Hi guys,
I'm back from the dead after suffering for three days with a GI infection.
Probably the Norovirus contracted from my mothers retirement home.
I had gone over on Monday to clean up some nastiness and got sick the next night.
Anyway, I am working on my 100w Dumble ODS clone and had a question.
If I bias the power tubes while connected to a 8 ohm dummy load (8 ohm 100w), will it be the same once connected to the actual speaker load of my 2 x 12 cab?
The cab has two EVM 12L 16 ohm speakers in parallel.
I realize one is a resistive load and the other is a reactive inductive load.
So you guys that bias an amp on the bench, is there a difference in bias readings when you compare the two loads?
Thanks!
Last edited by Structo on Sat Feb 02, 2013 6:09 pm, edited 1 time in total.
Sounds like you had a rough couple of days! When you're setting bias it's the current at zero-signal, so it won't make any difference. Theoretically, there's no power going into the load.
martin manning wrote:Sounds like you had a rough couple of days! When you're setting bias it's the current at zero-signal, so it won't make any difference. Theoretically, there's no power going into the load.
+1 What Martin said. Since the Bias current is a steady-state DC current, there can be no current flowing in the secondary as a transformer needs an alternating magnetic field to do what it does. In this instance, the transformer can be though of as a capacitor, in that it passes only AC, and blocks DC from primary to secondary.
while were talking bias
I am also working on a dumble 100w #124 almost
ready to fire it up. ( started a year ago!) I have 4 bias jacks on the back
I have never biased an amp this way. Usually I use a bias king type device
that hooks up between the tube socket and the tube. This Gizmo only does two tubes at a time. So is it better to use the bias meter vs the 4 jacks on the back. If i use the bias meter which tubes should be measured simultaneously? If I use the pins any recommendation never having done this I guess you place the positive lead from multimeter into red jack and teh black lead goes to ground? Than what? Also when powering up amp for first time which, may happen today, should i power up initially with no tubes and no speakers just to check for smoke? Also should I use a variac?
Finally any recommendations for brands of fire extinguishers.
I assume you have a 1 Ohm resistor in the cathode path to ground on each tube and that each red jack is connected to the non-ground side of the each resistor? If so, the put the black lead of your multimeter, set to read DC volts, into the black jack. Then move the red lead of your multimeter into a red jack. That reading will be the the bias current in mA for a single tube. Switch the red lead through all four red jacks and take a reading, while leaving the black lead in the ground jack. Tweak as needed based on tube dissipation and what your ear says. Easier than the bias meter IMO.
I have been biasing amps for at least 25 years and I am always thinking of new ways to do it. The Manzamp was particularly easy to bias because the aural transition out of the crossover distortion zone was easy for anyone to hear. So it lets me set it to the lowest cathode current that sounds good. Of course I'm also watching the cathode current value on the VOM as I do it.
I have also been checking tube temp with a temp gun. The wattage dissipated by the tube is in the form of heat. So, my low end current threshold is set by ear (just after it clears up) and the high end limit is set by tube temp. I have only been checking tube temp for about a year, so I'm still getting a grip on how hot is too hot. Right now I like to see the tubes below 150 centigrade.
So, I'd be concerned something is wrong if I don't get a good clear sound and the tube temp stabilizes over 160C. This takes time because you need to let the tubes equilibrate about 10 or 20 minutes to get to the steady-state temp.
Anyone else using a temp gun? Maximum temp you will tolerate?
"You feel like you're floating on a football field filled with marshmallows." -Dumble
ic-racer wrote:
Anyone else using a temp gun? Maximum temp you will tolerate?
Yep, I will check a few areas at start up and about 30 mins into burn in. The last MM build with the JJ 6CA7s I took temp of all the tubes because they emit so much heat at 700+V. I decided it was fruitless as it wasn't controlled enough to establish a valid baseline.
I had been biasing the 700v setup at around 20-22mA, but I didn't like the JJ fat bottle that cold, I thought it sounded better at 26-27mA. Thus, the reason for the space heater
