The downside to the KX-2000 is that you have to deal with ton of tubes and maintain them (check bias every now and then). FWIW, the meter died on this KX-200. Mods were performed so that one would set bias with a multimeter.

    I'm not exactly sure of the power of the KX-200. The manual says 35W at 0.4% THD per channel. I will see if I can find time to take some measurements. Curious of the power curve and harmonic patterns.

    I'll post a bit more on all the features of this KX-200 and the mods performed on it.
     
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    Last edited: May 11, 2021
  2. k4rstar

    k4rstar Britney fan club president

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    yup, that amplifier is 'the shit'. I have heard a Fisher X-101-C amp and 800 receiver which also use the excellent sounding 7591 output tubes but different OPTs (bell-end instead of tower style). your KX-200 would've been a kit version of the X-202, which is not saying much because kits back then were built by serious enthusiasts - retired engineers who had the money and time to play with this stuff in the 50s/60s.

    7591 is unfortunately getting very difficult to find in vintage examples, Fisher used this tube in their integrated amps and receivers for the early 1960s before switching to the 7868 (7591s in a miniature envelope). modern owners of vintage Fisher and McIntosh amps basically buy up all the fleabay stock. the coin-base 7591s are also not quite as sweet as the OG fat bases.

    here's an excerpt from a late Fisher StrataKit catalog:
    [​IMG]

    the only thing I can't agree on is EL84s! I have yet to hear a vintage EL84 amp I didn't like, and I happen to have three at home right now - one of which is the 'best' amp I have owned yet. the only thing is a different power envelope, with a standard Williamson circuit we get ~12 watts which will require something like your JBLs. the Fisher can actually work with some modern speakers.

    good to talk about vintage amps though - this stuff basically smacks modern Williamson derivatives (99% of tube amps sold in stores today) up and down the street, at any price. you have to put up with servicing, etc. but it is well worth it.
     
  3. purr1n

    purr1n Desire for betterer is endless.

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    Here is a photo from the top. Top left 12AX7 are the tubes for driver and EQ (more on this later). Top right 12AX7 are for the MC phono. The x4 7591 power tubes are at the bottom.

    PXL_20210511_234905788.jpg

    The bias meter is broken. This is fairly normal for these amps given their age. The amp has been modded with four test points marked A and B. The instructions I was provided indicate to balance the bias for each side and then adjust the bias to 0.32VDC. I'll open it up and figure out what was done inside.

    The original MON and AUX2 inputs were removed to make room for higher quality RCA jacks for AUX1. The original comes with speaker taps for 4, 8, and 16-ohms. These were the screw terminal on deals using a flat blade screwdriver. (LOL, they weren't into thick audiophile cables back in the way). It appears the options were removed and the 8-ohm tap is being used with a better speaker connector.
     
    Last edited: May 11, 2021
  4. purr1n

    purr1n Desire for betterer is endless.

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    Here is the front panel. The major modification is that the volume knob no longer serves as a power on / volume. It's purely a volume control now. The loudness contour switch now serves are the power switch.

    PXL_20210511_224325444.jpg

    Now here is the cool part: Tone controls. Yesss!. I think something was lost during the audiophile purist movement of the late 80s where tone controls got taken away. You can see that I have the bass dialed down a bit. The living room here in TX is slightly bassier than the room in my old place in CA. And no, I'm not gonna move those big JBLs away from the wall further into the room to get less bass if I can adjust it with knobs. Also note the stereo dimension knob. Yup, expand or compression the stereo width.

    Sure, the signal needs to go through an extra stage for the tone controls, but the number of active devices is still way less than any solid-state power amp.

    I haven't tested the center speaker outputs yet. This could come in useful for TV use.
     
  5. dmckean44

    dmckean44 In a Sherwood S6040CP relationship

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    Loudness contour is equally useful. I miss the quiet night time listening I often did as a teenager using a loudness switch.
     
  6. purr1n

    purr1n Desire for betterer is endless.

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    For future reference in case I need it.
    KX-200 schematic.gif

    I wanted to switch the speaker output to the 4-ohm tap and was trying to figure out why the stock hookup has the 4-ohm tap to ground.
     
  7. purr1n

    purr1n Desire for betterer is endless.

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    I am not familiar with the original amp parts. Looks like some boutique power supply related caps, small ceramics in audio path changed to film, interstage caps all changed to Auricaps, and ultrafast soft recovery diodes. The flanges on those diode are also tied to the cathode pin (checked with DMM), so that makes me a bit uncomfortable given how those flanges may accidentally touch the chassis, bottom cover, or each other, so I will put heatshrink over them.

    PXL_20210512_004855517.jpg
     
    Last edited: May 12, 2021
  8. purr1n

    purr1n Desire for betterer is endless.

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    That's fine. It will be a problem that I do not worry about.

    The KX-200 is supposed to eat tubes, runs them hard. Will get a better idea after I measure power output vs distortion. I run music (or TV shows) via speakers from 5pm to midnight on the weekdays, so I'd rather not have to worry about running expensive tubes into the ground every 8 months or so.
     
  9. Josh Schor

    Josh Schor Friend

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    Dougs Tubes has the 7591 new stock JJ's for $18.95 each, that's what's in the amp now. The amp has 18.5 watts per side. I put a 200kohm pot in to try and improve clarity, the original was a 500kohm and the amp had 35 watts per side. Its a beauty and am glad it is in a good home
     
  10. Donald North

    Donald North Friend

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    The McIntosh MC225 also used the 7591 and some believe its the best sounding of the original stereo tube Mac power amps. Go 7591!
     

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