Why is there a cathode to ground resistor on a KT66 power tube in an amp kit?

In summary, the cathode to ground resistor allows the plate current to be measured directly as a voltage.
  • #1
Planobilly
440
105
There is a cathode to ground resistor on a KT66 power tube on a amp kit. The value is 1 ohm. I ASSUME the only reason for it being there would be to be able to measure the current flow. I ASSUME 1 ohm is not going to change the cathode bias. Is my assumption correct?

The KT66 is a beam power tetrode and this amp kit is based on a Marshal JTM 45 schematic. The original schematic has the cathode going directly to ground.

Thanks,

Billy
 
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  • #2
The cathode to ground resistor introduces a slight negative feedback. At a cathode current of 100mA, it will change the grid bias by 100mV, which is not very much. The famous Williamson amplifier (http://www.preservationsound.com/?p=3823) uses a complex cathode resistor network, but assume an equivalent of 120Ω per tube, giving a cathode bias (at 80mA) of ≈ 10V.
 
  • #3
Do i perhaps remember discussing that amp ?
If i recall it was for measuring cathode current when you adjust bias ?
While amp is off , make sure it's still one ohm.
 
  • #4
No Jim, not the same amp. This is a new kit I am try fix for a kid to keep him from killing himself. You would not believe how bad he had it messed up. Solder running everywhere. Really dangerous stuff. I had to re-do everything.

I am now looking it over to see if the instructions are correct. I really dislike these kit projects. No bleeder resistors. No cover on the AC line. On and On...not good.
 
  • #5
  • #6
Thanks Jim

Billy
 
  • #7
I am learning this at the moment but can I assume that a 1 Ohm resistor in the cathode circuit allows the plate current to be measured directly as a voltage because a 1 Ohm resistor gives V=A. Once I have my plate voltage I can work out my plate dissipation and then adjust bias accordingly.

So if my plate voltage is 400 V, the voltage drop on the 1 Ohm resistor is 25 mV (0.025 V = 0.025 A) then my plate dissipation is 10 W.

By measuring the current in this way I guess I am ignoring the small current added to the plate by the input signal on the grid. Or am I?
 
  • #8
Bandit127 said:
By measuring the current in this way I guess I am ignoring the small current added to the plate by the input signal on the grid. Or am I?
grid current is microamps
and the electrons came from the cathode so plate current will be a teeny bit less than cathode, negligible when using analog meters of tube days.
 
  • Like
Likes Bandit127

1. What is a cathode to ground question?

A cathode to ground question is a concept in electronics where the negative terminal (cathode) of a device is connected to a ground point in a circuit. This is often done to stabilize the circuit and reduce the effects of noise or interference.

2. Why is it important to consider cathode to ground connections?

Cathode to ground connections play a crucial role in ensuring the proper functioning and stability of electronic circuits. Without a stable ground connection, the circuit may be susceptible to interference, which can cause errors or malfunctions.

3. Can a cathode be connected to any ground point in a circuit?

No, a cathode should only be connected to a designated ground point in a circuit. This ground point is usually the point of lowest potential or the reference point for all other voltages in the circuit. Connecting the cathode to a different ground point can disrupt the circuit's functionality.

4. What are the potential consequences of a faulty cathode to ground connection?

A faulty cathode to ground connection can result in unstable circuit behavior, increased noise or interference, and potentially damage the electronic components in the circuit. It is important to ensure a proper and secure connection to avoid these consequences.

5. Are there any alternative methods to grounding a cathode?

Yes, there are alternative methods to grounding a cathode, such as using a decoupling capacitor or a virtual ground. However, these methods should only be used if the proper grounding point is not available or if there are specific design considerations for the circuit.

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