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!

Just as a reminder, I configured the jig for tubes fitting the 5CE/6BG, 9A/9AJ/9LP, and 8BD base pinouts. This is a significant list of tubes. Here it is.

Now, in all fairness, this list contains a lot of overlap. For example, the tubes listed under the 5CE and 6BG headings are all basically just 6AB4s and 6C4s with numbering variations between systems and manufacturers. This should also be readily apparent under the 9A column with numerous examples of electrically identical tubes listed under different numbers and series. So it’s not quite as complicated as it may seem at first.

There were lots of additional signal triodes to be discovered. Many designed for what the manufacturers call “on/off”, “relay”, just “cutoff” service. These include such tubes as the the 5963, the 5965, the 6211, and the ECC186. These tubes are designed to spend a great deal of time at cutoff without electrode coating problems. But what this means to those of us using them as audio amplifiers is that, because of the special coatings and binders used, these tubes can have exceptionally long life in audio amplifiers. And since they are the more “unknown” varieties, they are plentiful and cheap on the resale market. Buying a single pair may well be enough for the lifetime of an amplifier.

There were also some interesting tubes to be found in the octal 8BD pinouts. There were some dissimilar triode sets like the 6GL7 that might make a very good option for something similar to the 6EM7 amplifier. But there was also a dual power triode, the 6BX7. I had never heard of this tube before so I decided to take a look. It is a dual triode that is good for 10W of plate dissipation per plate, but is limited to 12W for both plates together. Since I was looking at it as a dual output tube I did a load line against the 6W (i.e. 12W/2) dissipation limit. Here is what that initial load line looks like.

It requires about an 8kΩ output transformer and I’m running the triode fairly hard in this configuration (i.e. Pd=5.7W verses a 6W dissipation limit). But the design gives about 2W into the output transformer with a fairly robust 2nd order harmonic content. This would be a very warm and rich output stage for someone looking for that particular sound. If one were to go to a 10kΩ primary and a higher plate voltage, one would likely get a little more power and lower distortion. I may play with this tube design a little more.

Overall it was a productive morning. I learned some things I didn’t know before, played with a new power stage tube, and expanded my notes on equivalent tube types. As soon as I get a day that’s not so cold, I’ll make some progress on the SET as well.