Disconnecting the NFB wire and no sound?

[titser_marco]

Hi all! Working on a build now and for some strange reason, there is no sound when I remove the NFB wire. This is strange because in my previous build, removing the NFB wire just resulted in a rawer tone.

Thoughts?

[JMFahey]

[img:252:357]http://www.tarot.es/blog/public//blog/i ... 413547.jpg[/img]

SHE can answer with your supplied data.

All others require a schematic

[titser_marco]

Hahahaha! Anyway, I based the PI off a Trainwreck Express circuit (A1) but I was wondering what could be the cause, theory wise.

I know it's not the output tubes because it makes a sound when I tap on pin 5, and it's not in the preamp section because it works with a guitar when the NFB wire is CONNECTED.

[Phil_S]

Tell us exactly what you are disconnecting. As you've already been told, you aren't telling us enough. My WAG is that you are lifting the ground for either the speaker or the PI when you do whatever you do.

[Leo_Gnardo]

Have you observed the output with a scope? I'm guessing (along with the cute gypsy fortune teller) your amp goes into ultrasonic oscillation, blocking audio signal. Without a crystal ball, also known as oscilloscope, how do you know?

[titser_marco]

Tell us exactly what you are disconnecting. As you've already been told, you aren't telling us enough. My WAG is that you are lifting the ground for either the speaker or the PI when you do whatever you do.

So the NFB wire is connected to the speaker ground, then goes into an 820ohm NFB resistor. I disconnect the wire from the speaker ground going into the said resistor.

Sorry I have no scope on hand.

[pdf64]

...So the NFB wire is connected to the speaker ground, then goes into an 820ohm NFB resistor. I disconnect the wire from the speaker ground going into the said resistor...

I can't make head or tail of the above!
If the 820 NFB resistor is going to the speaker ground output, then there can't be any NFB in the normally accepted meaning. Voltage sampled NFB does it's thing in the signal path.
A photo may be helpful.

[titser_marco]

...So the NFB wire is connected to the speaker ground, then goes into an 820ohm NFB resistor. I disconnect the wire from the speaker ground going into the said resistor...

I can't make head or tail of the above!
If the 820 NFB resistor is going to the speaker ground output, then there can't be any NFB in the normally accepted meaning. Voltage sampled NFB does it's thing in the signal path.
A photo may be helpful.

I'll take a photo of it later. Yeah, you're right. Based on schematics, NFB should be connected to the tip of the speaker, right? I have a nagging suspicion that this could be caused by an error in the PI section.

[Phil_S]

Yeah, that miswired NFB circuit is doing little more than providing the ground for the tail resistor in the PI. Disconnect the NFB and then check your tail resistor to see if it's grounded. IOW, one proble on the ground side of the tail resistor, the other side to chassis ground. If it reads open circuit, you have your explanation.

When you are done fixing the PI, then you can wire your NFB properly to the tip side of the speaker jack.

[titser_marco]

Yeah, that miswired NFB circuit is doing little more than providing the ground for the tail resistor in the PI. Disconnect the NFB and then check your tail resistor to see if it's grounded. IOW, one proble on the ground side of the tail resistor, the other side to chassis ground. If it reads open circuit, you have your explanation.

When you are done fixing the PI, then you can wire your NFB properly to the tip side of the speaker jack.

Rechecked the wiring, and indeed it's a grounding issue. The cap going into the bottom triode had no reference to ground. Thanks all for your help!

[JMFahey]

DID we help????

The nature of your problem was never clear to us, so ......

[titser_marco]

DID we help????

The nature of your problem was never clear to us, so ......

You guys actually did, whether you admit it or not!