heater center tap through cathode resistor?

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embotone
Junior tube assistant
Posts: 12
(2/14/04 5:41 pm)
 heater center tap through cathode resistor?
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 I have 2 older amps ('60) I have been working on. In one amp the 6.3 v center tap is connected to ground through the 350 ohm cathode resistor of the power tubes. In the other, they used a hum balance pot (50 ohm) between the two sides of the 6.3 v wondings with the with the wiper connected to ground through the catode resistor , I think that one was a 200 ohm. Even though there is a center tap, they left it unused.
Whats up?
 
GroundhogKen
Forum Moderator
Posts: 2265
(2/14/04 8:30 pm)
 Re: heater center tap through cathode resistor?
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 Hum can be reduced slightly if the 6.3vac heater voltage rides on top of a DC offset which is above the ~1.5v level of the preamp cathodes.

Therefore, the heater cender tap ( or resistors or balance pot ) is sometimes connected to the cathode resistor of the power tubes which is usually in the neighborhood of 10-25vdc.

Ken
 
Lucid Alice
Senior tube assistant
Posts: 157
(2/14/04 8:55 pm)
 Re: heater center tap through cathode resistor?
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 Those are tricks I learned though sites like this one and others like it.

On the point of the heater circuit being tied to the cathode:
If an amp is cathode biased, it is technique to further reduce any 120hz hum that could be picked up from the heaters. The theory behind it is that some electrons do break free from the heaters. If the heaters are tied to ground, the cathode, which will be around 8 - 12 volts, looks like a mini-anode to the heater circuit. The cathode will attract those electrons. With those electrons the cathode will pick up some of the 120hz signal, and amplify it. If the heater center tap, or two resistors creating an artificial center tap, are tied to the cathode of the power tubes, the cathode will appear neutral or more negative than the heater circuit. This situation will not attract the free electrons from the heaters.

The point of the amplifier not using the heater center tap on the PT and creating an artificial center tap using two resistors is a safety mechanism. If you use the center tap and a tube fails by shorting the heaters to the cathode you will most likely toast the 6.3 volt windings in the PT. If you create an artificial center tap using two resistors and the same tube failure occurs, you will fry the resistors. The difference being 50 cents worth of resistors lost, or a entire PT!

The amp with the pot endeavored to reduce hum by balancing the two sides of the 6.3 volt heater circuit. The more perfectly balanced it is, the quieter it will be.

It sounds to me like both amps took their own route toward reducing hum from the heater circuit. You could do both if you really wanted to get that carried away with it.

On the amp using the center tap tied to the cathodes: If the same tube short occurred, my take is that the cathode resistors would fry and save the PT.



 
embotone
Junior tube assistant
Posts: 13
(2/15/04 12:59 pm)
 heater tap through cathode resistor
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 If I replace the common cathode resistor with seperate cathode resistors, can I still run the center tap through one of the cathode resistors, or will this cause some sort of imbalance?
 
 
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