Can a blown screen grid resistor cause this?

Outlaws

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Could a blown screen resistor in a 4 output tube A/B amp cause one set of power tubes to draw more current than the other pairs tube that is still working?

Working on a Krank Rev 1. The board is gonna require all 4 tube sockets to be desoldered to lift up if I am to properly R&R this resistor. I really don’t want to be replacing 1 known bad part and reassembling unless there is a chance this really is all that is bad.

Everything else seems to be checking out from what I can tell.
 

Jon Snell

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If a screen grid or G2 supply resistor goes open circuit, you will have no current flow through the valve that has no G2 supply.
(For 6L6/EL34/KT66 /6V6etc G2 is pin 4 and G1 pin 5. For EL84, G2 is pin 9 and G1 pin 2.)
Have you compared the voltages on Pin 4 of the output valves? They should all have approximately the same reading.
So in a nut shell, no G2 voltage means no current and the other pair may increase in current flow but only slightly, not significantly.
Check the G1 voltages, pin 5. There may be your issue.
 

Outlaws

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Are you sure you have the required expertise to work safely on that amp?
If a screen grid or G2 supply resistor goes open circuit, you will have no current flow through the valve that has no G2 supply.
(For 6L6/EL34/KT66 /6V6etc G2 is pin 4 and G1 pin 5. For EL84, G2 is pin 9 and G1 pin 2.)
Have you compared the voltages on Pin 4 of the output valves? They should all have approximately the same reading.
So in a nut shell, no G2 voltage means no current and the other pair may increase in current flow but only slightly, not significantly.
Check the G1 voltages, pin 5. There may be your issue.
5881
Without power tubes, every pin on all four tube sockets see the same potential except pin 4 on the blown resistor one, which is zero. All four 750ohm resistors that feed pin 4’s see the same potential on the other side of the leg.
 

Jon Snell

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5881
Without power tubes, every pin on all four tube sockets see the same potential except pin 4 on the blown resistor one, which is zero. All four 750ohm resistors that feed pin 4’s see the same potential on the other side of the leg.
In that case replace the resistor and fit a new balanced quad of output valves. (5881 is similar to 6L6GC).
It may be tempting to fit the new resistor underneath the board but it is better to fit the new resistor on the correct side.
I would replace all four and use 1k 2W MO type. Metal Oxide (or Metal Film) are physically smaller for the same power handling, fire proof and are less likely to damage the board if one blows again.
 

Ken Underwood

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Are you sure you have the required expertise to work safely on that amp?
I doubt it very much, so many on here take on repairs that they have no clue on what to do and expect others that do know to tell them what needs to be done. Once they have done that then they get tempted to do other repairs until something goes very wrong
Leave to those who know
 

ELS

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yes, but usually not by a lot, an open screen resistor will stop a tube from conducting which can increase the plate voltage on the other tubes because theres less load, and subsequently increase current passed trough them.

btw this is probably obvious but you dont want to run an amp with a tube on one side not working, that huge mismatch will saturate the output transformer and burn it up, as well as most likely sound like crap.
 

Outlaws

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In that case replace the resistor and fit a new balanced quad of output valves. (5881 is similar to 6L6GC).
It may be tempting to fit the new resistor underneath the board but it is better to fit the new resistor on the correct side.
I would replace all four and use 1k 2W MO type. Metal Oxide (or Metal Film) are physically smaller for the same power handling, fire proof and are less likely to damage the board if one blows again.
Thank you.
 

Outlaws

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Update: The screen resistor didn’t fix the imbalance, but the old tubes going back in did. Also, upon closer inspection they used top mounted pads for those screen resistors so no pulling of the board is needed in these.
 


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