Do all tube preamps leak DC?

Almost all preamps and amps have a tiny bit of DC offset. Maybe 5 mV or less for a pre, 60mV or less for an amp.

A preamp with 5 mV of DC at its output is rare. Our preamps have direct-coupled outputs and they have less DC Offset than that!

With **any** preamp, tube or solid state, it is advisable to turn on the preamp first and let it stabilize before turning on the amplifier. All preamps can exhibit turn-on thumps unless the manufacturer installed a mute relay.

Krell started the 'DC at the output' rumor with tube preamps; it was entirely false. Tube preamps usually have less DC at their outputs than solid state! This is mostly due to the output coupling capacitors used; tube preamps usually use a film cap at their output as stated earlier; solid state preamps often use electrolytic capacitors which are far more prone to leakage and failure.

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.