I'm brand new to the RoundTable.

I have a Presonus BlueTube DP mic preamp that uses a 12AX7 in a dual channel configuration. There's nothing "wrong" with the unit. The existing tube doesn't have any visible markings, so I'm not sure about the OEM brand.

I wanted to try some different tubes to fine tune the sound and perhaps derive a little more gain. I'm looking for a smooth, not gritty/grainy, warm sound since I will be using this exclusively as a mic pre for a pair of AT 3035s. The unit has a 16V power supply and I assume that the tube is powered at that voltage (which would affect the choice of tube?). I'm looking for some suggestions, including using a slightly different tube - 12AU7, etc.

I have done business with BOI Audioworks before and have been very happy with the experience. I say this because I expect to buy from them and get matched triode tubes for consistency between the two channels.

Thanks to all for your help and suggestions.

I quickly looked at the manual for your pre and I doubt they are running that 12AX7 at 16V, but I may be wrong. Typically they are gonna run at a much higher voltage (100-250V) than that to optimize it's performance. You should stick with 12AX7's if that's what the manufacturer recommends. 12AU7's and their variants are gonna give you considerably less gain, plus the circuit could damage them or the whole pre if not set up to run them properly. A 12AX7 is a high gain tube. And since we don't have a schematic of the pre, it's really hard to recommend anything. FWIW, other 12AX7's may very well give your pre a different sonic flavor--and they are usually not all that expensive unless your going for Telefunkens or something.

My suggestion: call the manufacturer and discuss with them tube rolling in depth. Nobody like the designers to let you know what you can use and what you can't.

Go get'm.

• Duncan's