Background noise

I use a MC Sumiko Blackbird through the MM phono section. The Blackbird is 2.5 mV, and the Shindo dealer says the minimum MM should be 3 mV.

OK, now we are getting somewhere. You probably have a noisy tube or tubes that is exacerbated by being below the minimum recommended input voltage, therefore needing to crank up the gain. Its also possible that you may have a loading mismatch by going into an MM phono stage (typically 47k ohms), but I'm not sure what that would sound like. See if you can get some phono grade/v, low noise tubes from a reputable dealer and report back.

I noticed a fairly high level of background noise when I turn up the volume without playing music (and this doesn't change by unplugging the interconnects). When the volume control is at its maximum level, you even listen to a little whistle. Is it normal for a tube preamp or do you think there is a tube problem or another issue?

My tube preamp is absolutely silent with the volume all the way up and putting my ear against the speakers. Of course, everything is turned on. Considering that my preamp has six tubes, I was very surprise by this total silence.

I'm not sure what the problem would be, but it is not normal to have such level of noise. Since yours is a second-hand unit, I would probably start by replacing the tubes.

Best,

iSanchez

Swampwalker, actually there is no loading mismatch, but I think your diagnosis is correct -- noisy tube, exacerbated by being below the minimum input voltage.

I'll look into it and let you know. Thanks.

I agree with Swampwalker.

First, a MC cart is not intended to be used in a MM phono circuit.

Second, if the minimum is cartridge output for the phono section is 3mV, and if your cart outputs 2.5mV, then there is definitely a mismatch which will be exacerbated by the phono circuit's amplification.

I'd suggest trying a MM cart, even an inexpensive one, to hear what improvement in the background noise might be gained.

Tvad sez, "First of all a MC cartridge is not intended to be used in a MM phono circuit"

To pick a nit....there is no difference between an MM and an MC circuit except for its ability to respond to the loading and gain demands of the cartridge. There are MC cartridges that are high gain and load at 47K (or up to). A flexible phono stage with adequate gain can handle both types of cartridges (my SP10 does it quite well). But as with Ggavetti, using my .9v output Benz produces tube hiss when you run up the volume to LOUD. It is basically noise free with high output cartridges, MC or MM. Low output cartridges need not apply.

BTW, while tube selection is critical, noise free tubes will not overcome tube rush caused by inadequate gain.

At least I think so this morning. :-)