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
2. A foot switch for the trem
3. A p to p board.
Has anyone done any of these things?
ampeg j12t
Moderators: pompeiisneaks, Colossal
ampeg j12t
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Re: ampeg j12t
To get more headroom, I'd try to reduce gain on V1 a bit. See about changing the cathode resistor on the input section of V1 by reducing the value below the stock 1.5K. You'll have to experiment to find your sound. I think you might go as low as 820 ohms. It might also neuter the amp, so you'll have to see.
To f/s the tremolo, find the on/off panel switch. Route the contacts on the panel switch to the footswitch if hard wired, or to a jack if you want to do it that way. When you short the jack by closing the foot operated switch the tremolo should turn on. If you use a jack, isolate it from chassis contact.
All of the above goes out the window if you are set on replacing the amp with a PTP board. That's a lot of work and/or you can change everything about the amp when to do that.
To f/s the tremolo, find the on/off panel switch. Route the contacts on the panel switch to the footswitch if hard wired, or to a jack if you want to do it that way. When you short the jack by closing the foot operated switch the tremolo should turn on. If you use a jack, isolate it from chassis contact.
All of the above goes out the window if you are set on replacing the amp with a PTP board. That's a lot of work and/or you can change everything about the amp when to do that.
Re: ampeg j12t
Mods to these are pretty severely limited because everything, including the tube sockets, is mounted on the PCB. If you stripped the amp down in order to go point to point, there would be nothing left except the tranformers. Much better to start from scratch in that case.
If you limit yourself to changing component values, it's much simpler, but you still have to be careful not to damage the PCB traces.
There's only one preamp stage and then the phase inverter, so those are the two places you can manipulate gain. You won't get much effect changing the value of R4 because it's bypassed by C1. You could, of course, remove C1 and drop the gain roughly in half, but this would cut the balls off the amp. Instead, change R5 from 100K to 82K or 68K. This will drop the gain of the stage and reduce the signal to the PI.
OR you can reduce the gain of the PI itself; change R15 and R16 from 100K to 47K. OR try changing R17 and R18 from 221K to 150K or even 100K. This may alter the sound of the tremolo, though.
I don't think you want to get involved with making the trem footswitchable. It doesn't look like the amp has a tremolo switch, so you would have to cut traces on the PCB in order to wire one in.
If you limit yourself to changing component values, it's much simpler, but you still have to be careful not to damage the PCB traces.
There's only one preamp stage and then the phase inverter, so those are the two places you can manipulate gain. You won't get much effect changing the value of R4 because it's bypassed by C1. You could, of course, remove C1 and drop the gain roughly in half, but this would cut the balls off the amp. Instead, change R5 from 100K to 82K or 68K. This will drop the gain of the stage and reduce the signal to the PI.
OR you can reduce the gain of the PI itself; change R15 and R16 from 100K to 47K. OR try changing R17 and R18 from 221K to 150K or even 100K. This may alter the sound of the tremolo, though.
I don't think you want to get involved with making the trem footswitchable. It doesn't look like the amp has a tremolo switch, so you would have to cut traces on the PCB in order to wire one in.
Re: ampeg j12t
for the effort needed to make the amp P-to-P just sell it and buy a vintage Jet. The original Jets don't break up as early so it's probably more what you want anyway.
A quick fix for the volume / breakup issue without doing any modifications would be to roll down the guitar (instrument) volume and bring up the master.
the tremelo can be as easy as wiring a shorting jack into the send or return side of the tremelo.
A quick fix for the volume / breakup issue without doing any modifications would be to roll down the guitar (instrument) volume and bring up the master.
the tremelo can be as easy as wiring a shorting jack into the send or return side of the tremelo.
My Daughter Build Stone Henge
Re: ampeg j12t
Yeah, sell it and buy something that does what you want.selloutrr wrote:for the effort needed to make the amp P-to-P just sell it and buy a vintage Jet. The original Jets don't break up as early so it's probably more what you want anyway.
A quick fix for the volume / breakup issue without doing any modifications would be to roll down the guitar (instrument) volume and bring up the master.
the tremelo can be as easy as wiring a shorting jack into the send or return side of the tremelo.
Alas, there's no master on this one. And no send/return on a trem. I suppose you could put a switch between the wiper and ground tabs on the intensity pot so you could switch it to be always turned down, but it wouldn't be actually off, so if it makes noise, that would still be present.