Tube Radio
Super Member
I'm thinking of building my own vacuum tube preamp as I cannot afford most of what's out there. What complicates this is the fact that I need to have a cathode follower output in the signal chain before the volume control for use with an equalizer which shouldn't be too hard to work in. I'm thinking maybe a combo of 12AX7's for the amp sections and 12AU7's for the cathode follower sections. Two of each should suffice as I'm only seeing the need of four audio stages to make this work properly. I may also add input switching if I can find the right kind of multi position switch. I would like input as how to go about doing this without spending a lot of money while making it sound good. I'm not seeking perfection just something that's flat from 20 Hz-20 Khz. What I may do for the outputs that go to the equalizer is select the cathode resistors of the cathode follower stage to equal the input impedance of the equalizer to insure most efficient signal transfer Which is why I'm making the output impedance of the preamp 100K if possible to match the impedance of the amp which is a 100K dual gang audio taper pot. I don't need tone controls since I have the equalizer. I'm thinking maybe two 12AX7's and two 12AU7's unless there are better tube recommendations. I would prefer to use current production tubes and not tubes no longer being made. I have a new Hammond 270CAX power transformer to work with which will save a little on cost and I'm pretty sure with all the stuff I have around here that I can dig up a good chassis. With a few flea markets antique shops around here if I don't have a chassis I'm pretty sure that I can find one preferably with controls mounted on the front so I won't have to drill any holes in the front to keep a nice appearance. I'm thinking of using 1/4"" T/S jacks for the connections simply because when I've used RCA jacks that are held to the chassis by a nut I've always had a problem of them eventually coming loose. I do plan on using a gain control after the first stage before the cathode follower output to the equalizer which is also where I will put an output and input for my computer's sound card which will be similar to a source/ tape monitoring switch found on most three head reel to reels with the addition of a switch to interrupt the signal going to the computer if so desired. Basically that will allow me to listen to my sources going through the computer or just straight to the equalizer and also I will be able to play my music on my computer as well. I have a homemade box that does just that so I may take that citrcuit and copy it for the preamp. Also should I put the volume and gain controls after the gain stages before the cathode followers or before the gain stages. Impedance on all inputs will be 100K except for equalizer input which will be 600 ohms. Nice feature about my equalizer is I can view either the input or output signal level (Beringher Ultragraph FBQ Pro FBQ 6200) which will make setting the gain pot on the preamp and output level pots on the equalizer a lot easier. In the brief glance around my 12' by 12' building I didn't see anything usable as a chassis without destroying a good piece of electronic equipment so off to the flea market this weekend to see what I can find. What would be ideal is something rack mount as I have a rack that a Hammarlund SP-600 communications receiver sat in (receiver is now on shelf above workbench) which is where the equalizer is mounted in. I'll probably play around in Microsoft paint and get a rough schematic drawn up. When done I will post it here. Any ideas would be a big help as it's my first attempt at a DIY preamp. I'm planning on using it with this amp. read about it here http://www.audiokarma.org/forums/showthread.php?t=103661 This amp requires higher than normal input voltage for full output as it was made for a ceramic phono cartridge. If this is a success I will post another topic about the preamp so others can build it if they so desire.
so anythings possible. First I would need to find a chassis with a few 9 pin miniature tube sockets already mounted. For the tubes in the MM phono preamp section I will use a regulated 6 volt supply so as to minimise hum. I find it easier to build the basic design and try that out to see what components I need to change to higher or lower values. Given I'm most likely using the 12AX7 and the variable resistors I will use for the gain control and volume control are 100K I will most likely use 100K plate resistors on the line level preamp tubes and a bypassed 1K cathode resistor also. On the cathode follower outputs to the equalizer I will use 600 ohm cathode resistors to match the equallizers input impedance which means I will have to use a lower voltage for that dual triode so not to excede its cathode current ratings.