sanity check? EL34 pinouts and bias probe points

[dms]

Hi - trying to finish the Ruby this weekend, and looking for someone to verify/correct this. All I have for the bias test point information is the photo in our files section, and I think I see clearly enough how it is connected, but I'd feel better if someone with more knowledge of EL34s can validate. It looks like the cathodes (pin 8) are tied together and go to a single 1 ohm/5W at the bias point, the other end of the resistor, of course, to ground. The pin1's are tied together and go directly to ground, and not through the resistor. Can someone confirm that this is a valid way to connect, and also, since there is clearly only 1 resistor shared by both cathodes, won't I get a bias reading in mV that is double?

Thanks! Davids

[Robert]

That may not be quite right, although it might work. I followed the Two Rock schematic and united pin 1&8 on each power tube, then connected each tubes 1&8 connection together at the bias point, then bias point to 1 Ohm resistor to ground. You are correct, your measurement at the bias point (appr. 70-100) is double the current at each individual power tube (35-50). The Two Rock forum mentions just setting to 70 for both tubes. That is to cold for mine. I use Ken Fishers suggestion to bias to pin5=31mV. I don't appreciate any tonal difference if I set it slightly colder than that, which for my amp is a bias point reading of 85-95, and do so to increase tube life.

[dms]

Hi Robert - there you are! Thought you might be able to help since you've been there... The drawing I have doesn't show the 1 pin connect explicitly, only the 8 pins, and the photo seems to corroborate that but you have to look hard and sort of trace the wires as they cross and twist. If it worked for you to route pins 1 and 8 through the resistor, then I'll take that advice too. By the way, thanks for the notes on the biasing; saves me having to ask!

Davids

[Robert]

You have sure been working on that amp for a long time!! It's about time you get to finishing it. I remembered too the advantage to connecting pins 1&8 is that you can swap in 6L6's with only a bias change. I prefer EL34's but it's nice for experimenting with. Somewhere on this forum I posted a pic of mine but I can't find it now. I will try to post one this weekend.