Do all tube preamps leak DC?

atmasphere, thats a remarkable statement and it makes sense as well. If this is true then the reviewer over at 6 moons is full of it.

He might have got that from Krell. Its a rumor and nothing more.

It's been a very long time since I touched a tube preamp, but as I recall, the film caps do have a smidgen of DC leak. Maybe 50uV was closer to the truth, and yes, film caps will tend to have less DC than SS state pre's.

I work on tube preamps all the time. Right now I'm restoring a very nice condition Marantz 7c. Like most tube preamps, it employs a film coupling cap at the output. There is not even a 'smidge' of DC Offset. If there were, the problem of DC Offset would have been documented a long time ago, but it was not until Krell put that comment in their manuals that this rumor got started! Similar caps are often used in the output section of tube power amps, where small amounts of leakage can cause a lot of problems with the bias on the power tubes.

The cap at the output of any tube preamp has a resistor to ground at the output. Typically this is about 1 meg ohm, and is there to discharge the capacitor due to the effects of the voltage applied to the other side of the cap during warmup. It can be rather high in some cases (several hundred volts, although not in the case of the Marantz), and if there were no resistor to discharge the cap it would sit indefinitely at the B+ voltage or at whatever voltage the cathode follower at the output of the preamp runs. The resistor discharges this voltage (as does the input resistance of the amplifier) and as a result, the preamp after it has warmed up has no DC at its output at all. If you measured something, it was because of a measurement error or the preamp was not stabilized or the cap was actually bad (the only real case of this I have seen is in Fisher preamps, the ones that have those black coupling caps that otherwise look like Orange Drop capacitors).

Some preamps (tube or solid state) don't have regulated power supplies. If the AC line voltage were to vary slightly, such preamps might appear to put out a slight amount of DC but if one were to study the phenomena a bit closer it would be found to be a bit low frequency AC noise. To be clear here that's not a phenom of tubes, its a design issue.

Well then, I stand corrected! :) 

As I said, it's been a very long time, and quite possible I'm confusing the behavior of electrolytics. :) 

Good to have those with more and more current experience here to correct me. 

Best,

Erik 

Hi Paul,  3 years ago Ralph Atmas-phere convinced me to open up my tube/solid state hybrid digital player that I run directly to my modified krell 700cx amplifier to prove that I had no DC leakage,  as a matter of fact, I have two coupling capacitors behind each tube,  krell once accused my player was the fault of on going problems with my amplifier,  it took Dan D’Agostino himself to tell me by speed dial on my phone to share with me what was actually wrong with my amplifier after modifications,  it was two heat sensors on the left side of the amplifier,  the amplifier has a total of six,  three on each side,  when I told krell how to fix the problem with the amplifier,  they wanted to know how I knew that,  of course I could not divulge who told me,  any way,  Ralph is a good friend of mine,  when it comes to this subject here of DC leakage,  I learned every thing I know on the subject from him,  I assure you Paul,  you can take what he has said as the gospel! 

Hi Keith, I believe you are correct about Ralph & would agree that he is very knowledgeable. It is nice to know that we have people like him to provide reliable info on most given subjects (questions) but you are also in the "A" Class, an asset to the audio community and a hell of a good guy to boot.