Discussion in 'Fisher' started by craig25, Aug 24, 2018.
I can't wait to get this one done!
Rock n Roll, Craig ! May your adventure be fruitful.
Looks like you're good to go!!!!!
Are those teflon caps????
You have the will, the desire, and the parts, too! Good luck with it. I have the factory-wired X-200 and it is just about my favorite amp.
Yes. Russian teflon. Philip's NOS 5751's. Multi sections from Hayseed Hamfest. New diodes and grid resistors from McShane. I'm pretty sure I'll be adding some additional parts as I get into it.
I thought so. I've always been interested in those teflons but decided against using them due to size/fit. Please let us know how they turn out!!
Watch out with the 5751s. Collectively, they will draw 75 mA more heater current from the DC Heater/Bias supply, loading its output down more than the original tubes do due to their increased current draw. Against this, the Tung Sol 7591's are known to typically require notably more bias voltage than the original American tubes did just to achieve the original operating point, and even more if you back the tubes down to sane quiescent current levels. You may not have a problem at all, but it is something to be aware of with the combination of tubes you are using. The comment is made with that intention. Never install a 5751 in a socket whose heater is wired in series with another socket that has a 12AX7 family tube installed, as the dissimilar heater current requirements of these two tubes will starve the 5751 heater, and over-voltage the 12AX7 heater. When series heater strings are employed like they are in this unit, all the tubes of a given string must be of the same type so that the heater voltage will be evenly distributed between all the tubes in the string.
If your on board bias/balance meter is in good shape, and you plan on using it to bias your tubes, you might consider installing the alternate value resistor I've recommended for the meter circuit (240Ω versus 390Ω originally) to achieve a reasonable quiescent current setting for the output stages. The original design has the output tubes drawing stratospheric quiescent current levels, and the alternate resistor value brings this down to a very reasonable level that maintains a good balance between the performance characteristics of the design, and tube life. Also, with the installation of your new power supply can caps, the installation of Screen Stability resistors is a must in the model to protect the output tubes against arcing.
Finally, also be aware that with the reduced gain of the 5751 tubes, it will alter the original RIAA curve produced by the phono preamps, slightly reduce the sensitivity of the unit to below that specified by Fisher, and also increase distortion somewhat due to the reduced open loop gain produced in the power amplifier section.
Just some thoughts to chew on as you get started.
The KX-200 is a wonderful amplifier -- good luck with your rebuild!
I've got a KX-200 that I've restored a couple of years back. I'll be interested to know what your take is on the lower-gain of the 5751s.
Edit - cross post with Dave
Thanks so much Dave! More than enough food for thought to digest on using the 5751's.
I'm leaning back towards 12ax7's now.
dcgillespie good morning. Im about to pick up a kx-200 I bought on the bay auction that the seller said it sounds great. I understand after all the research I have done that you are the Fisher Master therefore I am hoping to get some help/advice from you. I understand that a rebuild will be necessary and have read about some upgrades to make tubes run cooler. Also I read that Jim is a great guy to get tubes from and If I recall he is also a go to guy to get a rebuild kit from too. However I do not know how to contact Jim as Im pretty new on this site and do not know the ins and outs of this site. Any advice on this rebuild and on how I can contact Jim Mcshane would be appreciated. I have a lil bit of experience replacing caps and stuff like that but I am not a pro at reading schematics. Thanks for all your great posts and any of your help advice, Ben
Hi Lopez -- Thanks for the kind words, and welcome to the AK Fisher forum! First, you can contact Jim through his website here:
The very best modification you can do to your KX-200 is change the meter resistor to 240Ω, to produce a sane current level through the output tubes when using the onboard meter to set the bias and balance.
For the remainder of your work, I suggest that you start your own thread on your rebuild project, so that it doesn't overrun that of the original poster of this thread. There's plenty of great folks here that will want to follow your project and provide any help from their experience with their own Fisher equipment. That way, we can all comment and help as you need it, and it will be compartmentalized to your own project.
The KX-200 is arguably one of Fisher's best integrated amplifiers so congrats on getting one! It's worth your best work, and will return years of enjoyment!
Thank you Dave, I will start my own thread once I pick up my amp possibly this weekend. Sorry craig25, I just didn't know how else to get to dcgillespie. Good job guys
No apology necessary! We're all at different levels of knowledge and the great thing is we can help and rely on each other to learn more here.
craig25 I am about to restore a kx-200 and am leaning towards using the Hayseed cans also. Did you finish the restoration on your kx-200 and are you happy with those can caps? And thanks for not getting all over me for posting on your thread. Ben
Unfortunately, I have not begun the restoration yet, because there's been too much work going on.
I used the hayseed cans in a Fisher 400 exclusively substituting exact values and it came out very nice. Some believe we can use better quality parts today than were available at a price point back then and have suggested substituting higher capacitance parts. That's your own call but you might want to research that a little bit.
craig25 one reason Im leaning towards the Hayseed cans is to keep my restoration as easy as possible for now. However I did order Jim's kit without the replacements for the big can caps. Maybe down the road I might change them if Im not happy with them. Thanks for the great advice. I also have been very limited on time too unfortunately. Jims kit should arrive soon. Thanks again, Ben.
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