• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Advice and Suggestions on Amp designs

cjmurphy87

Member
2011-06-10 3:25 am
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?

TL:DR
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.
 
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!
 

cjmurphy87

Member
2011-06-10 3:25 am
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.
 

prairieboy

Member
Paid Member
2010-11-22 2:31 am
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.
 
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?
 

prairieboy

Member
Paid Member
2010-11-22 2:31 am
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.
 

wlowes

Member
2007-03-18 1:44 pm
Toronto
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.
 
The DynaClone Z565 O/P transformers are price/performance champions that work with 4 and 8 Ω speakers. Given your budget and sonic goals, they are what you should use.

An "El Cheapo" configured with ultralinear mode "finals" will yield approx. 12 WPC. The O/P tube operating point can be tweaked to squeeze more power out, but sound quality will suffer. Name your poison. Any 6V6 family tube works with the parts values shown. Employing the 6BQ5/EL84 is easy enough, but the 6BQ5 does not make as nice a triode as the 6V6 and having UL/triode mode switches in the build allows for hearing "intimate" performances with peak pleasure.

A full set of power "iron" can be ordered from Allied Electronics and I wouldn't be surprised if they take care of Canadian customers, just like Mouser and DigiKey do. In any event, everything but the B- trafo can be ordered from Mouser".

Jim McShane offers kits of parts and tubes for the "El Cheapo" project, at fair prices. Jim is also a font of knowledge you can draw on.

BTW, the uploaded schematic shows triode wired "finals", as the cheapy guitar amp O/P "iron" called for can't take UL power in the deep bass. To date, the Z565 has proved to be the "Gold Standard" for builders of the project.
 

Attachments

  • EC big.gif
    EC big.gif
    38.8 KB · Views: 166
CJ,

The 7247 seems to be preferred in NOS, but a better way is to use the separate 12AU7 & 12AX7 as diytube does with there PCB.

It's exactly the same circuit, because that's what the 7247 is made of, but you can sub the not so well liked 12AU7 part for better sonically to many 12BH7 olr 12BH7A or ECC99 or with rewiring 6FQ7/6CG7. Even the 12AX7 can be subbed for lower gain versions if desired.

Some want a really compact amp and look of the stock ST-35 so stick with the 7247. A 12AT7 might replace it as I read somewhere and that's what the Tubelab EL84/6BQ5 PP amp uses in pretty well the same circuit also.

Here is a thread showing the gain of different common driver preamp tubes.

New tube combos help on VTA70 mod - Page 2
 
The " EL Cheapo" is another to look at and is to many even a better amp than the simpler circuits on the Dynaco's etc. It's called the differential or LTP , but is more complicated with needed the CCS with it's SS parts to work great.

There is also the popular Music Machine 6V6 amp circuit with the same style circuit with a non production driver tube.

Baby Huey is another popular LTP one with production driver tubes and SS CCS.

All the above are P2P with some mini circuit boards for the CCS
 
The " EL Cheapo" is another to look at and is to many even a better amp than the simpler circuits on the Dynaco's etc. It's called the differential or LTP , but is more complicated with needed the CCS with it's SS parts to work great.

Somebody with ambition could replace the cascoded FET CCS with a 6AU6. ;) Ground g2 and rectify the B- with 4X Schottky diodes, the filament winding of the B- trafo would energize the 6AU6s. The power for the 12AT7 heaters would come from the same place O/P tube power comes from. This sort of arrangement relies on the warm up of the 6AU6s, instead of an 'AL5, to prevent the 'T7 grids from going positive with respect to the cathodes, at turn on time. Every last V. the B- supply can provide is needed here and the B- PSU filter needs to be reworked. However, that's NBD, as a 100% SS bridge can work into a good sized 1st filter capacitor.
 
In my previous post, I mentioned the 6AU6. That type is needed in both FM tuner IF strips and some audio circuits as a voltage amplifier. I don't want anybody pressuring the available NOS supply for something like a CCS. Fortunately, there is a semi-remote cutoff type, the 6BZ6, that will be fine in the CCS role. As a CCS, its fundamental non-linearity is of no consequence. :) Always look for types that seem to be useless, when (in fact) they are just fine in specific jobs. Money gets saved and the limited supply of highly useful NOS only types is not impacted.
 
Thanks again for all the suggestions, I'm going to go with the St35. The simple nature of the circuit appeals to me, and i've already learned a bit from studying it's circuits. Also the wide variety of modifications out there is a big plus to me as I like doing mods and experiments. The 12au7/12ax7 change is something I'm definately considered. Though lower gain I've read some good things about using a 5751 inplace of the 12ax7.

1 quick question, I'm looking at the original circuit here http://www.tnt-audio.com/ampli/dynaco_st35_manual.pdf. The input path is made up of C1(.1uf) and R1(47k) in series then C2(33pf) followed by R2 (470k) in parrallel to ground. This gives a total input impedance of 517k with C1 and and R1+R2 providing DC coupling, and making a highpass filter at around 3hz, C2 and R1 provide RF attenuation at around 103Khz. Because R2 is parallel with C2 for the low pass filter it basically isn't founded, as for the frequencies we are concerned with they would see it's resistsance as the R where 1/r = 1/ESR of C2 + 1/R2, which would be negligible with a modern film cap. So far so good?

So if I want to add volume control without using a preamp, wouldn't my attenuator mess up the input impedance as a pot is conceptually a series resistor in the input path (R between in and wiper, Rwiper) followed by a second resistor between input and ground (resistance between wiper and ground, Rgnd), where the 2 resistors always add to the same value as you change the attenuator value. R1 and Rwiper are not in series, and R2 and Rgnd are now is parallel- so if I just added a pot the input impedance now become (R1+Rwiper) + R [where 1/R=1/R2+1/Rgnd]. So my input impedence is now substantially lower, and varies a bit with volume with volume. For the purposes of the the low pass filter since as mentioned above the resistors in parallel with it should be negligible to relevant frequencies (the ones we want to attenuate) the relivant resistance would be just R1 + R wiper

Now for my basic questions. Does it matter for these filters the some of the impedance is after C1, in otherwords do I only actually use the resistance after C1 for calculating input impedance for the purpose of the filters and ignore the pot before C1. I don't think this is true, but just wanted to make sure.

So my solution would be to use a 500K attenuator preceded by C1 [actually C1 could go before or after] of .1uf (I've read the theoretically ideal value for the LPF is 2hz, so .15uf actually puts you closer) then eliminate R1 and R2 and change C2 to 3pf (likely 3 parallel 1pf caps) giving us a 106Khz high pass. This gives me a constant input impedance (always Rwiper+Rgnd now) and filter -3db frequencies that are basically the same. If the pot was before C2 the ESR would cancel out Rgnd as they would be in parallel, and as the volume approached max Rwiper would approach 0, and the LPF cuttoff point would approach infinity, meaning it wouldn't be doing anything. With the pot after C2 Rwiper, and Rgnd are always in series to ground, and add.

Thanks, sorry for the basic question! Also sorry if my work is a little messy, I had to change it a couple times as I was typing and realized I'd overlooked something
 
Ah, crap ignore everything above!

I just realized with C2 before the pot, even though Rwiper and Rgrnd are in series with eachother , they are in parallel with C2, so no good.

So What I need to do is put R1 and C2 infront of the pot, (C2 esr cancels out pot at relevant frequencies)and it doesn't matter where c1 is. Basically I just replace R2 with a 470K [500k] pot and everything is good.

I shouldn't do physics problems before having my morning coffee!!!
 
Hi Folks,

TL:DR
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.

Yes, your post was long, I read some of it at the start in detail and skimmed over the rest.

Why not an RH84 amp?

You wanted low cost (people have said that this amp can be made from leftover parts in a parts bin, leftover iron from console stereos, and even re-using old tube sockets again with some people quoting $30 -- I'd find that hard to do, but then again my parts bin is not overflowing), you are powering small bookshelf speakers at 8 ohms and 90 dB sensitivity (the RH84 will do that nicely), parts are easily found and the EL 84 tubes are relatively cheap (if you want, you could reuse those tubes in a Baby Huey or another OddWatt type amp later on down the road), there is a lot of information on diyaudio.com (search for it up there) and on the web (Google is your friend and you can find the BOM from the designer), you wanted a circuit you could understand (RH84 isn't overly complex or complicated).

The designer's website is here... not much to look at but the schematic.

RH 84 - Tube Audio ...... RH DESIGN

You want bass? Upgrade output transformers with some more cash and you can reach down a little more in tone better.

RH84 has been said to be the first tube amp that any diy'er should build.

And for the price it is hard to beat.


And here is a link I just clicked on from earlier today...

http://www.diyaudio.com/forums/tubes-valves/233729-rh84-amp-plate-300v-what-voltage.html
 
Last edited:

Shoog

Member
2002-08-15 10:16 pm
Eire
There are far better designs than the RH84, and driven to the necessary wattage it will distort badly.Here is a cheap and fantastic design for you to consider which will wipe the floor with the RH84;
The Mighty Midget

PP is definately the way to go if you really need that much power;

http://www.pmillett.com/tubebooks/807pp_amp.htm

Quads of Russian 807's can be had for absolute peanuts.

Shoog
 
Last edited:
The might midget looks interesting. Do you have to use a preamp with it? I don't see a preamp tube like all of the other designs I've looked at have.

The 807 looks like a beast! I'll definately do more reading on that, tubes with anode (i think?) caps definately get a +1 for cool factor... but a -1 for the fact my cat likes to occasionally bite onto and try to disconnect plugs like that... I don't think my partner would forgive me if my home made amp made that idiom about cat's and curiosity come true in a spectacular high voltage fashion.

Back to the ST35, if someone wanted to maintain closer cutoff values to the original for the input filter you would obviously drop R1 to 17k, and raise C2 to 90pf if replacing r2 with a 500k attenuator.

On the matter of input impedance, I've noticed most more modern tube designs are based around 100K, while a lot of the older ones are using 500k. Solid state is often as low as 25k or 50k. I've read that higher input impedance makes the amp "easier" to drive for your source, but is there a down side to using the higher impedance?

My phonostage is opamp based, and my dac will use opamps for the I/V stage, atleast at first. How do I determine what is an appropriate input impedance for my system?
 
Last edited: