One of the main purposes of coupling capacitors at the output of tube preamps is to block DC. A properly designed tube preamp running at spec should be putting out very little DC. I measured mine and it is less than 5 millivolts when fully warmed up. Best advice is to let the tube preamp warm up 1/2 hour before taking it out of mute. When cold, the DC voltage is around 25 millivolts. So, do measure the voltage at the RCA jacks on whatever model you wind up getting.
Krell FPB 600c 700cx 400cx Heatsink noises.
In 2001 I bought my complete new Krell cast music system: Krell FPB 600c amplifier, Krell KCT preamp and Krell KPS28c cd-player .Siltech cast interlinks. Speakers are JM Lab Utopia with Transparent ref.XL loudspeaker cables. However I certainly have the feeling that all the manufacturing dollars have been spent on sonically critical design elements,there is a mechanical problem,which is bothering me since the moment of purchase.Two or three times during a listening session the metal construction of the heated amplifier cabinet in operational mode makes a sound (tsssjakkk).Probably because the difference in heat of the heatsinks and/or heavy gauge alluminium chassisplates. When I bring the amp in stand by position after listening,this mechanical sound is there also ,several times. I phoned the Krell service department in 2002.Irving Gross told me that it was a diffraction problem.(traction and contraction of the heated amplifier cabinet.) The service department advised me to tighten all the bolts of the heatsinks. It did not help. During the upgrade from 600c to 700cx,end 2002, I told the German importer about the problem. He admitted that this diffraction problem was a tough one to solve.I had the hope that the problem would have been gone after the X-Upgrade but unfortunately it did not. In the english Hi-Fi News magazine of december 2005 (page 28/Howard's Way) I read an article about a former owner of the Krell 400cx."I sometimes got noise from the heatsinks through heating and cooling." He finally could not live with it and sold the amp. My question is ;does anyone has the same problem with this Krell FPB amplifier(600c,400cx or 700cx)? More important, which owner has a smart suggestion to eliminate this undesirable mechanical'vibration'. Otherwise...I figure I have to live with it, which I can because I simply love the sound of my complete Krell cast system.