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5 w tube practice amp

11/18/1997 3:47 PM
ryan 5 w tube practice amp
hi. does anyone have a schematic for a 1-5 w practice amp that would have common parts like transformers and 12ax7 type tubes? maybe with 12ax7's as class a output tubes? is that possible? thanks, ryan
11/18/1997 4:12 PM
I don't think a 12AX7 will quite do it, but a 12AT7 will. You want to make an output stage from the reverb driver transformer of a Fender. This will put out 1-2W into 8 ohms, driven by a an AT7.  
This is one of the things I need to get cased up into a project and never can get to.  
Another possibility is a single ended 6AQ5. These were very common, and put out about 3W.
11/18/1997 9:48 PM
John Kos

I've thrown this idea around for a while. How about using a 12AT7 (or any other such tube) in a cathode follower configuration with capacitive coupling to the speaker/headphones? That would eliminate the output transformer.  
Also, with such a low power amp, could the high voltage power transformer be eliminated by using a more common transformer and voltage multipliers?  
What do you think?  
John Kos
11/19/1997 5:23 PM
it would be super cool to do this without an output x-fmr. would this also be possible to do with a wall wart power supply? how do the blue tube type boxes get the plate voltage from a wall wart?
11/19/1997 8:49 PM
Steve M.

Hi John,  
I built a little OTL headphone amp for a CD player using a 6SN7 and a 6V6 tube. The 6V6 is triode connected and is a cathode follower. I connected the 6V6 cathode through a 270 Ohm resistor which gives a 500 Ohm output impedence. It drove a set of Sony headphones (DC resistance 25 ohms) to very loud levels, and will drive a power amp. I have yet to hook up a guitar preamp, but that is my next project. I plan on hooking it up to a stereo power amp and using a modern high gain preamp to drive it. This might work well for guitar. The only reservation I have is that 6V6 tubes tend to "fart out" when driven too hard. An EL-84 might work better in this amp.  
11/19/1997 9:26 PM

I was just wondering, did you connect the headphones in series with the cathode resistor, or did you couple the headphones from cathode thru a capacitor? What size was the capacitor?  
11/19/1997 9:50 PM
Steve M.

Hi Doc  
(thanks again for the info on adding an adjustable  
bias pot to my Marshall)  
I connected the cathode resistors (I made it stereo) to the ring and tip of a stereo jack, so the headphones are plugged in they are in series with the cathode resistors.  
Originally this was a power tube cathode follower hifi amp that had a 500:8 ohm OT connected in series to the cathode resistor. I couldn't find a tranny in that ratio, so it hit me that I could leave off the OT and connect the cathode directly to the headphones through the cathode resistor.  
Would there be any reason to couple the cathode directly to the phones through a cap?  
BTW, the project text stated that this amp puts out about 1 watt when building it as shown (with the cathode follower OT)  
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