12A Input jack install

[Tbone55]

Hi All,

I know this is a dumb question but I have to ask. Is this jack grounded to the chassis when you install it or do you have to use insulating washers so it isn't grounded when installed? In my Blues Jr. Build I had to install a fibre washer in the chassis hole for the jack to be centered when installed. I also used a separate ground for the ground/shunt pins on the speaker jacks which I grounded to the transformer screw. The reason I'm asking is I used two of these jacks for the speaker jacks of the amp I'm building now and wonder if I've connected them incorrectly which may be why I'm not getting any sound at all from the amp. Could I have grounded the signal? Is there a recommended grounding point for the jacks?

As always, thanks for your help.

[galaxiex]

Hi All,

I know this is a dumb question but I have to ask. Is this jack grounded to the chassis when you install it or do you have to use insulating washers so it isn't grounded when installed? In my Blues Jr. Build I had to install a fibre washer in the chassis hole for the jack to be centered when installed. I also used a separate ground for the ground/shunt pins on the speaker jacks which I grounded to the transformer screw. The reason I'm asking is I used two of these jacks for the speaker jacks of the amp I'm building now and wonder if I've connected them incorrectly which may be why I'm not getting any sound at all from the amp. Could I have grounded the signal? Is there a recommended grounding point for the jacks?

As always, thanks for your help.

It could be either, it depends on the circuit behind it.

A schematic would help.... :)

[shooter]

here's the datasheet

https://datasheet.octopart.com/12A-Switchcraft-datasheet-24266.pdf

[2deaf]

Is this the Trinity Tramp?  If it is, don't connect the shunt on either jack.  Those jacks are grounded to the chassis by contact.

[acheld]

This is the "schematic for  12A:

[Tbone55]

Is this the Trinity Tramp?  If it is, don't connect the shunt on either jack.  Those jacks are grounded to the chassis by contact.

Yes. This is the Trinity amp. The plan shows just regular 2 contact jacks but I wanted to use the shunt jacks to help with hum. So, I should remove both wires that connect the ground terminal to the shunt terminal on each jack and just have a ground wire connected to the jacks going to chassis ground? Should I remove the fibre washers from the jacks as well?

Thanks

[PRR]

YOU PICK where you want the circuit to connect to the chassis.

The Switchcraft jack is a fairly reliable chassis ground. My preference is to ground the circuit at the input jack, then have no (or selected few) other chassis grounds.

The reverse is perfectly fine and has served many builders. Connect to chassis at the power supply end, then make the input jack "floating". This works well with the plastic jacks which are naturally floating. With the classic Switchcraft you nominally need insulating washers.

Yet another plan is to ground the preampery at the input jack and all the big stuff at the power end. Now the preamp ground return is *only* through the chassis. While signal currents in chassis are not the best idea, for the teeny currents from preamps it is often fine.

[2deaf]

Yes. This is the Trinity amp. The plan shows just regular 2 contact jacks but I wanted to use the shunt jacks to help with hum. So, I should remove both wires that connect the ground terminal to the shunt terminal on each jack and just have a ground wire connected to the jacks going to chassis ground? Should I remove the fibre washers from the jacks as well?

Yes, remove both wires that connect the ground terminals to the shunt terminals and see what happens before you change the ground scheme.

[Tbone55]

YOU PICK where you want the circuit to connect to the chassis.

The Switchcraft jack is a fairly reliable chassis ground. My preference is to ground the circuit at the input jack, then have no (or selected few) other chassis grounds.

The reverse is perfectly fine and has served many builders. Connect to chassis at the power supply end, then make the input jack "floating". This works well with the plastic jacks which are naturally floating. With the classic Switchcraft you nominally need insulating washers.

Yet another plan is to ground the preampery at the input jack and all the big stuff at the power end. Now the preamp ground return is *only* through the chassis. While signal currents in chassis are not the best idea, for the teeny currents from preamps it is often fine.

I had the jacks grounded to the input jack pre-amp ground. The PT is grounded to the chassis at the other end of the chassis. The fibre washers I used then aren't for insulating the jacks?

[sluckey]

The fibre washers I used then aren't for insulating the jacks?

That's exactly what they are for. Keep the shoulder washers and run a short wire from the sleeve lug to your preamp ground point (hopefully located very near the input jack). If you remove the shoulder washers you will find that the 3/8" switchcraft bushing is a very sloppy fit in a 1/2" hole!

[Tbone55]

The fibre washers I used then aren't for insulating the jacks?

That's exactly what they are for. Keep the shoulder washers and run a short wire from the sleeve lug to your preamp ground point (hopefully located very near the input jack). If you remove the shoulder washers you will find that the 3/8" switchcraft bushing is a very sloppy fit in a 1/2" hole!

I removed the two wires that were connected between the ground and the shunt tabs of the output jacks. I kept the fibre washers as you suggested. My preamp ground point is very close to the input jack. I have to run a ground wire from the sleeve to the preamp ground point. Not sure by short how short you mean. It's probably going to be around 4" long. I probably should have just used the standard 11 jack for the outputs. Live and learn. Thanks.

[2deaf]

I removed the two wires that were connected between the ground and the shunt tabs of the output jacks. I kept the fibre washers as you suggested. My preamp ground point is very close to the input jack. I have to run a ground wire from the sleeve to the preamp ground point. Not sure by short how short you mean. It's probably going to be around 4" long. I probably should have just used the standard 11 jack for the outputs. Live and learn. Thanks.

So did the amp work with ground-to-shunt connection removed?

[Tbone55]

I'm getting some sound now but I have to move the ground connection from the jacks to the preamp ground. Right now the jacks are grounded to one of the PT mounting bolts. When I try to turn up the master volume I get a really loud noise that I'm not sure how to describe. I'll post back after I've moved the ground connection to the preamp ground. In the meantime, here are the layout and schematic. Thanks for your help.

[Tbone55]

Okay. Sorted out the ground for the jacks. I connected it to the input/preamp ground. Also found I had the negative feedback connected to the wrong pot.  That's what was giving me the feedback sound when I turned the master up. Fired up the amp (it was connected to a speaker  ) and took some voltage measurements as indicated on the schematic. Plugged in my guitar and played for a while then the volume just died off. Shut the amp off and waited a bit. Turned it back on and had volume again. Played for a bit then happened to look at the power tube (6V6). It seemed to be glowing pretty good. Is the outer metal portion that's encased the plate and the cathode? This is the part that looked pretty red. I'm going to take a picture and post it. I do have a higher voltage reading at B+ 3 (393) vs what's on the schematic (340). That seems to be the only reading that's out of whack. Bad resistor?

[shooter]

This is the part that looked pretty red

do the math;
voltage at top (cathode) of R15 / 750 = tube current
tube current (in AMPS) * 393 = ~~ tube dissipation

[Tbone55]

This is the part that looked pretty red

do the math;
voltage at top (cathode) of R15 / 750 = tube current
tube current (in AMPS) * 393 = ~~ tube dissipation

I re-checked the location where the voltage should have been measured. Looks like I measured at the wrong location. The layout shows it should be measured at the junction of R10 and C7. Voltage indicated as 340, my measurement is 323v so it seems to be within the acceptable range (5%). The voltage at the top (cathode) of R15 (750ohm resistor) is 29v, current is 29/750 = 0.0386 amps. Tube dissipation, 0.0386* 393 = 15.196W.

[shooter]

14W is closer to the target.  I get ~ 110% plate dissipation at >15.  the most I ever set up a SE amp was ~ 105%.  the tubes would last ~ 10months but the musician was cool with it 

[Tbone55]

Found the culprit causing the red plating. Me.  I had incorrectly wired up the bias switch. My understanding of how the switch actually works mechanically was wrong so I had the wrong resistor wired to the Lo setting. Fixed that and no more red plating. My cathode voltage is 27.5 and working the math my plate dissipation is 14.41W. That's approximately 103% dissipation. Tube should live a good long time I think.

Thanks.