4-4-0 build - done 2-14-15...happy Valentine's Day!

[RJ Guitars]

Got it... yeah that helps. I am still laying some ground work toward building my low watt Express using a pair of the 1626 octal triodes for the outputs.

I was mostly wondering about the bias circuitry, specifically whether I needed that dual control or if a simple single control Express bias was adequate. I want to try to keep everything essentially the same as my typical "Econo Express" parts package except the parts that are directly affected by my output tube choice.

I haven't ever done fixed bias on a triode and I have no idea whether to expect them to be matched very closely. Ultimately it's just a matter of less parts and less holes in the chassis. If I am lucky enough to have a matched set of tubes (I have 4 each 1626's to pick from), then I can keep things simpler... is there more to it concerning triodes and matching them up?

[johnnyreece]

I'd say if you can get ahold of some decently matched tubes, go for the single control. I was unable to do so in my small stock of 6J5, so I found the dual bias to be handy. Oh, that does bring me to my next point...I did have to modify R8 from 180k to 150k to get appropriate bias voltage. I may even go a touch lower, as I'm still a little low on bias voltage. A couple of my tubes still won't bias up, even at 150k.

If you want to stick with one bias control, I'd just be sure that you can at least get the highest-draw tube in the safe range. The other might be a little cold, but it should survive. I don't know that it will result in an optimum sound, but if they aren't too far apart, I can't imagine it will make a huge difference.

[RJ Guitars]

This brings up an interesting issue - how to you know when a steel bottle tube is red plating?

[johnnyreece]

This brings up an interesting issue - how to you know when a steel bottle tube is red plating?

Indeed! I've got some glass versions I've not tried yet.

[romberg]

I'd say if you can get ahold of some decently matched tubes, go for the single control. I was unable to do so in my small stock of 6J5, so I found the dual bias to be handy.

Yea, I found the dual bias pots on my 4-4-0 useful as well. I only have one 6SN7. So, maybe the norm is for each triode to be better matched. I'm not sure.

The tricky part with a 6SN7 is that each triode has a rating of 5watts for the plates but 7.5 for the whole tube. So, running each triode at 70% is running the whole tube at 93%. And running the whole tube at 70% puts each triode at only 52%. So, I decided to just put each triode at 3watts (60%) which puts the whole tube at 80%.

Doing this with my tube I found that I could get there with a bias voltage around -13V. But there is a difference between each triode of around 200-300mV. And this small voltage difference does make for a larger one with my bias power calculations. Since you kinda have to dance between the lines of each triode being cold and the whole tube being too hot the two pots help.

This might work better with one triode per envelope as you can swap around tubes to find a better match. If I ever build another myself, I think I'll just use two bias pots again.

Mike