Cascade Tubes (Test Site)

Remember back in the beginning of January when I traced the failure of the Lacewood amplifier to a failed JJ power tube? Well, here we go again.

Lately I’ve been looking at a revised amplifier design based on the Marblewood project. The intent of the redesign effort is threefold. First, a different and more widely available rectifier tube. Second, an improved, if slightly more complicated, power supply design. And third, some potential improvement in the low frequency response. As it turns out, you can’t always get what you want. But also, two out of three ain’t bad.

There are lots of practices in the DIY world that are more than a little out dated. I’m obviously not talking about using vacuum tubes but rather the methods used in building and testing the circuits we build with them. Many of these test methods came about for very good reasons; perfectly applicable at the time. But as technology has advanced the reasons for many of these practices are no longer valid. And there is really no better example of this than the practice of testing audio amplifiers with square wave (or various sawtooth or triangle wave) signals.

After a busy Christmas season, including a brief trip out of state between Christmas and new year, I have finally managed to get the Lacewood amplifier back up and running. As part of this effort I also identified the root cause of the initial failure. And frankly I’m a little angry about it.

Remember a few weeks ago in the post When it Rains, It Pours! I said that the Lacewood V2 reservoir capacitor had likely failed? Well, I was wrong.

Have you ever made a mistake? Not a typical everyday little mistake like forgetting to run an errand, but a blunder of such proportions that you’re shocked by your foolishness? One that leaves you thinking you couldn’t have been that naive, that foolish? Well I did. And it left me with significant embarrassment, as well as some smoke coming out of an amplifier.

Would you believe it? Today while listening to the 6V6 Lacewood V2 I heard a funny little popping sound and a short duration whine. It only lasted a few seconds, then it was gone. But when I walked over to the amp there was a distinct smell of “magic smoke”. Once again, the game’s afoot!

I have witnessed a lot of electrolytic capacitor failures. Most of these failures have occurred from improper wiring polarity or a significant over voltage condition. They generally involve an explosive emptying of the capacitor shell. These failures usually leave a smoking husk of a component and a spray of foil bits, dielectric insulation, and liquid electrolyte spewed across the circuit, chassis, and sometimes, the test technician. But recently I came across a much different failure. One which seemed to be specially concocted to sneak by without notice.

When testing an amplifier, the first thing I generally do is to check voltages. This is what I advise others to do as well. However, the real trick is in determining if the measured voltages are acceptable or not. And herein lies the difficulty.

This topic comes up a lot! Someone will email me with, “I want to build {insert project here} from your web site. But…”. Sometimes it’s just that part availability varies. Sometimes people want to use parts on hand which are slightly different. Sometimes people want to substitute tubes, transformers, or whole assemblies. And sometimes people want to actually change the topology! So at what point is the amp being built no longer the design on my website?

Please read the post “That Didn’t Go Well!” before following the power supply change detailed in the later half of this post.

Sometimes a design which seems fine at first can later exhibit some marginal behavior. This exact issue recently reared its head with the design of the 6L6 “Spalted Alder” amplifier.

I know it’s been a while since my last post. Things have been a little hectic for my family this summer. Hopefully I’ll be able to get back to more regular posting in a few of weeks.

But that’s not what I want to talk about. I want to talk about the major changes currently taking place in the US economy and our supply chains, and some things we as hobbyists can do to help protect ourselves from problems.

I guess that some folks have been wondering what I’ve been up to the last month or so. Well, I’ve been trying to take it easy to let a shoulder injury heal. If physical therapy can be considered “taking it easy”. In the last month I’ve learned all about labrum tears. However, today something arrived in the mail and that means it’s time to get back to work.

As I said the other day, “It is my estimation that the EL84 would greatly benefit from a lower percentage UL tap transformer.” Whereas there is much to be learned from the current data set taken with the Edcor GXSE10-5K transformer, I have decided that a different configuration needs to be tested as well.

So I’ve spent the last week looking at the data from the initial effort. I’ve retaken some data, analyzed the performance and setup, poured over data sheets and characteristics curves, listened to some music at a lot of different operating points, and I’ve drawn some initial conclusions. And I must confess I’ve gotten some interesting surprises.

Well, all the data for the aforementioned optimization is collected and in the computer. I’ve been chewing on the data and my notes for the last couple of hours and it’s safe to say, there is a lot more here than first meets the eye.

For anyone who is interested in a little more of the history of feedback development, I have posted an article from the Proceedings of the IEEE from 1999. It’s a relatively quick but informative read about the topic.

This may seem a non-sequitur to some. After all, I don’t post designs on this website that make use of feedback. This is simply because I don’t like the way feedback affects the sound of an audio amplifier. But that doesn’t mean that I don’t understand feedback and make use of it in other applications.

We’ve had a bit of a cold and rainy snap here which has prevented me from getting to the chassis work for the new 807 amp. But that doesn’t mean I’ve been totally idle.

My last post concerning unused and little used test equipment got me thinking. I decided that maybe some of the equipment I’ve accumulated over the years should really be put to better use.

It never ceases to amaze me how much test equipment I have seemed to have accumulated in the last 40 years or so. And some of it has been largely overshadowed by the continuing march of technology. It seems that everything now is so feature packed that lots of the old equipment is (nearly) totally obsolete.

I’ve been moving some amplifiers around today, listening to some that have been idle for a while and doing a little side-by-side comparison. And I’ve been reminded of a very important fact about amplifier performance.

There is truly nothing like the smell of a freshly wound transformer.

When looking at schematics and builds of vacuum tube amplifiers employing output transformers some people notice a curious inconsistency. On some amplifiers, the output (i.e. secondary) side of the transformer is grounded, on others, it is not. This can lead to a dilemma for amplifier builders: To ground, or not to ground? Luckily, there are some simple guidelines which will almost always yield the correct answer.

In my last post, I talked about the changes I’ve made to my 807 prototype jig so that I could investigate the 400v, triode strapped, plate voltage limit. Now I’ve managed to spend some time playing with the high voltage prototype power supply and my newly modified jig. As a result, I’ve learned a surprising amount about the 807 operating as a triode.

A comment by “Dave K” a few days ago got me thinking about how I intend to run my 807 power stage and the limitations listed on the data sheet. So it shouldn’t be a shock to anyone that I’ve decided that a little more testing is required. And that required an update of my 807 test jig.

I have the initial set of data with the power supply prototype using the Edcor XPWR011 power transformer. It is enlightening and clearly shows which amplifier I need to build next.

Be sure to look carefully at the environment where you do your work. Failure to look too closely may result in you sacrificing time and effort to the gremlins in the machine. At least that’s what just happened to me.

One of the risks of becoming too familiar with vacuum tube electronics is that it’s easy to become a little careless when working around high voltages. And because of this learned carelessness, sometimes we don’t take as many precautions as we should. Especially when prototyping.

In the modern western world there is this pervasive, and slightly bizarre, idea that the advance of all things (e.g. technological, medical, political, personal comfort, success, etc.) is a monotonic increasing trend from awful to awesome. And that just because something was once done or existed, then that thing is now “in the books” and there is no reason to ever revisit that thing, whatever it was. It’s a belief that once it’s been done, it’s easy to do again. But the universe is ruled by entropy. And if you stop fighting to reduce it, it just keeps growing.

I was looking at some web page data this morning and I think I may have given a false impression in my post “You Never Really Know…” back in early November.

I’ve finished modifications to my test jig to allow the testing of the 6BQ5/EL84. It’s really a shame that so few other 9-pin power pentodes use the 9CV pinout. The 6CW5/EL86 is the only other really common one.

So as I said in my post The Next Amp, I kind of settled on the triode strapped KT88 based on the power supply. Now I’m not so sure.

So I’ve had an experience recently that surprised me a little. And I’m wondering if this is what other people are experiencing.

Just in case anyone was wondering, I have made a decision about my next power tube optimization.

So I’ve been thinking about my next amplifier project. And I thought I had made up my mind. Then things changed… again.

So I’ve been reviewing EL34/6CA7 data sheets preparing for an EL34 SE-UL optimization study like the two I’ve done before for the 6V6 and 6L6. However, I’ve run into a little snag.

Whenever I make a decision to start a project for this website, I never really know how that particular project will be received. And sometimes I get surprised. This is definitely the case with the new Source Selection Preamp.

Now that the Source Selection Preamp writeup is complete, I took a little time today to prepare for another effort.

The project page for the Source Selection Preamp (previously known as the Source Selection Unit) is now posted.

As always, comments and questions are more than welcome.

I’ve been pretty busy for the last month. But things are starting to move forward again. I’m still working on the project page for the Source Selector Unit and something new as well.

It really is amazing how much I can accomplish when suddenly I can’t work on all the projects which have been keeping me busy for the last month. The SSU assembly is complete and I’ve done some testing.

As it turns out, there is a crew working on painting the exterior of my house today. Since I can’t work on many of my outdoor projects around the house, I thought I’d take the opportunity to get a little done on the source selector unit.

No, I haven’t fallen off the face of the Earth. And I realize that people are waiting for the completion of the Source Selector Unit. I just got unexpectedly busy.

It took a while but the DC checkout is complete and I’m finally ready to finish assembly on the Source Selector Unit.

The resistors for which I’ve been waiting showed up this afternoon. Time to get back to work on the Source Selector Unit.

We’ve all done it or at least thought about it. It’s so easy. It’s really no big deal. It feels good. No one will ever know. Well, I’m here to tell you that eventually, you’ll regret it.

I think that most people understand that the internet is a low engagement medium. The vast majority of those browsing the internet are data consumers not contributors. But I don’t think most people really have any idea of the scale upon which this is true.

As I said before, I decided perform the DC testing of the tube assembly for the source Selector Unit in the clamping fixture. But just as I started testing, I ran into a little problem.

I got some time today to finish out the vacuum tube assembly of the source selector unit.