S.E.X. or Crack2A - which is better for learning and experimenting?

[Deke609]

Although the S.E.X. is the "experimenter's" kit, I wonder whether the C2A might be more fun to experiment with. 

The biggest question I have is whether the C2A could be modded (perhaps heavily) to eventually use OTs (i.e., after all the experimentation with OTL is done).  It would be really interesting to learn what's involved in going from OTL to OT.

All thoughts/advice welcome.

Thanks,

Derek

[Paul Birkeland]

I think it depends a lot on what you mean by mods.  If you want to put in fancy capacitors, a nice attenuator, and roll tubes, then the C2A is a great choice with lots of space for all those parts.  If you're wanting an amp with output transformers to drive lower impedance or power hungry headphones, then the SEX is the better choice.  Adding output transformers to the C2A is possible, but for the expense and effort, the SEX amp is going to be a better choice.

There is plenty of experimenting that can be done with the SEX amp, especially since the other compactron tubes floating around out there that will plug into the amp aren't really in demand at all.

[Deke609]

Thanks PB. I'm not interested in upgrading components, but in learning something about actual amp "designs" (e.g., OTL versus OT) and their workings.  I figure, perhaps wrongly, that an OTL design is simpler than a OT design - leaving aside shunt regulation and all the other things that might be done with an OTL to improve upon its fundamental design. 

And I'm not really looking for an amp to listen to, per se - my S2 has me covered in that department, which is soon to be joined by a BeePre, and next year I have my eye on  Kaiju.  I'm literally looking for something to learn with - and learning the basics about what's involved in going from OTL to OT is high on my list of things I'd like to do. 

Should I be looking elsewhere for this?  Like DIY Audio - or maybe some early Valve schematics for simple amps? 

Many thanks,

Derek

I'll add: If anyone can point me to a simple tube amp design that they think would make a good starting point for learning, particularly one that they have built and learned from, that would be great.

[Paul Joppa]

OK then, that clarifies your objective.

So here's lesson number one: OTL tube amps are usually cathode followers for headphones (like Crack), or push-pull with tons of feedback for speakers (pretty complicated for a starting point). Amps with output transformers are usually common-cathode amplifiers (SEX, Mainline, and our various speaker amps). It's not just OT vs. OTL, in other words.

The starting point would probably be the Quickie, which is a common-cathode preamp. It's cheap enough to experiment on without worry, you can't shock yourself because the voltage is pretty low, and it's simple enough to make the changes clear and understandable. You can probably drive 600-ohm phones with it, though it won't be optimal sound. Several people have added an output transformer, enabling it to drive sensitive low-impedance phones. At least one or two have converted it to a cathode follower. And the follower version can also be connected to an output transformer. And you can experiment with series feed vs. parallel feed, and try resistor plate loads or choke loads or current-source loads.

[Deke609]

That's fantastic.Thanks PJ. Quickie it is. 

I have a number of texts -- Morgan Jones' "Valve Amplifiers" and "Building Valve Amplifiers", and Merlin Blencowe's "Designing High Fidelity Tube Amps", and others - each of which is great but very challenging to work through - but they're doable. My problem is that I can no longer retain things that I've only read.  My job requires me to quickly process a lot of  information and concepts (all non-mathematical/non-scientific), and I've unintentionally trained my brain to absorb, analyze, solve, purge from memory, and move on to the next problem. Tabula rasa, over and over. Things only stick in my head if I do something I enjoy with them - ideally by solving a problem that I encounter in trying to do a personal project.  My hope is that working with a physical apparatus (amp) that I am trying to make do X, Y, or Z will make stick at least some portion of all the stuff I've been trying to jam into my head about tube amplifier circuits. Tiny example of success: implementing your schematic for an impedance switch for the S2. Before you shared your schematic, I had spent two very late nights trying to solve the problem myself - and could only do so with 4 switches, in contrast to your 2-switch solution.  But b/c I had worked the problem on my own, when you shared your schematic I was able to appreciate just how ingenious it is.  And my understanding of the problem made it easier to turn your schematic into a pair of physical, working switch boards that are now happily installed in my S2.  Whereas I can't remember the work problems I dealt with last week, my sketches of the OT-2 wiring combinations for different output impedances are burned into my brain. I could probably build those switches again from memory.  And, I guess more importantly, as much as I enjoyed assembling my Crack and S2, making those simple switch boards and knowing why they would work, was the most fun I've had with a soldering iron to date. 

Tonight's reading: "common cathodes"   

cheers and thanks,

Derek

[Paul Birkeland]

My recommended next step based on what you posted would be to scratch build something super basic that doesn't come with an assembly manual.  The mistakes you make and problems you run into along the way will be where most of the learning happens.

[Doc B.]

What Peebs says. You gotta walk before you can run.

[Deke609]

Thanks PB and Doc.  I've seen a lot of references in this forum to the original "spud" amp - would something like that qualify?  Or do you mean, literally start from scratch by working up my own schematic? THat's where I want to get to -- thinking up simple designs, building and troubleshooting them -- but I'd like to get one or two simple builds under my belt before designing something from scratch. With the Quickie and possibly spud amp, my plan would be to take parts of the schematic, study up about what they do and how the do it, before actually building them. Then I could test the various "parts" to demonstrate to myself how the math either lines up with reality b/c I've built it properly, or doesn't, in which case I'll try to use the learning I've done to troubleshoot.

cheers and thanks,

Derek

[Paul Birkeland]

Yeah, I wouldn't jump into design just yet. 

This is a very simple 1 tube per channel preamp that's not too hard to build:
http://wardsweb.org/LSB/11_06_04/6j6.jpg
The Hammond 124B transformers aren't all that cheap, but you should be able to build this with basic parts for under $250.

[Deke609]

Thanks PB. That's very cool. I really appreciate it.  I quickly reviewed the schematic and the parts/groups are few, so I'm in. And it's a "push-pull"!  Neat.

[Deke609]

PB - I can't seem to find the PT listed in the schematic as Allied part # 227-0001.  Doesn't appear to correspond to any part # on the Allied website. Any ideas? Or this something I should be able to figure out from the schematic?

Thanks,

Derek

[Paul Birkeland]

This is what you want.

[Deke609]

Awesome. Thanks PB.

[Deke609]

@PB: I need a little help understanding part of the circuit. I think I've figured out the basics of the DC/B+ part of the power supply and the AC heater supply, as well as how the input signal controls the grid of 1/2 of each 6J6, and then gets amplified by the OT (spent many hours on the latter point: I had no idea that AC and DC can co-exist without interfering with one another in the same wire, and even move in opposite directions - never would have guessed that).  But what I can't figure out is what is happening with the other side of each 6J6. I've circled what's confusing me in the attached pic of the schematic.

I just need a hint about what's involved here so I can go learn about it - Is there a term of art I can read up on to figure this out?

Many thanks,

Derek

[Paul Birkeland]

It's a differential amplifier.  There's some reading available here, with parts 1-3 available beforehand. 

http://www.enjoythemusic.com/diy/1113/vacuum_tubes_part4.htm