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Power tube questions

pennylink

Member
Messages
1,865
  1. Could someone please explain the advantage of having matching power tubes in your amp?
  2. Ideally how close should they match?
  3. When one power tube goes bad, should you put in a whole new set and, if so, why?
 

VaughnC

Silver Supporting Member
Messages
17,752
With the inconsistency of modern tubes, it is possible for one tube to be conducting so hard it actually turns red (red plating) with the other(s) barely conducting. Back in the day, tubes were pretty much plug & play ;). So, having matched output tubes is just so they're close enough in plate current so one doesn't go into meltdown.

10ma difference would be about the most I'd be comfortable with IF the single pot bias was set properly to kinda split the difference....unless the amp had a separate adjustment for each tube.

I see no reason to replace all power tubes if one goes bad...as long as they are within about 5-10ma of plate current of each other and biased so the hotter one's aren't too close to their safe maximum plate current.
 

pennylink

Member
Messages
1,865
With the inconsistency of modern tubes, it is possible for one tube to be conducting so hard it actually turns red (red plating) with the other(s) barely conducting. Back in the day, tubes were pretty much plug & play ;). So, having matched output tubes is just so they're close enough in plate current so one doesn't go into meltdown.

10ma difference would be about the most I'd be comfortable with IF the single pot bias was set properly to kinda split the difference....unless the amp had a separate adjustment for each tube.

I see no reason to replace all power tubes if one goes bad...as long as they are within about 5-10ma of plate current of each other and biased so the hotter one's aren't too close to their safe maximum plate current.
Thanks for the explanation. I have a NOS Brimar 6V6GTY with test result marked 34/3.2 and another marked 30/3. Is that within the range you mention or are they too far apart to use as a pair?
 

Blue Strat

Member
Messages
30,075
It's impossible to know for sure without knowing what tester and parameters those numbers represent. They COULD be Maxi Matcher readings (current/transconductance) in which case they would be about 4mA apart which is fine. Only the person who took the readings knows the details.

If used in a fixed bias amp (the type you must adjust) a bias reading will reveal whether they're close enough or not.
 

pennylink

Member
Messages
1,865
It's impossible to know for sure without knowing what tester and parameters those numbers represent. They COULD be Maxi Matcher readings (current/transconductance) in which case they would be about 4mA apart which is fine. Only the person who took the readings knows the details.

If used in a fixed bias amp (the type you must adjust) a bias reading will reveal whether they're close enough or not.
Unfortunately I have no idea what tester was used.

How can I do a bias reading for each tube in a fixed bias amp? I have a basic digital multimeter... can I use that?
 




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