Adding a second channel

[dunner84]

A while back I built an ab763 lite inspired by sluckey. Its the ab763 normal channel without trem or reverb. The schematic is as pictured below, but with standard hi/lo input jacks, and an added raw control, and type 2 master volume. I've been unhappy with the chassis, so I have finally found a suitable replacement to transfer it into.

The new chassis has a lot more room, and I have always wanted to build a SLO clone, so I decided add one to it. I picked up a slick PCB that is more or less the SLO overdrive channel. I also picked up an electronic relay kit that utilizes jfet shunt style switching.

I still haven't decided how I want to switch the two circuits. I have the radial ABY box, so I could use separate input jacks for each channel, or I could use the switching I picked up. I have never implemented channel switching before, so I'm unsure how to proceed.

If I go with the electronic switching, I was planning to use the guts from a 12vdc wall adapter for the -12v

I have read a bunch about shunt style switching, and it seems as though I would need to mute the circuit in two spots.

I am really not sure of the best way to proceed, and would love some feedback.

[sluckey]

I would just disconnect the bottom leg of R14 from ground and reconnect it to the wiper of the level pot in the new preamp. Then use your AB/Y box to switch channels.

[dunner84]

That was my initial thought, because I already have those mixing resistors in place. The retailer did a good job of talking me out of it. It all made sense when he explained it to  me, but never sat right. Probably because I am unfamiliar with channel switching.

[VMS]

I'm building similar amp with the z-pre and erk-kit.


I think i'm going to put those shunt jfets on the volume and drive potentiometers.


And try to get the -12V from plate winding. Not completely sure yet if it works because i'm using bridge rectifier but it should work with little mods to the circuit.

[dunner84]

What are you combining with the z-pre? I haven't seen any builds or videos of any of his kits, so I am excited to see how they turn out. Do you have a build thread?

[VMS]

No build thread yet.
Here is rough schematic of my preamp, crunch channel is the standard lead channel.


I might need to move one of the mute points if there is too much treble bleeding to ground through the 470pf cap. But i like the idea of putting jfets on the potentiometers so i'm going to try it like that first.

[VMS]

Also Kevin is pretty helpful so you should email and ask him too.

[dunner84]

So I decided to go with two separate inputs, and I'll be combining the two channels like Sluckey suggested.  I'll be using isolated jacks, and one ground point for both preamps to keep the noise down. The PCB for the second channel I am adding has one input for the B+ and it's own dropping resistors. Is there any reason I can't share a node between the preamps? What I mean is, can I just parallel off my node A or B to the Vin on the board?

[shooter]

parallel off my node

I don't like more than 2 gain stages on 1 node or tap.
I believe you can parallel off any node, but put a "filter" before you use it.

[VMS]

Is there any reason I can't share a node between the preamps?

Plenty of filtering on the z-pre board so i can't see any reason why not.

I wouldn't use A tap. C tap makes most sense if you think the signal flow but B tap would probably work too giving little more voltage for z-pre stages.

[dunner84]

I finally got everything together in one chassis, and I have good news and bad news.

Good news: it didn't blow up, volume controls maxed produce a normal amount of hum, both tone stacks control the tone of said hum, and the nfb switch produces a noticeable increase in output, and all the voltages look good.

The bad news: no music when guitar is plugged in.

I think it's the input jacks, and I am hoping it's the input jacks. They are cliff style jacks and the first time I have used them. They are also the 6 pin variety, but I am only using them as mono jacks.

Here is a pic of the jacks.

[VMS]

yes you're using the wrong pins on jacks, move wiring from the middle pins to the pins near chassis

[VMS]

also can't see from the picture which side of pins is making the connection when plug is in so you might need to flip the wiring 

[dunner84]

Thanks for the response. I had a feeling that was the case. The two sets of pins closest to the chassis look mechanically similar. I dont have a data sheet for these jacks(cheap knock off), so I had to make a guess.

Here are some pics showing how the pins interact with the plug.

[dunner84]

I think I see now why the pins nearest the chassis are correct. It looks like the metal but makes contact with those pins, and the washers are to adjust for chassis thickness.

[VMS]

Actually both should work with mono plug but usually middle pins are meant for the other ring when using stereo plug.


Can we see where the wires go from the jacks?

[VMS]

Use these pins

[PRR]

> I dont have a data sheet for these jacks(cheap knock off)

Don't buy cheap jacks, there's always regrets later.

You don't need no silly data-sheet for a "Cliff". The entire action is visible! Take them out where you can see the contacts. Put a plug in. What touches what? You want to touch Sleeve, and Tip, and you want the contact that 'tip' falls back against when the plug is removed.

[dunner84]

Well that's what I thought when I looked at them, but it didn't seem to work.