fisher 500c explain my problem?

[jstraw]

I have one tube that start's to glow red.I think you would call it the grid It's the inside part.It glow's brighter than my other 3.The tube though is cooler to the touch.Their 7591's.3 type's jj, nos and eh.The only one that does not look odd is the eh.
So i meausre the pin's and the exact same pin on my other 3 tubes read 450v.The bad side of one pair read's 0 to 3 v.This is a blue and white wire that come's off the output transformer.
How does this thing even work?
I think i need a new output transformer if anyone has one for a 500c?
I just don't understand why it even work's?
A odd thing i noticed with the jj 7591 is at 0 to 40% volume the inside is much brighter.At 50% it's normal but the glow flicker's to the beat and the tube is colder than the other 3.
The receiver actually sound's ok but only to 50% volume.
I measured everything the pin and the voltage not comeing from the output is the problem.Every other pin is pretty much equal.

 

[seta1]

First off, here's the pinout for the 7591A
1. not connected
2 and 7 filament
3 plate (should read around 450 volts on a 500C Fisher
4 G2 Should read between 400 and 450 volts
5 G3 and Cathode (on a 500C, should read zero volts)
6 G1 (on a 500C should read between -18 and -22 volts)

If you are missing the voltage on pin 3, you probably have a bad output transformer. The output transformer can fail such that one tube does have voltage on it's pin 3 while the other in the push pull pair does not. In fact, this is the most common failure.

If you are missing voltage to pin 6, your tube's plate will glow red. This is what you will hear called 'red plating' and it is fatal to the tube and many times, the output transformer. Loss of voltage to pin 4 can also cause this.

It sounds like you have a bad output transformer that was caused either by a faulty 7591A and or a bad coupling capacitor causing the negative grid bias voltage at pin 6 to be incorrect.
You can measure the resistance of the primary of that transformer from the red wire (center tap ) to pin 3 of each output tube the transformer feeds. You should read around 100 - 150 ohms depending on your meter. If the transformer is bad, you can check Ebay for one. I see them up for auction on a pretty regular basis. Expect to pay around $100 or so for one.
Oh, your receiver can still work with half of the output transformer's primary blown. It runs essentially like a single ended amp. Any push pull amp will run with only one output tube. But, you get a loss of power, higher then normal hum and higher distortion, especially as you crank the amp up.

 

[jstraw]

Great explanation.Especialy on how it might work.I have a 24 watt fisher x 100-c.It is double the power Of my 36 watt 500 c.My 500c has alway's hit a wall then distorted badly.My x 100-c will go into distortion for forever and still sound sweet.Now i know why.Maybe this thing has been sick for awhile and i rebuilt the bias supply long ago.
It figure's the time i need a 500c output transformer ebay has none.i swear every time i look there seem's to be atleast two