Remember when I said that you can’t always get what you want? Well, it turns out you can. At least in this case. Especially if you are willing to shift some priorities.

As I said in the post above, I’m playing with the idea of an updated and improved design based on the 6V6 Marblewood amplifier. I had three major updates in mind and, at first, I didn’t think I’d be able to do anything about one of them, the low end frequency rolloff. Not that there is anything wrong with the Marblewood design. I feel like I have to scream this point from the mountain tops. THE MARBLEWOOD IS A GREAT AMP! The Marblewood was designed as a first power amp for people new to building tube amplifiers. And it fits that roll perfectly. I just want to expand on that design in a few areas.

So what changed in the time since I wrote that first post? What changed was priorities. The fact of the matter is that I generally have several projects in the planning stage at any one time. And this is no different. For those who haven’t been following along, there are now three amplifier projects near the top of my list. The first is an 807 based triode mode amplifier. The second is a dedicated KT-88 triode mode amplifier. And, now, the third is this new upgraded Marblewood design.

So on my parts shelf right now I have six Edcor output units; four GXSE10-5k and two GXSE15-5k transformers. Two of the GXSE10-5k transformers are going to be used on the 807 triode amplifier. The other two GXSE10-5K units were leftover from the 6L6 Spalted Alder project from when I decided to go with the bigger GSE15-5k transformers on that project. Finally the two GXSE15-5k transformers currently on my shelf were intended for the KT-88 triode mode amplifier.

Of these three projects I had originally intended to build first the 807 amp, second the KT-88 amp, and third this new amp. However, as I said above, my priorities have changed. I really want to get this new design in process. I was also unhappy with not being able to get the improvements in the low end frequency response which I wanted. When I saw the two GSXE15s sitting on my parts shelf, I decided that they could be used in this new project to give me that improvement. When it comes time to build the KT88 amp, I’ll just have to scrape together enough funds for a couple more GXSE15 transformers. It may slow things up a little, but I think it will be worth it to make this amp what I wanted. It also frees up the two leftover GXSE10s for a couple of “spud” mono-blocks I’ve been thinking about.

So let’s talk about what changes I hope to get with this change of output transformers. Below are the two transformers sitting side by side on my work desk for comparison.

The GXSE10 is on the left and the GXSE15 on the right. The most obvious difference between the two is the size. The GXSE15 is considerably bigger than the GXSE10. This is to account for both the larger size of the magnetic core required to handle a full 15 watts and the increased number of turns necessary to properly magnetize that core. A consequence of those differences is that the primary inductance of the GXSE15 is significantly larger than that of the GXSE10. According to my measurements about 50% larger.

Before anyone asks, no I am not going to publish my measurements. Every time I talk about output transformer primary inductance I end up in arguments with individuals over values and methods. It is sufficient to say that the one is significantly larger than the other. And frankly, the exact numbers really don’t matter.

The net result of this larger primary inductance is that the low frequency rolloff of the amplifier will be lower. Since I will be operating the power tubes at the same operating point as in the marblewood, the improvement in the low frequency rolloff will be entirely due to this increased inductance. The low frequency -3dB point on the Marblewood is about 33Hz. Using the larger outputs with the 6L6, the Spalted Alder low frequency -3dB point is about 17Hz. I hope to split the difference and achieve something around 25Hz. When I get the amp completed and characterized, I’ll overlay it’s bandwidth plot and the Marblewood bandwidth plot so people can compare for themselves.

The results should look similar to the overlaid plots for the two 6CY7 amps comparing the XSE and GXSE transformer performance. That plot from the 6CY7 designs looks like this.

The performance will be different obviously. But this plot does serve to illustrate the difference that changing output transformers can have. Now that I have made my decisions about what parts to use, it’s time to get back to work on the actual design.

As always, questions and comments are welcome.