Since this project was posted, several people have suggested that I post a version of this preamp for other tubes. There have been a lot of suggested tubes and, for various reasons, I had put this off for a long time. However, Chris65 gave me the suggestion recently that I post a version for the 6v equivalent dual triode tubes. These are the 6AU7, 6GU7,
6CG7, & 6AX7. The thing is, all these tubes will work in this preamp with the same circuit values. There is only one hitch; the heaters are 6.3v, can be up to 600mA, and don't use pin 9 in the 6.3v configuration. Here are the various basing diagrams.
Attachment:
basing diagram 2.jpg
So with a little rewiring of the tube socket, these tubes can be accommodated in this design with no other changes. Pursuant to the 600mA heater requirement, of course.
Here are the performance numbers for these tubes:
Attachment:
Variant Performance.png
Finally after looking at this, I thought that since the components are all the same, a simple DPDT switch could be added that would allow the preamp to be used with
both sets of tubes. All that needs to be done is to switch pin 9 out and switch pin 5 from one side of the heater supply to the other in the 6**7 version. So here is what that switch wiring would look like.
Attachment:
Heater Switch s.jpg
When the switch is in the left hand position, the heater is wired for the 12A*7 family of tubes (i.e. pin 9 to one side of the heater and pins 4&5 to the other), and when the switch is in the right hand position, the heater is wired for the 6**7 series of tubes (i.e. pin 9 open and the heater applied to pins 4 and 5). Simple.
Now a single preamp can be used with the following tubes: 12AU7, 12AV7, 12AY7, 12AT7, 12AZ7, 12AX7, & 12BH7 in heater switch position 1 and 6GU7, 6AU7,
6CG7, 6FQ7 (electrically equivalent to the
6CG7), & 6AX7 in heater switch position 2. That's a total of 12 tubes in the same preamp without any circuit changes.
How's that for "Universal"?
Well, what do people think?
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