12au7

I took some time yesterday to finish up the 6V6 Curly Maple assembly. Everything is complete and in place except for the main dropping resistor, Rd.

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.

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

So last night I posted about a mistake I made in wiring the latest project. And because of when and where I made that mistake, it took all afternoon to correct it.

I can’t believe I did that! How could I have been so thick headed to make this kind of mistake?

Between regular chores, gardening, and yard work today I managed to steal a little time to make some progress on the latest project. The gain stages for the source selector unit are essentially complete and I’ll begin working on the buffers as soon as I can.

I’ve been catching up on all my spring chores and managed to steal a couple of hours in the shop this morning. The Source Selector Unit chassis is now all sanded. I also took the time to cut and fit the aluminum for the top and bottom plates. Now my attention will turn to getting all the holes laid out and drilled on the aluminum.

Before I perform the chassis glue up, I decided I wanted to get the two pieces of metal for the back drilled. This means I can make the pilot holes for the mounting screws in the inside of the back without having to work with a drill inside the glued up chassis. It’s a little thing but it makes the first assembly go that much smoother.

So I had another productive day working on the Source Selector Unit / Preamp chassis. The chassis has all the major woodworking complete and is ready for glue up.

I got some time today to make grain decisions and cut some basic joinery. It finally feels like I’m making progress.

After a difficult and focused three months or so, I am finally getting back to my normal routines. And that means I’m returning to work on my Source Selector Unit/Preamp project. Thank you to everyone who has had patience with my absence from the website.

Since I picked the wood for the Source Selector Unit/Preamp project yesterday, I decided to spent some time in the shop today getting the chassis pieces milled to thickness and cut to size.

Over the last several days I’ve made some progress on the layout of the new project. This is about the point where it’s time to start looking at wood.

I just posted the project page for the 12AU7 “Color” Preamp which I completed a couple of months ago. Why don’t you go check it out.

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.

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.

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.

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.

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.