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There are a variety of sayings, axioms, proverbs, or quips which could apply to me this afternoon. A few which come to mind include:

“How wayward the decrees of fate are” - Vanitas Vanitatum - William Makepeace Thackeray

or this one:

“Pride goeth before destruction, and an haughty spirit before a fall” - Proverbs 16:18

or just the simple axiom:

“It ain’t over till it’s over” - Yogi Berra

In any event, it would seem that “Final Chapter” or no, I have one more post concerning the Color Preamp.

So I decided today that I really needed to close the book on the color preamp. This means that I couldn’t just prototype a new buffer and leave it at that. The preamp needed an end-to-end test in it’s final design configuration. So that’s what I did. The results were even better than before.

Lots of work has been going on the last couple of days working on an alternate buffer design for the 12AU7 “Color” preamp. As readers should know, I was not particularly pleased with the performance of the previous buffer circuit. It was bottoming out at too low a voltage. Now however, I have a simple change which yields much better results.

This afternoon I completed the “color” preamp and tried it out. I am very happy with the results.

The “Color” preamp has been checked out on the test bench and everything is working per design. Bias points are good and the gain stage is operating exactly as designed. However, the buffer design I used was from a different project, and it operates differently in this project than the last one.

I had thought that the coupling capacitors I ordered on Tuesday would not arrive until this Saturday. However, the gods of DIY decided to smile down upon me and the capacitors showed up in today’s mail delivery. In my opinion, two days from order to delivery is exceptional. I spent some time this afternoon with a soldering iron in hand and the Color Preamp is now fully assembled.

As you may know I’ve been waiting on components to finish my “Color Preamp”. I thought that I’d have my output coupling capacitors by now, but I was mistaken. It seems that my parts supplier, after promising to get the parts right out to me, spent three days trying to find the parts in their warehouse, then sent me a refund. But I thought I’d outsmarted them.

Since I’ve been at a standstill waiting for parts, I’ve been going over my parts supply and getting things organized again. It seems we’re all constantly working against entropy. While doing this, my bagged and sorted capacitors reminded me of a short article I wrote back in 2013 about component tolerances. I’ve decided to reprint it here for reference.

I made significant progress today. Most of the signal stage wiring is done. I could finish it up in about half an hour, but there are two little blockages in my path; one due to a supplier problem, one due to me.

I’ve been working on the color preamp over the last few days as I’ve had time. My volume control and output capacitors are supposed to arrive today so it shouldn’t take too much longer to finish it up. However working inside the small chassis is a challenge.

Since I’m still waiting for appropriate weather to finish the 6AS7 SET chassis, I’ve been making progress on a small project for the aluminum chassis from the last post. I decided to put together the “Color” preamp from the post a few days ago. This project will make use of one of the Hammond discount power transformers about which I posted a couple of weeks ago. It’s coming along well, but VERY slowly.

While waiting for appropriate temperatures to paint and apply oil finishes, I’ve been working on a simple preamp project. It’s only a handful of parts and to avoid complications with finishing a handmade chassis, I decided to use a preformed aluminum project box I had sitting on my shelf. I quickly regretted that decision.

For a while now, ever since the “Universal Preamp” was posted on the site, I’ve been getting steady requests for a good tube preamp to provide some color to solid state amplifiers. My initial answer to this problem was to design a good quality cathode follower for the Universal Preamp. There was one problem, the volume control.

Ignoring the wood chassis for the time being, I made significant progress on the triode test jig today. I just thought I’d post a quick update. Parts are now mounted and some wiring is underway.

I’m working on two projects simultaneously right now; the 6AS7 SET and the Triode Test Jig. In the last couple of days I’ve made progress on both. However now I appear to be at an impasse.

It’s been a tense couple of days waiting for the patina to set on the 6AS7 SET top plate. But I just placed the copper on the chassis and it looks great!

Regardless of how much planning you do and how many test runs you accomplish, sometimes things simply don’t go as planned.

As everyone who’s tried to buy one knows, power transformers for tube projects can be expensive. Even small transformers for small projects. But today I stumbled on a great deal for a small preamp power transformer made by Hammond.

With the return of some more normal weather here I got the chance to work on the SET chassis today. Specifically I got the bare copper plates chemically treated for the patina for which I was looking. They look good. Or, more accurately, they look just the way I imagined when I first thought of doing this. I’m very happy with the results.

Since I first introduced the concept for the “Universal Preamp” over ten years ago on the DIYAudioProjects forum, there has been almost constant discussion of my decision to place the volume control on the output of the preamp. I have been asked questions about it numerous times over the years. I have also had some “spirited discussions” with some who vehemently disagree with that design decision. Well, I thought it was high time to explain that decision once and for all, and to discuss exactly what this configuration means for preamp performance and output impedance.

As I was going through some things today I stumbled upon an email discussion I had recently on the topic of toroids (i.e. toroidally wound transformers and inductors) and shorted turns. Given the popularity of toroids, and amount of misinformation floating around on the internet on the topic of shorted turns, I thought that a short discussion of the phenomenon was in order.

The weather has finally returned to normal here and I got to spend a little time in my shop today. I decided that I wanted to be able to finish the base of the SET and my new triode test jig at the same time. So I needed to get going on the test jig base.

I have added a side bar widget on the Blog page labeled “ARTICLES OF NOTE”. I decided that rather than having links floating all over the posts and comments, when I found something of interest I’d post the link in this block.

The first item is a web page from the Lampizator site discussing Audio Heresy. I found it fairly humorous.

With my shop temperature hovering around 38°F (≈3°C) and with the outside temperature hovering around 20°F (≈-7°C) I though that today might be a good time for a little “inside” amplifier discussion. And today’s discussion is about SETs; BIG ONEs.

One more thing concerning Front Panel Express; with a tie in to the SET project. Just as an experiment, last night I opened Front Panel Designer and duplicated my 6AS7 SET top plate design. Now that I’m familiar with the software it took less than an hour to complete.

When I talked about designing the Dual Triode Test Jig in the “More on Tube Testing” post I mentioned an indulgence. Well the results are in.

We’ve had a chilly rain/snow mix today at CascadeTubes so it’s a little too cold to work on the SET chassis in my unheated shop. Instead I’ve been thinking about my new triode test jig and have been going over the tube list that it covers. There are lots of tubes on the list with which I am not intimately familiar so I’ve been pulling data sheets and doing some investigations. And I’ve received some pleasant surprises!

The discussion on tube testers and their usefulness (or not) got me thinking about my own tube testing needs and how I go about it. I already have a solution for octal power tubes with the 7S base pinout which I’ve mentioned before. But what about those ubiquitous signal triodes I seem to use in almost every project? What should I do about them?

So I’ve been playing with the copper tarnish/patina process and I think I’ve got it figured out. After doing lots of small parts with various approaches I’ve learned enough to start the amp pieces. Among the things I’ve learned is that the washing process both before immersion in the chemical bath and after tarnishing while washing off the chemicals is very important. I’ve also learned that an even buff with a steel wool is better than taking the copper to a high gloss shine. And finally, the initial tarnish is very fragile. After doing the bath and letting the piece dry, it really should be left sitting in the open air for between three and five days. After this, the oxide layer toughens up and it can be handled and clear coated.

As I mentioned back in June, I have been looking at different finishes for the copper portions of my 6AS7 SET amp. At that time I had mentioned that I was interested in a “patina” type finish for the copper. I learned that to many people copper patina means green tones, blue corrosion, and wild colors. I was looking for something more like a tarnished penny look. Something that would make the unit look more like an older piece of equipment. Unfortunately this was to be more involved than I first imagined.

I get a lot of questions from people looking at my website. Most questions are about the posted projects or requests for recommendations for test equipment. However a topic which comes up regularly is about vacuum tube testers. The questions range from simple a “Should I buy one?” to complex requests for information on types, operation, “accuracy” and the ever present question “Are they worth it?”. Unfortunately, the topic is actually much more complex that most people realize.

I took some time yesterday to perform a performance check on the driver for the 6AS7 SET. As some may recall, this driver was developed and prototyped back in 2015, and brass boarded in 2016, as a driver for the 6336 SET Amplifier. In the 6336 design it uses a 300v B+ to produce a swing of 82V peak to fully drive the big triode. However, in the unique power supply topology of the 6AS7 SET, there are some additional considerations.

To anyone trying to get in touch with me for the last couple of months, I apologize if I didn’t get back to you. It’s been a couple of bad months.

Back in June, in the Competing Priorities post when talking about the 6AS7 SET, I made the following statement:

I also had another idea this morning which may lead to a little more prototyping work to check out a theory I have (another potential delay). But if the experiment turns out well, I may move bias points around a little on the 6AS7.

Well, I spent this morning with some prototype circuits and a stack of tubes investigating the bias points for the 6AS7 SET power stage. And I learned a couple of important lessons.

This last week I was thumbing through my very dogeared copy the RCA RC-30 Receiving Tube Manual. I was not looking for anything in particular but just browsing and a data sheet for a particular tube caught my eye; the 6DQ5 Beam Power Tube. The only description was “Glass octal type used as horizontal-deflection amplifier in color and black-and-white television receivers”. As I began looking through the specifications I began to wonder how I had missed this tube up to now.

It’s been a month since I posted about the new SET and I just thought I’d let everyone know that I haven’t forgotten about the project. The truth is, as the title says, life has gotten in the way. Most specifically, this month I started on a major project; I’m helping my daughter build a house.

“Peck”. This is a term with which I don’t expect most tube amp builders to be highly familiar. However, it is a term that is exceedingly important to my work this morning on the 6AS7 SET amplifier. Let me explain.

I’m now just shy of ten weeks into my new routine and there are a lot of projects vying for my time and attention. This situation has made it difficult to make serious progress on my 6AS7 SET project.

It’s been several months since I’ve talked about my new 6AS7 SET and my guess is that some folks are wondering what’s happening. Don’t worry, it’s still in process. However there are some extenuating circumstances associated with the amp’s delay. Let me explain.

For me, it really started about six years ago. I had been using an iPod for a while before that, but the real transition took place sometime in 2015. I had been notified that I was moving cross country for work and things were getting boxed up and ready. All my CDs and vinyl were carefully packaged up for the move. I was left with an iPod and my computer for playing music. That was when I started my slide into bad musical habits.

I often get emails and pictures from builders that have built some of the projects I’ve posted here at CascadeTubes. I like seeing all the different ways that people have taken what I provided and modified it to put their own spin on the project.

The Christmas break saw me very busy with tasks other than amp building. So the new SET is a little behind schedule. In spite of it supposedly being a “quiet” Christmas with just immediate family, I ended up tackling several small projects which I had been putting off and I helped my daughter move. And of course I was back at work this week as well. Now that doesn’t mean that I got nothing accomplished, it just means that things are progressing slower than anticipated. This is where I’m at:

I just wanted to touch on the subject topic this afternoon. I have recently been helping out an individual who is assembling an amplifier with a purchased PCB. And he’s having a few issues.

No, really, it is. I swear. This time I’m sure!

So I took some time the other day and got all the purple heart planed to thickness and ready to go for the new SET amp. Then, when I was checking all the layouts, I discovered a problem.

I often receive some variation of the question “Are you going to write a book?”. While the answer to this query is almost universally “no”, I thought I’d take the time to highlight what I think are some of the best books available on tube amplifier design and why I think so.

Well, it took a while but I’ve finally settled on the wood for the new amp chassis. I had narrowed the decision down to either the Purple Heart as I used in The 6DJ8 Headphone Amp or the Wenge that I used in the Dummy Speaker Load. So I took a careful look at both previous projects.

I have been doing internal layout work and making some design decisions concerning the new SET amplifier. I have also been looking at different materials and I decided that I wanted to make the top plate holding the transformers and tubes of brushed copper. While this is an simple decision to make at this point, it raises the issue of what wood to choose for the chassis frame.

I have been working on the new SET chassis design but that’s not all I’ve been doing. This morning I got a little surprise when I checked on my sump pump under the house. It’s starting the rainy season here and I wanted to make sure everything was ready to go. Unfortunately, I found myself replacing a GFCI outlet and the sump pump that had failed and taken the outlet with it. Since the morning was shot, I decided to finish up a couple of other little projects this afternoon. Last week I mentioned another project, the portable class D amp. We’ll, I took a little time this afternoon to finish off that one.

The arrival of the output transformers today gave me the impetus to finalize a few things in the 6AS7 SET design. So now I have a complete schematic with all the important information noted on it.