Just bought a KX-200 - WOO HOO !
- Thread starter thornev
- Start date
thornev
Super Member
Dave - I measured the resistance from the red lead at can cap C1 and at each pin 3 of the 7591 tubes. Results:
V7 = 56 ohms
V8 = 55 ohms
V9 = 55 ohms
V10 = 50 ohms
What do you think? Thanks, Thorne
Perfect! Those are all within +/- 3Ω of what I typically see with these transformers. Looking good so far!
Dave
Dave
thornev
Super Member
Well, Folks, I decided it was time. I installed the precautionary components to ensure that if something blew up, it wouldn't take out the whole damn thing. You would have laughed at watching me sit there - in fear - second guessing my decision driven by excitement to hear this baby sing. Finally after about 3 minutes I just turned the volume knob on... just to minimum, just to let it warm up, just to listen for loud bangs in the first few seconds, indicating there is a serious problem. BUT !!!...
Oh man ! This thing sounds absolutely amazing ! The clarity ! The punchy bass ! And it's deep ! The crystal clear highs. And it fills out my AR 2Ax speakers very nicely. I've never seen the wind from those woofers move the cloth speaker grille ! And this thing is DEAD QUIET ! I'm just listening to my mobile phone's 1,000 songs so I expect even better sounds when I connect some high(er)-end audiophile gear. I don't think I'm going to get any sleep tonight ! I'm listening to songs I've always hated listening to, but now they sound stunning - I want to see what I've been missing. Too bad there's no headphone jack. I know I could add one. I might just reach out to you all for that (I think I saw a schematic or a picture of one during my KX-200 research this week).
Anyway, I'm very happy with my KX-200 and am excited to complete the restoration. Stay tuned for more of my journey. I'm psyched to see what sound improvements the EFB can do !
PS - I have encountered several problems just after 1 day. The face plate is bent (but not so badly that it affects anything), several screws are missing that hold it all together, whoever cleaned or repaired inside did a lousy job, a lot of the writing on the metal is worn away, the bias meter doesn't seem to work, etc. But that's part of the fun, right? Making this beauty whole again !
Oh man ! This thing sounds absolutely amazing ! The clarity ! The punchy bass ! And it's deep ! The crystal clear highs. And it fills out my AR 2Ax speakers very nicely. I've never seen the wind from those woofers move the cloth speaker grille ! And this thing is DEAD QUIET ! I'm just listening to my mobile phone's 1,000 songs so I expect even better sounds when I connect some high(er)-end audiophile gear. I don't think I'm going to get any sleep tonight ! I'm listening to songs I've always hated listening to, but now they sound stunning - I want to see what I've been missing. Too bad there's no headphone jack. I know I could add one. I might just reach out to you all for that (I think I saw a schematic or a picture of one during my KX-200 research this week).
Anyway, I'm very happy with my KX-200 and am excited to complete the restoration. Stay tuned for more of my journey. I'm psyched to see what sound improvements the EFB can do !
PS - I have encountered several problems just after 1 day. The face plate is bent (but not so badly that it affects anything), several screws are missing that hold it all together, whoever cleaned or repaired inside did a lousy job, a lot of the writing on the metal is worn away, the bias meter doesn't seem to work, etc. But that's part of the fun, right? Making this beauty whole again !
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Good score,Thorne! Longevity mods good for anything running 7591's, 7868's , same thing regardless of make/model. I've managed to crack the inner panel of the closest window to my AR powered subwoofer, can only attribute to the sub. (Yes, i like to crank the shiite out of it at times). Have fun, just don't burn it up!
P.s. you likely don't need to piss around with the diodes , they either work, or dont. For headphones jack, just take a pill bottle, put a couple of 10k 1 watt or bigger resistors inline with the hot leads, add a
Tip-ring-sleeve 1/4"jack in it, hook up to b-side speaker out terminals, plug in & jam-in private. Good to go. I'd send pics of my home brew unit, but too many Molson xxx beers tonight. Catman
Tip-ring-sleeve 1/4"jack in it, hook up to b-side speaker out terminals, plug in & jam-in private. Good to go. I'd send pics of my home brew unit, but too many Molson xxx beers tonight. Catman
What was that beer? Bradoor?
thornev
Super Member
Well, after several tries, it looks like the bias adjustment feature on this unit does not work for all tubes. It came with JJ 7591S tubes and I cannot get the bias adjusted according to the instructions. That would mean I have to use the old technique of measuring with values in P = I*V using the pin 5 10-ohm resistor at the grounded end to measure voltage. So measure pin 3 voltage and pin 5 voltage, divide pin 5 voltage by 10 (for 10-ohm resistor where I measured pin 5 voltage), multiply pin 3 voltage times current (pin5/10) to get wattage being generated. If I'm not around 70% of 19, I'll have to install an IBAM (think I'd rather do IBBA this time).
I tried with the old Sylvania 7591's I have and the adjustment didn't work with them either. I still have the Tung-Sol 7591's to try, but I'm guessing they won't work either. Of course the best fix is to find out why the meter doesn't work with these tubes as THAT might be the real problem that needs to be fixed. Any thoughts?
(1 hour later...) Someone must have screwed around internally, messing up the bias feature, because I would think the old Sylvania tubes were close to what was stock in the kx-200. And I've got the unit getting electricity whose voltage is maintained at around 115VAC. So maybe the bias resistors were incorrectly replaced or some other partial circuit changes are affecting bias. I believe this is a "fixed bias" system.
The transformers run cooler with the JJ tubes than with the Sylvania tubes.
I tried with the old Sylvania 7591's I have and the adjustment didn't work with them either. I still have the Tung-Sol 7591's to try, but I'm guessing they won't work either. Of course the best fix is to find out why the meter doesn't work with these tubes as THAT might be the real problem that needs to be fixed. Any thoughts?
(1 hour later...) Someone must have screwed around internally, messing up the bias feature, because I would think the old Sylvania tubes were close to what was stock in the kx-200. And I've got the unit getting electricity whose voltage is maintained at around 115VAC. So maybe the bias resistors were incorrectly replaced or some other partial circuit changes are affecting bias. I believe this is a "fixed bias" system.
The transformers run cooler with the JJ tubes than with the Sylvania tubes.
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KX-200 is fixed bias.
thornev
Super Member
Just measured pins 3-6 on all 4 power tubes. Had to play with balance and bias controls to get the numbers close to each other and here's what I measured:
V7 pin 3 = 433VDC
V8 pin3 = 432
V9 pin3 = 433
V10 pin3 = 431
V7 pin4 = 381VDC
V8 pin4 = 380
V9 pin4 = 391
V10 pin4 = 389
V7 pin5 = .467VDC
V8 pin5 = .452
V9 pin5 = .426
V10 pin5 = .430
V7 pin6 = -14.49VDC
V8 pin6 = -14.71
V9 pin6 = -15.11 (this number was way high like -15.80)
V10 pin6 = -15.11
With a 10 ohm resistor at all pin 5's, with P=IV I get for V10:
P = 431 * (.435 / 10) = 431 * .0435 = 18.7 watts
For V7:
P = 433 * (.452 / 10) = 433 * .0452 = 19.6 watts
That's over the 19 watts max. Yikes.
Gotta get this under control. Don't think the BIAS and BALANCE controls on this thing will do it.
V7 pin 3 = 433VDC
V8 pin3 = 432
V9 pin3 = 433
V10 pin3 = 431
V7 pin4 = 381VDC
V8 pin4 = 380
V9 pin4 = 391
V10 pin4 = 389
V7 pin5 = .467VDC
V8 pin5 = .452
V9 pin5 = .426
V10 pin5 = .430
V7 pin6 = -14.49VDC
V8 pin6 = -14.71
V9 pin6 = -15.11 (this number was way high like -15.80)
V10 pin6 = -15.11
With a 10 ohm resistor at all pin 5's, with P=IV I get for V10:
P = 431 * (.435 / 10) = 431 * .0435 = 18.7 watts
For V7:
P = 433 * (.452 / 10) = 433 * .0452 = 19.6 watts
That's over the 19 watts max. Yikes.
Gotta get this under control. Don't think the BIAS and BALANCE controls on this thing will do it.
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Dig into the KX-200 threads. There is a resistor change in the bias circuit that helps get the bias under control with old and new tubes. One of Dave's Bag of tricks.
thornev
Super Member
This post seems like the one to which Larry refers:
"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."
I measured the impedance on that resistor and got 409 ohms. So the bias adjustment problem is made even worse by the higher value.
"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."
I measured the impedance on that resistor and got 409 ohms. So the bias adjustment problem is made even worse by the higher value.
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thornev
Super Member
Should I replace these .047uF 400Vgold caps? They are so old and cool looking, and I don't see any leaks. But every amp I've seen, people always replace them and with .068uF. Sonicap seems to be the popular replacement brand. I've got .068uF 600V Sonicaps. Would the 600V work? They are connected to pin 6 (plate) of 12AX7 tubes.
Attachments
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Hold on to the ERO-FOL's.Check them for electrical leakage. . If you really want to put in .use .047uf.Leave the Grid return resistors as is The Grid Return resistors are BELOW the MAX VALUE for Fixed Bias (300K). So no need to change the coupling cap value.
thornev
Super Member
I piggy-backed a 510 ohm across a 406 ohm to get about 587 ohm, and used that "parallel circuit" across the bias meter posts to expand the range of bias adjustment. With the one resistor I couldn't get the bias adjustments anywhere near their red line on the meter. The 587 ohm (temporary) did the trick.
-=-=-=-=-=-=-=-= THANK YOU, DAVE G. =-=-=-=-=-=-=-=-
Using 598 ohms and adjusting, on V8 I measured:
pin 3 = 437 ohms
pin 4 = 390 ohms
pin 5 = .340mV
pin 6 = -17.06vdc
Using P=IV, P = 437 * (.34 / 10) = 437 * .034 = 14.9 watts which is 78% of 19 max watts. Much better !
All the 7591 tubes' pin values are similar so that's an improvement. And wow, the temperature difference of the transformers to the touch is significant ! So much cooler.
PS - While I was poking around in there I noticed that the bias and balance adjustment pots are dated. The 4 dates I could read are 6608, 6431, 6424 and 6527 (date format on pots is yyww where yy=last 2 digits of year and ww = week number in that year). The only explanations I can think of for the spread between 1964 and 1966 is that defective pots on a purchased unit had to be replaced (which I doubt after so few years), or that the older pots were just lying around at the factory for a couple of years, got mixed in with newer ones, and were picked when the unit was built.
The date prefix on all the pots is 134 which means Mepco/CentraLab, Inc.
Also, I hooked up a turntable to the MAG2 jacks and the sound was horrible. Lacked the punch and full frequency spectrum as the AUX1 jack. I don't believe a phono preamp is needed for the moving magnet cart I am using.
-=-=-=-=-=-=-=-= THANK YOU, DAVE G. =-=-=-=-=-=-=-=-
Using 598 ohms and adjusting, on V8 I measured:
pin 3 = 437 ohms
pin 4 = 390 ohms
pin 5 = .340mV
pin 6 = -17.06vdc
Using P=IV, P = 437 * (.34 / 10) = 437 * .034 = 14.9 watts which is 78% of 19 max watts. Much better !
All the 7591 tubes' pin values are similar so that's an improvement. And wow, the temperature difference of the transformers to the touch is significant ! So much cooler.
PS - While I was poking around in there I noticed that the bias and balance adjustment pots are dated. The 4 dates I could read are 6608, 6431, 6424 and 6527 (date format on pots is yyww where yy=last 2 digits of year and ww = week number in that year). The only explanations I can think of for the spread between 1964 and 1966 is that defective pots on a purchased unit had to be replaced (which I doubt after so few years), or that the older pots were just lying around at the factory for a couple of years, got mixed in with newer ones, and were picked when the unit was built.
The date prefix on all the pots is 134 which means Mepco/CentraLab, Inc.
Also, I hooked up a turntable to the MAG2 jacks and the sound was horrible. Lacked the punch and full frequency spectrum as the AUX1 jack. I don't believe a phono preamp is needed for the moving magnet cart I am using.
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