Just bought a KX-200 - WOO HOO !

Discussion in 'Fisher' started by thornev, Jan 13, 2019.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thorne; Like Dave said, the 1n5408's are fine for the KX-200. Both the 1n5408 and the STTH3R06RL are rated for 3A with the 5408 voltage being 1000V. I've used 5406's in one or two units (600V piv) in the past, but it's really not enough headroom, due to surge voltages, especially if Filter capacitance is raised. I like a 150% safe area above the highest voltage spec'ed from the transfomer for Diodes. For example, say your highest B+ is 525V. 150% is 787.5V. Therefore a 1000V would have the headroom, but a 600v could possibly be blown from overvoltage upon turn-on(especially if the amperage is at or above rating). Luckily FISHER's gear usually doesn't draw more than 2.5A on turn on (at least none of them that I have seen) so there may be some fudge room. For now I stock just the 5408 and 4007 (1000V) for rectumfriers, and the 8A table bridges.
     

     

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  2. thornev

    thornev Well-Known Member

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    To all and especially to Dave G and Larry...Thanks again for your willingness to share what you so expertly know about our gear. I was just thinking this weekend how far I've come in 1 year in my ability to work more comfortably (and knowledgeably) in the electronics/tube restoration world. I actually enjoyed studying the kx-200 schematic this weekend and reveled in being able to identify stages, components, signal versus power flows, and using components to adjust both signal and power flows. Although I'm still at the point where I need to follow others' step by step instructions and choices of which components to add, replace and remove, I'm making my best effort to UNDERSTAND WHY I'm replacing components and the effect of their different values.

    I replaced diodes/rectifiers SD1 to SD 4 with 3A 1000v editions. I'm not sure, but I seem to have lost a little of the deep bass I so enjoy. And it seems I've lost some volume too. I'm having to turn the volume up to get the same punch I had before. The super clarity is still there. Overall I just love the sound of this kx-200. My poor 500-C has been sitting idle. I don't yet have a permanent location for the KX. Table space in my studio/office is extremely limited. I have to start thinking vertically !

    This weekend I also added a second CL-80, this time on the fuse side of the incoming electric. I replaced 2 caps, C13, C14, in the tone stack with orange drops. They are .047uF, were 400v, I upped the voltage to 600v. I also replaced 2 caps in the phono preamp stage, C7, C8. One of them was pushing its upper tolerance limits. I replaced 2 lytics C25, C26, that were 50uF 70v, gave them some head room since they're right up front before everything - 100uF 100v. Like I said, somewhere in all of this I seem to have lost some deep bass. I would bet it has to be in the caps values. I know in guitar amps the combination of values of caps and resistors affect the cutoff/slope frequencies of bass, mids and trebles.

    Still more to come... Replace can caps with Hayseed Hamfest products. Replace some missing screws, replace some damaged RCA jacks. Find a nice cabinet (I have some web sites marked). Adjust the phase inverters since I changed all the tubes. Maybe try some changes some other folks have done in other forums. Add EFB !!

    Thorne

    PS - I just replaced the bias meter resistor. Was 390 ohm. Replaced with 240 ohm 1/2 watt 5%.

    PSS - I've been playing the KX all day. Missing deep bass? Not anymore. Tubes need to warm up? Just so happened the source material earlier today was weak? I love this KX !
     
    Last edited: Jan 22, 2019
  3. Tim D

    Tim D AK Subscriber Subscriber

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    Thorne,

    I do exclusively LP's with my KX-100 and I find that source material (how it was recorded and cut to the LP) makes all the difference in the bass. I've got some LP's with real punch and some that are just "meh".
     
  4. thornev

    thornev Well-Known Member

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    Tim - Absolutely true... The board engineer, producer, band members, even onlookers, have everything to do with how the final mix sounds. I have a handful of songs I use to evaluate how good speakers are in reproducing what I consider to be excellent mixes. I didn't use them when I made the comment this morning. My comment was based strictly on playing my Android "Poweramp" playlist over again. Not a good methodology since I'm not as familiar with those songs.

    Plus I had burned myself not once but twice within 10 minutes of each other and was not in a very good "evaluative" mood. I'm full of excuses !

    Tim - I'd be interested in knowing a summary of changes you made to your KX-100 especially the changes where you increased or decreased the component values and why you made those choices. If you don't mind. Thanks.
     
  5. Tim D

    Tim D AK Subscriber Subscriber

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    Hi Thorne,

    For my KX-100 did a bunch of the usual mods associated with Fisher amps. It's all listed in this thread I did on my KX-100. There are some mods I didn't do that time around such as EFB, speaker ground mod, the separation of the small signal heaters from the output tube cathodes and a couple of others. I figured I had taken that first project far enough. Here is that thread for your reference.

    http://audiokarma.org/forums/index....-100-tube-amp-will-need-some-guidance.765247/

    In general I didn't try to vary components for purposes of improving the sound so much as I was trying to extend the life of the unit and get it running reliably. For those goals I succeeded and the KX-100 is running very well. I have a Fisher X-100-B that I picked up from AK member primosounds on BT. (That's the factory built version of the KX-100.) For that one I'll probably do all of the posted/recommended mods. I don't have plans to deviate and try my own designs for any particular areas - I'm just not good enough to improve on the original design with proven mods.

    BTW, the KX-100 has a good thump to it. I think part of the key is the synergy with the speakers and source. I am using KLH Sixes with a Dual 1019 w/ Shure M91ED and it is a sublime combination.
     
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  6. thornev

    thornev Well-Known Member

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    Tim - In all the KX research I did last week, I do have your thread bookmarked. I copied your list of parts in my notes (I always keep notes about my projects).

    I'm a little more daring about trying suggestions about which I read, but I like your philosophy about "not being knowledgeable enough to make improvements."
     

     

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  7. Tim D

    Tim D AK Subscriber Subscriber

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    Well, I wasn't completely correct. I did change out a few items....

    - I used a 2A fast blow fuse. The unit was spec'd to have a 3.2A slow blow. I wanted the extra safety.

    - I believe I adjusted a resistor in the small signal tube heater circuit to get the right Bias for the output tubes.

    There might be some other minor items I did on the KX-100.... If I think of any I'll post them here.
     
  8. fred soop

    fred soop Super Member

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    The peak reverse voltage across each of the diodes in a voltage doubler is twice the PEAK voltage of the transformer. So, a 100 volt secondary would have a peak of 141.4 volts and the maximum inverse voltage on the diodes would be 282.8 volts, or nearly 3 times the transformer RMS voltage.
     
  9. thornev

    thornev Well-Known Member

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    fred - There are 4 diodes coming off the secondary. The pair attached to the red with yellow lead, according to the schematic, generates up to 405 volts. I don't know which pair to which you refer that is the "voltage doubler". Is there a problem because I upped the voltage of the diodes to 1000v? I just looked at the old diodes and besides the part number, the only other markings are 66-5297 and not all of them have any other number. The old ones are not Schottky's. This is what the old ones look like: https://www.flickr.com/photos/lenore-m/7579250558/
     

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    Last edited: Jan 22, 2019
  10. thornev

    thornev Well-Known Member

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    Calling Dave G (or anyone who knows)... I'm ready to install EFB on my KX-200. There are two EFB schematics, written by Dave G, floating around:

    1. "Universal EFB w/BIAS Supply and Screen Grid Regulators adpater for Fisher 400" dated Mar 24 2013. This image has been seen with a brown background and a white background.
    2. "Modifications for EFB Operation of Fisher SA-100" dated Nov 19 2012

    Which one of these schems, if either, would be closest to a KX-200? I would guess the Fisher 400 because it also takes 7591 tubes. I know neither will be exactly like my KX. In that case, if you wouldn't mind writing something about what considerations I need to ponder. Thanks, Thorne
     

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    Last edited: Jan 23, 2019
  11. thornev

    thornev Well-Known Member

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    I did some research on voltage doubling using diodes. Apparently fred was referring to SD1 and SD2 which appear to be arranged as a voltage doubling circuit while SD3 and SD4 seem to be grounding a lot of AC noise (half of C2 grounds even more noise!). Forward and reverse biasing is a new concept to me. These states are apparently induced by the positive and negative cycles of the incoming AC (sinusoidal) signal. As AC signals keep coming in, C1 gets filled up, is blocked from discharging so that C2 gets whatever C1 contains plus an additional amount of voltage up to the peak voltage coming in, hence C2 gets the doubled voltage. DO I have that right?
     

     

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  12. thornev

    thornev Well-Known Member

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    Today I replaced all 6 of the ERO-FOIL .047uF 400v caps (C13, C14, C19, C20, C21, C22) to orange drop .047uF 600v 5%. Love that deep bass of the KX.
     
  13. Tim D

    Tim D AK Subscriber Subscriber

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    Thorne,

    Did you happen to mention what speakers you are driving that give you that wonderful bass?
     
  14. thornev

    thornev Well-Known Member

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    Oh yeah, I'm always touting my restored, beloved Acoustic Research AR-2Ax', an early pair with the woofer cloth surrounds. Acoustic suspension rules !

    PS - And when I'm feeling like they are a little too much, I have an old pair of B&W DM601's that also throw amazing bass for their size (6" woofer).
     
    Last edited: Jan 24, 2019
  15. Tim D

    Tim D AK Subscriber Subscriber

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    Hmmm. There are some AR-2AX's listed in CL locally. I wonder how the KX-100 would sound with them?
     
  16. thornev

    thornev Well-Known Member

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    I just replaced 3 of 4 Hayseed Hamfest can caps (the last one's going to be a bear because of all the wires), turned it on and the fuse blew. I did something about which I'm unsure. I took one of the ground leads in the chassis and soldered it to one of the outside tabs of a cap can (the tabs that go through the chassis and are bent to hold the can in place). I did this because I wasn't sure there was a good ground connection for the can covers. But now I'm thinking that's the cause of blowing the fuse. I was very careful to take pictures and verify I hooked up everything correctly. Anyone? Thorne

    PS - Something else I noticed... R76 and R77 are each connected to one side of the 5k bias pot and the other side of each is grounded. The schematic calls for 8.2k, but both resistors measure 3k ! I can understand that one could go out, but both out of tolerance the same amount?
     
    Last edited: Jan 26, 2019

     

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  17. Linehand

    Linehand AK Subscriber Subscriber

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    I'd check again to be sure caps are orientated proper. If so, maybe theres a faulty component. My question is why was there an available ground lead?
     
  18. thornev

    thornev Well-Known Member

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    There was an available ground lead because when I unsoldered it from a can I was replacing, it wasn't long enough (the lead broke) to reach the can. So instead of removing the lead, I used it to reach another can and added a new, longer lead to replace the one that broke.

    As for orientation, I oriented the cans using the shapes on the bottom (the triangle and semi-circle and square) so unless Hayseed screwed that up, I have to assume that's not the problem. I don't know how I would verify that the orientation is correct anyway.
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Post Pictures of the unit as it is NOW. Get some good closeups of the area the Can's are in from a couple different angles so we can see ALL of the wiring.
     
  20. thornev

    thornev Well-Known Member

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    Thanks, Larry. Here ya go. If you need more pics, let me know. The cans I changed are C1, C2 and C4 (they are marked in sharpie on the chassis next to each can). C3 has so many wires that I am leaving it for last when I have several hours to do the right job. The extra ground lead I mentioned is black and connected to C1 at its bottom tab. I also posted the 2 pages of schematic.
     

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    Last edited: Jan 26, 2019

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