Quickly setting bias with new components

[brad44]

Ive been going thru my amps looking for resistors that have drifted or failed. I found some in a bias circuit. I know changing them back to spec will change the current bias setting. Not knowing how that will effect the voltages I am a bit nervous.

How fast do I have to be on the trigger dialing the bias back or forward when I power it back on?

By figuring out the percentage the resistors have drifted, can I assume the voltages will be that same percentage off with the new resistors?

 

[Dave451]

Is the bias voltage adjustable on the unit? If so, just adjust it to the maximum negative voltage setting (produces the least current draw in the tubes), power up, and adjust the bias per instructions for the unit. Knowing what you're dealing with (guitar amp, stereo amp, receiver, etc with model number) might help. Depending on the type of unit you have, you may be able to measure the bias voltages prior to putting B+ on the output tubes, rather than try to estimate.

 

[brad44]

turn it down obiously! People have been telling me to do that with my stereo for years. Thanks

 

[brad44]

All's well after component change, but almost not.

Prior to installing the new components I accidently forgot to hook speakers to it and I powered it up to check and lower the bias. My mind was wandering and it ran for almost 5 minutes with no load (no source input either). I lowered the bias and switched it off and could kick myself.

I know its likely a common question here, but how much damage could I have caused running it without a load for 5 minutes?

Nothing smoked or sparked and after I installed the new components and reset the bias (with a load this time) it seems ok.

 

[gadget73]

if its not an unstable amp, none. No signal in means no signal out, no damage to worry about. If its a badly behaved amplifier that oscillates with no load, or if you had input signal then *maybe* the output transformer could be damaged.

You're probably just fine.

 

[primosounds]

Ive been going thru my amps looking for resistors that have drifted or failed. I found some in a bias circuit. I know changing them back to spec will change the current bias setting. Not knowing how that will effect the voltages I am a bit nervous.

How fast do I have to be on the trigger dialing the bias back or forward when I power it back on?

By figuring out the percentage the resistors have drifted, can I assume the voltages will be that same percentage off with the new resistors?

Tubes are very sturdy and even if the current draw of the underbiased tube was much more you still had a few minutes before something bad would have happened. So, even if you did not dial the bias voltage up you had a few minutes. Also the resistors in the bias circuit don't affect the bias voltage as much as the pot. So, even if the resistor value was 10% off the effect on the bias voltage would be small, about 10%.
Also, i will mention, in case you did not know, that measuring resistors in circuit is sometimes misleading. If a resistor is connected to a capacitor the resistor measurement will drift. To get an accurate reading you would need to lift or disconnect one of the leads. If the resistor is connected to ground then you would get an accurate reading.

 

[brad44]

Thanks, it didn't smoke, spark or die at the time and I did not have a signal going into the amp. In general, its silent at idle too.

I have always been obsessive of having a load on the amp when messing around with it on the bench.... it really hurts that I spaced out this time. I really hope it doesn't come back to bite me.

 

[gadget73]

if something were damaged, you'd probably already know it.