JTM45 internal jumper noise

[stevewdewitt]

I have read a lot of posts about wiring V1 pin2 to V1 pin7 rather than using a patch cable from the low treble to the high normal inputs. I tried to add a spst switch for this but it made a lot of noise when it was off. Sounded fine on. I used short shielded cable. The noise was coming from the cable on pin2. Replaced that. Same thing. Very loud hum/oscillating noise. Got worse as the volume was increased. If I turned the normal channel all the way down the noise was gone as well. Remove the switch and the patch cable worked great again.

My question is, can this be done with a switch? I based my circuit off of the schematic from tubedepot.com

TIA
Steve

[bradicusmaximus]

I've done this in a number of amps without any issues what so ever. Generally, I've used two approaches. First, if the switch is mounted on the front panel near the input jacks (as in a combo configuration) I used shielded cable. If I put it on the back, quite close to V1, I've used regular wire. In both configurations, I've never had any problems.

Are you certain this is the actual cause and not just something that is bringing a different issue to the surface?

[sunnydaze]

+ 1

This can work. Another approach, which I like is to use one of the input jacks as the "jumpered" input.

How is your switch setup - where located and how is the shielding grounded?

This is a great circuit, probably my favorite, although I more partial to the original 5f6a version, with some tweaks here and there....

Simplest is still just the patch cable at the input.

Thanks
Mike

[stevewdewitt]

The switch is right behind V1 on the backplate. The shielding is NOT grounded. It is a simple mod. I will try grounding the shielded cable.
Thanks
SWD

[sluckey]

My guess is that using a SPST switch has left the unused grid floating when in the off position. If this is the case you could use a SPDT switch to ground the unused grid when in the off position.

[pinkphiloyd]

My guess is that using a SPST switch has left the unused grid floating when in the off position. If this is the case you could use a SPDT switch to ground the unused grid when in the off position.

This was my first thought as well.

[stevewdewitt]

Thanks I am going to give this a shot when I get home tonight. I will let you know how this works out.

[Richie]

If you jumper inputs, its best to use a good quality cable,and small in length. Some people Jump or bridge channels in different ways. I guess whatever one likes. Hendrix used a Y cable. A little different ,but still much the same.

[stevewdewitt]

grounding the cable used to connect v1 pins 2 and 7 seemed to do the trick. Sorry it took so long to reply. I am busy building more amps and we had a Guitar Show here in my area that we attended as a vendor.

thanks to everyone.