Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on May 10, 2017, 08:12:42 am
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I saw a peavey 6505 piranha at my local music shop and noticed it was just a single 12ax7 inside. No schematics online (curse the corporations) but the operating manual says it takes a +/- 30vDC supply. I also noticed there were no big death caps (saw mabey one cap looked like 2200uf) so im assuming its running the 12ax7 off the 30v supply with a SS power amp.
Can someone point me in the direction of a high gain single 12ax7 design like this? I want to use it with a 5w LM384 power amp chip.
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I know there are 12v projects that use a 12A*7 family tube, but seems they are used in pedals, not for preamp
however I think that is reasonable to think that they use a quadrupler so the voltage that feed the plate of the
tube can be a normal Haigh Voltage we see in many preamps
In your project you can adopt the back to back transformer configuration that is used on Doug's tube pedal
Doug also sell the small transformer and wall supply that are required
here you can see how the thing is arranged
http://el34world.com/projects/tube_box_3.htm (http://el34world.com/projects/tube_box_3.htm)
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... high gain single 12ax7
On a single 12ax7 you have 2 triodes, for a real High Gain I think you need, at least, 3 triodes, 1 more than what you have in a single 12ax7
EDIT:
May be you can obtain what you want using a 12ax7 in cascode configuration followed with a mosfed configured as Source Follower to drive a Tone Stack
http://www.valvewizard.co.uk/cascode.html (http://www.valvewizard.co.uk/cascode.html)
Franco
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I've find this schematic (very far to be a single tube preamp :dontknow: )
https://images6.static-thomann.de/pics/atg/atgdata/file/diagram/180482_schaltbild.pdf (https://images6.static-thomann.de/pics/atg/atgdata/file/diagram/180482_schaltbild.pdf)
Franco
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I agree with Franco, that "high gain" one tube preamp is not really a common design. Most of the one tube devices I have seen are running low voltage and are aptly named 'overdrive' or 'distortion' because that is what they do to your signal. I followed the link to Doug's two tube design and it looks quite nice and it has plenty of tone shaping for preamp type usage. If it were me, I would build a small SS amp using a complete design with preamp and power amp chips, then put a one tube overdrive pedal in front. That way you are building two proven designs which should produce stable, usable amplification.