#124 Questions

[Bombacaototal]

Hi Martin, I just fixed the wiring. I reverted back to your comment and I just wanted to double check if you are suggesting that I change my current mains switch to a dpst?

[martin manning]

It’s up to you. Dumble seems to have used SPST like Fender.

[Bombacaototal]

Great! Many thanks again. I will leave as is.

One question on the wiring of the speaker jacks. On the layout of #124 it looks like the grounding of the 2 jacks are jumpered together, even if the jacks are grounded individually to the chassis.

Is the best approach adding the extra lead jumpering the grounds of each of the two jacks together?

[dorrisant]

Great! Many thanks again. I will leave as is.

One question on the wiring of the speaker jacks. On the layout of #124 it looks like the grounding of the 2 jacks are jumpered together, even if the jacks are grounded individually to the chassis.

Is the best approach adding the extra lead jumpering the grounds of each of the two jacks together?

The safest way is with a ground jumper between the two jacks. Just in case one of the jacks would loosen from the chassis, you'd still have connection to the OT secondaries. If it were to become disconnected while pushing signal through it, your OT would be at risk. Just one short wire goes a long way towards peace of mind.

[martin manning]

The safest way is with a ground jumper between the two jacks. Just in case one of the jacks would loosen from the chassis, you'd still have connection to the OT secondaries. If it were to become disconnected while pushing signal through it, your OT would be at risk. Just one short wire goes a long way towards peace of mind.

I would jumper the speaker jack sleeve lugs together to insure a solid connection to the extension jack (doesn't rely on the bushings), and that gives you a redundant connection to the chassis (two bushings instead of one). The OT secondary ground reference is needed to eliminate noise, and for the feedback voltage, but if you were to lose the chassis ground, that doesn't remove the load from the secondary, and so there is no danger to the OT.

[Bombacaototal]

Great! Many thanks again. I will leave as is.

One question on the wiring of the speaker jacks. On the layout of #124 it looks like the grounding of the 2 jacks are jumpered together, even if the jacks are grounded individually to the chassis.

Is the best approach adding the extra lead jumpering the grounds of each of the two jacks together?

The safest way is with a ground jumper between the two jacks. Just in case one of the jacks would loosen from the chassis, you'd still have connection to the OT secondaries. If it were to become disconnected while pushing signal through it, your OT would be at risk. Just one short wire goes a long way towards peace of mind.

Many thanks Dorrisant and Martin for the explanations. I had wired with the jumper so no need to amend that!

[Bombacaototal]

I am having a weird issue on my amp. After starting it, without the input jack plugged in, if I turn the volume past 2 o’clock the amp goes into oscillation and squeals, but if the guitar input jack is there I can set the volume to max without a problem.

My input jack has isolation washers on both sides and it is grounded at the buss bar. Any ideas of what the issue may be?

I already tried a few different tubes and the result is always the same

[ayan]

Maybe a silly question, but, when nothing is plugged into the input, the Jack is grounded via the jack switch, yes? Assuming that is the case, the thing to do is get your scope out and see where there oscillation pops up in the signal chain. For example, scope the input jack to see that it has no signal. Scope the wiper of the preamp volume pot and verify it has no signal. Scope the output of V1A, etc., etc. Based in the symptom, you may see something early enough since I take it you mean the squealing begins when you turn the preamp volume up. Then it’s detective work, checking all the grounds are solid, that you have no cold solder joints, etc. You may want to you a wooden chopstick and start poking around and see if touching something will make some kind of noise.

How high do you have the Master volume turned up when you do this? You may have something getting into the power tube grids. What kind of power tunes do you use and what are your power tube grid stopper resistors? If you turn the master up and down, does the pitch of the squeal change?

Another thing, use a short patch cable to connect the FX loop send and return jacks. That connection can be bad or get bad, creating problems.

I am having a weird issue on my amp. After starting it, without the input jack plugged in, if I turn the volume past 2 o’clock the amp goes into oscillation and squeals, but if the guitar input jack is there I can set the volume to max without a problem.

My input jack has isolation washers on both sides and it is grounded at the buss bar. Any ideas of what the issue may be?

I already tried a few different tubes and the result is always the same

[Bombacaototal]

Many thanks for the reply Gil, so when nothing is plugged to the input the jack is grounded to the pre-amp star ground, given they are isolated from the chassis with washers.

The master volume was fairly dimmed, around 3 o’clock. Power tubes are 6V6 from TAD and the grids are 3.3k. Let me experiment with the master while the squealing is happening. I will also experiment with the jacks on the FX loop.

The scope is a great idea as well, I need to get one sooner than later

[Bombacaototal]

Question about the wiring of the socket of a 50W amp with 2 6L6 Power Tubes (not the amp with the squealing above by the way):
I have the 470R hooked on one side at the pin 4 (with nothing else connected there) and the other side of the 470R hooked at the pin 6 which then goes to the screens and the other tube.

The 2.2k grid resistor is connected on one side to pin1 and the other on pin 5. I have hooked my incoming signal to pin5 but I think it should have been to pin1 instead? Fender schematic shows Pin5 but Ceriatone layout shows Pin1, so I am confused.

A pic of how it is now

[talbany]

Question about the wiring of the socket of a 50W amp with 2 6L6 Power Tubes (not the amp with the squealing above by the way):
I have the 470R hooked on one side at the pin 4 (with nothing else connected there) and the other side of the 470R hooked at the pin 6 which then goes to the screens and the other tube.

The 2.2k grid resistor is connected on one side to pin1 and the other on pin 5. I have hooked my incoming signal to pin5 but I think it should have been to pin1 instead? Fender schematic shows Pin5 but Ceriatone layout shows Pin1, so I am confused.

A pic of how it is now

If you pull up the tube data on a 6L you will see pin 1 has no internal connection. Fender uses the extra pin to mount the grid resistor from pin 1 to pin 5 (Grid) So your now hooking your grid wire to pin 1 (Dummy) to the input of the grid resistor which then goes to pin 5 (Grid)
Some builders might not use pin1 as a mount for the grid resistor and mount it to a terminal strip or fly the grid resistor separately like dumble did with say 183. This is normally done with EL-34's since pin 1 has an internal connection and needs to see gnd.

Tony.

[Bombacaototal]

Thanks a lot Tony, I will swap it around!

What about pin4 and pin6, is my approach correct?

[talbany]

Thanks a lot Tony, I will swap it around!

What about pin4 and pin6, is my approach correct?

Same type of deal. pin 4 are the screens and pin 6 is no connection.Screen resistor goes across pin 4 to pin 6 DC input should be pin 6

BTW..On the amp with the squeal?. Unplug the cable from the input jack and take your meter and measure the input to the grid resistor to V1a should be a dead short to gnd. If not either the switching contacts are faulty or dirty or wired wrong?. If it is grounded pull V1 and see if it stops.if it does it could be in the stack somewhere. If it still squeals pull V2 and see if it stops. If it does it's in your OD section. If not pull the PI. This will at least help to narrow down what section of the amp the oscillation is being generated (input/stack/OD/PI output) since you don't have a scope.
When you narrow it down to a section,check all you voltages in that section then look for either lead-dress issue,,cold solder joint or something not wired correctly. go over each connection including the OD relay on back to the power supply B+:D

Tony

[Bombacaototal]

Many thanks for the help on the PT socket wiring.

Also thanks for the tips on how to troubleshoot the squealing amp. I will work on it this week. Hopefully it will be something obvious and easy to fix

[Bombacaototal]

The safest way is with a ground jumper between the two jacks. Just in case one of the jacks would loosen from the chassis, you'd still have connection to the OT secondaries. If it were to become disconnected while pushing signal through it, your OT would be at risk. Just one short wire goes a long way towards peace of mind.

I would jumper the speaker jack sleeve lugs together to insure a solid connection to the extension jack (doesn't rely on the bushings), and that gives you a redundant connection to the chassis (two bushings instead of one). The OT secondary ground reference is needed to eliminate noise, and for the feedback voltage, but if you were to lose the chassis ground, that doesn't remove the load from the secondary, and so there is no danger to the OT.

Just wondering if it would be safer to besides jumpering the 2 speaker jacks together if one should add an extra lead to the power tubes ground?