tube pre amp and Krell FPB 600

I've used lots of tube preamps with my 700c and never used the internal jumper and never have had a problem. The jumper defeats one of the better aspects of the amp by putting a capacitor in line with the signal. If a preamp leaks DC, I wouldn't want it. You can easily check the outputs of the pre by using a meter.

Rwwear I’ll defer to your expertise as you’ve actually used a tube pre with your 700cx and I decided to look elsewhere. But this was my exact dilemma just last month. I was looking at a tube preamp, either the AR Ref 3 or VTL 6.5/7.5. I contacted the preamp manufacturers as well as Krell and was told by both that in order to properly protect my FBP700CX I should absolutely engage the coupling capacitors when using a tube preamp. Unfortunately placing these caps into the signal path degrades the sound and largely defeats the design purpose of the FPB amps. It wasn’t that the preamp was going to leak DC constantly and at a constant rate that would necessarily allow an easy meter read at the outputs. It was that tubes are not the most stable device in terms of consistency; they change constantly in small amounts over their operation period and more radically over longer periods of time. This can cause spikes to occur during lets say turn on until the circuit settles out, this can be enough to fry an amp or a pair of speakers in the right circumstance. I was told that the later designs have much better turn on, biasing, and dc offset circuits then they used too and to that end the risk has certainly been minimized, but not eliminated.
If you do want to engage the coupling caps I have the procedure from Krell for all the FPB series amps. I can send it to you offline or you can contact Krell directly. They were very helpful and had lightning fast responses.
Let me know what you decide and how it works out. I may revisit the tube preamp dilemma in the future but for now I opted for a KCT.
Dave

Thanks GR. I am no longer using a tube pre. I am also friends with the Krell service manager.

When I talked to Krell about using the VTL 7.5 with my FPB650M monoblocks they said no problem at all. I did for awhile and the sound was great, and no problems. I used two other tubed preamps with these amps. Again no problems. Right now I am waiting for a custom made tubed preamp(Singlepower Audio) to go with the Krells. I have never changed over the Krells to "adapt" them for tube preamps. If your thinking about the 7.5 with your Krell, IMO they go very good together.

I compared two preamps over a local audiophile home who owned a FBT 300 amp. One was the tubed Modwright SWL 9.0 (w/ upgrades)and the other was a Belles 28a reference preamp solid state. There were no issues running the Modwright with his amp, but I did have to have caps installed in my Pass labs X-150 amp which is also servo I believe. All Pass amps have an input cap in them now to be safely used with tubed preamps. I don't think you'd have a problem. For what it is worth the Belles sounded better in his system. More depth, dynamics, soundstage and control over the Modwright, and you would not have to switch in any caps (if that is the case) in your amp to use it. Which would result in a cleaner sound.