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Old 31st January 2011, 03:35 AM   #1
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Default HiFi Tube amp project


I have from now until mid-may to design, test, refine and complete a 10-15W hifi stereo tube amp, and was hoping to get help, suggestions, and comments for this project.

Me and a friend built a pair of bass-reflex enclosures with Pioneers B20FU20's in them last semester - I'd be designing that amp for specifically for those. He's building me a guitar in exchange.

Requirements are pretty basic:

-cost kept in the 400$ range
-be able to work with a variety of sources (from record player to ipod) and have multiple ins
-basic tone control, with maybe a bass & treble boost pots, but nothing complicated
-be very sturdy (foolproof). We all make mistakes (forget to plug in the speakers - I'd like to know how to make an amp that can survive anything).

I've been doing research in more "unusual" tube cicuits, which led me to be considering some combination of the following options:

-ultralinear output stage
-parallel triodes for the preamp
-gated beam discriminators ( la Pete Millett's Magicbox, very uncertain - would it actually be useful at all?)
-solid state power supply
*triple triodes for the preamp (6AC10; 6AK10,6MJ8...)
*double pentodes for a self contained push-pull output stage (6BN11, 6BV11)
*Or double triodes + pentode, such as the 6AS11, to contain one whole channel in one or two tubes, although apparently the interactions within the tube make this unrealistic (AX84.com - The Cooperative Tube Guitar Amp Project)
*maybe a 6Z10 if I actually end up wanting a 6BN6

Experimentation will be a big part of this project, but I'm going to be making refining schematics of different versions in order to get parts and start testing around 3 weeks from now.

I'll add many many details as this project kicks in. Comments, suggestions, questions are very appreciated! This project is supposed to be an opportunity to understand how to design a product from start to finish - I'm learning here, all help is appreciated!


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Old 31st January 2011, 04:11 AM   #2
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An "El Cheapo" using UL mode "finals" will yield an honest 12 WPC. You should have little trouble in meeting the $400 budget limit, if you build with Edcor O/P trafos.

Tone controls can easily be "a can of worms". A truly decent tone control setup is fairly complex and implies some expense.

The 'AQ5 shown in the schematic is, for all intents and purposes, a 6V6 in a 7 pin mini package. Any member of the 6V6 "clan", including the 7C5 and 12AB5, can be used.

Read the LENGTHY saga of "El Cheapo" here. It's a lot to wade through, but you'll get some ideas about circuit design, necessary compromises, and parts selection.
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Old 31st January 2011, 04:29 AM   #3
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compactatrons immediately bring you to Pete Millett's Engineers amp...
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 31st January 2011, 05:22 AM   #4
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Originally Posted by Satellite Icarus View Post

I have from now until mid-may to design, test, refine and complete a 10-15W hifi stereo tube amp, and was hoping to get help, suggestions, and comments for this project.
Doesn't the assignment disqualify you from just copying an existing design?

My suggestion would be to 'hit the books' - Morgan Jones would be one place to start - and get working on your design, 'from the ground up'.

Keep us posted with your progress. It should be an interesting project.

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Old 31st January 2011, 03:02 PM   #5
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The assignment is as hard as I make it for myself. Of course, chances are each stage is going to resemble something that's been made before, but I'd like the overall schematic to be mostly my own...

I'll be taking a look at the Jones book very soon. I'm trying to avoid the "cookbook" approach though - my goal is not only to design something functional, but also to understand exactly what every component does and why it's a certain value.

The El Cheapo Power Stage is somewhat in the range of what I'd need, and I was actually looking at using 6AQ5As. Thanks for suggesting that.

I was looking at the Engineer's amp, mostly because it advertised itself as fairly accessible and *built with a big red PCB*. However it's way more powerful than I need the amp to be. I guess I could spend this whole semester designing a preamp for it, but that would make me go over my preferred investments and end up with something probably to beasty.

I'll be uploading schematics for different stages as I work with my instructor on those and figure out how to use eagle (or get annoyed at computers and draw everything by hand then scan) - thanks already for the interest and replies!

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Old 31st January 2011, 03:41 PM   #6
SY is offline SY  United States
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Originally Posted by Satellite Icarus View Post

I'll be taking a look at the Jones book very soon. I'm trying to avoid the "cookbook" approach though - my goal is not only to design something functional, but also to understand exactly what every component does and why it's a certain value.
That's exactly where Morgan's book excels. The Bevois Valley is a nice design exercise and could be a model for your eventual design path.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 31st January 2011, 03:52 PM   #7
DF96 is offline DF96  England
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Four months is not a long time to learn about valve amps, even if you are already an expert on sand circuits. I assume you need time to eat, sleep, watch TV etc. too? I think you need to decide now whether you would prefer to complete a glorious failure with lots of badly working bells and whistles, or achieve a more modest success and then try something more challenging next time. I realise this will make me sound like a boring old f**t!
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Old 31st January 2011, 04:42 PM   #8
taj is offline taj  Canada
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Sounds like a fun project.

I completely agree with DF96. Your time is short! Make it count.

My suggestion would be to take a basic, yet known good, amplifier circuit (such as a Mullard 5-20 as one example), and use different tubes. This forces you through the design/calculation process and should get you a good working "product" in the end, without all the time spent deciding between all available topology options at each turn.

Solid state power rectification is a good detour. Take that fork in the road.

There are lots of options you can add later if time permits, to tweak things. (CCS, tone controls, etc.)

Start at the end; pick the output tubes, and work backwards to determine what everything else needs to be. If you're going push-pull/Ultralinear it will be really easy to exceed the design goal of 10-15 watts, so think small. Using push-pull Triode mode will keep that goal within easy range.

Ask for direction, not answers, or you won't learn much.

Good luck,


Last edited by taj; 31st January 2011 at 05:05 PM.
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Old 1st February 2011, 12:36 AM   #9
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If you want to keep yourself honest use output tubes that are not commonly used so that there are few if any designs to copy. If you really want to test your self chose one for which no plate curves exist so you have to trace them yourself or experiment (requires more test equipment such as distortion analyzer etc).
Generic Signature
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Old 2nd February 2011, 04:51 AM   #10
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Thanks already for all the advice, everyone!

here's what I'm thinking so far: SE output section, with an EL34 (or any other similar power tube), into an UL transformer such as:

EDCOR Electronics Corporation. XSE25-4-7.6K

And a simple preamp stage with a pair or trio of parallel triodes. Why parallel triodes? I'm not sure. I think the idea is pretty interesting, it seems to have convenient advantages for hifi, and I really want to use triple triode compactrons, all paralleled, as a line stage, out of curiosity. comments concerning my lack of logic concerning this are more then welcome.

More than the UL operation, I'd really like to make this design my own by cleverly using compactrons. At this point, seems like I'll be using triple triodes with an EL34 power stage, unless I finally find a double triode+pentode that could work in UL and preferably provide enough power without P-P. Horizontal Sweep tubes were recommended but I'm incapable of finding one appropriate for UL operation because of their g2 max voltage ratings.

opinions? chances are, I'm going to give a shot at a few of those ideas and realize that there's no way I'll have time to make that work and resort to more classic options. I'll give it my best try though.

I've been using this website Compactron Tubes: A Junkbox Guide to browse compactrons. Anything better I should know about?

If a distortion analyzer is basically an FFT tool, then I can have access to one, it might simply not be sturdy enough (not sure it can take the voltage swing, although to be honest I might be worrying for nothing). I'm not sure I'll have the time to trace plate curves for this project, even if it's something I'd like to learn how to do soon.

Definitely planning on solid state rectification. Will have to get to that soon.

I'm going to have to make some quick decisions, as the deadline for funding requests is in 2 weeks and requires a fairly detailed list of parts.

just remembered I also liked beam tetrodes, but I'll keep decision-making for the snowy tomorrow.

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