Bottlehead Forum
General Category => General Discussion => Topic started by: Deke609 on November 01, 2018, 10:27:31 AM
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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
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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.
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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.
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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.
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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" :D
cheers and thanks,
Derek
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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.
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What Peebs says. You gotta walk before you can run.
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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
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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 (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.
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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.
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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
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This is what you want.
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Awesome. Thanks PB.
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@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
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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 (http://www.enjoythemusic.com/diy/1113/vacuum_tubes_part4.htm)
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Thanks PB. That's a good article.
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Sorry - I have another question/problem.
As I "understand" it (I am taking some liberties in using that term), the differential use of the 6J6s results in a complete signal, but with the first half of every waveform being inverted - so you end up with 2 camel humps, instead of a sine wave. So something needs to be done to fix this.
Are the OTs being used "backwards"? I.e., the secondary is used as the primary, and vice versa? And the B+ wire acts as a center tap that inverts 1 half of the AC signal putting the the whole signal back into phase? If so, is that the only purpose of the OTs? Or have I completely missed what's going on?
Many thanks,
Derek
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As I "understand" it (I am taking some liberties in using that term), the differential use of the 6J6s results in a complete signal, but with the first half of every waveform being inverted - so you end up with 2 camel humps, instead of a sine wave. So something needs to be done to fix this.
You get one sine wave on one plate and one sine wave on the other plate, but out of phase. There's nothing to fix.
Are the OTs being used "backwards"? I.e., the secondary is used as the primary, and vice versa?
The nomenclature of "primary" and "secondary" windings gets into a gray area with a part like the 124B.
And the B+ wire acts as a center tap that inverts 1 half of the AC signal putting the the whole signal back into phase?
No phase inversion is done in the output transformer. It is feeding B+ to each plate (DC) and acting as a load for each plate (AC).
If so, is that the only purpose of the OTs? Or have I completely missed what's going on?
The 6J6 has too much gain to be useful as a linestage (38x or 31dB approximately). The combination of the differential circuit and the output transformer dramatically cuts down the gain even more. The output impedance of a 6J6 on its own is also a bit higher than you'd want for a linestage, but the step-down ratio of the output transformer takes care of that for you as well.
Many thanks,
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Thanks PB.
You get one sine wave on one plate and one sine wave on the other plate, but out of phase. There's nothing to fix.
Hmm. I got that completely wrong. So you get the original signal and its inverse superimposed on it? Doesn't that muck with sound?
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If you had the original signal and its inverse superimposed, then you'd have no signal at all.
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Yeah, true! So are you saying that the signal from both cathodes are in phase with one another, but out of phase (inverted) as compared to the original?
Edit: I think I see what you mean about using the differential circuit to keep the gain down. I had been thinking (without actually checking) that the 6J6 was grid-biased with -ve DC voltage, but now see that it is cathode-biased in an intentionally weird way to lower the voltage gain. Whereas a capacitor would normally be added to bypass the cathode resistor to send AC signal on the cathode to ground, in this circuit it is intentionally left out with the result that the cathode acts as a signal source and lowers the output the tube in doing so -- I don't yet understand the "mechanics" of the power lowering effects -- but only that this is something that one usually seeks to prevent by adding a capacitor bypass. This seems a very strange amp. Is there a sonic purpose to its design? Or was it born as a result of someone smart having a two 124bs and two 6J6s kicking around and deciding to see if they could make something work? Either way, it seems pretty unique and clever.
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Yeah, true! So are you saying that the signal from both cathodes are in phase with one another, but out of phase (inverted) as compared to the original?
There's only one cathode in a 6J6.
I had been thinking (without actually checking) that the 6J6 was grid-biased with -ve DC voltage, but now see that it is cathode-biased in an intentionally weird way to lower the voltage gain.
It is biased with negative grid voltage. The grid is at DC ground and the DC current of both plates flows through the bias resistor, so the cathode is at a positive DC voltage with respect to grid.
Whereas a capacitor would normally be added to bypass the cathode resistor to send AC signal on the cathode to ground, in this circuit it is intentionally left out with the result that the cathode acts as a signal source and lowers the output the tube in doing so
This is the case for the common cathode stage, but a little more complicated for the differential circuit. If you bypass the cathode bias resistor in a differential circuit, it's no longer differential (the side with the grounded grid is no longer acting on the signal, but rather just sitting there sipping down power supply current).
-- I don't yet understand the "mechanics" of the power lowering effects -- but only that this is something that one usually seeks to prevent by adding a capacitor bypass. This seems a very strange amp. Is there a sonic purpose to its design? Or was it born as a result of someone smart having a two 124bs and two 6J6s kicking around and deciding to see if they could make something work? Either way, it seems pretty unique and clever.
I don't recommend using "power" in the discussion of preamp design, it can get to be a problem. I'm not so sure how the design originated, but it's a really easy circuit to lash together and listen to (other than the lack of phase indicators on the 124B, but you will clearly hear if the channels aren't in phase and you can flip one pair of output wires from one transformer around).
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Thanks PB - Sorry, my message was sloppily worded - this is all new terminology for me. It's going to take me while to get the lingo down, and eons to become proficient with the math! I'm not going to tackle the math until after I get the amp built - for now, I'll just trust that the values are correct and focus on trying to understand how the different components within blocks of the schematic function together.
Edit: And just to be clear: I think you meant to write that the cathode becomes positive relative to the grid.
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Oops, yeah, fixed that typo.
The problem with talking about power and preamp is that something like the BeePre will push a lot of voltage into loads from 1.2K to 100,000K. This makes nailing down any kind of "power output" pretty difficult.
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Recommended prototyping chassis material?
@PB - the last of the parts for the Nickel Wonder just arrived, and I need to decide on a cheap chassis. I don't want anything expensive or difficult to work with since I want to be able to move things around and add/remove components. I am leaning towards perforated pressboard - the stuff with holes every inch or so that you sometimes see in stores for displaying/hanging wares. I figure I can fold up some copper sheet and use that as chassis ground since it looks like the schematic calls for a ground plate to complete some circuits. Would this potentially work?
If that's a no go, my fall back is to use aluminum baking sheets - but cutting holes in this will be a little more involved. So I'd like to avoid metal if possible.
Your thoughts?
Many thanks,
Derek
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Screw t strips down to a piece of plywood.
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Thanks Doc! I have a bunch of plywood scrap kicking around that would work nicely.
Full metal terminal blocks or the kind with metal tabs and mounts that BH kits use?
Edit: I am assuming that "t strip" stands for terminal strip and not t-profile aluminum used for carpeting, tile, etc.
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Assuming that Doc meant that I should screw terminal strips to the plywood, should I add a copper strip to act as chassis ground, or just solder all ground terminations to a common lug that is then wired to the ground of the IEC inlet?
I ask b/c I don't know whether surface area or mass of the chassis ground is important.
many thanks,
Derek
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Try a cake pan.
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Ha! Thanks PB. That would work nicely for a permanent build, but I think it would make it difficult to experiment with different components and OT positioning relative to PT.
So I like Doc's idea of using plywood to prototype and experiment. What I don't know is whether it's better to add a copper strip as ground plate or simply wire all connections to ground to a common grounded lug.
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Second choice. Star ground it.
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Many thanks Doc.
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I actually built one of these back when. I'll dig it out and post a pic...John
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John - that would be cool. Even cooler would be if you could help me understand the circuit. That way I'd not always be pestering PB - I prefer to spread my pestering around ;D
From the so-called "conventional" view of positive electricity flow, I think I Have a really basic understanding of the circuits. But when I analyze it in terms of negative flow, the circuit seems more complex - but at the same times makes more sense this way.
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Here you go, the case is a Stainless Steel tissue box IKEA use to sell. The power supply is in a separate except for the big oil cap you can see which is the last cap of the supply.
To better understand the circuit, redraw it a little. Remove the transformer and add 2 resistors, one end to each plate, the other to B+. Now you have a conventional differential amp stage, good circuit descriptions can be found in most any tube book...John
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Thanks John. That tissue box makes for a pretty good looking amp! Do you use it?
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@PB - How do I wire the pot? I am using a dual deck A100K that I took out of the S2 when I upgraded to the MourQuiet.
Please see attached diagram with pot (bottom view) and pot wiring schematic. I have labeled the 3 lugs 1, 2, 3, (left to right, bottom view) and the pot connections on the schematic as A, B, C.
I am confused by the schematic b/c I would have thought that the signal input to a pot is wired to the wiper and would be represented by the wire with the arrow pointing to the middle of the resistor. But that doesn't seem to be the case here.
Can you tell me which solder tabs goes with which schematic wire?
Many thanks,
Derek
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If you wire the wiper to the incoming signal, then the pot will present a variable input impedance to your sources and a variable output impedance to the grid of the 6J6.
I would suggest reading up on resistive voltage dividers, as a potentiometer is just a variable voltage divider.
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Thanks PB. I'll do that.
More practical question: Is the center solder lug on the pot connected to the wiper? I can't find a datasheet that specifies which lug connects to what inside the pot.
Many thanks,
Derek
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In order to figure out which terminal is the wiper, set the potentiometer to the center. Now measure the DC resistance between various lugs.
The DC resistance between two of the lugs will be the value of the pot (100K), then you'll see a different lug that measures two lower resistances to that other pair of lugs, and this will be your wiper.
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Thnaks PB. I should have thought of that.
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The "Nickel Wonder" is built and it works!
Thanks PB for suggesting it. I've already learned a lot from the experience ... but about 95% of that was moving things from the "unknown unknowns" column to the "known unknowns" column. The actual differential circuit still confounds me, but that's ok, at least now I know what I find confusing and it gives me something definite to figure out.
In more practical terms, I learned that layout is H-A-R-D. I failed to take into account all the ground wiring that is involved in "star grounding" when planning the layout; and trying to find a good path for the signal wire away from the transformers is all but impossible! But I had a lot of fun. And now I have something to experiment with. The first thing I want to look at is DC rectification and ripple smoothing -- in no small part b/c it's the only important part of the circuit that I sorta understand. The Nickel wonder uses 2 diodes to strip out -ve current, but I've seen that many amps use 4 in what I think is called a "bridge" structure. I might play around with that and additional/different ways of smoothing the DC. This will give me a reason to learn to use my o-scope. It should be pretty simple to run a sine wave tone through the preamp and actually see the rectification and smoothing -- which will be cool.
Listening wise: I was expecting the Nickel Wonder to sound pretty poor, but it is actually quite listenable when put in front of the S2. I particularly like what it does to treble -- it seems fuller-bodied and livelier -- but also a lot less detailed. The two observations -- fuller/livelier and less-detailed -- may be related. I've previously noticed that upgrades/mods that give greater detail often make the sound seem smaller and thinner. The mids of the Nickel Wonder are fine. But the lower bass is absent. I have high hopes for the BeePre in front of the S2.
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A bridge rectifier will change the voltage output of the power supply. Once you blow up an electrolytic capacitor and have to live with that stench, you'll tend to tread fairly carefully on stuff like that in the future!
One of the features of the differential circuit is that it's less picky about power supply noise.
This is a powerful tool that you can use to simulate power supplies:
http://www.duncanamps.com/psud2/ (http://www.duncanamps.com/psud2/)
One hint that is helpful is that when you use a full wave rectifier in PSUD, the transformer voltage is half the total winding. In the instance of the Nickel Wonder, you would use a full wave solid state rectifier and the transformer voltage would be 125V.
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Once you blow up an electrolytic capacitor and have to live with that stench, you'll tend to tread fairly carefully on stuff like that in the future!
Very good to know! I will start treading carefully now.
And many thanks for the link to PSUD2 and tips.
cheers,
Derek
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Pick up an old Radio Amateur's Handbook for an approachable discussion of power supply design.
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Thanks Doc. Amazon sells a new reprint, so I should have a copy of it it a day or so; and it's available electronically for free. The miraculous convenience of modern tech!
I too remember the days of libraries: go the library, search the catalogue index, copy stack number, go to stack location .. only to find that the book you want has been borrowed by someone else. So you speak to a librarian who tells you that it isn't due back for another 2 weeks. So you put your name on a waiting list and a librarian calls you on your home telephone when it is returned. All of which seemed reasonably convenient at the time! Hah!
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It should be pretty simple to run a sine wave tone through the preamp and actually see the rectification and smoothing -- which will be cool.
I just realized that what I wrote above about sending a sine waveform through the amp makes no sense. Signal input doesn't get rectified and no signal passes through the PT (other than 60 HZ AC waveforms). I just need to put scope lead A on the pre-diode output and lead b at various points post-diodes.