DIY All-Tube Phono Preamp Project
Part 3, Metal-Working for Dummies
By Steve Graham, chief dummy
Working with fire and steel? Would you believe alcohol and aluminum? Bonus points to those who can name the 80’s pop group that issued the album shown above. No cheating searching the title online.
Before we start torturing some metal: If you are sitting on the fence, wondering whether or not to build one of these for the following reasons:
- You think this preamp won’t have enough gain because it uses a 6DJ8 for the first stage (a 12AT7 is used for the second stage): Most line stages have plenty of gain so it takes some of the gain burden off of the phono stage. The preamp my audio buddy built is the same configuration we are building here. His has lots of gain even when used with my system which has only moderated line stage gain.
- You just don’t like tubes in the 6DJ8 family and would prefer to use a tube in the 12A?7 family: I’ll accommodate you, sort of. I’ll describe the option of using 12AT7s for the input tubes. See the supplementary parts list attachment below.
For those who would like to use a 12AX7 for the first gain tube, my advice would be: Don’t go there. You only “gain” about 2.5dB more gain going to a 12AX7 versus a 12AT7. In my experience you “lose” reliability and “gain” a lot of unwanted noise going the 12AX7 route. I’ve found both NOS and new production 12AX7 tubes to be noisy – of the hiss and spit variety – and short-lived. My prototype shown in pictures at the beginning of this series uses Russian Tung-Sol reissue 12AT7W/6201 tubes and they have, so far, been quiet and reliable. I’ll make those wanting the “all-12AT7” option their own board assembly instructions in the next installment of this series.
One more thing before we start. One possible source for internal wiring might be the Connex house brand of cables sold by Partsconnexion.com. I haven’t actually used it, and the pictures on the website don’t show much detail, but it looks similar to some cable I’ve employed to good effect in the past. There are two PTFE (Teflon) insulated conductors inside a shield and it’s available in two sizes, 24 gauge, that might be suitable for signal wiring, and 18 gauge, that might suit AC wiring and power supply connection to the amp board. At between $1.80 and $2.50 US per foot, it might be worth a shot – and Teflon-insulated wires, though slightly trickier to strip, are far less bother to solder.
Open and save the four, “Metal-Working for Dummies”, attachments. Perform these in order. This is where we’ll start to make some metal chips. The tools required are specified at the beginning of each attachment. If you wish, print these and all subsequent attachments and check off the steps as they are performed.
Next time we’ll heat up the soldering iron and assemble the circuit boards.