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Old 9th April 2013, 07:35 PM   #1
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Default Advice and Suggestions on Amp designs

Hi Folks,

This is probably going to be a longs post, I'll put a "too long; didn't read" summary at the end.

I want to build my first tube amp this summer.

My background: I have some experience with DIY audio, and a bit more experience with DIY electronics in general. I'm handy with a soldering iron, I can do surface mount, and enjoy fabricating my own PCBs. I'm well versed in the safety practices required for working at the voltages involved in a tube amp. Physical construction of the amp and chassis are not a concern, complex circuits are fine.

My approach is to pick a design/circuit, and then develop a reasonable understanding of what each portion of the circuit does. I don't actually build until I at least understand why various components are there and what they do. I may not understand why a particular value was chosen, or why particular topologies were chosen over others, but I atleast want to be able to draw a functional diagram accounting for every component. I want to get what I've built, not just assemble it.

This wasn't too hard with chipamps based on my past experience, from there speakers, phonopreamps, and a dac were all pretty easy to get my head around. I'm at the point were I can start to spot weaknesses and alternatives in some of those circuits.

Tubes are a different animal all together. I've reached the point where I feel the most useful thing would be to settle on a design and dissect it to further my understanding of how tube amps work. However, I really don't know enough to pick a good design right now, and I don't want to waste my time with a circuit I'm either not going to build, or has deficiencies that will be a detriment to my understanding of these systems. That's where you all come in

What I like:

As far as sound goes I like a slightly warm but clear amp. I like well defined bass. I like low noise. I dislike overly bright, airy, or harsh top end. I hate muddy base that just thumps. The amp should be clear, but not tiring to listen to.

I listen a variety of music types, not a lot of classical but I do like some instrumental, particular solo cello, I like folk, blues, jazz, classic and modern rock, etc. Sometimes I listen to clear, concise, beautifully recorded jazz, sometimes i listen to low-fi garage rock.

Requirements:

Predominately the amp will be fed from a USB dac, right now the rest of my gear is on the other-side of the continent though eventually I'll also be using a turntable with it. It is likely that it will occasionally be hooked up to an MP3 player when I have friends over.

Initially it will power 8 ohm bookshelf speakers with sensitivities in the low 90db range. Listening will be done at low to mid volume in a 11'x13' home office. Low volume performance is important to me, but I guess that depends more on the speakers. Eventually the amp might find it's way into a small living room, but if that happens it would get new speakers built for it.

Biggest limitation is cost, this is a hobby, and there's always a chance I won't like the way the amp sounds. Plus I can totally see myself dropping a tube or making some other stupid mistake at some point. Luckily I have pretty good stores of caps, resistors, etc, access to samples, left over components and old projects I can scrounge from. Chassis costs, jacks, plugs, etc aren't being factored in right now. I'm located in eastern Canada, so ordering parts from the states is an option, though shipping and duty can easily increase costs 50-100%. I have no local sources for audio components, and very limited local supplies for general electronics components. Parts will be sourced from:
Digikey
Mousesr
PartsConnexion (Canadian)
Tube Depot
Antique Electronics Supply
Etc.

So what i've come up with:

-Sounds good!

-DIY not kit based.

-$250-$300 for tubes, transformers, and shipping for transformers. I'm willing to go to about $350 if its really worth it.


-Ignoring costs of resistors, most caps, jacks, wiring, volume attenuator, voltage regulation, chassis. After scouring stocks, begging and trading with friends, abusing samples programs, salvaging old projects, etc these costs will likely be very low.

-New production tubes, I may use NOS, but new production tubes must be available.

-Solid state rectified, though having read up on rectifier tubes I'm confident I can adapt a design intended for tube rectification to SS rectification

-I'm open to SE or PP, though SE seems to cost more.

-Not Monoblocks.

-Not so big that it can't sit ontop of a dresser, or corner of a desk.

-Relatively low power is fine.

-Stable, reliable, decent tube life.

-Potential for modification, experimentation.

-Performance/quality, and cost are the main priorities, although aesthetics are relevant to some extent, all things being relatively equal cool looking tubes get bonus points.

Most single ended designs I've looked at blow my budget, as does anything using 300b's, KT88's, etc. 45's seem to only be available NOS, so they are out too? Most/all NOS tubes blow my budget anyway.

Designs I've considered:

Poddwatt: Poddwatt: Class-A Stereo Push-Pull EL84 (6BQ5) Vacuum Tube Amplifier (most recent schematics at bottom of page)

Meets all requirements, under budget. Lots of support, helpful DIY friendly designer. Transformers available as package deal from manufacturer.

Tubelab SE

Well regarded, but not sure if I can make fit in budget. However, designer's website is a mess and I'm uncertain whether documentation is complete.

Various K12 designs

Inexpensive, lots of community support. Not sure if its really the best I can do in my price range though.

DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic or the Dynaco 12AX7/EL84 it's based on.

Simple circuit, could adapt for SS rectifier. Specified transformers blow budget apart, would have to sub edcors or similar. Not sure how to evaluate other aspects of circuit and powersupply. +1 for cool looking 6v6's.

Single Ended Tube Amp project

I've seen a number of favourable references to this design, however the unknown transformers give me pause.

Restoring/Moding a vintage amp

Not sure what to look for, not a lot for sale locally, and people seem to want a lot of money for even non-functioning or incomplete ones. Likely many of the ones I've seen were poor quality to begin with so cost and effort savings over build from scratch would be questionable or uncertain.

So that is where I'm at. What are some thoughts on these options, and what are some other options I haven't covered?

TLR
I'm looking for a first tube amp project. I have intermediate skills with DIY electronics, so complex circuits are not a problem, however I'm a beginner with tubes. I have about $300 to spend on tubes, and transformers, independent of most other costs. I've listed some possible designs above, but am not sure how to evaluate them, and am looking for other design suggestions.
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Old 9th April 2013, 08:21 PM   #2
JMFahey is offline JMFahey  Argentina
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Hands down, go for the DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic and use the specified transformers.
Best of the bunch by far.
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Old 9th April 2013, 09:00 PM   #3
rmyauck is offline rmyauck  Canada
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With 90 dB speakers I think you need PP power of at least 17W/CH that you could get from EL84/6BQ5 tubes. Hard to beat the sound of those tubes also! Maybe the tubelab SE design with KT-88's would work also. You could get up to 12-14 W out of it.

He also has a DHT SET amp board which can take 300B tubes , but costs would be out of your price range.

He more recently came out with a simple PP EL84 PCB that can be had with a kit of parts to populate the PCB. All his amps are popular and have been well liked on the forms and well proven over many years so don't worry about building one of his designs1

I would save up more and try to budget a bit more $ as the Output Transformers determine the sonics of a tube amp so it's money well spent for your long term listening pleasure. I would stick to tubes that are in production so that may rule out some designs. The K12's etc. are kit based and use non production tubes also. Plus most replace the Output Trans!

I think the reproduction Dynaco Z-565 Output Transformers are a great start for EL84/6BQ5 PP amps (& others like PP DHT) and Magnequest & Triode Electronics are the originators of these well regarded transformers.

For any PP low to med power amp consider adding Dave Gillespie's fixed bias EFB mod as an upgrade for output tube life, sonics, and a little more power.

If you want SE you would have to start with Edcor GX 15W and if you could budget it the bigger 25W CX may be a good choice. You should be able to build in the $350 range to start.

One vintage EL84 PP integrated that uses the Output Tansformers mentioned is the Dynaco SCA-35 and plenty were produced so you may find one for a good price and recap it. All the boards and mods are available for it. Some convert it to the hard to find ST-35 for sonic reasons as it used the same transformers.

Check out diytube for more info on ST-35's as the have a version on PCB with the driver tube split into 12AU7 & 12AX7 for more tube choices. Triode has a kit with their PCB also.

Tronola site with EFB mod and other info:

Dave's Lab

Anyway for your first diy the the PCB's mentioned above should give you a more sure way of success and they are easy to wire and repair.

There are a number of P2P EL84 amps if you go through diyaudio , but they are more complicated with the SS CCS etc. so may not be for a first timer.

Here's what it could cost you for a really nice PP EL84 which may be a great start for most people.

Tubelab PCP board & parts $100

Z-565 @ $85 ea $170

PS trans $65

JJ El84, NOS 12AT7, $85

Wire, RCA Inputs and Speaker $20

diy case or chassis from scrap $20
wood aluminum etc.
Terminals etc.
New production 5AR4/GZ34

Even buying a used Dynaco would add up to the above if you recaapped and replaced tubes.


Hope that helps!
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Old 9th April 2013, 09:57 PM   #4
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Hmm, I'll have to look into the ST-35 some more, I just took a quick look at it and it looks/sounds very nice. Reproduction Dynaco transformers and JJ tubes fit the top end of my budget. My only complaint is that i would prefer 4 and 8 ohm output windings to 8 and 16. Scratch that, the original 565's were 8 and 16, the reproductions are 4 and 8.

I think I've found my amp!

Also, I'll be etching my own boards, doing a mixture of through hole and turret mount (to facilitate repairs and experiments down the road), maybe even a little surface mount depending on what components I have on hand. So no need to buy PCB's, I've done many diy electronics projects, including a few solid state amps, just nothing tube based before.

As for costs, its really isn't a matter of saving up more money (within reason), but a matter of what I can justify spending. I can spend more, but past a certain point I would rather spend the extra money on albums and scotch to enjoy with my amp. From comparisons I've read between Hammond and Edcor transformers I really hand justify double the price for equivalent Hammonds.
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Old 9th April 2013, 10:23 PM   #5
rmyauck is offline rmyauck  Canada
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The Magnequest Z-565 is the only production one that is wound just like the original it should be noted so it has 8 & 16 ohms. The other copies as you mentioned changed that detail.
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Old 9th April 2013, 10:31 PM   #6
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I won't comment on designs, but can give some suggestions as to parts since I'm in Western Canada and have had to deal with the cross border shipping/duty nazis.
Both Digikey and Mouser ship free if you purchase over $200 worth which would probably blow your budget. However, Digikey charges $8.00 for Fedex shipping on orders below $200, and Mouser charges $20.
If you buy Hammond transformers, chassis, or chokes check out A1 Electronic Parts - Serving the Toronto area for 30 years! in Toronto. They also have heat shrink and other odds and sods.
For NOS tubes, try Pacific T.V. Online Tube Catalog. Or www.thetubestore.com - Your online source for audio tubes.. Both are in Canada.
If you want an alternative to making your own boards, and still want the adventure of DIY, and not a kit, try Classic Valve Design: Audio Design Where It Matters - Sound!. They're located in the lower mainland (Southern BC for those who don't understand Canadian) They offer an assortment of boards to upgrade Dynacos or to make your own, offer manual downloads with schematics so you can see what your'e getting. I've dealt with them and found them always helpful and friendly. Gregg (Geek) used to be on these forums but I don't know if I've seen activity from him lately.
And if you want a cheap alternative for chassis, there is always Fat Daddios Square Cake Pan - 12 12 4: Amazon.ca: Home & Kitchen. Note that this is just one example of the aluminum bake pans Amazon.ca carries.
For resistors, and some caps, try Antique Radio Schematics and Capacitors for Tube Radios, also in Canada. They ship quickly and are easy to deal with.
There is, as you mentioned, Parts ConneXion - The authority on hi-fi DIY parts and components, but I've always found them pricey and too audio tweaky for me. But, to each his own, though I do like, and have purchased, their PRP resistors.
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Old 9th April 2013, 11:14 PM   #7
rmyauck is offline rmyauck  Canada
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Here's another way of constructing which would allow changes.

diytube.com • View topic - I did it!
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Old 9th April 2013, 11:53 PM   #8
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Hi prairieboy, I used to live in Victoria, good to know there are some other Canadian suppliers I hadn't head of. I completely forgot about A1, i used to order from them for a store I once worked in. I didn't realize the tube store was Canadian, that helps. The prices at PcX are quite variable, I've used the PRP resistors in the past as in small quantities they can be cheaper than equivalent general purpose resistors, same goes for Nichicon Capacitors, sometimes they are more and sometimes less than Digikey, etc. That said their customer service could definitely be improved, and some of their capacitors must use unicorn blood as a dielectric based on the price!

I know Digikey's shipping practises quite well, good to know about Mousers rate, I haven't actually ordered from them in the past. I find with shipping from the states try to stick USPS, less or no brokerage and I've rarely been hit with duty, etc. I refuse to use UPS or Fedex from the states.

Perfboard is great for 1 offs, although for this project I want it to look professional when all is said and done. Also when one of my friends inevitable asks "how can i make one of those?" after a project is done giving them a PCB generally prevents the question from becoming "can you make me one of those?".

Looks like the options for 7247 are electro harmonix, JJ, or NOS. Any preferences?
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Old 10th April 2013, 12:28 AM   #9
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I know well the aggravation with shipping out of the US, and generally, like you, prefer USPS. However .... much to my very pleasant surprise, when Digikey and Mouser ship FedEx they cover ALL (seriously!) customs charges. Additionally, since I live outside of a city, on an acreage, and have a PO box, FedEx 'hands of' to Canada Post and I get to pick it up at my local PO with NO additional charges. Compared to the usual gigantic hassles from UPS, and 'normal' FedEx, both Digikey and Mouser have made my life much, much, easier.
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Old 10th April 2013, 01:12 AM   #10
wlowes is offline wlowes  Canada
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Another reference source would be Transcendent Sound. Designer Bruce Rosenblitz designs and sells kits. He also caters to the DIY market by publishing books that contain some level of theory and projects which provide the circuits and a detailed description of operation for his products. The quality meets all your objectives. All but one of his published designs are OTL which eliminates the cost of the transformer. The BOM cost will be outside your budget. If you decide to allocate more like $500-600 you might enjoy his stuff.
Tubes and Circuits by Bruce Rozenblit
Current book has lots of theory and a 15w amp that gets rave reviews and would meet your needs.
Prior book, Audio Reality has a bunch of projects. I built the monos described in his patent and love the result.
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