Since there was at east one other person interested in the response, I've posted what Ron emailed to you here as well:
I recently purchased a very lovingly cared for F1X preamp. It was everything I had heard about and more! I absolutely love the tone! I was in heaven jamming with it for a few days prior to gigging it this past Sunday. That's when I discovered a problem: when we ran the DI to the PA, there was a very noticeable hiss. The hiss was not present when playing the F1X through my rig, only after running to the house.
the signal path is that the DI buffer is fed from the full-range output so if the signal-to-noise ratio is ok on the full-range output there should be no difference on the DI output.
It's a white noise sort of hiss, not a 60 cycle hum or anything like that. Lifting the ground did nothing. We then unplugged the F1X and tried my SansAmp and Stewart DI boxes, and the noise disappeared from the PA in both cases. Plugged the F1X back in, and the hiss was back.
the signal level on the F1X DI output is much greater than some other DI boxes. so you'll hear more noise if you just change from one of the boxes you mentioned to the F1X. just checking here, since you mention below that you tried it with different basses, but you need to play the bass and set the levels in the PA based on the output level from the different boxes, and then listen for noise...both the signal and the noise may be considerably greater in the case of the F1X, but the signal-to-noise ratio should be acceptable. that is, you'll be running the PA input at a lower gain with the bigger signal from the F1X and so the noise will also be turned down...the noise level at this lower gain should be satisfactory.
I tried both of my basses (Fender Jazz/passive pups, Custom 5 string w/active humbuckers), same story. The eq settings on the preamp are at 3-8-3, with no high or bass boost. I really love the sound I get with it, but the noise through the PA is pretty noticeable.
it was reading this that made me think that you had covered the situation properly and that you did have the PA gain adjusted to the proper level for playing the basses and were not comparing the noise without adjusting the gain on the PA input.
Is this just a tradeoff for using ! a tube preamp (it's my first tube anything), or is there anything I can do to eliminate the noise?
no, the noise can be greater or less with tube gear. some circuits have more noise than others, and sometimes there are circuits that are inherently more noisy when they give a particular tone. but in general, some tube gear and some solid-state gear is quiet and some is more noisy.
to understand the set up you were using, did you have the direct boxes you were comparing with patched in between the F1X and the power amplifier? this is the place that you should have them if you want to have the effect of the tone controls included (and the F1X of course in the 'POST' position)
if the F1X is working properly you should not have a poorer signal-to-noise ratio at the DI ouput than at the full-range output. there is a dual op-amp in the F1X where one of the op-amps amplifies the signal after the
front-panel volume control and feeds the direct output. the other op-amp in the package is a buffer amp with only a gain of 2 that feeds the transformer that is connected to the XLR connector on the front panel.
usually if there is a problem with the op-amp, it will just be dead, but it could be defective and cause noise. if it needs replacing, it is fairly easy, since it is socketed, so you don't have to do any soldering to replace it.
let us know if you feel the signal-to-noise ratio is worse on the XLR output compared to the full-range output, and then we can either supply you with a replacement op-amp or if you'd rather, you could send the unit back in and we could replace it for you.
