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.
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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. Continue reading →

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? Continue reading →

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. Continue reading →

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. Continue reading →

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. Continue reading →

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. Continue reading →

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. Continue reading →

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.

If ever asked the question about why you do tubes, please let me recommend this demonstration. It’s truly awesome.

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 (http://www.cascadetubes.com/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. Continue reading →

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. Continue reading →

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. Continue reading →

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.” Continue reading →

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. Continue reading →

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?”. Continue reading →