Posted by BrianM23@msn.com (A) on July 14, 2006 at 18:45:22

In Reply to: The 4 20K Bias Pot Mod That Could NEVER WORK! posted by BrianM23@msn.com on July 14, 2006 at 15:46:55:

Yes, the readings just seem to be independent and totally unto themselves... Sometimes they are in millivolts and sometimes volts - but then sometimes I can get a reading of say .6 volts on ONE tube (of the R/L pair) while the other one is at say 1.00 volts... After messing around enough with ALL of the pots, I was able to get them ALL to read "roughly" .974 volts or so.

Obviously this is not what I wanted... One of the MAIN (perhaps THE MAIN) reason(s) I did not like the original bias setting scheme is that when you would adjust ONE PAIR of EL34's, the OTHER PAIR's setting would also change - and you would have to go back and forth like this over and over again! In essence, I DIDN'T WANT one EL34's bias settings to have ANYTHING TO DO with the other(s)... And with this CURRENT scheme (as I sort of KNEW it would), they are ALL seemingly dependent upon one another! This is MUCH WORSE than it was before, especially since I can't get them to bias correctly AT ALL. So NOW, EVEN IF THEY WORKED CORRECTLY, it looks like I'm going to have to go back and forther over the 4 test points and pots over and and over again... I can just imagine the permutations!

So listen... I DON'T CARE IF I HAVE TO HAVE FOUR SEPERATE BIAS SUPPLIES. IN FACT, I'M STARTING TO THINK I SHOULD HAVE FOUR SEPERATE POWER TRANSFORMERS AT THIS POINT!! I DON'T CARE HOW COMPLEX THE SCHEME HAS TO BE AND I REALLY DON'T CARE HOW MANY PARTS ARE REQUIRED. I JUST WANT THESE DAMNED EL34'S TO BIAS AT .5 VOLTS! WHY IS THIS SO DIFFICULT?

Obviously, if you have ALL four pots WIRED together (i.e. all of the external terminals of ALL the pots are strung together in series with one another) the tubes are NOT INDEPENDENT of one another! They ARE in fact, TOTALLY dependent upon the settings of each other... So I guess I went from having a scheme that worked well enough, although EACH PAIR affected the other one when biasing - to having a scheme where ALL the tubes are now totally affecting each other and NONE can actually bias properly... Is this REALLY the best we can do? Or is something ELSE wrong again, causing these issues? ? ?

I really think maybe it's time for me to move on from this. I THOUGHT I was ONTO something, but now it just seems like more of the same...

How about THIS: Can someone just tell me how to bias a SINGLE EL34? ? ? Just the ONE TUBE... Than (without telling anyone) I will find a way to duplicate THAT scheme FOUR TIMES and maybe FINALLY get the results I'm looking for! Unfortunately, the reason I need to ask this is because I can't make sense of Rozenblit's description of how this works - or maybe I CAN "make sense" of his DESCRIPTION, but he just never tells you how to implement it... I think THAT's actually it! So I will need to reinvent the wheel yet again. Hey, at least I'm really LEARNING about it this way - but it sure is a hell of a lot more trouble than I was looking for.

Just to reiterate (on the above), I will PROVE that I understand HOW this WORKS... but state (once again) that I CANNOT figure out how to IMPLEMENT IT!

HOW BIAS WORKS (I think this MAY actually be from Joe Curcio's Manual... By the way, Joe Curcio does a very EXCELLENT job when it comes to describing how things work and how to DIY). So I MAY be paraphrasing here or I may not, but it's what I wrote down based upon what I read:

1. Output tubes are meant to conduct a little "bias" CURRENT when the amp is idle. The bias current is CONTROLLED by placing a NEGATIVE VOLTAGE at the CONTROL GRID (PIN 5). [IF THIS NEGATIVE VOLTAGE WERE TO DISAPPEAR, THE CONTROL GRID WILL FALL TO ZERO VOLTS AND THE TUBES WILL CONDUCT WAY TOO MUCH CURRENT, CAUSING RED GLOWING PLATES!]

The NEGATIVE VOLTAGE is PRODUCED in the BIAS SUPPLY and is DELIVERED TO THE CONTROL GRID (PIN 5) via the Tube Socket...

2. ADJUSTMENT is made via the BIAS ADJUSTMENT POTS... As the user adjusts the VOLTAGE ON THE CONTROL GRID, the CURRENT flowing through the tube (which supposedly flows THROUGH the tube, from PLATE to CATHODE), flows into the "CATHODE BIAS RESISTOR" (15.6 Ohms), causing a "VOLTAGE DROP" to appear "across" this resistor...

QUESTION: How can two opposing currents (or "forces") flow through a vacuum simultaneously and reach their intended targets without "bumping into" one other and therefore opposing EACH OTHER?

I can understand how say a suppressor grid works - by using the concept of the attraction and repulsion of electrons to prevent electrons that bounce off of the Plate (due to secondary emission) from making their way BACK to the Cathode (or more specifically, from bumping into the ORIGINAL electrons coming FROM the Cathode), hence providing for greater efficiency and "Kinklessness" (i.e. the Pentode). But as far as I'm AWARE, the laws of Chemistry and Physics do not allow for two opposing forces to simply GO THROUGH each other... So I'm quite confused about that.

3. Anyhow, "The USER then MEASURES the Voltage 'across' (i.e. you stick the red probe lead on one side of the resistor and the black probe lead on the other) this resistor as an indicator of the CURRENT flowing through the tube." (i.e. ohms law: V = I x R)

4. Since the CURRENT flowing through EACH TUBE SHOULD BE ~ 50mA, if you use the equation above to make these measurements with the STOCK ST70, you would DOUBLE the Current you're looking for to 100mA... And though nobody ever really tells you this, you will actually need to be working in AMP units rather than MilliAmps, so FIRST OFF you will need to DIVIDE mA by 1000.

So divide 100mA by 1000 to get .1 Amps. If you're measuring for a SINGLE tube, than you would divide 50mA by 1000 to get .05 Amps.

ORIGINALLY, you would take the .1 Amps and MULTIPLY it by 15.6 (Ohms) to get the intended 1.56 Volts. And if you were trying to solve for a SINGLE tube using a 10 Ohm resistor, you would multiply .05 by 10 to find the voltage you were looking to find(i.e. .05 x 10 = .5 Volts).

But as alluded to above, with the ORIGINAL setup, you have BOTH tubes Cathode currents flowing through a SINGLE ("COMMON") Bias Resistor - therefore you need to use .1 Amps for CURRENT (I).

So if I'm getting values of .9 volts at my test probes, doesn't that mean I'm putting 90 (NINETY) milliamps of current "through" the tube, which is essentially opposing the flow of "idle" current WAY TOO MUCH???