Jadis OR bias value for Mullard RL34 and KT88 help

They are still breaking in I believe. They did seem to start filling the room nicely a few days ago ( about 100 hrs ago), but they have paused short of the kt88's at this point. I have tried swapping out the TungSol with Mazda 12ax7s's and EH12ax7's and the Mullards seem to still be shy of the holographic richness I am looking for.

I am going to continue burning them in for a while more. But I am considering playing with the bias a little. I have it at 110mv now and was thinking of going to the recommended extremes of 90 and 120mv. What do you think of this plan?

If you've reached the 100 hour mark, I think you're now hearing what the tubes are going to do.

"I am considering playing with the bias a little. I have it at 110mv now and was thinking of going to the recommended extremes of 90 and 120mv. What do you think of this plan?"

Well, I've been resisting saying this since the beginning of the thread, as I've seen the pocket protected high-end audio safety police go nuts in a "we're going to DIE!!!" scream whenever anyone suggests going outside of the manufacturer's recommendation. But I have experimented with this, and I have discovered that as with most tube amplifiers if you are interested in sonics, you want to bias the tubes hot (if you are bent on staying within the lines, think 120mV, not 90mV). This was demonstrated most starkly to me by an older gentleman who really knows tube amps, and biases his amplifier to the point where the plates of the tubes begin to glow, then (maybe) backs off a hair. The increase in everything - be it imaging, low end, warmth, and overall musicality is obvious. Of course, tube life suffers. Perhaps, dramatically. It's no concern to him, as he's not on the audiophile train, and uses a tube that no one has interest in, so he would have enough for his grandchildren that he acquired for nothing. But I don't want to suggest anyone try that as standard operating procedure.

Anyway, I can shoot an e-mail off to my friend at Jadis to find out if their novel (pioneered by the Quad II) transformer connection plays any role, or if we can simply figure plate current straightaway from the 5.62 Ohm bias resistor before giving you any hard and fast recommendations in terms of numbers.

That would be excellent. Thanks.

Trelja-

I tweaked the bias up to 120mv and everything opened up immediately. I backed it down to 115mv and it sounds great. Lots more midrange detail and air. Bass seems crisper also but not as strong as kt88's as you mentioned.

Should changing the 12ax7's affect the bias at all?

Mattzack2, your experience perfectly mirrors my experience when it comes to bias. As I said, as you increase the current flow in the output tube, outside of tube life, EVERYTHING gets better.

At this point, I don't want to encourage you to stray far from the recommended values. But I will say that if one calculates the JOR's plate current in the traditional manner, 120 mV divided by the 5.62 Ohm bias resistor would yield 21.35 mA, an incredibly (low) cold figure.

In comparison, my Dynaco ST70, which uses the same circuit, aims for a bias of 50 mA. The rule of thumb in the guitar industry shoots for 70% of maximum (25 watts for an EL34) plate dissipation, which obviously comes out to 17.5 watts. Though so many maintain the Dynaco's biased way too hot (415V B+ X 50 mA = 20.75 watts), I argue that after all these years you find so many with their original Mullard EL34, the assertion proves false. Figuring on the JOR's approximate 450V B+, at 21.35 mA, we're only at 9.6 watts, not even 50%.

I did send an e-mail to Patrick on Friday, but as of yet, have not received his reply. He's usually very prompt in these matters. We'll hope to hear from him soon. Again, I want to get his answer on the matter before saying to aim higher, as Jadis does employ a novel twist on connection to the output transformer, and the usual way of calculating these things might be very, very wrong. But, there's no need to back off from 120mV.