Finished Tweed Twin - Input Jacks Connected?

[Stovemananon]

Hello all,

I recently finished a Tweed Twin 5f8A build. I am very pleased with it. However, I noticed that when the bright volume is maxed out along with the normal volume, I get a 60 cycle hum. I think this is because the low jacks are not grounded to the switch (ring), so all signal is allowed to be picked up by the low jacks. You can see in the photo below that the ground terminal of the low jacks remains unused. If it were connected to the switch, all signal picked up would bleed to ground. Of course to do this you would have to sever the tie between the high and low jacks.

My question is:

Why are the tips of the high jacks connected to the switches of the low jacks?


Wouldn't it make more sense to not connect the jacks in this manner, and instead connect the switch of the low jack to ground (just like the high jacks) so that it does not act as an antenna and create hum when both volumes are turned up?

Thanks for your input!

Picture of the finished build:

https://photos.app.goo.gl/wMoH2Km6CiseexNi7

[sluckey]

Why are the tips of the high jacks connected to the switches of the low jacks?

They are wired like that to provide the high and low options. The low jack is much lower level than the high jack. See page 11 of my amp scrapbook for a good explanation of how those jacks work.

I doubt the jack wiring is the issue, provided they are wired according to Rob's layout.

[Stovemananon]

You are right. The issue was that the ground wire shown in Rob's schematic coming off of the board and linking to the ground terminal on the high jack was actually grounded on the potentiometer ground bus. I don't know how this is any different, since they all link to chassis ground, and the jacks are grounded by touching the chassis. But it made a big difference.

I will definitely take a look what you have provided. I appreciate it!

[sluckey]

Just curious, why would you turn both volume pots all the way up if you're only plugged into one jack?

[LOUDthud]

Why are the tips of the high jacks connected to the switches of the low jacks?

That connection does two things. One, if you are only using the High jack, it connects the 68Ks in parallel and that lowers the noise. Two, it grounds one of the 68Ks so you get 6dB of attenuation when you use only the Low jack. When you use both jacks, the 68Ks isolate the two guitars so changing the Volume on one guitar doesn't affect the other guitar as much.

Wouldn't it make more sense to not connect the jacks in this manner, and instead connect the switch of the low jack to ground (just like the high jacks) so that it does not act as an antenna and create hum when both volumes are turned up?

If you did that, it would be just like using the Low input, 6dB of attenuation with 120K (136K in parallel with 1Meg) input impedance.

Try connecting both 68Ks to the tip on the Hi Jack with the 1Meg and not connecting the Low jack to anything.

Now this gets tricky. Instead of grounding both Hi jacks at the jacks, run the ground wires from each Channel both to the tip of one of the Low jacks and ground the switch of that Low jack. Now you can use that Low jack to connect to Both Channels.

Both_Input.GIF