100 watt head, one pair output tubes pulling way more current?

Outlaws

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TLDR question: 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?

The board is gonna require all 4 tube sockets to be desoldered to lift up. 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.




I am looking at a Krank Rev 1. It has 4 power tubes, all 5881WXT. It appears from the board the tubes are paired left two and right two. No matter what all are way too high of an idle current draw. The lower of the two pairs current draw has one tube doing nothing. Upon opening, that screen grid resistor is toast. Both tubes on that pair show 522 VDC with tubes. The one tube working on that shows 39ma. However the other pair shows like 505 VDC with a 58ma and then I killed it.

Pulled all the power tubes and they all are getting the exact same VDC to plate. Measured all the 750ohm screen grid resistors (one OFL) and they are right at 750. The 4x 1.5k grid resistors for pins 5 are all measuring the same. The 150k pair of grid leak resistors also meter near perfect.

All four sockets at pin 5 show -56.5vdc.

Would that one blown screen resistor cause the other pair to get a much higher current draw? Swapping tubes doesn't cause the current draw to follow, and they are supposedly a matched quad.

Supposedly this amp was working for years with no issue until new tubes were put in, nothing biased. Same make and model of tube as before.

The layout best I can tell is very similar to this
Supposedly these are heavily based on Sovtek Mig 100s.
 
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2L man

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install new screen resistor before you test amp more! When you get 58mA do you measure tube pair current or individual tube current?
 

Outlaws

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install new screen resistor before you test amp more! When you get 58mA do you measure tube pair current or individual tube current?
I have two bias probes from Euro so I can see two at once. I’ll get two more one day.

That resistor is gonna need the board pulled which is a real chore since the tube sockets are top mount on the chassis, but then lugs bent over and soldered direct to board. Like an old Mesa PCB.
 

Wally

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What do you see if you pull one of those two tubes in the side that has both tubes working?
 
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Wally

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Measured all the 750ohm screen grid resistors (one OFL) and they are right at 750.

Does this mean that you have three screen grid resistors that are good and one that is not? I do not know what ‘OFL’ means?
 

2L man

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I have two bias probes from Euro so I can see two at once. I’ll get two more one day.

That resistor is gonna need the board pulled which is a real chore since the tube sockets are top mount on the chassis, but then lugs bent over and soldered direct to board. Like an old Mesa PCB.
If it require lots of work look if you can solder new resistor to the original resistor legs above the board so you dont have to pull the board? Then crush burned resistor away with pliers.

Screen resistor burn concerns me that the tube might be bad?
 

Wally

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Okay…so an open…infinite resistance. Bad resistor….IF one has the meter set to the correct range..as you obviously have because you have three resistors that read correctly.
Thanks…I learned something today…OFL…overflow With regard to an ohmmeter reading.
 

dan40

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The socket with the bad screen resistor will likely contain a bad tube also. Something caused that resistor to burn up.
 

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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