Biasing using output transformer resistance or 1ohm resistors

[jammied]

Ok I have been playing around with the KLD JTM25. I originally was taking plate voltage then reading across the 1ohm resistors to calculate bias current.


Decided to try out the nos rca vt107a grey glass 6v6's. Took voltage readings on both tubes and are as follows.


B+ = 402v
1st power tube


Pin 1 and 8= 23.9ma


Pin 3= 396v


Pin 4= 367v


Pin 5= -29.7


2nd power tube


Pin 1 and 8= 23.9


Pin 3 = 395


Pin 4= 367


Pin 5= -29.7


So 23.9ma bias current using the 1ohm resistors


Then I used the output transformer resistance method


Tube #1 ohms between output transformer center tap and pin 3 primary is 327ohms. 402-396=6. 6÷327=.0183 so 18.3ma current


Tube #2 had 304ohms resistance. 402-395=7. 7÷304=.0230 so 23ma current


Now tube 2 is very close to what the 1ohm resistors read for current. But tube #2 reads 18.3ma so a little bit lower than the 1ohm resistors.


Could this difference be do to no current flowing through the output transformer during the resistance measurement? Or which one of the 2 methods is more accurate?

[pdf64]

Maybe by the time of the 2nd tube the OT had warmed up a bit, thereby increasing the OT’s winding resistance.
Or the 1 ohm resistor has overheated sometime and is now a bit off?

[shooter]

any time you test under "operational" conditions I consider bestYou will find most OT's are not "balanced" so the variation in R will affect your variation in Iwhere it comes in handy is having an idea of what to expect when you light it up - ALWAYS a good thing.If you are clueless as to expected outcome, you can't really know what's "normal"

[jammied]

I took the live readings at the same time . After letting the amp warm up for 20 minutes or so. Then I turned it off to take the ohm readings of the output transformer.


Took the readings with my el'cheapo gardner bender gdt-311


The 1ohm resistors measured .9ohm


Just seemed strange that the one socket was really close to matching and the other off a bit

[shooter]

Pin 1 and 8= 23.9ma

2nd power tube  Pin 1 and 8= 23.9

you can't get much closer in real time

the OT being off by 7%(?) makes math more a variable than a fact

[sluckey]

You have to realize that cathode current does nor equal plate current. Cathode current is the sum of plate current plus screen current.

[jammied]

So is it better to set bias with plate current? Just trying to figure the best way to bias? Everything I have read says either across the 1ohm resistors or transformer resistance method.

[shooter]

look at the cathode R as the "canary" in the mine
it's also quick n easy

[trobbins]

You may also be seeing a build up of 'tolerances', which will be difficult to reconsile with the multimeter you have.
One tolerance is due to the 1 ohm sense resistors. It is a hassle, but you could connect them both in series with one cathode and see what your meter reads across each resistor for the same cathode current.
Some meters don't give accurate results for winding dc resistance when there is a large winding inductance, and they may also be a concern when measuring dc voltage that has some ac ripple.
Plus your meter may not be too accurate when measuring low mV levels.
All up, if you know your sense resistors measure the same when connected in series, then i would have more confidense in using them to confirm balanced cathode currents, but as mentioned, that doesn't imply balanced anode currents.

[glass54]

Hi Jamied,
Running with Shooters and Sluckeys advice, that is if you want more accuracy and have lots of time whilst sitting in front of your log burning fire  on a cold wintery night, enjoying a Schnapps, you could acquire a supply of OHMITE 1R 1% resistors (http://www.farnell.com/datasheets/1759515.pdf), fit accordingly into Plate circuit and Screen circuit as well as Cathode Circuit. You would then use you Fluke 87 (V) or in my case Fluke 79 MK4 (Both reasonably accurate instruments) and compare your Plate and Screen Currents which should theoretically add up to Cathode currents (Per tube). I also suggest two meters, so you can do a real time comparison. Be warned!! You are dealing with interruption of high voltage circuits, ie one hand in pocket while making measurements and no temporary connections. You could get fussy and use 0.1% resistors but I'm with shooter "use the canary in the mine" method.
As one who has performed many calibrations of serious Sound Reinforcement Systems and ElectroAcoustics Measurements (for some decades), the 1R Cathode resister really makes life simple and certainly gives me great confidence "that all is well" 
Kind Regards
Mirek

[jammied]

I would just drink all the schnapps then get crazy with adjusting 


Guess a canary is close enough for rock and roll!

[pdf64]

look at the cathode R as the "canary" in the mine
it's also quick n easy

And far safer.
And much lower likelihood of inducing instability.
And though cathode current doesn’t precisely equate to plate current, it’s plenty close enough, more than adequate for our purposes.
Kinda like how a 22uF cap is a completely acceptable choice when a schematic calls for 20uF.
The point was that if measuring them both simultaneously, cathode current will inevitably be a bit higher. The discrepancy being due to the screen grid current, which can usually be easily verified.
Consider that for decades, outside of the prototype bench (and often perhaps not even there), individual power tube current typically wasn’t checked in an amp.

[Latole]

Use 1 ohms resistor. That is what I do before I bought Amp head Dual Bias Probe, no more avilable

[HotBluePlates]

... Could this difference be do to no current flowing through the output transformer during the resistance measurement? Or which one of the 2 methods is more accurate?

What everyone else said.  Plus the resistance of the transformer primary is different hot vs cold.  And screen current varies among tubes. And, and, and...

You wouldn't be worried about it if you never bothered to investigate, and your amp would sound just as great.  So there's that.

They're expensive, but the Eurotubes Pro One is excellent, and directly measures plate current for octal tubes with the plate on Pin 3.  I've got 2 of them and monitor both sides of the output stage at the same time.

OR you could just stick with the 1Ω resistors and know there is a slight cushion built in due to screen current.

[jammied]

Actually not to bad of a price imho. Seems to be a good quality accurate unit.


Probably worth it if someone is going to dabble in some of the high dollar nos tubes.