Fisher KX-200 rebuild project starting...

gkargreen

Well-Known Member
Just found this via Dave Gillespie's threads, and I thought I would post some questions regarding a rebuild of this amp I recently picked up. This particular unit is pretty clean, apparent issue is a cracked resistor (12 ohms on the 7591) and leaking electrolyte from one can capacitor. Some readings here talk about the use of film resistors in these Fishers, I see a lot of maroon-colored resistors in this, I assume these a films and should be left in place unless bad, correct? I also see a few of the Ero-film caps, should they also be left in place? What about the other film caps, the blue .047 and the so-called "brown turds", replace them? And the silicon diodes, replace them with FREDS? Of course the can caps will have to be replaced, any suggestions there? I am also looking for a schematic, a cursory search is not turning up anything yet, but I have further searching I can do. I saw a pic that The Don did of a KX-200 rebuild, Don, where did you get those screw terminal banana plugs? They remind me of the Pomona 5699 but better, love to get some for my various vintage amps! Thanks, all!
 
Please do tell folks! I still have the KX-200 that I built while I was in high school and it is on my short list of items that need to be rebuilt. I will be watching and reading with piqued interest!

Good luck with yours gkargreen. :thmbsp:

Thank's in advance!
 
A great place to start is reading two sticky threads at the top of the Fisher forum.
Deep Cleaning a Tube Amp
and
Most Common Parts for Fishers.

You've asked a lot of good questions, but before answering some of them, I'll just comment that you can find the bulk of your answers in those two threads.

I suggest reading and absorbing the many takeaways in those threads. While there is some conflicting information or information that's changed with the passage of years, much of what you need is there.

Good luck.
 
thanks, Don, I have been reading a lot of yours and Dave's restoration tips and techniques, I will also look at the two articles you mentioned. Is the listing for the terminals strip banana jack contained within those articles, it is so pretty!
 
thanks, Don, I read both articles (as well as am reading numerous others) and I have yet to see anything related to my questions other than the 5699 pomonas, which I am already using. The questions were the erofilm caps, the brown turds (which I am assuming from the nomenclature should be replaced), the purple/red resistors (which I am guessing from something else I read are carbon film and don't need to be replaced as long as they are of the proper value), the diodes (Freds would be the ticket in my experience), schematic (which I found in the AK library), and the speaker terminal jacks shown, the gold terminal screw 6-32 that you are now using in place of the pomonas... thanks
 
I replace the brown caps. They tend to get stress cracks near the leads as they dry out. Cap replacement is a subject of controversy. Some replace all. Some replace the .047 outputs with .1uf outputs and change the grid resistors from 330k to 200k to ease the burden on the output tubes.

I don't replace resistors willy-nilly in a Fisher ever. They rarely drift in value. It has been known to happen, but isn't common.

I used some Freds early on in my restorations, but the form factor is wrong and the hype is far greater than any improvement you'd get based on my experience. If you already have some hexfreds, it won't hurt to use them. I simply no longer seek them out.

I don't know what gold terminal screw you're discussing, or I've forgotten. I might have fixed one once with some banana receptacles, but if the connectors are solidly anchored in the phenolic for the speakers, I'd suggest using the Pomonas to replace the screws.
 
Thanks, Don, I had read on McShane's site that to use the reissue 7591 that those resistors would have to be reduced in value, as well as changing the cap value to retain the time constant...
 
These are great amplifiers. I have a 500C, 500B and 800C and the KX-200 sounds just as good to me. Plenty of power as well.

I've read where there is a mod to remove or disable the center channel which improves sound. I've never done this to my KX-200 so I'm not sure of the amount of improvement. Someone else may chime in on how it is done.
 
These are great amplifiers. I have a 500C, 500B and 800C and the KX-200 sounds just as good to me. Plenty of power as well.

I've read where there is a mod to remove or disable the center channel which improves sound. I've never done this to my KX-200 so I'm not sure of the amount of improvement. Someone else may chime in on how it is done.

Still learning but how would removing the center channel improve sound? Seems to me it's just another way to connect a 3rd speaker? I dont see how this would improve sound imho..
 
Still learning but how would removing the center channel improve sound? Seems to me it's just another way to connect a 3rd speaker? I dont see how this would improve sound imho..

It looks to have a floating ground on the speaker outputs which may have something to do with it. Looking at the schematic, the 4 ohm terminal is grounded. This may be a good idea for a true "center channel" but maybe not so good for stereo listening.
 
Please do tell folks! I still have the KX-200 that I built while I was in high school and it is on my short list of items that need to be rebuilt. I will be watching and reading with piqued interest!

Good luck with yours gkargreen. :thmbsp:

Thank's in advance!

Bruce, adpopulum did a resto on the KX-200 that I am planning on using, and will combine that with Dave Gillespie's EFB circuit. I will try to keep you posted on the progress, right now I will need to put together a parts order to Jim McShane & Mouser to get this project going!
 
Thanks, Don, I had read on McShane's site that to use the reissue 7591 that those resistors would have to be reduced in value, as well as changing the cap value to retain the time constant...

Even reducing those resistors is somewhat controversial. Some believe the 7591 reissues are a glorified 5881 (6L6) which call for 100K grid resistance in fixed bias. They're pretty robust tubes and seem to handle 200-220K well. Tube data for the old stock 7591s calls for 300K in fixed bias.
 
Vendo81 pointed out that, with the KX-200's bias/balance circuit, the grid resistors R51, R52, R57, and R58 are not 330k, as in fixed bias Fishers, but 270k in series with a pot, the situation is different.
If the pot is set near full to properly bias the tubes, you're still close to 330k.
So, changing those 270k resistors to 200k and changing the value of the coupling cap is not mandatory, but a choice with new production tubes.

The schematic is in the digital docs forum.
 
Vendo81 pointed out that, with the KX-200's bias/balance circuit, the grid resistors R51, R52, R57, and R58 are not 330k, as in fixed bias Fishers, but 270k in series with a pot, the situation is different.
If the pot is set near full to properly bias the tubes, you're still close to 330k.
So, changing those 270k resistors to 200k and changing the value of the coupling cap is not mandatory, but a choice with new production tubes.

The schematic is in the digital docs forum.

Don, does that not add up to over 300K on g1? thanks
 
holy crap, this thing boogies!

for those who have a KX-200, or something similar, this may not be a shock, but coming from a Scott 299A (El-84), this amp has some serious bass kick, at least mega bass in the lower regions, finally my system has the bass that it had been lacking for so long! I mean, maybe +6 dB in the bass! Make me wonder what is going on with the Scott. At any rate, finally finished the rebuild, ps caps are 560 ufd x2, and 100 ufd x4, and a pair of CL-80 in the primary, no kick in the turn on (but they sure get warm/hot!). The couplers where changed to .1 ufd K42y and the bias feed changed from 270K to 220K. The other caps are 716P, and added 100 ohm to the screens. But I am in shock wrt. the bass response, the kick and fender just punch out there, anyone know what is going on here?
 
Ya done good all around.
Yes, the KX-200 is one punchy sounding amplifier. Great iron, simple circuitry, good bias/balance controls.
I also love the K42Y. Highly overlooked cap much smaller than the K40Y. You beefed up the power supply, right?
What'd you use for diodes?
 
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Ya done good all around.
Yes, the KX-200 is one punchy sounding amplifier. Great iron, simple circuitry, good bias/balance controls.
I also love the K42Y. Highly overlooked cap much smaller than the K40Y. You beefed up the power supply, right?
What'd you use for diodes?

Hi, Don, I used UF5004 for all the diodes, and for the power supply, I used 560 ufd x2 (replaced the 200 ufd) and 100 ufd x4 (replaced the 20 ufd caps). I am now hearing some turntable rumble, something I did not notice before, it pretty apparent that I am now getting significantly more energy in the bass. Does the KX-200 share the same PI "noose" that DG spoke of in his 400 rebuild?
 
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