frank9310
Full Member
Now dig this baby...
Posts: 146
|
Post by frank9310 on May 10, 2008 10:46:32 GMT -7
On the Route 66 per Doc Zs notes he has on the Bias Notes and said the DC voltage from the red & white wire on the can cap should be at 475v based on his shop wall voltage of around 125v and to subtract 4v for every volt less at your location to determine the correct DC voltage.
So here I have read my wall voltage at 119v
When I used a Bias tool and set my KT66 Shuguang tubes at 36mA I get a red & white wire voltage reading of 447v at the same time using another meter.
However if you take 119v (my wall voltage) x 4 = 476v
So could there be something wrong with my output transformer or something to be reading that low but tubes still reading at 36mA? It read the same way when I used JJ KT66s.
If I lower the mA to around 30, the DC voltage seems to raise up to around 452 but if I raise it to 40mA it stays at around 445-447v. Also the tone is kind of ice picky on any setting between 30-40mA. Any idea if something could be wrong?
|
|
|
Post by benttop (Steve) on May 10, 2008 11:06:42 GMT -7
You're doing the arithmetic wrong.
What the Doc was saying is you subtract 4V from your B+ for every volt your wall voltage is lower.
So here's how that would play out:
His wall voltage is 125, yours is 119. That's a difference of 6 volts.
6 X 4 = 24 so you will expect your B+ to be 24 Volts lower than his.
His B+ is 475, so you subtract that 24 V from there for a voltage of 451. That's a lot closer to the 441 you got - it sounds like if you set it to 451 you'd have a bit more bias current, but I think this is what Doc Z was talking about.
|
|
|
Post by DRZ on May 10, 2008 11:16:27 GMT -7
+1 Benttop
I think I'm going to remove that section from my web-site it causes more confusion then help I feel.
Just get Bias Rite tool or similar product and set the bias that way.
I thought it would help but varying wall voltage and varying rectifiers will cause differences that some can't understand.
A bias tool or doing a shunt bias measurement ( like we do at the shop , but very dangerous to the untrained ) is a better way for most.
Z
|
|
frank9310
Full Member
Now dig this baby...
Posts: 146
|
Post by frank9310 on May 10, 2008 18:51:14 GMT -7
Thanks benttop & DRZ. Just one more question on that. If I set the bias to 36-34mA the DC voltage seems to decrease from 447 downward to around 440v but if I lower the mA to around 32mA then it increases to the 451v range. Would the 32mA then be the ideal setting in that case rather than raising it up to a higher mA like 36-38ma??
|
|
|
Post by dixiechicken on May 11, 2008 10:52:08 GMT -7
DC here! Just my two cents you understand... There's no clear cut right or wrong I think in a situation like this. Long as your bias current is within lets say 28 to 40 milliamps . regardless of the B+ voltage you're within specs. It becomes more a taste thing than a technical thing - if it sounds good it is good. Cheers: Dixiechicken!
|
|
|
Post by Jim @94 Amps on Jun 16, 2008 20:34:55 GMT -7
We usually set the bias at 36mA on the medium range KT-66Cs. Anywhere between 28-40mA could work for you depending on the tubes you're using. Don't worry about the B+ voltage.
|
|