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I'm preparing to tryout building a small 1-5W amp out of junk. I don't have many suitable high mu triodes, but I've got loads of med-low mu triodes, and lots of small signal RF pentodes. I read on valvewizard.com about how you can get different distortion in the preamp triodes depending on whether they are hot or cold biased. I also noticed you appear to get more distortion out of high current, low gain triodes like a 12AU7.
So, I was thinking of a clean pentode first stage that would overdrive a med-mu triode. I'd try to keep the pentode totally clean and undistorted so it doesn't color the sound, then pass the signal through 3 stages of cold biased low mu triodes, dumping most of the gain and driving each successive triode just barely into clipping so I get a nice bluesy overdrive. Then, for the output, a single ended 6GC5 with a gain control pot that I can go from clean to overdriving the hell out of it. I can play with the bias to get the right amount of edge out of it. Unfortunately, I don't have a SE OT handy. I'm going to get a little Hammond 125BSE in the future, but for now, I have a 12.3 VCT power transformer. I calculated that with 8 ohms on this side, the mains side would present 2.2k to the 6GC5, which should work well for it. I'd connect it resistance coupled with a DC blocking cap.
Once I get that up and working, I'd like to add a cathode follower stage from a med-mu triode to drive a three knob tone stack. So, if I use twin triodes, 2 tubes will give me 3 distortion stages and one tone driving stage.
Tube complement 1 6688A or 6AU6A, 2 6GU7 or 6BQ7, 1 6GC5
I'd appreciate any comments at all, or leads of known schematics to look at for inspiration. I know the AC15 and the Gibson Les Paul Jr used pentode input stages.
I'm especially interested to know if anyone else has tried a clean pentode first stage for gain to overdrive a small signal triode.
Finally, what do you guys use to draw schematics?
Thanks!
So, I was thinking of a clean pentode first stage that would overdrive a med-mu triode. I'd try to keep the pentode totally clean and undistorted so it doesn't color the sound, then pass the signal through 3 stages of cold biased low mu triodes, dumping most of the gain and driving each successive triode just barely into clipping so I get a nice bluesy overdrive. Then, for the output, a single ended 6GC5 with a gain control pot that I can go from clean to overdriving the hell out of it. I can play with the bias to get the right amount of edge out of it. Unfortunately, I don't have a SE OT handy. I'm going to get a little Hammond 125BSE in the future, but for now, I have a 12.3 VCT power transformer. I calculated that with 8 ohms on this side, the mains side would present 2.2k to the 6GC5, which should work well for it. I'd connect it resistance coupled with a DC blocking cap.
Once I get that up and working, I'd like to add a cathode follower stage from a med-mu triode to drive a three knob tone stack. So, if I use twin triodes, 2 tubes will give me 3 distortion stages and one tone driving stage.
Tube complement 1 6688A or 6AU6A, 2 6GU7 or 6BQ7, 1 6GC5
I'd appreciate any comments at all, or leads of known schematics to look at for inspiration. I know the AC15 and the Gibson Les Paul Jr used pentode input stages.
I'm especially interested to know if anyone else has tried a clean pentode first stage for gain to overdrive a small signal triode.
Finally, what do you guys use to draw schematics?
Thanks!
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