The 6V6 Lacewood Amp

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A Design For All Seasons

This amp was designed and built with several different goals in mind. The first was that I needed a small amp for my work/listening area. This is a small space, about 8’ x 10’, so I didn’t really need lots of power. I thought that a nice single ended UL 6V6 design should be just about right.

The second goal was to test a hypothesis. I had been seeing lots of talk on the tube forums about “heater hum” and how DC heaters supplies were required and that heaters should always be “lifted” above the potential of the cathode. Now there is much truth to this when using tubes with directly heated cathodes, but with unipotential cathode tubes (i.e. tubes with a cathode electrode separate from the heater) the isolation between heater and cathode should be sufficient to prevent audible hum. My hypothesis was that the hum problems people were discussing arose mostly from B+ ripple and poor lead dressing inside the chassis. So I would design a power supply with B+ ripple below the limit of my test equipment, apply proper build techniques and see how I did.

Lastly, I wanted this amp to have just a little bit different visual aesthetic. So often all the attention goes into the electronics and little into the look of the amp. I wanted to avoid that trap. So I decided to use a nice piece of Australian lace wood for the base and I chose a black and brushed silver accent theme.

Overall I think the design worked out quite nicely.

The Electrical Design

As I said in the introduction, I decided that I wanted to go with a single ended 6V6 design run in ultra linear mode. To do this I simply took the design point for a single ended class A stage directly off the 6V6 data sheet. This called for a -12.5v bias, a 250v plate and screen voltage, and a 5kΩ plate load. I then chose an Edcor output transformer that had a UL tap, calculated a resistor for cathode bias, and my preliminary output stage design was done. Now what about the driver?

I only needed 12.5v peak to drive this output stage. I also wanted a good performer sonically. So I decided to use a 6SN7. With it’s low µ and very low noise it fits the needs for this driver very well. The 6V6 family (6V6, 7408, 6AQ5, 6005, etc.) can be a little cool in the UL mode and I wanted to preserve some warmth as this amp will be used mostly for soft jazz and light classical music. As such, I decided to use a fairly small plate load on the 6SN7. This preserves some of the 2nd order harmonics and keeps the sound both warm and very smooth. Here is my load line for the driver stage.

6SN7_load_line

I decided that the unbypassed cathode gain of -8 was just about perfect. This means that I only need a little over 1.5v peak (1.1v RMS) input to fully drive the amp. This is well within the capabilities of most line level outputs, iPods, CD player, etc. Also, the -4.25v bias point of the input stage means that the probability of every overdriving the input stage is very low.

I decided to put the volume control after the input stage to minimize it’s effects on amplifier noise figure. This led to the following schematic for the amplifier.

Amp Schematic

Now it was time to worry about the power supply. As I said, I wanted a very quiet B+ supply. I also wanted to use tube rectification so that I could avoid using a “standby” switch. This necessitated adding an additional stage of LC filtering to the supply as well as additional filtering for the driver stage. (This is the 2.58K / 47µf pair in the schematic above.) Here is the resultant power supply schematic.

PS schematic

My goal for the supply was less than -90dBv ripple at the inputs to the power stage. This equates to ≈ 8.6mV at 272vdc. Ripple at the output of the rectifier is approximately 5.5v so my filter section needed a ripple reduction factor of approximately 640.

The primary ripple frequency into the filter is 120Hz. As a rule of thumb, for any series-shunt LC pair, if the reactance of inductor is greater than about 20 times the capacitor reactance, then the filter stages can be treated separately. This is clearly the case here. For the first stage, the ripple reduction factor is ~181. This puts the ripple at the output of the first filter stage at 30.4mV. The ripple reduction factor for the second filter stage is 283 for a total ripple output of 0.1mV. But the voltage was still too high at this point and a dropping resistor was required. Here the resultant RC filter stage has a ripple reduction factor of 27.4 resulting in a final ripple into the power stage of ≈ 7.5µv (for a total ripple level of -171dBv @ 272vdc). Clearly this B+ supply will not be introducing any extraneous hum into the audio path. The additional RC pair on the driver stage supply gives an additional ripple reduction factor of 92 giving a theoretical ripple voltage of 81nV at 250vdc.

Obviously these theoretically small ripple voltages are not realized in the actual circuit. But the values are small enough to virtually guarantee no B+ induced hum on the audio outputs. Now with the electrical design complete, it was time to turn to the layout.

The Prototype

I had already done a fair amount with the 6SN7 and the 6V6 design point came off the data sheet so I didn’t feel like I really needed to prototype the amplifier circuit itself. The power supply on the other hand was something I really needed to test out.

I needed to confirm that I could get the very low ripple numbers I needed from my design. So parts in hand, I turned to my prototyping station and went to work. Below is a picture of the entire power supply connected to a 2kΩ load resistor.

ps_prototype

The only thing omitted from this prototype is the 470kΩ bleeder resistor and the 5v filament supply for the 5U4GB is coming from the prototyping station. The total ripple measured at the resistor in this setup was about 100µV or -128dBv. Not quite the -171dBv calculated above but quite remarkable for a prototype of this nature. In fact, so remarkable that I checked the reading several times. It was however VERY sensitive to lead placement.

So my power supply design was good. Time to layout the amplifier.

The Layout

It took a while playing with all the components to arrive at an acceptable layout. The top plate of the chassis would carry the major components with the point to point wiring underneath. The only compromise on this decision was to place the 5H power supply choke inside the chassis. This was done for two reasons; one, putting it on the top plate resulted in a much larger build and, two, I decided to use an open frame choke I had on hand so for aesthetic reasons I wanted to hide it. Here is the layout for the top plate showing all the major component locations.

top_layout

This layout has several advantages over the others I tried. First, it places the power transformer and the rectifier tube as far as possible from the main driver tube. This minimizes interaction there. It also puts the driver tube close to the last power supply filter stage. This minimizes the possibility of additional hum being induced in the run from supply to driver. You’ll also notice the dashed gray line across the center of the amp. This represents a metal shield between the power supply and audio sections of the amplifier. This is to help minimize any interactions inside the amplifier.

The Build

In addition to wanting this amp to look good, I also really wanted this amp to be quiet and fairly well shielded. Since I had already decided that the base would be lacewood, this meant lots of metal. In addition to the top plate and base plate, it has a metal divider, a hum shield inside the front wall, metal plates for the inputs and outputs, and a metal mounting plate for the AC power input, fuse, and switch. This translated to a large amount of 80mil 6061-T6 aluminum plate. Here are all the aluminum plates cut and marked for drilling.

metal

Once drilled, painted, and installed on the wood chassis it was time to actually start the build. Here is a picture of all the smaller metal pieces installed on the chassis.

bare_chassis

To assemble the amp, the top plate was secured in place so that the wiring could be accomplished from below. With the top plate installed all the major components were mounted. Here is a picture from the underside.

underside

From this point, assembly was fairly straight forward. Just follow the schematics and do the wiring point to point. Every metal plate in the chassis has a point to tie all the parts together. This ensures a continuous electrical shield. The signal ground for the amp is tied to the chassis ground at only one point to avoid ground loops. Here is the finished wiring.

wired_chassis

There are several features that help make this amp work well. The first is the bare copper wire ground bus. This serves as the main signal ground for the amp and connects to the power supply behind the shield. By tying all signal grounds to this bus, it helps to prevent ground loops. The second feature is the filament wiring. This is solid 20 AWG twisted pair that is very carefully laid out. Note how it follows the corners of the chassis whenever possible and when it feeds the sockets it actually loops up and comes down on the socket. This places it largely near perpendicular to the other socket wires helping to minimize interactions. Third, the signal inputs and the interstage signals are carried by twisted pair wring. Again, this helps to minimize interactions.

The Test

With the amp all wired up it was time for some testing. It turned out even better than I thought it would. First, the amp is dead quiet. With my iPod connected and paused, at full volume I could not detect any audible hum on the output. An oscilloscope hooked across the speaker terminals confirmed this as well. The amp produces 3.2W at full power into an 8Ω load. This is about three quarters of a dB below the data sheet number for the pentode wiring, but I expected a little loss when going to UL mode.

The amp was designed for flat response from 20Hz to 20kHz (even thought my hearing is nowhere hear that good anymore) but the Edcor output transformers are only rated down to 40Hz. They are still very clean at 20Hz but the signal rolls off below 40Hz and is about 4.5dB down at 20Hz.

Impressions

I think this amp was a success for me on several different levels.

First, the amp sounds great. Whether playing soft jazz, swing, classical, opera, or blues, it all sounds excellent. The amp sits in my work area and is alternatively fed by my computer, my iPod, or my portable CD player all with great results. This is exactly what I wanted for this space.

Second, the aesthetics of the amp work well. It is still traditional and understated in design, but the materials and theme work well together. The black and brushed silver create a nice feel and the lacewood is definitely something out of the ordinary. Here is a close up of one corner of the amp showing how it all comes together.

close_up

Lastly, I think I have sufficient evidence to support my hypothesis that AC heater supplies on unipotential cathode tubes do not induce hum in the audio chain. This amp uses 6.3 volt AC heaters with the transformer winding center tap tied directly to the single point ground. If there was going to be any interaction between the signal path and the heaters, I would have heard it in this amp.

Conclusion

other_view

This is a success. It looks good, it sounds good, and if does exactly what it was intended to do. With several hundred hours of listening now behind me I find myself liking this amp more and more each time I listen to it. I’m finding things in old familiar music that I never knew were there and I find myself exploring new music with ever greater enthusiasm. I couldn’t ask for more from an amplifier; nor anything I’ve built with my own hands.

31 thoughts on “The 6V6 Lacewood Amp

  1. Hello! Thanks for this project and great details! I’m thinking to build this one. My speakers have 98db of sensibility. Do you think I will get nice (mainly bass) response with this project?

    • This amp has a very good response for music. I suggest you use the V2 schematic as this uses optimized bias points and has a better soundstage. In general UL amps are a tradeoff between musical response and deep bass. For example, the V2 Lacewood amp has a -3dB frequency of almost exactly 40Hz. This is more than sufficient for almost all music. In addition, this amp has excellent transient response. This amp keeps up with the fastest scores I can find with no problems. But if you’re looking for rumbling bass for your home theater, this is not the amp you want. If you want an amp that tackles the toughest musical score with ease, this amp should make you very happy.

  2. Matt,
    I am about to start work on building a Lacewood amp. Is there any design for the power supply without using chokes. Also I would like to use solid state rectifiers. Will I have to make any changes in the PS filter design if I do so?
    Sharat

    • There are two major changes to make in the power supply.

      The first is to replace the 5U4GB with two good quality UF4007 rectifiers. Without chokes in the power supply to tame the high frequency noise, you really need to use UF series rectifiers. Do not skimp here. I recommend Vishay/Dale or Fairchild as the manufacturer.

      Second, you’ll need to adjust the filtering. RC filter sections provide far less ripple reduction than LC sections. This means increasing the size of the capacitors after the first 32µf cap following the rectifiers. Don’t increase the size of the first capacitor or the conduction angle will get too small and cause transformer heating. You’ll likely need two RC sections to replace each LC section. Don’t worry about making the caps extra large. You’ll get far better filtering with more stages than with overly large capacitors. Manage the series resistance elements to get the output voltage around 270v-290v.

  3. Hi again Matt,
    I just completed the Lacewood amp build, and am having a low output volume problem. I’m driving a pair of small Polk audio Monitor 30 speakers, and the amp input is an Ipad at max output level. I use this setup with another low wattage tube amp and it provides good strong volume, so I’m comfortable with this arrangement.
    I did some basic voltage tests, and found the B+ is 20 volts low at the power supply, and subsequently throughout the amp. I assume that could cause a low volume condition. Any ideas why this voltage would be low? The only difference in my build from yours is I used a 330 ohm 5W resistor in the PS. Otherwise, I copied your build exactly. I did test the PS before I built the amp, and it was strong, well over 300 volts with no load.
    Would appreciate any thoughts you might have on this problem.
    Thanks for everything you do here, Ron
    voltage

    • A 7% drop in B+ voltage should not be causing your problem.

      The first step in diagnosing something like this is to gather all the basic operational data on the amp in question. So you need to record the following DC voltages with the amp powered up and with no audio input: 6SN7 bias voltages (i.e. cathode to ground, left and right), 6V6 bias voltages, 6SN7 plate voltages (i.e. plate to cathode, left and right), 6V6 plate voltages, 6v6 Screen voltages (i.e. screen to cathode, left and right). B+ at the output transformer primaries and the driver stage B+ across the 47µf filter capacitor. You should also verify that output transformers are wired correctly (Red = B+, Blue = Plate [pin 3], Blue/white = Screen [pin 4]). I know it sounds crazy, but I have systemetically wired circuits wrong more times than I can count.

      Reply with your voltage readings and we’ll go from there.

      • Thanks for your help Matt. Before I give you the meter readings, I should explain that when I when I was reading B+ 20V low I was powering the amp from a variable iso and it was set a bit low, around 116 AC. The amp was plugged directly into house outlet (about 124V) for the following readings:
        Driver bias – 3.93, 4.03
        6V6 bias – 11.70, 10.56
        Driver plate to K – 130, 127
        L 6V6 Plate to K – 237.5
        R 6V6 Plate to K – 239.8
        B + at xformer primary – 259
        B+ at 47 mf – 238.5
        I checked the xformer wiring, and I took all measurements with no input. Ron

        • All your bias voltages look to be in acceptable ranges. Since you’ve confirmed the output transformer wiring, the next place to look is the 6V6 cathode bypass and the volume control wiring. I think to really help, I’m going to need some pictures. Why don’t we move this discussion over to the DIYAudioProjects support page for this project; located here: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3274
          I’m sure we’ll be able to get to the bottom of your issue.

  4. Hi Matt,
    May I ask where you purchased the dual can electrolytics for this project? I’m having trouble locating them. Thanks, Ron

  5. Hi Matt could I use a GZ37 tube as rectifier in place of the 5U4 tube in the 6V6 Lacewood amp, as I have many of these tubes
    best regards
    Laurie

    • Yes. You can use any of the comparable octal base rectifiers (5AR4, 5AS4, 5AU4, 5R4, 5T4, 5U4, 5V3, 5V4, or 5Y3). All will produce slightly different B+ voltages. Just be aware that going from a 5U4GB to the GZ34 will likely increase the B+ voltage by around 30v. This is ok so long as you don’t use the old metal can 6V6s or the shouldered 6V6 tubes. These have a B+ limit of 250v which will be exceeded. I routinely use different rectifiers in the Lacewood design when trying things out.

  6. I was noticing the comment about hum. It has been my experience, from over 50 years of tube electronics, design and repair, that the 120 Hz hum is due to not having the choke tuned. By this I mean a capacitor usually between .1 a,d .15, in value, shunted across the first choke. In the older Thordarson literature they used to talk about this subject in detail and how to tune a choke. Thordarson had two service manuals where this was discussed in detail. These manuals were from the 40’s era.

    The comment about using a specialized amplifier for use with headphones was right on topic and is the only way to go.

  7. Hi Matt,
    I really appreciate your thoughtful designs and excellent builds. This Lacewood amp is truly a showpiece.
    I’m wondering if it’s possible to add a headphone jack to this amp, and how you would do that. My goal is to build this amp, and would like to include the ability for a headphone.
    Thanks for what you do here, it is inspirational.

    Ron

    • Ron;

      Just adding a headphone jack is tricky in a tube amplifier. Due to the sensitivity of most headphones, you can’t tap off the power stage because the noise, inaudible in speakers, really comes through in the headphones. This usually means the taping off the signal driver and then employing an impedance matching stage like a cathode follower or a white follower.

      My experience is that the separate headphone amplifier is really the way to go when working with tubes. Take a look at the Recovery Amp for a really fine sounding headphone amplifier.

  8. Hi ,

    I found your website while searching for info on how to reduce power supply noise. Did you ever put an oscilloscope on the output, and see if there is any hum visible? How about if you hook it up to a sensitive speaker, and put it directly beside your ear?

    I am currently fighting a losing battle with a commercially made amplifier, trying to get rid of hum. Hum is coming from ground loops, poor layout, and inadequate filtering – basically everything you paid attention to. It was below the threshold of audibility with my Linn speakers, but with my new Klipsch La Scalas it is extremely (annoyingly) loud. My plan is to build an external power supply according to the best possible design principals, and connect it via an umbilical cord. I was planning to use DC heaters too, but you have convinced me otherwise.

    • Glad you like the site. I hope you find it helpful.

      I actually use three methods to look for hum. One is the oscilloscope, the second is a Leader LM-185A high sensitivity AC millivolt meter, and the third is my ear with shorted inputs, max volume, and my ear against the speaker. Experience tells me that if there is any num, one of these three is going to show it. I don’t trust my ears alone because at my age my hearing is not what it used to be.

      • The hum I am chasing looks very weird to me, maybe it has a signature you are familiar with it. When you look at the oscilloscope trace of the amplifier output, there is a spike which appears at 120Hz. The spike is a sharp transient, and the rest of the waveform is entirely flat.I am wondering if this is something the power transformer is emitting as it charges the filter capacitors.

        • This is classic power supply hum. The primary ripple frequency out of a full-wave rectified power supply will be at two times the mains frequency. If f-mains = 60Hz then f-ripple = 120Hz. This is almost always due to a filter problem. This can mean a design issue such as inadequate filtering in the design, a component failure (usually a bad capacitor), or a construction problem such as cold solder joints in the filter section. Personally I would check each of these in reverse order (i.e. construction, then components, then design).

          By the way, if it was transformer leakage, then it would be at the mains frequency and not at twice the mains frequency. Actually, hum from power transformer leakage is pretty rare in well made transformers.

  9. Matt,
    I regularly read diyaudio projects.com and I enjoy reading the build threads. I really like this 6v6 lacewood amp and I am thinking I will build it in the next 12 months.

    2 questions
    1. What is the size/dimensions of the top plate?
    2. If you were to build this 6v6 amp again would you change anything in the power supply?
    I have read through the build thread.
    I have read through your article on power supply design.
    I have read through the thread on tube rectifiers.
    I have read through Mark’s 6v6 black lace build with a split power supply.

    Would you still go with a hammond power transformer or consider an edcor?
    Would you still go with the 5U4 rectifier tube?
    Would you go with the split power supply or not?
    Or any other changes?

    Thanks for your time,
    Todd

    • The top plate is 11″x14″. If I were to rebuild the amp there are a few changes I would make but, yes, one of them would be to use the very nice Edcor XPWR163 in place of the Hammond 270FX. I would also go with an Edcor main choke for an “all Edcor” build.

      If you post your questions over on the DIY Audio Projects page 6V6-6SN7 Humless “Lacewood” Amp I can go into design details.

      Looking forward to hearing from you.

  10. Thank you for the great write up–you have a great site! I have several guitar amps under my belt and was looking for a simple amp that I could build for my office–this one looks like a perfect weekend build.

    I have a PT on the shelf that is 150ma @ 550 v–an Edcor XPWR163 I had from another project. By my calcs, that should work, correct? I’m figuring 47ma max plate, 7 ma max screen for each 6V6 and 40ma max (20 x 2) for the 6SN7 (although I don’t think it will ever draw that much) …do you mind double checking me?

    Also, do you recommend I just take the plunge and build the universal preamp while I’m at it? Based on your impressions, the two sound like a great combo.

    Thanks!
    Kevin

    • Your transformer should work fine. I would recommend building just the amp first. They when it’s complete, put together the preamp if you like. In my opinion, doing multiple projects at the same time is just an added frustration that can take away some of the enjoyment of doing the builds. Not to say that I don’t do just that fairly often, but that’s why I know.

      • Thanks for the feedback, Matt!
        One other question: what is the purpose of the banana jacks next to the input jacks? I tried to trace the wires, but I can’t make it out from the picture…

        Cheers,
        Kevin

  11. I liked the 6v6 lacewood design so much i wanted to build my first amp from scratch from parts salvaged from a 1954 RCA victor black and white floor model tv. i used 6aq5 power tubes, it took some head scratching with the socket pins, although im still working on refining my power supply i had it working on first power up thanks for the inspiration you got me hooked on a new hobby and i have gained alot of knowledge about vintage electronics and i crave more knowledge on the subject.

  12. Being 12 years your senior I too remember the tube amps and am curious to try to build one myself. My resume includes a great deal of interface in the manufacturing of electronics. 8 years as material manager with Cray Research and 15 years owner of a wire and cable manufacturing firm. But alas my technical knowledge is limited at best. Although the magic of osmosis is a wonderful thing. Where can I go to get an education in such matters as these?

    • Sounds like you had an interesting career! Well, there are couple of ways do go about gaining knowledge, but the best is usually just diving in. As I told Jim above, the forum over at http://diyaudioprojects.com/ is a great place to start. The forum is excellent and there’s also a fair number of project pages with projects for all skill levels and experience.

      Are you interested in building, designing, or both? People over on the forum can suggest some first projects to get you started once you decide what you’d like to tackle first. I also have some links to some text books on tube design if you are interested in learning how to do design. Do you currently have any knowledge of electrical principles? If you think you could use an introduction or refresher, I can suggest some texts for that as well. Welcome aboard. It’s a great hobby.

  13. i want to build a vacuum tube amp and a vacuum tube preamp for my stereo system (NAD integrated amp/pre amp with a Rotel phono amp). i don’t know quite where to begin, so i am networking to learn about kits, books and to get advice. my system is in my garage and i like to listen to jazz when i putz on projects. i am retired and 70ish and smart enough to learn. thank you you for any advice. sincerely – jim

    • Jim;

      I’m always glad to help. The best suggestion I can make is to get involved with the forum over at http://diyaudioprojects.com/Forum/. If you post your question there myself and the other regulars can help guide you. I will say that the limiting factors are really your skill set and how much you want to spend. There are some bottom up designs that are very good (this 6V6 amp for example) as well as some good kits. I look forward to hearing from you on the forum.

      Matt

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