6as7

I added a note to the end of the 6AS7 “Purpleheart” SET amplifier page concerning power tube variability and their use in this amp.

Yesterday, after setting the B+ and installing the power supply dropping resistor, I tried out the 6AS7 SET. To say that I am pleased would be an understatement.

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.

With all the distractions this week, I’m not quite as far along as I would have hoped. But all the top plate wiring is done, the power supply sections are complete, and the 6AS7 power stage is complete.

I really haven’t had as much free time this week as I thought I would. But, in spite of other tasks taking priority, I’ve managed to get the iron for the SET all wired into the top plate.

I managed to make good progress on the assembly of the 6AS7 SET today. In addition to getting all the hardware and iron mounted in the chassis and on the top plate, I’ve actually started wiring.

I managed to eke out a little time to work on the 6AS7 SET top plate this afternoon. I didn’t get a lot done but at least I’ve finally started.

After many delays and distractions I have finally finished the 6AS7 SET chassis and have begun assembly.

It’s been a productive few days. Two different amp projects are coming along nicely and today I’ve reached a milestone.

This afternoon I’ve been working on another amplifier project. This particular one is the 6L6 SE-UL for which some readers have been looking since I published my Optimization of the 6L6 SE-UL page back in March of 2019. I recently realized that it had been three years since I published that data set and that it was probably time for me to get the project started. So this afternoon I’ve been investigating driver stages for the amp.

It looks like the weather has finally broken here at CascadeTubes and spring has arrived. Yesterday, the 6AS7 SET chassis got its first coat of oil. The purple heart used for this chassis is definitely much deeper in tone than that used on the The 6DJ8 Headphone Amp.

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!

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.

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 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.

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.

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.

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:

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.

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.

One simple sight that really gets me excited is to see that very heavy box sitting on the front porch. Today, the output transformers for the 6AS7 SET amp arrived from Edcor.

As I discussed in my last post, there is a new SET amplifier on the horizon. This SET will sit midway in power and performance between the little SETs (6CY7, 6CY7 V2, and the 6EM7) and the big 6336 SET Amplifier.