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What is the difference betweeen a matched quad and two matched pairs of tubes

Metatron_X

Member
Messages
280
Is there any difference in performance between a matched quad and two matched pairs of tubes in an amplifier with only one bias adjustment?

I recently asked an online tube dealer for a tube recommendation for my amp and he suggested a particular tube that he sells. When I go to his site I see that he only sells matched pairs of that particular tube. I understand that I will have to find a comfortable balance between the pairs (One pair running hot + one pair running at 70%) or (One pair running cold + one pair at 70%), etc

I asked for a pair of tubes that would give me more gravel and grit in the low mids, smooth mid-mids, grainy upper mids and a dimensional, warm and fuzzy high end that isn't shrill. My dislike for the current matched quad of EL34's in my amp is that they sound overly balanced, "transparent" and sterile in an already, very, dry and controlled amp. I'm guessing that having a slight mismatch to the pairs can give me a little bit of grit, gravel, chaos and character but are there any other differences in performance - like less sustain, etc
 

DrainBamage

Member
Messages
2,401
I just recently learned from Carol Ann amps that a mismatch will have more noise however the harmonic content of a mismatch should be more pleasing to the ear.
 

Metatron_X

Member
Messages
280
I just recently learned from Carol Ann amps that a mismatch will have more noise however the harmonic content of a mismatch should be more pleasing to the ear.
I can see how the mismatch might do that as each pair will produce a different dynamic.
 

Carol-AnnAmps

Member
Messages
4,853
I can see how the mismatch might do that as each pair will produce a different dynamic.
Actually its because the closer the tubes are matched the more even order harmonics generated by the power tubes are cancelled. A slight mismatch gives the amp a little more width. Even order harmonics generated by previous stages are unaffected by power tube match. Its a fine balance , too much mismatch and the amp loses focus , large amounts and you will have mains hum start to come through. At what point depends on the amount of regulation the power supply has.
 

drbob1

Gold Supporting Member
Messages
28,303
It's also an issue of how the rest of the power amp is balanced. If you have an off the shelf transformer from the lowest cost provider, and unmatched caps and resistors, the output stage can be pretty unbalanced to start with (or if your output stage uses a cathodyne phase inverter like the Princeton Reverb or the vintage Oranges). Putting balanced power tubes in there isn't going to do much except make biasing easier, although if you tuned by hand with an oscilloscope you could pick and chose power tubes to optimize 2nd order harmonics. All of that is irrelevant to the OP...

With 4 power tubes, they're set up in pairs. If you balance the pairs, then you don't really need the two pairs to be identical to each other-it won't affect harmonics or hum. It might make biasing a bit more fraught since, as you say, one set might bias up at 70%, while the others a bit cold at 40%.
 




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