introduction and circuit mod question

[Jason C]

Hey guys,
I'm new to this forum and have learned a lot so far just by poking around. I would like to build an Ampeg Jet (j-12-a) but I would like to ditch the tremolo and maybe have treble/bass instead of tone only? How hard is that to do? Here's the schematic http://members.aol.com/portaflex/schems/j-12-a.gif

[J Martin]

Its an easy circuit to build. Here are specs for the transformers to get you started.

Output transformer 6600 ohms to 8 ohms

Power transformer 120 volts primary to 280-0-280 volts at 135 Ma., 6.3 volts at 3 Amps, 5 volts at 2 amps if using a tube rectifier. Any transformer close to these values can be used.

[Jason C]

Thanks for the transformer info. I'm still trying to get my head around altering circuits. I figured this would be a simple enough circuit to learn to tweak. Also, when altering the tone stack how much is the over-all sound of the circuit changed?

[J Martin]

Instead of using the "stacked" tone circuit, try using the Baxandall tone controls. It would be more in the spirit of the old Ampegs. Since you are not building the trem circuit, the other tube section can be used for recovery after the tone controls. Just look at some of the other Ampeg circuits. I own 5 Jets, from 1960 up to the reissue. they are fun to crank up.

[Jason C]

Thanks J, So you have experience with the different Jet circuits? Which one would you say has the most clean headroom and best suited for a small jazz practice amp? I chose the version with the 7591's just because my Gemini I has them and is the best jazz amp I've ever played through. A friend let me borrow his reissue Jet and that got me hooked on Ampeg now he needs it back so that's why I want to build one.

[J Martin]

The Jets are almost all the same power although you may get a little more headroom with the 7591 tubes. Its just a matter of the cathode bias resistor value and the socket wiring. I think the 6V6 tubes have a sweeter sound, especially for a practice amp. One of my old Jets came to me in pieces from different places and I had to strip the circuit board and build from scratch. I also own two Gemini amps and at least two Reverb-O-Rockets. And also a Rocket which is my favorite. The Rocket is just a Jet in a wider cabinet with an intensity control for the tremolo.

[Jason C]

Would you say the 6v6 version has more or less clean headroom than the reissue? I might do the 6v6 version after all. Would the transformers be the same? I am also more than a little envious of your Ampeg collection

[J Martin]

They all use the same transformer set. It does not matter which version you build because its simple to rewire the sockets and change the cathode bias resistor. I think I mentioned before that the 7591 seems to have a little more power. This is just by ear, no electronic measurements. I should mention that I also have a B12, B15, B18 and a 1957 era combo Ampeg bass amps.

[J Martin]

I misread your question, you were asking about the reissue. My reissue uses 6BQ5 power tubes and sounds a lot different. Never compared headroom. When I occaisionally play live I use the reissue and the old one I restored and play in stereo and almost always in a clean mode. My tastes lean towards jazz also.

[Jason C]

So where do I start when removing the tremolo? I'm having a hard time figuring out what can go and what has to stay in a circuit. Yeah I wanted to know if the old Jets are as loud (clean) as the modern reissue.

[J Martin]

The trem section on the schematic you posted a link to shows the trem section right below the preamp. Don't build that part and ground the line that goes to the middle of the two 270K resistors in the output section. Time to stop talking and start soldering. It will be easy once you start building.