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previous: Bobby All of the tones from the past that... -- 12/12/2000 2:01 PM View Thread

Re: forget about matched pairs.

12/12/2000 5:11 PM
Shea
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Re: forget about matched pairs.
I wanted to bring up this topic this week too. Many techs advocate taking every possible precaution to ensure that the two sides of the output stage are perfectly matched, like using 1% resistors in the driver, replacing the 82k plate load resistor with a 91k, or taking three matched sets in order to put together a set with even closer matching. It's supposed to just put the icing on the cake tone-wise.  
 
 
 
The extreme-matching camp doesn't talk about the advantage of an unbalanced output stage: 2nd order harmonics. Those are the sweet, musical harmonics that can sustain into beautiful ringing feedback. A perfectly balanced push-pull amp cancels them out, which is why many players prefer single-ended/class A output.  
 
 
 
Here's my experience: Once, for a retubing five or six years ago, I insisted that this one tech get me NOS tubes instead of Groove Tubes. His answer: "Uh, they don't make American tubes any more." My response: "Well, some guys know where to get them. Can you or can't you?" He ended up putting a pair of GE 6L6s in my Bandmaster Reverb. They sounded pretty good.  
 
 
 
This year, I got a silverface Twin Reverb up and running, and started to learn some mods. I blackfaced both amps, which involved changing the Bandmaster from bias balance to bias adjustment. I started to notice that the Bandmaster made a lot more nice harmonics and had a depth to it that the Twin didn't.  
 
 
 
The other day, I put that pair of 6L6s in my home-built Tweed Twin copy. I dialed in those nice harmonics as I biased it. But, as I set one power tube to about 37 ma, I noticed that the plate on the other one was starting to glow. Then I measured that one, and found out that the tubes are SERIOUSLY mismatched - the other one was making 59 milliamps when the first one was at 37. I ruled out any problem with the amp. I tell you, this is one of my favorite-sounding sets of tubes.  
 
 
 
Sometimes mismatched tubes just sound muddy. I found that out when I put two different pairs of tubes in my SF twin. Also, the Bandmaster never had the sharp-edged high end that the SF twin does.  
 
 
 
The technical problem with mismatched power tubes is that they strain the OP transformer. I read a paper online somewhere that suggested matching the DC in the output stage, but putting the AC out of balance, in order to get the 2nd-order harmonics while protecting the op tranny. In other words, use matched output tubes and deliver equal negative voltage to the grids, but put the signal out of balance somewhere. I guess the way to do that would be to replace one of the plate load resistors or cathode resistors in the pi with a resistor and trimmer in series. I really wonder if you could get a radical enough effect that way to make a noticeable difference, considering how badly mismatched my 6L6s had to be to do it.  
 
 
 
I would love to hear from some of our experts here like Randall Aiken, Bruce, and Tim Schwartz on this topic. I was going to put up a post asking for opinions on putting a trimmer in the pi, like I suggested above.  
 
 
 
Shea

 
Replies:
Rick Erickson Most interesting topic. I recently ... -- 12/12/2000 7:22 PM