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Power Tube Cathode Resistor "distance"

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M Gregg

M Gregg

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Joined 2010
#1
Hi,

Here is a question I have never thought about until now.

I know it will effect pre-amp output, this is a power tube question!

Is there any maximum distance between power tube cathode and cathode resistor? I know we normaly put them directly by the power tube.


Regards
M. Gregg
 
#2
Is the resistor bypassed? Will there be any signal riding the wire?
The cathode can be viewed as an input, but if the resistor is bypassed, and low in value, I can't see how it matters if the wire between cathode and resistor is the long one, or if the wire from cathode resistor to gnd is the long one.

Then again if you wanna nit-pick, the cathode is the tube's reference, and all good engineering wants all paths to the cathode to be short and direct. But for 'our' BW I can't see the problem with either or...
 
#3
SemperFi said:
Is the resistor bypassed? Will there be any signal riding the wire?
The cathode can be viewed as an input, but if the resistor is bypassed, and low in value, I can't see how it matters if the wire between cathode and resistor is the long one, or if the wire from cathode resistor to gnd is the long one.

Then again if you wanna nit-pick, the cathode is the tube's reference, and all good engineering wants all paths to the cathode to be short and direct. But for 'our' BW I can't see the problem with either or...
Click to expand...

The thought was for cooling a chassis why not move it to a different location. Away from the tubes and vent somewhere else!

Bypassed resistor with cap! Just a thought why not bolt it to a heatsink on the back of the chassis?

Regards
M. Gregg
 
Last edited:
#4
Audio doesn't mind, provided that the wire to the resistor doesn't pick up interference from the PSU or radiate feedback to an earlier stage. If bypassed it is the capacitor wire which is relevant, not the resistor. In any case, you still have to think about where the stage is grounded.

RF might mind, so long wires might create instability. To be on the safe side I would loosely twist the wires to the resistor so that any loop is minimised.
 
#5
DF96 said:
Audio doesn't mind, provided that the wire to the resistor doesn't pick up interference from the PSU or radiate feedback to an earlier stage. If bypassed it is the capacitor wire which is relevant, not the resistor. In any case, you still have to think about where the stage is grounded.

RF might mind, so long wires might create instability. To be on the safe side I would loosely twist the wires to the resistor so that any loop is minimised.
Click to expand...

Thank's for your thoughts,

It would seem then that if the bypass cap is put by the tube " To Keep the signal path short", then the resistors can go any where on the chassis within reason. As you say the wire would cancel if twisted. However do think the point of return to Ground would be any more critical than a standard build?

In the simple single ended the cap is used to give feedback from the output Tx, just thought it was worth a mention!

Regards
M. Gregg
 
#6
The cathode should be a relatively low impedance point. I wouldn't expect long leads here to cause too much trouble, but personally, I prefer to keep the signal path -- and loop area -- as short and small as possible. Why invite EMI if you have the option not to.

~Tom
 
#7
tomchr said:
The cathode should be a relatively low impedance point. I wouldn't expect long leads here to cause too much trouble, but personally, I prefer to keep the signal path -- and loop area -- as short and small as possible. Why invite EMI if you have the option not to.

~Tom
Click to expand...

I agree with the EMI thoughts, however the cathode can have feedback windings on the output Tx so this is a large loop also.

Regards
M. Gregg
 
#8
You still have to think about the ground point for the resistor as below the audio region the bypass will take less current and the resistor more. For the sake of simplicity I would ground the resistor wherever you would have grounded it had it been near the valve. This ensures that the grid circuit and cathode circuit are referenced to the same ground potential.

Personally, I would not bother to move it away. In most circuits the power dissipated is not huge, as it represents wasted power anyway. I guess the exception is DC-coupled SET which have a huge hot cathode resistor, but I would never build one of them anyway!
 
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