Ok trouble in paradise!

Ken Boyd

Super Member
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As the title states, I think I have a problem. I purchased a wood case for my Fisher the KX200. But I was using a quad of EH's for the output tubes and the case wouldn't clear the one tube on the left. So I found a quad of Fisher branded Westinghouses on the auction site, the seller stated all tested good and included a picture of one tube on his tester with the needle all the way into the far good zone, and stated all tested about the same.

I installed the tubes, and tried to bias them everything was good, but channel A would not balance correctly, by the meter. Then I saw one of the tubes getting really red, so I shut it down. I tried again after letting things cool, all the tubes glow equal, and the amp has equal sound out of both channels but the right channel has some noise like static when your turning a pot, but it comes and goes without turning anything, it gets louder when you turn up the volume and goes away when turned down. Is that one tube bad and thats why it wouldn't pass the balance test, as the EH's had no problems, and is that where the noise is coming from? Any idea's?
 
Ken, you've got an output that's red plating. My guess is that your Westinghouse's aren't matched for current.

Not sure of the bias scheme on the KX-200 but if you bias for pairs then you might try switching one of the 7591s to other channel to see if those pairs match.
 
Ken, Make sure the pins on the tubes and sockets are clean and tight. The new EH's with the new pins may have stretched the retainers slightly making for a poor fit with the old tubes with worn pins. I haven't had problems switching EH's with OS 7591's but I've probably been lucky. A dental pick works well to tighten the retainers inside the socket.

If you can, you should be able to check the bias MA @ the 10ohm resistors directly off the tubes each on pin 5. Also check the NEGATIVE Voltage on pin 6. Should be about 16-17V per the schematic. When you check pin 5 check across the resistor, in mv and move the decimal point 1 to the left to get ma.

Write all these down along with the voltages of pins 3 and 4 and post. We can figure out how much your tube are dissipating and whether or not they are being run in spec.

Larry
 
It sounds to me like you might have grid bias runaway on one Westy 7591. That shows up after the unit has been on a while as a change in the dissipation on the 10 ohm resistor. Best to test it while the unit is upside down with the bottom chassis cover off. Test it a couple minutes after start, again in 15 minutes, and again at 60 minutes. Keep the output tubes in a place you can see them to avoid red-plating.
Larry covered why and how to check in the previous post.

A separate but related issue: Your bias/balance settings could be off.
Use the bias/balance and meter to check. A couple things:
1. Sometimes the meter sticks and you have to tap it with your finger to move it.
2. Sometimes the bias or balance pots are off and you can only adjust them with a thin screwdriver from under the chassis with the bottom cover off.
Remember that the meter is not an absolute, where you have to have the settings exactly to the marks.
 
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