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.

Tube preamps have a blocking capacitor on the output. That should not leak any DC, or TINY amounts. However..... when you first turn a preamp on it will take a few milliseconds for the cap to charge, and there will be a very loud thump.

Preamp makers deal with this usually by relays on the output to mute it for a few seconds. Otherwise, they may deal with it in the manual: "Please let the preamp warmup for 10 seconds before turning on your amps".

The way an AC coupled amplifier would deal with DC is usually by an input cap, but since you already have a cap in your preamp, it is redundant.

If you are worried, measure it. Put a 30K resistor on the outputs of your preamp and use a volt meter set to DC. See what you find after the relays click off. .

Best,

Erik

Next you can get into the world of cap upgrades...:) :) :)

Best,


Erik

Talk to them, I am not sure how their caps are arranged, and it's most likely that these caps are in addition to the main cap. There's no way I'm aware of to make a single-ended preamp without a blocking cap. :)

The voltages inside the pre are around 300V and would easily fry electronics expecting a maximum range of +- 2V.  This is part of what a DC blocking cap would prevent.

Like I said, if in doubt, measure and listen.

Best,

Erik

So I just found a picture of the guts. As expected, it has two very large caps and a relay at the output. :) I think you are all set, but caps do fail, film caps VERY VERY rarely, so by all means, spend $20 on a multimeter with 100mV resolution and you can see it yourself. :)

If you measure any DC offset at all (relative to connecting the meter’s pins together) then contact the amp maker and ask them if it’s within tolerance.  Remember to use a 30K to 100K resistor while measuring, as this will be more realistic than the voltage across an open circuit.

Best,

Erik

Actually, if you wanted to get a very nice bypass cap, here's one

https://www.parts-express.com/audyn-true-copper-cap-010uf-630v-copper-foil-capacitor--027-150

You'd put it in parallel with your existing caps. Give 48 hours of play time.

I've not tried them in electronics but in speakers they are excellent. :)

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've never seen an AC coupled solid state preamp in the high end, certainly not one with an electrolytic coupling cap. 

I _HAVE_ seen lots of bi-polar electrolytics as coupling caps in pro gear though, especially amps. 

But my experience is all pretty random, and from a small sample. I'm sure examples of all of these exist. 

Best,


Erik 

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