DC issues using a Krell KSA 150 w/ tubed preamp?

Not true Atmasphere, your amps may be of a better quality but I experienced a reasonably high DC offset on a modern preamp that retailed 8 grand; the coupling cap was indeed the culprit. So always measure with a VOC meter which can be had under 20 bucks.

Atmasphere explain that to C/J. I just got done working on a Prem140 that had Polystyrene film caps at the input and output of a 440Vdc regulated power supply. Every single semiconductor device including 15 zener diodes failed dead short because one of those film caps instantaneously shorted and healed. I've also had to replace coupling caps in McIntosh and Dynaco amps and preamps due to either shorted or leaking coupling caps. It happens far more often than you think, especially in older gear.

Polyglot, if you encountered such a thing then the preamp is in need of service. No preamp, tube or solid state, will have any DC at its output. The simple fact is that as a designer you can't count on the amp being used to have a coupling cap at the input. Many amps have direct-coupled inputs so any DC could result in DC at the output, or at the very least, excess distortion.

So all preamps if functioning correctly will have no DC at the output. If otherwise, the unit is either malfunctioning or a bad design, and in either case should not be used. If a manufacturer tries to convince you otherwise, turn around and run as hard as you can.

HiFiGeek1, FWIW, film caps don't short and then 'heal'; that is a paper and oil thing. It should be noted though that if subjected to a voltage above their rating, any cap can arc.

However your example is not one that should be included (although I am sure it must have been a disappointing event). It sounds like you had a power supply failure, not a failure of a coupling cap at the output of the preamp.

I've worked on a lot of vintage gear and shorted or leaking **coupling** caps are quite rare (unless we are talking about vintage solid state- the coupling caps there are often electrolytics). OTOH this is quite common with filter capacitors.

Any preamp, tube or solid state, is capable of failure... what this topic is about is whether a tube preamp in good nick is going to put out DC, and the answer to that is no.

Dynaco and Mac preamps, if vintage, may well be over 50 years old and are excused, as if one is serious about using such gear, it has to be properly serviced out (as I am guessing you now know).

Hifigeek1: what brand were the film caps in your Premier 140? Did your tech report that the faulty origin was indeed the cap? My preamp was a CT-5 and luckily that was still under warranty. Should I worry for my LP140s?

Polyglot I am a tech and a Warranty Repair Station for ARC. The caps that failed in the C/J were C/J's own branded polystyrene caps. I'm sure they were not made by them. Their tech advised me that this series cap do indeed short and then heal themselves. Their update includes removing all those particular caps from the amp and replacing them with updated parts. I would contact C/J and ask them. My friend and great guy Ed who worked their for decades, retired and no longer works there in customer support. I don't know if you will be able to get any information from them.