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Fisher KX-200 Bias circuit problem

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G

Glowin Plates

Member
Joined 2004
Hi All.
I am working on a Fisher KX-200 with 7591 outputs.
These are a u-build "Strata Kit" which have a built in bias meter.
They are biased for original tubes at -16VDC which I have read is not great for tube life.
So.. the solution (I read) is to change the 390 ohm resistor to 240 ohm which will allow you to use the meter
to bias to a better value for tube life.(mine came out to -19Vdc 450V on plates with this mod.
I also replaced the 1000 ufd (35Wvdc) can in the neg bias circuit with new 65V WVDC electrolytics as original was leaking goo.
The big issue I am having however is the 15 ohm 5W resistor between the pins of the
1000 ufd cap is getting super hot and smoking after only a few minutes.
It did not do this before.
The volts on the cap are around -32v and (-25 on other side of resistor) so it seems to be dropping about 7volts across the resistor
which is 433mA by ohms law which would seem to only be about 3 Watts across
The resistor Which should be ok for a 5 Watt power resistor.
I am unsure wether I did something wrong or
this is a result of bias change and I need to increase wattage of this resistor.
I did observe polarity on caps with positive going to ground as this is a negative supply.
Any thoughts?
Click to expand...
 
6

6A3sUMMER

Member
Joined 2016
A schematic with voltages and currents is worth 1000 words, and helps analysts to analyze.

Perhaps a very experienced Fisher KX-200 expert can answer your question without a schematic.

A resistor that gets very hot, usually means one of two things:
1. The current through the resistor is too large, caused by a fault that is downstream.
2. The original design tried to save the cost and space of a larger resistor.

Just my opinions
 
G

Glowin Plates

Member
Joined 2004
My Apologies.
Hopefully the schematic works.
I re-stuffed the original can with 1000ufd 65Vdc electrolytics that my local electronics store happened to have.
I felt the resistor and it is too hot to touch so I assume that's where smoke is originating...
 

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G

Glowin Plates

Member
Joined 2004
Yes.The strange thing is other than increasing the bias voltage the circuit is exactly the
same.(other than functioning capacitors that are almost double the Working voltage)
The voltage levels are good, amp sounds normal(at first)
I am afraid to just increase wattage of that resistor because it doesn't seem right that it should suddenly be drawing more current.
I thought increasing voltage should reduce current?
 
rayma

rayma

Moderator
Joined 2011
Making the grid DC voltage more negative will reduce the tube's plate current.

How much voltage ripple is present on the first capacitor in the bias circuit?
Excessive ripple voltage could cause the 15R to heat up more than it should,
since the 15R has to dissipate most of that power.
 
G

Glowin Plates

Member
Joined 2004
That’s a good point.
I don’t know how to measure ripple but when the cap blew it took out
The first 1000ufd and shorted one of the new 8ufd that I put in.
So I put the originals back in just to test.
So maybe those originals have excessive ripple?
I was planning on replacing the 8ufd (again) but need to order more.
 
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