Dc voltage sent to my speakers

If a preamp is outputting a little DC, is it always because of faulty output coupling caps?  Should those caps always be expected to block all DC?  I had one tube preamp that would trip the protection circuit of one of my amplifiers due to DC leakage, and I'm wondering if replacing the output caps would have fixed the issue.

@erik_squires @ketchup Unless the coupling caps are really old, its unlikely there's any DC at all at the output.

However, when you start up a tube preamp, there is a voltage rise on the input side of the coupling cap. A corresponding rise will occur at the output which follows an exponential curve. Usually a resistance is supplied at the output of the preamp to discharge the part as the tube warms up.

So if the preamp is turned on after the amp is already on, the amp is in danger! That is why the common protocol is the preamp of any system (tube or solid state) is always turned on first and allowed to stabilize prior to powering up the amp! It really doesn't matter if the preamp is tube or solid state in this regard.

@atmasphere  My experience with tube preamps is very limited, but the one or two that I played with had a tiny bit of DC on the output.  In the range of 5mV or less.

So if the preamp is turned on after the amp is already on, the amp is in danger! That is why the common protocol is the preamp of any system (tube or solid state) is always turned on first and allowed to stabilize prior to powering up the amp! It really doesn’t matter if the preamp is tube or solid state in this regard.

@atmasphere , this is pretty much what I do now-a-days, but how much time should it take the preamp to stabilize?

As far as understanding electrical theory, much beyond which wire goes to where is beyond me, but here is what I have:

the separate power supply of my SLP-05 has two meters on it, and one is supposed to read "210 vdc nominal" but usually reads ibetween 230 to 240, and the other is supposed to read "60 mA dc nominal" and usually reads between 55 to 60. After turning the preamp section ’on’, it takes a couple of minutes for the meters to come up to within the paramters I just listed and then a couple of more minutes for a red light on the front panel to illuminate which theoretically means it is ready to play.

(My amp has two switches on the front panel, one power switch I am assuming provides power to the four large ower caps; according to the manual "left and right bias voltage and high voltage" and the second switch is what gets the tubes glowing. After the preamp meters and red light say it, the preamp, is ready to go, I normally turn the first switch of the amp on, but then I usually wait about 20 to 30 minutes before turning the second switch on & lighting up the tubes.)

Is my amp at risk the minute I hit that first switch, and would you think that I should wait longer on that? Thanks.

My experience with tube preamps is very limited, but the one or two that I played with had a tiny bit of DC on the output.  In the range of 5mV or less.

@erik_squires Some preamps like the Dynaco PAS3 make a small amount of DC like that but its not on account of the output coupling cap. The circuit employs feedback to the cathode of the input stage so there is a small amount of DC on the output as a result. As you already know though, its harmless.

this is pretty much what I do now-a-days, but how much time should it take the preamp to stabilize?

@immatthewj Normally about 30 seconds; your preamp is an exception. But if you're talking about a tube power amplifier its not a concern since the tubes can't be damaged that easily. 

Thanks, @atmasphere  , I guess I am going a few extra miles then.  I will worry no longer.  Thanks again.