Another bias question

[daz]

Sorry to start another thread, but if i add a second related question to the old thread people tend not to look when they know that thread has been answered. But i have a question related to the other one about sag resistors. I decided to bias the amp (it's cathode biased for those who don't know) for the sag resistor, because as you may have read i have the resistor switchable in or out of circuit. And it was biased ay 100% with the resistor OUT of circuit. Problem was when i switch the sag resistor in the bias goes very cold. So i biased it to max with the sag resistor in. this equated to about 1/2 the cathode resistor value i had. But now the sag resistor get extremely hot. It's mounted to the chassis and only got a bit warm before. It gets so hot it's worrysome. But i am not sure how this relates to the change of bias. Why does this happen and is there no way to bias this thing anything more than very cold w/o the sag resistor getting burning hot? Maybe the value is too much (270R) and when using a sag resistor for some reason you cannot bias hot at all? I don't understand this at all. This sure sounded awesome tho !

EDIT: reduced the sag resistor to 180R and adjusted bias resistors so that i am now about 5 MA under 100%. sag resistor is still getting much hotter than it should. Even the chassis where it's bolted too gets very hot.

[Wakculloch]

Because as you lower the cathode bias resistor the amp is becoming more and more biased towards class A, and the sag resistor becomes less effective at creating sag and more effective as a constant supply line dropper resistor. If you take some voltage readings on either side of the sag resistor on no load and full load, this should become apparent (or not).

[daz]

But what you are saying basically then is that a sag resistor must be used with a cold biased PA. Because the only way it doesn't get so hot is if it's biased very cold. So is that it? Are all amps that use a sag resistor biased cold ? I now have it biased at probably 90-95% and once the resistor is fully hot i can't touch it for more than maybe a seond or possibly a 1/2 second w/o burning myself.

So does anyone have any thoughts on what you would do if you really like the sound with this setup? Would you put 2 or more resistors in series to multiply the handling power and dissipate heat better? I figured i have two 270R's i could double up on for 135R and 60 watts. Or will there still be some issues. Can i even do this at all or will i have to run it cold?

[Chuck H]

But what you are saying basically then is that a sag resistor must be used with a cold biased PA. Because the only way it doesn't get so hot is if it's biased very cold. So is that it?

What he's saying is that if your amp is drawing 100% current at idle there is nothing left to "sag". While this isn't exactly true it is very close. You will still see your voltage drop on power chords. But not as much as if the amp were running cooler. So the sag resistor won't create sag because the amp isn't trying to draw as much VARIABLE current through it as it would if the amp were biased colder. resistance in the power supply only sags when there is a change in the current demand. In class A there is very little change in the current demand.

Are all amps that use a sag resistor biased cold ?

Thats a trick question since if the amp is biased hot enough the resistor ceases to be a "sag" resistor... But the answer is no. Many amps that are biased hot use a resistor in this position for one reason or another. I have a build that runs a pair of tubes at 90% and uses a sag resistor. It drops about 35 volts and still does soften the attack a little.

I now have it biased at probably 90-95% and once the resistor is fully hot i can't touch it for more than maybe a seond or possibly a 1/2 second w/o burning myself.

Resistors operate in different ways. Some are designed to dissapate heat over a larger mass and are not intended to get blazing hot. Others are designed to dissapate heat over a small mass and can get incredibly hot. As in "look, it unsoldered itself" hot. Here's a simple test that you can do to determine if you need a higher watt rated resistor:

Measure the voltage across the resistor (-lead on one end +lead on the other with your meter set for DC). Do this with the amp at idle and when you smash a power chord through it at full tilt dime. Use the larger of the two readings. Divide this voltage by the value of the sag resistor. Now multiply the result by the voltage. This is how many watts the resistor is dissapating. You should use a resistor rated for at least +50% of this finding.

So does anyone have any thoughts on what you would do if you really like the sound with this setup? Would you put 2 or more resistors in series to multiply the handling power and dissipate heat better? I figured i have two 270R's i could double up on for 135R and 60 watts. Or will there still be some issues. Can i even do this at all or will i have to run it cold?

You don't have to do anything. There is usually a way to force an issue. But in some cases there is a compromise. If you want more sag then you will need to run the amp cooler. But... If you like the tone that you have with the amp at 90% cathode bias using the sag resistor, and you don't get red glowing tubes, go for it. Just run the above test to see if you need a higher wattage resistor.

If you want to get the warmer tone of the lower voltage/cathode bias arrangement AND add more sag, you could swap out the PT for a lower voltage unit. Lower plate volts have a "browner" sound with EL34's. This combined with the cathode bias may get you closer to the tone you want without the need to bias soo hot. Then you could bias cooler and get more sag from your sag resistor. If you did this you would need to change the B+ dropping resistors for the PI and preamp to keep them close to where they are now. That way all else would be created more equal and the results wouldn't be unpredictable.

Chuck

[daz]

Hmmm....a lot to think about. But i hate thinking ! But i will think one more time.....i think i will put it back the way it was.

[black_labb]

keep in mind that the sag resistor will drop some voltage all the time, giving a colder bias unless you change the cathode resistor. in the 5e3ish amp i rebuilt recently i put a switch that switches the sag resistor out as well as switching some zener diodes in instead of the cathode resistor. you could do a similar thing and use a different value resistor instead of the zeners. i used the zener to get an aproximation of fixed bias. it worked out well as it had a 5e3 sounding setting and one with more headroom and a tighter sound.


do you have the preamp voltages varying with the sag resistor? or do the preamps not chain from the plates?

[daz]

I was thinking of making the cathore resistors switch when i switch the sag r in and out. But theres no sense in it because it heats up too much when it's not biased colder with the sag R in circuit. So i'm just going to leave it as is....near 100% without, cold biased with.