2 bias questions

[iknowjohnny]

I never really learned how to add screens into the equation. Or i guess i once did but can't recall because since then i have always just figured i could subtract around 5ma from whatever bias my meter shows across the 1 ohm cathode resistors, at least as long as i don't change from 1k screens and the voltages stay close. But a while back i went to a PT that gave me a higher B+. 5 Ma as i recall was what i always found back when i first learned, so since i always just subtracted 5. So if I want 35ma i set the bias to 40. However, i was experimenting the other day with various 6L6 and EL34's and the SED's which i never liked a lot somehow seem good with the sozo caps, and i sound that if i bias them up to around 70% they really take on a fantastic feel and tone, much better than the 5-7 Ma or so lower i had used them in the past. I never thought biasing hot was a big difference, and I may still be right about that but i read that SEDs really come to life biased hot so i tried it and wow. Awesome. However, i now have to be sure i'm seeing the right bias so the 2 questions are 1-how do i measure the screens to get a Ma figure to subtract from the bias i read, and 2-where can i find some extremely precision 1 ohm resistors?

[Firestorm]

Measure the actual resistance of your 1K screen Rs. Then measure the idle voltage on each side of the screen R and calculate how much current is flowing. Mouser has a slew of 1Rs with varying precision (and price). 1% should be fine.

[surfsup]

mouser has some 1% 1ohm resistors:

http://www.mouser.com/Passive-Component ... or&FS=True

I thought when you are biasing, you are really reading the cathode and cathode only, not the screen? On my HO I put the bias points between the cathode to ground and never saw any bias measurement referalls for the screen so this is new for me unless I read your post wrong. The screen draw is on the tube sheet and that's what I've been using for the math, I guess if you're biasing at 70% your screen draw would be 70% of the max as well? I hope to learn a bit from this thread.

[iknowjohnny]

Is there one of those online calculators for this? As i've said in the past, i'm literally retarded when it comes to math. you can tell me the equation and it will make as much sense to me as women. Er, i mean, except Jana.

Any other sources for the 1 ohms besides mouser? preferably a tube amp supply like tubesandmore or hoffman, mojo, etc? I can always find something i want or need at those places to make it worth paying $7-8 for shipping. I hate paying $8 for 2 or 3 resistors.

[surfsup]

Firestorm so the bias you read on the cathode includes the screen current as well? Yea that makes sense. the current runs from the anode to cathode and from the screen to cathode too. Like a transistor base/collector to the emitter. Okay so when biasing one should subtract the screen current from the bias reading. Or if you are shooting for X value, you want to bias to X+Is where Is = screen current

Won't screen current change as you rebias the tube though? So you are always shooting for a moving target. I guess with all the inconsistencies in a tube amp design close is best you can do

[Firestorm]

Firestorm so the bias you read on the cathode includes the screen current as well? Yea that makes sense. the current runs from the anode to cathode and from the screen to cathode too. Like a transistor base/collector to the emitter. Okay so when biasing one should subtract the screen current from the bias reading. Or if you are shooting for X value, you want to bias to X+Is where Is = screen current

Won't screen current change as you rebias the tube though? So you are always shooting for a moving target. I guess with all the inconsistencies in a tube amp design close is best you can do

Actually, the current flows from the cathode to the screen and the plate, but yes, when you change the idle point, the screen current will change too. But just a little; the screen current curves are pretty flat.

Some people suggest just ignoring the screen current since that adds a margin of safety by preventing you from biasing too hot. But it's simple enough to calculate: if the voltage from one side of a 1K screen R to the other is 5VDC, then 5/1000 = .005 (5mA). Subtract that from the reading across the 1R cathode resistor to find plate current.

[iknowjohnny]

But it's simple enough to calculate: if the voltage from one side of a 1K screen R to the other is 5VDC, then 5/1000 = .005 (5mA). Subtract that from the reading across the 1R cathode resistor to find plate current.

Ahh, thats right, thanks Firestorm. As to the margin of safey, thats what the 70% is for so i think adding more margin is fine if they want to be as safe as possible making the tubes live long and tonal perfection isn't the more important goal. But i just find those SED's are amazing at the bias i now have them at which is still i think lower than 70%. I'm going to check the screens and get some 1%'ers so i can be sure exactly where they sit and bias them right up to 70%. The feel gets so chewy and rubbery it's orgasmic. I was surprised to note such a great change just biasing up a bit. And so much for mismatching ! These sound so good like this i will no longer even need to do that. I think the sozos made a diff there too because they seem to darken the tone and add mids over the high end. And to those sozos....man o man. If the premiums sound better as jana and others have said i will be on a cloud. It's already just killin. Took the amp up yet another level.

[surfsup]

Unfortunately on my HO, they sell the kit with the EL34 option, without notifying you that the existing PT doesn't support the mA draw of the big bottle, as I've found out recently.

I plan to order a different PT from RJ for the HO (from Edcor) when we get closer to the champ build project...