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Old 10th November 2010, 04:46 PM   #121
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Originally Posted by desperateaudio View Post
how did you deal with your 6AS7 matching since so many of those tubes just do not have matched triodes in the same envelope?
True! Is the 6C33 any better?

I bought 100 tubes But I've had some amps with unmatched tubes, and honestly it's hard to tell.

But some mismatch is ok. When current is drawn, the weaker sections catch up to the stronger. This is where the relatively large cathode resistors come in, tho it happens even without them.

I've found that sections 1,2,3 in one bottle is easier to match with same section in another. So I use sections 1,2,3 in the upper triode, and sections 4,5,6 in the lower.
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Old 10th November 2010, 05:25 PM   #122
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Hi SemperFi, so for the untrained ear (mine), how would you compare the Mellow with the other OTLs you've built?
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Old 10th November 2010, 05:37 PM   #123
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I've found that sections 1,2,3 in one bottle is easier to match with same section in another. So I use sections 1,2,3 in the upper triode, and sections 4,5,6 in the lower.
SemperFi Thx again.

Are u saying that you just try to match the top sections with each other and the say for the bottom sections on a totem otl? Not sure I am gettin g you here?

Also I have tought about just paralleling the 2 sections of a 6as7/6080 and using that as 1 section. Thoughts?

As always your insight is greatly appreciated
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Old 10th November 2010, 07:24 PM   #124
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Semper,

You mentioned the EF86 as a reason not to build the amp. In my limited experience, the EF86 is a very, very nice sounding pentode. I wondered if it may be misapplied here, since it only runs at 12AX7 type currents, and for a driver, especially a 6C33C driver where grid current is to be expected, I just don't see the available current to keep the output grid supplied. I was thinking of something many times more robust, along the line of Tube Lab George's PowerDrive circuit....

Your amp design is very interesting. i must go back and read up on current mirrors again. I have to admit, although I use lots of sand in my tube equipment, bipolar junctions make me twitch.

Stuart
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Old 10th November 2010, 08:16 PM   #125
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U're right, EF86 didnt deserve getting on that list. I didnt mean to slander the Mellow OTL, its a cool design and I probably wouldnt hear any difference. But it gives me no reason to depart from the design Ive been using and have experience with.
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Old 10th November 2010, 08:33 PM   #126
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.... I didnt mean to slander the Mellow OTL, its a cool design and I probably wouldnt hear any difference. But it gives me no reason to depart from the design Ive been using and have experience with.......
For me, I have zero personal experience in building OTL amps, but they are definitely on my bucket list. I just want to do it right and am examining all options I can find. I hope this amp sounds great, be nice if someone had one in the area.

I wonder if Mr. Mellow reads this board? Really could use his input in clarifying his choices.

Stuart
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Old 10th November 2010, 08:39 PM   #127
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Originally Posted by desperateaudio View Post
SemperFi Thx again.

Are u saying that you just try to match the top sections with each other and the say for the bottom sections on a totem otl? Not sure I am gettin g you here?

Also I have tought about just paralleling the 2 sections of a 6as7/6080 and using that as 1 section. Thoughts?

As always your insight is greatly appreciated
Well, I cannot remember which, but in those tubes it seems one section always measures a little stronger than the other. And its the same in each bottle. I am likely to recall wrongly now, but I think section 123 almost always has a little more Ic than section 456. So I feel sections 123 in one tube should be paralleled with same section in another tube. Not sure it matters, but thats how I do it. It can seem like this gives grossly different bias for the upper and lower, but no since they are in series. The NFB provides reasonable offset.
Using all sections in one bottle (paralleling 123 with 456) may be better for avering out the mismatches. I dunno...
Dynamically I am sure it does not matter. DC match does not guaranty AC match. And like I said, I dont think matching is critical since it appears in average they even out. Not counting extreme mismatch that is.
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Old 10th November 2010, 08:46 PM   #128
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Hi SemperFi, so for the untrained ear (mine), how would you compare the Mellow with the other OTLs you've built?
Sorry man. I havent tried the Mellow OTL.
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Old 10th November 2010, 08:49 PM   #129
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Originally Posted by SemperFi View Post
U're right, EF86 didnt deserve getting on that list. I didnt mean to slander the Mellow OTL, its a cool design and I probably wouldnt hear any difference. But it gives me no reason to depart from the design Ive been using and have experience with.
No problem SemperFi, I was just wanting to get your opinion of this amp. I have 6C33C already on the shelve and always wanted an OTL. I also have a chassis that will work nicely. In your opinion, if I am tri amping, where would this amp shine....tweet, mid, bass?

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Originally Posted by sepolansky View Post
For me, I have zero personal experience in building OTL amps, but they are definitely on my bucket list. I just want to do it right and am examining all options I can find. I hope this amp sounds great, be nice if someone had one in the area.

I wonder if Mr. Mellow reads this board? Really could use his input in clarifying his choices.

Stuart
I've never need Mr. Mellow on any board....and I've looked.
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Old 10th November 2010, 08:50 PM   #130
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Sorry man. I havent tried the Mellow OTL.
OK, jumped the gun in my last post. As soon as I get these other amps done, I guess I will find out.
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