Hey everybody I have a reissue J12T that I really like but I wish I could do a few things to it.
1. More headroom it starts to break up at around 3
Not surprising, once you look at the test voltage chart on the lower right of page 1. TP4 shows the bias voltage present at the top of the cathode resistor, which is listed as 10.4vdc. TP2 and TP3 are the output voltages of the phase inverter, with the volume and tone full-up and a moderate input signal of 50mV; those voltages 37.5vac and 35vac. It's not indicated if those are RMS voltages (which they might be), but either way, they are about 3 times bigger than the output tubes can handle cleanly. Once the peak input signal equals the amount of bias, the tube runs out of headroom (and starts to distort mildly even before that).
So you should ask yourself, "what do I want to change about this amp?"
You probably will not get any more output power simply by biasing the tubes colder (using a bigger cathode resistor, to create a bigger bias voltage). Therefore, the sound pressure level at which you start noticing distortion will likely stay the same without dropping a "bigger engine" in the output stage.
You could add a cross-line master volume between the outputs of the phase inverter, to knock the signal down a little. That seems to not be quite what you'd want, because that would possible increase preamp distortion (or change the balance of preamp to power amp distortion), and more distortion doesn't seem to be what you're asking for.
This amp doesn't have feedback, but no-feedback EL84's is intentional to allow as much tubey flavor as possible. Feedback around the output stage would allow a bigger preamp signal before distortion. You might hear the effect as "a little cleaner, a little more solid" but there won't be a "wow" change of apparent power. You'll be less likely to hear any tonal difference when changing output tubes. Feedback will be tricky to apply, because the typical feedback input on the long-tail inverter is already being used by the reverb circuit (more on that in a bit).
Maybe you're happy with the volume from the speaker at which distortion starts happening, but you just want a little more sweep to the volume control before you get there. The simple way to make this change is to add a resistor between C2 and the top of P1 (the volume control). In effect, this means that even when you turn the volume control full up, you still have some reduction of signal strength before the signal is passed to the phase inverter. To find out what resistor value to use, you'll have to experiment to see what gives the best tradeoff. I would probably start with a 470k resistor, and see if it gave the results I wanted. I doubt a resistor as small as a 220k would noticeably move the point distortion sets in (due to the logarithmic nature of a volume pot).
2. A foot switch for the trem
Easy. Look at the
AA1164 Princeton Reverb schematic, where it says "vibrato pedal." What is drawn is simply a jack with the sleeve contact grounded, and a wire from the tip contact to a point in the tremolo oscillator. This point in your amp is the junction of C7, C8 and R10. The footswitch is made by using a shielded cable, with the shield connected to the sleeve of the plug, and the center conductor connected to the tip of the plug. One end of the cable is at the plug, the other running to a SPST footswitch in a housing. When the switch is closed, the point we noted is grounded, killing the trem oscillator. You can use an RCA plug, 1/4" or whatever floats your boat.
3. A p to p board.
This will be a problem if you like your reverb. An IC is used for the reverb circuit, so you'll need probably another tube (or two) to replace the effect once you ditch the pc board. Building a handwired board for the rest of the amp is not too complicated for an experienced builder.
Topic: ampeg j12t (Read 3867 times)