I'm into blues guitar amps that can cross that line from clean to dirty with how hard I pick and get complex overtones with chords and bending simple chords.
I have with this exact champ clone:
http://www.stf-electronics.com/
I'm drooling over this:
http://angela.com/images/Super_SE_6V6_Amp.gif
I started accumulating parts to add to hop-up the champ in that direction.
But instead I decided to just leave the little tyke what it was meant to be, and make this beefier single-ended amp a completely new build. I have a good 15 watt Hammond output transformer. Plenty of caps. Pots. 12AX7. Sockets. No chassis yet.
Instead of the parallel 6V6's I already have a single KT120.
Next step is to choose a power transformer. Some people rave about the "brown" tone at lower volumes from a low B+ and the compression of poor regulation. Others claim the best bluesy distortion is best when the B+ plus is high and so is the grid bias and drive. I have the wimp when I need it dirty at lower volume. I already have a nice solid state diode bridge and appropriate heat sink, so I was thinking I'd just use that for the B+. So I need 12v filament and B+ windings, and don't need another 5v winding for rectifier filament.
So what's a good B+ target voltage and grid bias voltage for a KT120 running single-ended? I don't want extreme screaming metal distortion with constant extreme compression, so I don't need more preamp stages, assuming the two stages of one dual 12AX7 will drive the KT120 decently.
I have with this exact champ clone:
http://www.stf-electronics.com/
I'm drooling over this:
http://angela.com/images/Super_SE_6V6_Amp.gif
I started accumulating parts to add to hop-up the champ in that direction.
But instead I decided to just leave the little tyke what it was meant to be, and make this beefier single-ended amp a completely new build. I have a good 15 watt Hammond output transformer. Plenty of caps. Pots. 12AX7. Sockets. No chassis yet.
Instead of the parallel 6V6's I already have a single KT120.
Next step is to choose a power transformer. Some people rave about the "brown" tone at lower volumes from a low B+ and the compression of poor regulation. Others claim the best bluesy distortion is best when the B+ plus is high and so is the grid bias and drive. I have the wimp when I need it dirty at lower volume. I already have a nice solid state diode bridge and appropriate heat sink, so I was thinking I'd just use that for the B+. So I need 12v filament and B+ windings, and don't need another 5v winding for rectifier filament.
So what's a good B+ target voltage and grid bias voltage for a KT120 running single-ended? I don't want extreme screaming metal distortion with constant extreme compression, so I don't need more preamp stages, assuming the two stages of one dual 12AX7 will drive the KT120 decently.
Any and all other advice appreciated.
I've also got a bunch of other big amps, so this is strictly for hobby fun.
I've also got a bunch of other big amps, so this is strictly for hobby fun.
Never seen one of these tubes before, 60W plate dissipation! How about just following the datasheet example, Vp=400V, Vg2=225V, Vg1=-14V and you get 20W already without even breaking a sweat ;-) But it's 6.3V filament not 12V...
Good point about the heater, before I order the wrong transformer. And that seems like a good clean place to start. But I'm not sure it's going to make for a great guitar amp. There's one sure way to find out!
Somebody on-list at diyaudio.com made some crude curves, apparently with either the load or supply very resonant and a sine signal, and ended up with some lumpy periodic waves in the resultant curves. So at diyaudio there are a lot of people who got the erroneous impression the KT120 isn't very good. However, nobody else with good technique has been able to reproduce these "problem" curves as any worse than a KT88, and the general consensus outside of diyaudio.com is that the KT120 is an excellent audio tube approximately like a KT88 but with nearly 50% more plate area. So there's more capacitance because it's like a KT88 and a half so it doesn't operated into RF as well, and the heater requires more current because it's longer. But the bass can be phenomenal for a tube. Some people realize that it can dissipate more heat and handle plenty of voltage, so they think you need 600V to get more power than a KT88, but a lower-impedance load seems to me like a nice way to get more current thru it instead.
Somebody on-list at diyaudio.com made some crude curves, apparently with either the load or supply very resonant and a sine signal, and ended up with some lumpy periodic waves in the resultant curves. So at diyaudio there are a lot of people who got the erroneous impression the KT120 isn't very good. However, nobody else with good technique has been able to reproduce these "problem" curves as any worse than a KT88, and the general consensus outside of diyaudio.com is that the KT120 is an excellent audio tube approximately like a KT88 but with nearly 50% more plate area. So there's more capacitance because it's like a KT88 and a half so it doesn't operated into RF as well, and the heater requires more current because it's longer. But the bass can be phenomenal for a tube. Some people realize that it can dissipate more heat and handle plenty of voltage, so they think you need 600V to get more power than a KT88, but a lower-impedance load seems to me like a nice way to get more current thru it instead.
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Wild idea: take a PT with, say, 400Vac CT. FWB will give you 560Vdc unloaded, and 280Vdc on the CT. Add to that an OPT with several secondaries: 2k to 8 ohm, hook the 8 ohm speaker to the 4 ohm tap and the KT120 sees a 4k load.
Take your pick. Want high voltage, high impedance? Low voltage, low impedance (high current)? Low voltage, high impedance (low power)?
Take your pick. Want high voltage, high impedance? Low voltage, low impedance (high current)? Low voltage, high impedance (low power)?
I've also got some 50 watt rheostats. Can't remember the resistance value... Maybe I could just use one as a drop. I don't think I'll get much good power supply 'sag' effect though because everything is class "A". But it would probably be a useful control.
I went in an unexpected direction. I had an old old teensie amp that had no power transformer (rectified wall voltage directly)...not cool. So I took the small power transformer and small output transformer and installed them onto that chassis and made it into a little champ-like circuit, and put a larger power and output transformer onto the old Champ clone chassis, which will go into a head cabinet I had laying around. Less work, 2 nice amps.
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