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One simple sight that really gets me excited is to see that very heavy box sitting on the front porch. Today, the output transformers for the 6AS7 SET amp arrived from Edcor.

There is one thing about vacuum tube amplifiers that sometimes gets in the way. They are not exactly what one might call portable.

As I discussed in my last post, there is a new SET amplifier on the horizon. This SET will sit midway in power and performance between the little SETs (6CY7, 6CY7 V2, and the 6EM7) and the big 6336 SET Amplifier.

So with the 6CY7 V2.0 Zebrawood amp finished and my 6L6UL project on the back burner, I’ve been working on a new SET amplifier. I have been thinking about an amp somewhere between my small SETs and the big 6336 SET amp. But I’ve run into a hiccough.

I was arranging some things today in preparation for an event this evening and happened to have a little Class-D amp in my office that I normally use in my wood shop. So I decided to try a little A/B testing.

So I was reading one of the tube forums recently and I saw an amp builder who, when talking about his latest build, said “To my ears sounds nice but I am not an expert”. Now I read or hear variations on this theme all the time. And frankly, it upsets me greatly.

So it’s been a while since I first grabbed the plank of Zebrawood in my shop and envisioned it wrapped around an amplifier. That was June 13th. Here it is a little over two months later and I now actually have a functioning amplifier. I have to say I’m pretty happy with how it turned out.

As some may already know, I’ve been working on a new version of the 6CY7 amplifier. There is a new chassis specifically designed for this amp and some circuit changes to improve performance. But here’s the issue, I can’t seem to get much done. Let me explain.

I have made a small change to the website which I hope will be helpful.

So I’ve had the chassis and metal done for the 6CY7 V2 amp since the 4th of July. It has now been a full 13 days and… the paint is not yet fully cured. It is hard for me to remember it ever taking this long for a semi-gloss enamel on metal to cure hard enough to begin assembly work. Obviously the manufacturer has changed the solvent makeup of the paint I use. Time to find another manufacturer. In the mean time, I’ve been working on other things.

Like everyone else all over the world, for the last four months I’ve been working from home and limiting my trips out out of the house. But the situation these last four months has not been entirely unwelcome (except of course for people getting sick). For one thing, the view from my home office of trees and mountains is immensely better than the windowless building in which I was working. And for another thing with no commutes and flexible schedules, I have gained several additional hours in my day for doing things I want to do. With all I new found time, I did what any responsible technical person would do in such uncertain times. I’ve been teaching myself some new skills.

Well, the new version of the 6CY7 amp is coming together. The wood for the amp has gone from a raw 11″ x 58″ plank of Zebra Wood {Microberlinia brazzavillensis} into a finished amplifier chassis complete with four coats of Tru-Oil finish. I think it looks pretty good.

I want to start by rolling the clock all the way back to March 6th of this year. It was Friday and I had just returned from a business trip the night before. That afternoon (Thursday) my employer had decided that everyone was to start working from home immediately. Not wanting to try performing complex engineering tasks with only a laptop screen, I immediately ordered two nice high definition 24" monitors. But the question was where to put them. The “answer” was, on my office desk where I usually do tube work, including prototyping and assembly. This gave me a good spot to do my job, but with two large monitors and their ample stands, it left little space for power supplies, oscilloscopes, signal generators, and other such trappings of my vacuum tube amplifier work.

So I usually have multiple amp projects going on at any one time. There is a rather large project that I’ve been working on but it’s going rather slowly and I was hungry to make some progress on something. As such, I decided to tackle a smaller project I first prototyped in 2011 and that has been on hold since 2015. I spent the better part of today turning an 11" x 58" plank of Zebra Wood {Microberlinia brazzavillensis} into an amp chassis.

So, I was sitting here on this rainy Sunday morning going through some amplifier designs and looking through my stash of tubes and various amplifier building supplies. While standing there looking at the shelves of vacuum tubes, transformers, and various piece-parts, my eyes drifted up to an amplifier sitting on the top shelf. It was my 6AS5 µ-power UL amplifier documented here. But why was the little amp sitting on a shelf collecting dust? Then I remembered.

For those that are paying particular attention to my amp builds it will come as no surprise that I think the #44 and #47 (0.9 and 0.5 candela respectively) indicator lamps are a little bright to be used on tube amps. I just think that they tend to overpower the look of the amplifier when the room gets dim. However, as these lamps are ubiquitous, it pays to use them in our builds. So, on the 6DJ8 Headphone amp and the 6EM7 vertical amp I included a small adjustment potentiometer to tame these power indicators. And on the Marblewood amp, I chose a fixed 25Ω 1W resistor to dim the indicator.

I keep getting questions about amp grounding so I thought I would consolidate some of my previous notes on the subject. The first is a reprint of a forum post I made back in 2013 on the DIYAudioProjects Forum.

Warning: It has been brought to my attention that some people reading this post mistakenly believe that I am advocating the override, disconnection, or elimination of the safety ground (or bonding) connection in some pieces of equipment. This is not correct! The safety ground or bonding requirement for every piece of equipment is a firm requirement. At no time should any signal cable or independent wire be expected to fulfill the safety ground requirement for any piece of equipment. The process of “ground lift” is simply to provide a different signal ground reference in a piece of equipment.

Being a proponent of single ended topologies, I am constantly on the lookout for singleton power tubes. Since I’m working on designs with all single ended signal chains, things like matching and bias points as less important to me than tubes which test strong and are reliable. One source containing lots of singleton “New Old Stock” and “Used” power tubes is Ebay. Often I can get a single power tube for far less than buying from one of the more established vacuum tube business on the web. But I have begun to wonder about whether these are really such a great deal after all.

Ever since I completed the Optimization of the 6V6 SE-UL I have been saying that I was going to do the SE-UL optimization process for more tubes. So yesterday I finally started to make plans.

I just got a serious lesson in room acoustics. Please let me explain.

I just had a little tussle with with the local blatherskites over at one of the less hospitable internet audio forums. The actual subject of the exchange is unimportant. However it got me to thinking about distortion and how today the concept of distortion has become seriously misunderstood.

Anyone who has taken a look around this site should soon realize that I really like my low power amplifiers. What this means is that most of my listening is at lower volume levels. Now the only problem with this is that the sensitivity of the human ear to different frequencies is dependent on volume level. So at low volume levels, the ear has relatively poor sensitivity to both low and high frequencies. And the quieter the music gets, the worse the mismatch becomes. Traditionally the answer to this conundrum  was the humble “loudness” control.

So I was sitting down today to prototype a new 6SN7 preamp with a buffered output. As I was getting equipment arranged and pulling components I looked over at my dedicated prototyping station and something didn’t look right. As I looked more closely something jumped out at me that was unexpected and frankly, more than a little surprising.

If you look over to at the top of my blog page you’ll notice a little change. At the top of the sidebar is a little yellow PayPal button readers can use to donate to CascadeTubes. There is also one at the base of the home page. And I thought that after all these years, a little explanation was in order.

In one of the page comment sections, Dave K asked this particular question:

You have both SET and UL pentode amps, from the listener’s standpoint, what are the audible differences in sound between the two types?

From a tech standpoint, I know that triode = even harmonic distortion, pentode = odd harmonic distortion UL pentode = ??

And this was the simple answer I provided:

A big part of building custom vacuum tube amplifiers is layout. Even when the schematics are finalized and parts are ordered (and even delivered) the preparatory work is not over. With this phase comes the arranging and rearranging of transformers, tube sockets, potentiometers, switches, jacks, terminal strips, and all the other miscellaneous parts that make up the final amplifier. And this is where some simple quality tools can make all the difference.

A little background. I am a thirty year Electrical Engineer. I have spent those thirty years woking on some of the most amazing pieces of technology ever built. I have worked with cutting edge technologies before most people knew they even existed. I have had a phenomenal career. And I’ve spent that entire career with one company. That company has treated me very well, and I am grateful. But times are changing.

So, CascadeTubes now has a Twitter feed. The link is “CaliSuncalc”. The link is also on the left side of the Blog page.

If you ask me what that means for the site, I can’t really say. There are a few tweets there already. This is going to be a journey of discovery. So I guess we’ll see how it goes.

Feel free to take a look or even follow my tweets.

There is something very satisfying about seeing a project really start coming together. I finally got some time today to start working on my 6V6 Marblewood amplifier project. It now looks like an amplifier instead of just a pile of boxes and parts.

So, this morning, I got a little reminder about the nature of the technology we all use. I currently have the 6EM7 Vertical Amp in my office at home connected to my computer sound card. When I reached over and turned it on this morning I got an interesting surprise.

I imagine that some folks have been wondering where I’ve been since the end of May. The truth is it’s been a very busy summer. Shortly after my last blog post, I learned that I was being transferred for the second time in less than two years. So far this summer I’ve bought a new house, moved ≈1500 miles (≈2400 km), totally packed and unpacked an entire house, a complete workshop, and an electronics lab, started a new job, and basically just tried to keep my head above water. However, in the process of unpacking things at my new house, I found something interesting.

A while back, I was at my local hardwood supplier and found a plank of wood of a species with which I was unfamiliar; marblewood (zygia racemosum). This is a South American hardwood with a golden yellow to orange color and large dark streaks of deep brown throughout. This, I thought, would make a very interesting chassis.

Well, my blog seems to be quickly becoming a list of things to avoid doing. I’ll get back to the more technical stuff soon but in the mean time I have one more blooper to share. I hope there aren’t too many objections.

So I have a new amplifier I just finished which will soon be added to the ever growing list of amplifiers on the web page. But when I was putting it together I managed to destroy a couple of expensive tubes due to a very simple error early on in the design process. Let me explain.

Sometimes I feel like an idiot!

This is a copy of a comment I made during a forum discussion on “improving” the response of an amplifier.

So lets talk feedback for a minute. There are lots of ways to apply feedback and all feedback performs the same function. Any differences in feedback response are usually due to the different impedance functions applied to the feedback loop. Contrary to popular opinion, there is no magic in any feedback function. The effects of feedback are to reduce gain, increase bandwidth, reduce circuit induced distortion and noise, and decrease gain function sensitivity to circuit parameter variation. And it works. We’ve been using feedback amplifiers widely since Harold Black introduced the concept back in 1934.

When writing for a webpage or a blog it is sometimes very difficult to get a feel for your audience. You never really know if you’re talking to a few friends over coffee, standing in front of a classroom of twenty, or lecturing to an auditorium of two hundred. I know there are a few people with whom I regularly converse, but my web traffic suggests that there may be more. So in order to help me get a feel for all of you, I am proposing a CascadeTubes version of Groundhog Day. Allow me to explain.

Everyone knows that you need to be careful when working with metal. Corners are sharp, shavings get everywhere, power tools can spin and throw pieces of metal; so it’s kind of obvious you need to be careful. But how about handling the other materials?

Lately I’ve been looking at a project that’s been sitting on the back burner for quite a while. It’s a stereo power amp for which the 6EM7 Vertical amp chassis was a proof of concept. This amp uses an 807 tetrode in a triode strapped configuration. Now the one thing that I didn’t care for in the load line design was the level of distortion. From the load line paper design, distortion peaked at around 8% at 4.31W output, yielding a Ds of 1.8%/W. This is larger than I generally like in my amps. Then I had an idea.

There are a lot of opinions out there considering what’s acceptable for low frequency amplifier performance. It seems like everyone has their own very strong opinion about how low the frequency response must go for an amp to be “acceptable”. And it seems that usually these opinions range from somewhere between 40Hz and 20Hz as the minimum acceptable “roll off” frequency with definitions of “roll off” ranging from “no loss”, to “-1dB”, to the Engineering standard of -3dB. And strangely enough, virtually no one ever addresses the technical performance of amplifiers at their low end rolloff frequency. Amplifier distortion is usually only measured (when mentioned at all) at 1kHz and not across the frequency band.

Lately I’ve been looking into drive topologies for Single Ended Triode (SET) power stages. The major benefit of power triodes is of course how great they sound. The major down side, from the perspective of the amplifier designer, is the fact that their power sensitivity is very low. What this means is that it requires a large voltage swing to drive these tubes. Some examples from my recent designs: a 6AS7 SET design that requires ±75v, a 6336 SET that requires ±81v, and a triode strapped 807 “SET” that requires ±72v. All from a line level inputs of between 1v and 2v peak.

So I was looking over the prototype work I’d done for a 6336 Dual Triode SET amp driver stage. This is the unit that I discussed on the “Brassboard Prototypes” page. However, when examining the pictures and performance, I noticed that something wasn’t quite right. So I took apart the brassboard and took a look. Sure enough, I had built the unit slightly differently than what was in the written schematic on that page.

As part of my fascination with vacuum tubes I am almost constantly laying out different amplifier chassis designs. I usually have between two and four different designs in process at any given time. This generally follows a very predictable pattern: design the amplifier schematic, choose a size and layout which is pleasing to the eye, then try to cram everything into the resulting (usually too small) chassis. It seems I am nothing if not predictable.

Because of this personal failing I am usually looking for component choices which are as small as possible. Which brings me to the point of this post; power filter chokes. I have gotten to the point where I simply will not build a tube amp power supply without one. There is too much benefit to performance to not have a good choke in the main filter. There is only one problem, chokes tend to be big.

Sometimes when you’ve been working on technical stuff and just listening to music in the background you can kind of forget why we do what we do. Then there are times like right now. I just reached deep down in my music collection and put on the original 1984 mix of Stevie Ray Vaughan’s “Couldn’t Stand the Weather”. Now I’m listening to 8 minutes of Stevie doing the Hendrix classic “Voodoo Chile (Slight Return)” through an all triode signal chain and I’ve got goose bumps it sounds so good.

Recently I have been doing some experimentation with 6V6 power tubes. As I was looking over some old designs I realized that I really wasn’t designing the 6V6 power stages so much as picking values and hoping for the best. This led me to develop an entire set of operation data on operating the 6V6 in Single Ended Ultra Linear (SE-UL) mode. You can read the results here (/optimization-of-the-6v6-se-ul-amp/) or over on the DIYAudioProjects site (http://diyaudioprojects.com/Technical/6V6-SE-UL-Bias-Optimization/). But it also got me thinking more generally about a common problem we all share, a general lack of resources to support the vacuum tube circuit design process.

I’ve been working on a new SET amplifier design. It’s a modern (well at least more modern) approach to an all triode signal chain with a relatively high power output (about 7.5w into the output transformer). But there’s a catch, I’ve never personally used this power tube before and there really isn’t much information about it floating around the internet. You see, instead of going with some huge transmitter output tube like the 211 or an over hyped DHT like the 2A3 or the 300B, I took a different approach. I decided on a very linear, high power, pass triode; the 6336A.

I’d like to take a few minutes to talk about about required amplifier output power. People who advocate low power single ended (SE) amplifier designs often get a bad time about the low output power of their amps. Due to the nonstop marketing forces of the consumer audio equipment industry over the last 40 years, most people think that 25W to 50W per channel is the absolute minimum acceptable power for any “decent” audio amplifier. They have been told repeatedly that this type of power is required to get any volume out of your system. But is this really true? Most of the are SE tube amps are from one to four watts peak output power. This seems totally inadequate to the “well marketed” masses but in reality, it’s much more power than you’d think. Matched to appropriate speakers, in a normally sized room, these amps can put out a surprising volume level and very dynamic sound. So let’s investigate how this could be.

In my post on how much audio bandwidth was really required, I posited that there was not sufficient benefit to justify the impacts of pushing the low end frequency response of our tube amps down to 20Hz. As part of that post I included the following statement.

“In order to pass really low frequencies the primary inductance of the output transformer must be rather large. Now the difference in frequency between 20Hz and 40Hz is a full octave. This means that a transformer designed to pass 20Hz at the same level as one designed to pass 40Hz must have a primary inductance which is twice as large. This equates to a much larger and more expensive coil and also introduces complexities at the upper end due to interwinding capacitances. This drives one to rather expensive solutions.”

After you’ve been building audio gear for a while you begin to acquire all sorts of miscellaneous test equipment and I am no exception. So today I was looking at a couple of audio function generators that I use to test amps and preamps. One is a relatively modern Tenma 72-455 and the other an older vacuum tube unit; a Precision model E-310. Neither of these units have been calibrated in all the time that I’ve owned them except for some rough checks involving a speaker and a tuning fork. So today I began thinking about calibration.

There is an ongoing debate in audio circles about just how much frequency response is really required from an audio system. Different systems for different uses may have vastly different requirements. The audio response required for a media room amplifier where various sound effects, explosions, noises, as such must be accurately reproduced may be far different that that required for orchestral music or jazz listening in a study or living room. So the pertinent question becomes, “For what conditions do we design?”.

So I get the question all the time “Why tubes?”. I usually answer something along the lines of “I just like them” but that really doesn’t sum it up that well. So I have been thinking about that very question; “Why tubes?”. After much contemplation, I think I have come up with one simple answer that sums up my feelings on the topic quite succinctly:

Why tubes?

Because vacuum tubes are like MOSFETs, but with class!