Pros and Cons of 3rd Stage in SE (6AV5/2A3 drive requirements) - diyAudio
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Old 4th June 2015, 03:48 PM   #1
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Default Pros and Cons of 3rd Stage in SE (6AV5/2A3 drive requirements)

I have a very large chassis and capable power iron that I intend to use for a 6AV5 SE build. However, when laying everything out there is too much empty space. So I want to add another tube per channel. Gah! Cringe! Yes, this is really about aesthetics, but I'm also hoping to learn something that I can apply to a 300B (or larger) someday. In addition, having another socket on-board would make converting this beast chassis to a PP amp down the road all the easier.

Can you guys point me to some reading or share your thoughts on adding a 3rd stage to SE amps? 6AV5 is pretty similar to 2A3 when triode-strapped, so 2A3 designs or experiences would be pertinent. Yes, I know I could get away with 2-stages.

I've got some low mu, high current octals (6Y6 trioded, 6CK4, 6SN7) that would be likely candidates in the extra stage and several different 9-pin drivers. Common design I've seen is the 6SN7>6SN7>300B, but that would be too much gain here (and wouldn't actually add extra tubes). Any discussion of how that middle stage is chosen/configured with relation to the preceding/following stages would be super valuable though.

Thanks in advance!
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Old 4th June 2015, 04:32 PM   #2
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Look for Aikido line stage. It uses two triodes for the first stage and two triodes for the second stage in follower configuration. Tubecad.com has enough single ended schematic for you to choose.

Or, add a pair of magic eye tubes to fill the space.
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Old 4th June 2015, 04:42 PM   #3
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Funny you should bring this up: I am converting an older amp I built that was three stages RC coupled to two stages direct coupled. The three stage RC coupled amp couldn't compete with ANY of my direct coupled two stage amps, but the chassis and output transformers are excellent. I couldn't just leave them in the basement mulching.

You never mention what the current topology is so that makes it hard to give recommendations. I would suggest that you refine the two stage amp by trying different driver tubes and by all means direct couple the driver to the output tube.

Attached is the three stage amp that I have disassembled.
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Old 4th June 2015, 04:44 PM   #4
disco is offline disco  Netherlands
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in general:

Read any decent book on amp building (ancient or modern) and you will find why it's desirable to have as few stages as needed to obtain your goal. That the needed gain; low distortion and noise; high BW; limited phase shift; low output impedance, or any combination. Losses are the most severe burdens to take into account.
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Old 4th June 2015, 04:45 PM   #5
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Here you go, Aikido SE Power Amp http://www.tubecad.com/2006/03/blog0057.htm
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Old 4th June 2015, 04:53 PM   #6
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Thanks, ballpencil. I did look at the Aikido designs and they're certainly under consideration. It would probably be a 12AT7>12AU7 just because I have plenty of those tubes on hand, though if I find a pair of 6SL7s, I might just try 6SL7>6SN7. I'd rather keep it octals if possible.

Magic eye is a good thought, too. Due to a lack of CT on my power transformer (and the need for higher voltage anyways), I'm going with SS rectifying. Thus no tubes in the PSU. I have one 0A3, but maybe I can scrounge up some more VRs...

Other ideas:
Mu-stage a 9-pin driver (I have several 6CB6s that might work as a load)
9-pin voltage amplifier into a 6Y6 cathode follower
add dummy sockets and stick a couple duds in them, haha
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Old 4th June 2015, 05:03 PM   #7
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Quote:
Originally Posted by Palustris View Post
You never mention what the current topology is so that makes it hard to give recommendations. I would suggest that you refine the two stage amp by trying different driver tubes and by all means direct couple the driver to the output tube.
This one is a scratch build:

Chassis is about 22" x 14"
PwTx is 315V-0 @ .5A, 205V-0 @ 80mA, 40V-0 @ 50mA, 6.3V CT @ 7A (monster)
Choke is 5H @ 300mA, 30ohm DCR (also huge)
OPTs are Hammond 125ESE (will upgrade if design warrants)

I'm building SE because that's what I have OPTs for at the moment (and it's what I'm most comfortable with). Because of the size of the chassis and power irons though, I'm thinking this will end up as a PP amp someday. That's part of the reason I'm looking at the extra sockets (and if they're there, I might as well experiment).

Quote:
Originally Posted by disco;
Read any decent book on amp building (ancient or modern) and you will find why it's desirable to have as few stages as needed to obtain your goal.
I'm definitely a K.I.S.S. believer. But I also hate feeling like I'm not making the most of the space and iron that I have on hand. Conflicting ideals. Certainly an opportunity to learn a bit more about tube design in any case.
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Old 4th June 2015, 05:27 PM   #8
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SiTubes

You can use the extra space for something that looks like a tube but offers some other purpose besides creating heat The SiTDR would be a good choice if you are using a SS rectifier.
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Old 4th June 2015, 05:52 PM   #9
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Quote:
It would probably be a 12AT7>12AU7 just because I have plenty of those tubes on hand, though if I find a pair of 6SL7s, I might just try 6SL7>6SN7. I'd rather keep it octals if possible.
Consider the Loctal 7F7, which is electrically equivalent to the Octal 6SL7. The Loctal tube costs less and, because of construction differences, is less microphonic prone. Sylvania (maker of Loctals, regardless of branding) used the same "cages" in both Octal and Loctal variants. Octal types bring the wires to the base out through a central stem, which are then soldered into the pins. OTOH, Loctal types have solid pins that pass through the glass base and the "cage" wires are connected by welds.
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Old 4th June 2015, 06:17 PM   #10
rongon is offline rongon  United States
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What about using the extra tube for an active load on your voltage amp/driver tube? Perhaps a mu-follower, or SRPP?

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