Just bought a KX-200 - WOO HOO !

I thought about it, saw it was at a relatively low price (which of course didn't stay low) and pulled the trigger... and won ! It is stock so I will be refurbishing it as I did with my 500-C although this time it will be from scratch. Naturally I'll be asking for some help. I'll start looking for a restoration list.

It doesn't come with a cabinet so I'll be looking for one of those. Scarce I imagine.

My first question... Since the kx-200 is also a 7591 amp, do the same safeties apply as they do to the 500-C? For examples, a 10 ohm resistor on pin 5 to ground (maybe not needed since bias adjustment meter is part of this amp - IBAM/IBBA won't be needed), moving the pin 8 screen grid wire to pin 1 and stringing all the 7591 pin 1's together? Also, adding a 100 ohm resistor between pins 1 and 4 to prevent a tube meltdown from taking out the other tubes?

Question 2 - Since I won't be using a center channel speaker, would it improve amp efficiency by removing its circuitry?

Thorne

 

Congratulations! Biggest iron outputs used for 7591s along with X-202-B. I love mine. Cabinets come around pretty frequently as lots of models like X100B, X-202-B and a lot of tuners use the same one. I look forward to your restoration thread.

Bruce

 

IBAM shouldn't be needed I believe, additionally do not remove the center channel as its a part of what the amp was designed for. Restore it as Avery would of wanted it, my 2 cents.

 

You could always run that center channel into a woofer. I did it with a 800B running a set of Klipsch Heresys. It's the fastest way to resolve the lack of base that some complain about with those speakers.

 

quad - Does that mean that the full spectrum of frequencies is available at the center channel posts? I would think that only the vocal range of frequencies would be available there and maybe the frequency range is adjustable with a capacitor and resistor.

 

In case anyone is interested in offering suggestions based on what can be seen, I've attached a picture of the underside chassis. Lots of old components that need upgrading.

 

Center Channel is FULL RANGE. Just like the 400,500b/c or 800b/c. Early on in Stereo, if the speakers were set too far apart (either by design or by WAF) you'd get a HOLE in the middle. Put a full range speaker in the middle on a separate amp, and turn it up to just fill the hole. You didn't want it to be equal to the L & R channels. More of a phantom sound to decrease the "hole" as the Center Channel is a Sum of the L & R Channels.

 

So it's great for a subwoofer. Cool.

 

Toss a powered sub at it and have some fun. Make sure you've got some plate glass handy, in case you crank it up past 9.

 

Congratulations on this acquisition! The basic chassis metal is similar to the 100 series, but the output transformers are the best of the ones for 7591 tubes. IBAM can still be beneficial with modern tubes and so can EFB. You may wish to try it without those improvements before making changes other than what is necessary for good operation.

Joe

 

Looks like some of the original .5 watt 10Ω cathode resistors have been replaced with a 5 watt and/or 10 watt device (!). If so, then this unit has seen its share of output tube arcing. The transformers in this unit are pretty tough nuts, but I'd still check for primary winding continuity first thing.

Dave

 

Dave - "primary winding continuity"... That's a term about the power and output transformers, right? The output transformer sends the signal to the speakers so I assume you don't mean the output transformer. I know there are input windings and output windings, but how does one test for continuity? I've got the schematic, so do I put my DMM on continuity and then test with DMM probes that the input windings connect to the appropriate output windings? And do I do this with the amp on or off? If on, I would think I put my DMM on voltage, not continuity. And then I'd hope the schematic shows what the voltages are supposed to be.

And then what about the center taps? Do I do anything special with them?

 

Jealous! Nice score.

Yes Thorn,
Dave is talking about checking that the primary windings on the output transformers are not open-circuit/blown. They should have some non-zero resistance (maybe a few hundred ohms). The center tap is in the middle of both legs of the primary winding and measuring from one leg of the primary winding to the center tap of the primary should be half of the full winding resistance.

 

The primary winding leads are blue, blue w/yellow tracer, and red. The blue and blue/yellow leads connect to pin 3 of each output tube in each channel, and the red lead (which is the center tap of the winding) connects to a power supply can cap. Measure the resistance from the red lead to each of the pin 3 leads and let us know what you get. No need to disconnect the leads.

Dave

 

Thanks so much, Dave. According to the schem, my secondaries are brown and orange with a white center tap, yellow and green go to the tube heaters, and red with a yellow trace and red. The primaries are only 2 black wired. The brown goes to pin 5 of all the 12AX7 tubes. The red with yellow stripe looks like it goes to the can caps.

UPDATE - You were talking about the primaries of the output transformers...DUH ! Sorry. I should be getting the KX-200 by Wednesday night... PSYCHED !

 

What Dave has pointed out, along with his comments, shows why it is not always a good idea to increase the wattage rating of a given resistor. There are often other factors to consider. It is far better to let a resistor burn open than to have it continue to conduct current until it destroys a far more expensive part such as an output or power transformer.

Joe

 

Got the KX-200 yesterday! Today I installed a CL-80 inrush limiter. I also moved the bias feed from pin 8 to pin 1 of the V7 7591 tube, cut the pin 4 to pin 8 connections between 7591 tubes, inserted a wire that connects all 7591 pin 1's together and inserted 100 ohm screen grid resistors between pins 1 and 4 on each 7591. 10 ohm pin 5 to ground resistors were already installed. I ordered the Hayseed Hamfest can caps though it looks like someone already replaced C1... very poorly. It looks like 99% of components are original. I plan to replace some other caps, lytics and then see what it can do. I will test the transformers before powering up.

 

Thorne,
My X-200 (factory version) is my favorite amp. You're going to love it. By the way, search Dave Gillespie's posts on X-200 or KX-200 and you'll find some very valuable info on the amps and the differences between the factory and kit builds. Good luck with it!

 

I need help with diodes in this unit. The schematic parts list reads "SD1 and SD2 are silicon diodes high voltage, and SD3 and SD4 are silicon diodes filament & bias". SD1 and SD2 come off the power transformer red/yellow lead and continue on to the can caps, and SD3 and SD4 come off the power transformer brown and orange leads, both go to ground. What would I order from mouser or whoever? Partial schematic attached. Thank you.

 

If they are working, I'd leave them (good quality silicon rectifiers in stock unit). Mine had been fiddled with and the replacements were under-rated. Something like a 1N5408 (3A 1000 V PIV) for the HV and 1N4002 or 1N4004 for the bias supply ought to work OK. The 1N5408's would be quite conservative, but robust. My thoughts, but others may approach differently.