Test

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.

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.

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.

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’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.

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.

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.

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.

Yesterday I was bottling mead but today I got a chance to wire up the buffers and tie in the coupling capacitors. This completes installation of all the signal chain components. All that’s left on the assembly is the final power supply wiring.

Here’s the salient point about putting your entire amplifier design and build process on the internet for all to see. Invariably, regardless of how well everything is going, things always arise which manage to keep you humble. And today it happened again.

I got a chance to do a little work on the amp today. I managed to install the top plate on the spalted alder chassis, install the filter inductors for the power stages, connect the output transformer secondaries to the speaker terminals, and wire in the power transformer primary and indicator light.

In the last few days I’ve managed to get most of the wiring on the 6L6 SE-UL top plate complete. This includes all the preposterously large power supply filter capacitors.

A while back in this post, “6L6 SE-UL Primed for Progress”, I mentioned that I would no longer be painting the exposed bottom plates of projects which won’t be seen. What that statement actually meant was that I would be leaving the aluminum in a bare “as delivered” finish. Or maybe with just a little steel wool polishing. But I’m now modifying that stance slightly.

I first announced this SET on September 18th, 2020. It’s only taken me about 19 months to get to this point. To be fair, that 19 months was fairly eventful, including both my retirement and much of the COVID madness. But tomorrow, the slow methodical process of testing can finally begin.

Today I was once again reminded of the perilous state of human fallibility and the risks of personal hubris. The cost of today’s lesson? One ruined KT-88. And a little smoke in the office.

About a year ago my television display died and I was forced to buy a new TV. I had been using the Lacewood V2.0 amplifier for the sound system and, when I plugged in this new TV, it formed a bad ground loop with the amplifier, introducing a nasty hum. This was not an insurmountable problem. I simply dug through my parts closet until I found a little line isolator I had built about a decade previous for some project I was working at the time. I installed this isolator and the problem went away. But that’s not the end of this story.

I took some time this week to get several coats of oil on the chassis for the Dual Triode Test Station. Even though I’ve already used it for the mystery tube investigation and to compare some 12AU7s, it’s good to have it finally complete.

I spend a lot of time experimenting with various vacuum tubes circuits. It is a past time which I really enjoy. It also gives me an outlet for my creative tendencies when the workshop is too cold to make custom furniture (and amplifier chassis). As a consequence I have amassed a fair amount of test equipment and miscellaneous cables, adaptors, probes, and other odds and ends over the last decade or so. And regardless of the fact that I want to concentrate my attention on my vacuum tubes, sometimes it’s the other things that demand my attention.

Back in November, in my post concerning Tubes and Tube Testers, I talked about how I was not really a fan of generic tube testers. In that post I sited a statement from the RCA “Receiving Tube Manual”, RC-30, about testing tubes.

The tube tester cannot be looked upon as a final authority in determining whether or not a tube is satisfactory. An actual operating test in the application will give the best possible indication of a tube’s worth.

So I thought that as a simple demonstration I would collect some data from a couple of tubes; one new and one rather ‘used’. The results are really kind of surprising.

This has happened to most of us at some point or another. You come upon a tube with no markings whatsoever mixed in with a bunch of miscellaneous tubes. Or, like I just did, you get a “lot” of tubes from a reseller and most are marked but a few are wiped clean. But how does one go about positively identifying the mystery tube? Especially with the numerous types of 7 and 9 pin miniature tubes in existence this can seem to be a rather daunting task. However, just like a game of “20 Questions”, it is actually simpler than one might think.

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.

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.

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.

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!

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.