SUT With a Manley Steelhead RC?

In all cases, at all settings, my Steelhead is very quiet.  So (Snackey), I am wondering whence comes the noise you say was bothersome when you set gain at 65db via the MM inputs. Try reducing the gain setting in 5-db increments until you get to a point where noise is not a problem but gain is still sufficient. Was the volume control over past the 12 o'clock position?  If so, that might suggest you won't have luck reducing the gain setting, as I just suggested. I cannot recall ever having the volume control past 1 o'clock, even with very low output cartridges, but that speaks to the input sensitivity of my amplifiers.  Also, try listening to a variety of LPs, to be sure that the noise you heard did not emanate from a dirty or particularly noisy LP. Before this turns into a SUT symposium. If you do hear noise, listen to determine whether it is in both channels or just one.  If in one channel, that might indicate a faulty tube,

The biggest improvement I made in my Steelhead (and it was huge) was swapping out the original tubes to Siemens CCA grey plates and Bendix 6900s. Granted, the Bendixes are hard to find these days (also sold as Mu, I believe), but those changes eliminated any tiny bit of grain I might have perceived in the stock Steelhead. And the gain of the Steelhead is phenomenal. 

Per the specs of the Manley Steelhead, the maximum rated input voltage is 50 mV. Your MSL cartridge with a 0.5 mV output is stepped up to 10 mV with the 1:20 MSL SUT. Maximum cartridge output voltage can be as high as ten-times the cartridge’s rated output when playing LPs. Therefore, your MM phono stage should have an overload capability of at least 100-mV for the 0.5-mV cartridge. I looked at the specs for an Audio Research Ref 2SE, the maximum rated input voltage is 250 mV and I used higher turns ratio (1:26) SUT’s with 0.4 mV output cartridges and never had any issues. I think most phonostages have a higher max input voltage capability than 50 mV. Also, I didn’t see a single Steelhead user in this thread indicate that they use(d) an SUT. I would poll the Steelhead users to see if they use SUT’s, and if so, the turns ratio and output voltage of the cartridge. This also seems like an appropriate question for Manley. In the meantime if you can get hold of a lower rated MC < 0.24 mV, and see if the issue persists.

I’m by no means an expert on this subject, but all this is based on what I’ve read from a Steelhead owner that was using a Cotter Mk2L (1:70 ratio) with a 0.28 mV output cartridge and was having similar issues as well as my own experience with SUT’s.

I've been demo'd through Tube Rolling the 50's CCa and the 60's Holy Grail, along side same Brand and other Brands Tubes through to the 80's Production.

All Tubes were above Factory Spec, with each Pair being almost identical in their Test Readings. 

There are a few Brands from the 60's that have equal impression with a slight difference to the end sonic.

The CCa and Holy Grails are very Similar, extended periods of time used for comparison might reveal a little more than I detected. 

As a result of the cost required to purchase at the time of my acquiring Vintage Valves, I got a very respectable matched pair of Holy Grails, via two different suppliers at a steal.

I also to date, have acquired a non Siemens Brand from early 60's Production as well, both flavours were seen as equal in value. 

rdk, where did you get the 50mV spec for phono overload? Since the Steelhead has both MC and MM inputs, and since MM inputs generally tolerate much higher signal voltages than MC inputs, there ought to be two different specs for phono overload, one for MC inputs and one for MM. The output from any SUT ought to be connected to MM inputs, and the gain set for the minimum 50db.

By the way , using a 0.5mV cartridge with a SUT that has a 1:20 gain factor into an MM stage that provides 50db gain is way overkill so far as total phono gain. Most MM stages proved 38 to 45db gain, typically 40db. 1:10 ought to be more than sufficient, with that cartridge into the Steelhead’s MM stage.