Nailing down bias...

I have an ST70 using the DIYTube driver board, solid state rectification and a Cyrus Chong cap board. My output tubes are a "matched" quad of Electro Harmonix 6CA7's with approx 150 hours on them. I have been biasing at 400mV per tube. I set my bias with the speakers connected and with the input sockets shorted. I notice that if my tubes are biased at 400mV after a short warm up, their bias will rise to 425 to 435 after a couple hours of play. If the amp is allowed to cool off, bias will return to 400mV by the next start up. My questions are: Is this normal? Should I reset the bias to 400 when it's hot, or leave it at my initial setting? All responses always appreciated! Thanks all.

Thanks Bob, That's all I needed to know. Just not sure when I wanted the amp at the target bias. Seems it spends just as much time getting warmed up as it does playing hot sometimes. Never enough time for recreation...

Hey Bob- While we're on the subject of bias, here's another question: Are there any benefits whatsoever to running output tubes at higher than normal bias? It's been recommended to me by a trusted associate that I bias these tubes at 400mV, but the designer of my driver board suggests in the manual that 500mV is more appropriate. I've chosen the lower bias so as to err on the side of caution and in the hope that my output tubes will last longer. Do you have any thoughts on this subject? Does higher bias have any effect on performance? Thanks.

Gotcha. Thanks for your time again. The more you know...
It also makes me feel better to know that my outputs have never strayed out of normal operating range.

Bob - What would be a conservative bias setting for a stock ST-70 with Electroharmonix or Winged SED EL-34 tubes?

There is one overriding reason for biasing output tubes. Every tube operates in three ranges, depending on how much grid voltage it is seeing. At the very low end, there is a range where the current through the tube does not vary linearly with the grid voltage. You obviously do not want the tube running in this range while playing music. At the very high end, there is a range where the tube also becomes nonlinear...it cannot deliver current linearly with the grid voltage any more because it is at its capacity. In between, the tube operates linearly...as you vary the grid voltage the current flow through the tube (and hence through its part of the output stage) varies linearly. This is the useful operating range of the tube for our purposes.

The bias presets the grid voltage so that with no signal applied, the tube is operating at the very bottom of its linear range. Set it too low and you will get distortion from the tube operating nonlinearly. Set it too high and you will be running more current through the tube at idle...and thus generating more heat, wasting electricity, and prematurely aging the cathode of the tube...with no benefit. That's why the actual bias setting is so critical.

Now, as in all things, tubes are not absolutely perfect, and neither are they all the same. The transition from the nonlinear to linear ranges are not precisely defined, and even the "linear" range may not be truly linear, with minor variances occurring. Often there will be regions near the low and high ends of the "linear" range, where variations occur. A particular tube may need a bit more idle current, and hence bias, to minimize distortion, than another; and generally if bias is set a bit higher than what is often "recommended", the distortion WOULD go down. In the shop I used to set bias and then do a distortion measurement at low power, then tweak the adjustment to see if the distortion would be reduced. Sometimes it was surprising how this kind of critical adjustment could actually make a difference. Other times, by doing this, I could detect tubes that were mismatched, poorly made, or nearing the end of their useful lives; in a lot of old tubes, even if they still sounded OK and "biased properly" the cathodes would be near-depleted and this would show up in distortion measurements. Granted...not everyone can do this. I can't anymore, myself, because the test bench setup had been meticulously tweaked for low noise and no ground loop hum; now that I've retired, my garage test bench is nowhere near as quiet and I sold the ultra-sensitive distortion analyzer, anyway.

I'm not meaning to scare anyone off. Dynaco allowed for this when specifying the Biaset procedure for the Mullard tubes it included with new amplifiers, and in fact most times I could set the bias on stock Dynaco amps lower than their specified 1.56 volts (whatever idle current that REALLY corresponded to would depend on which amp, and on the value of the cathode resistor). Other tubes, as noted, vary in their performance, and in their bias requirements. Worst of all: tubes with poor quality control, whose bias requirements vary from one tube to the next. On some of them, the internal element spacing can vary so much that a particular tube may HAVE to be biased much higher before it operates linearly. The best way an individual user can minimize this problem is to buy from a reliable vendor and have the tubes matched, in pairs or, in one of those huge amplifiers using four output tubes for one channel, quads.

Bob Latino wrote:

quadaptor wrote:Bob - What would be a conservative bias setting for a stock ST-70 with Electroharmonix or Winged SED EL-34 tubes?

You mention in your first post that you have a Diytube ST-70 driver board which uses a 10 ohm bias resistor. Your choice above of a 400 millivolt bias point which gives a bias current of 400 milliamps per each EL34 is a good choice. I believe this is the recommended bias point for an EL34 for this driver board.

Bob

Hi Bob- If you notice, you are responding to a fellow who is trying to bias a stock ST70, not my DIYTube unit. He's probably not capable of single tube bias. I think he's trying to figure out whether he should bias his pairs at the old D battery voltage of 1.56 volts or if his new Russian tubes need a lower bias point.

Bob, Kevco and GP49,

Thanks much for the very informative discussions and yes I'm trying to get the proper bias for modern tubes in a stock ST-70 amp. You've all been a great help!

Whoa Nelly! Bob meant to type 40 mA per tube, not 400 mA. Measuring across a 10 ohm resistor, .400 volts = 400 mV yields I=V/R or I=.4/10=.040A or 40 mA.

Well now I'm kinda confused. Is the bias setting for a stock ST-70 amp still 1.25 volts or is it a different value?

quadaptor wrote:Well now I'm kinda confused. Is the bias setting for a stock ST-70 amp still 1.25 volts or is it a different value?

The setting is still 1.25 volts DC to give 80 milliamps per each tube PAIR. 80 milliamps per each tube PAIR gives 40 milliamps for each individual tube which is what you want. As has been said earlier in this thread, Dynaco gave a recommended bias setting of 1.56 volts DC for each tube pair in conjunction with a 15.6 bias resistor (again for each tube pair - one bias resistor for two tubes). This gave an idle bias current of 100 milliamps for each tube pair OR 50 milliamps per each individual tube. This will lead to shorter tube life in an EL34 than using 1.25 volts DC as your bias voltage.
By the way, Dynaco chose this seemingly odd bias voltage (1.56 volts DC) because that was (and maybe still is) the output of a NEW standard D cell battery in the 1950's and 1960's. Back then there were no digital meters like there are today and the accuracy of many analog meter was not that great either. What you were supposed to do is get a NEW D cell battery and note EXACTLY where on your meter the needle pointed when you "tested" your new D cell. You then set your bias on each channel to that exact point on your meter.

Bob

1.56 volts is what a fresh CARBON-ZINC dry cell puts out, and still does today; the voltage is defined by the relative electrical "activity" of the anode and cathode and the chemistry of the battery. Alkalines are different, rechargeables based on nickel (NiCd, NiMH) are even more different, lead-acid rechargeables are different still.

The original Dynaco tube amp, the Mark II, at first didn't even HAVE a cathode resistor to enable the bias-measurement method we're familiar with. The cathodes were grounded and the bias had to be set by measuring the negative voltage on the grids of the output tubes. This method is still used by some amplifier makers...Jolida is one that I can remember. But the Dynaco method is better under non-testbench conditions and for kit builders because it effectively permits measurement of the actual current flow through the tube under quiescent conditions.

The addition of the 1% precision resistor between the output tube cathodes and ground (of a specific resistance value especially made for each Dynaco amplifier model according to the desired idle current in the tubes) and the wire to the octal socket for easy measurement, didn't come until later on. By the time of the Mark ((( and Stereo 70 it was standard; and it was available as the Dynaco Biaset Kit for retrofit into the early Mark II amplifiers that didn't come with it.