Question about NFB noise

[psychepool]

I made a 'SloRectoTwin' amplifier.
It is an amplifier that has Blackface clean channel and Mesa Recto drive dhannel.
Originally it's a preamp circuit, but I made it with 6L6 power tube and Hammond 35W output transformer.

With reference to the mesa rectifier circuit, the NFB is shorted with a toggle switch.

There is no problem when NFB is disconnected, but noise occurs when connected.
And when connected, the output does not decrease particularly.

I tried to connect the output transformer primary line backwards, but there was no particular change.

Except for this problem, there is no particular problem with the operation of the amplifier.

Attach a photo of My work.

nfb.jpg

Please let me know if there is anything to check.

[R.G.]

What kind of noise?

[xtian]

Your OT secondary common must be connected to chassis (or same circuit ground as your Presence circuit). I notice you have Cliff jacks for output, so not necessarily grounded to chassis.

[Stevem]

What do you mean by shorted?

The feedback should either be applied By the switch , or that feedback circuit should be open.

Is that what you have taking place or no?

[mhuss]

I've seen a few schemes where the small-value resistor at the bottom of the PI cathode chain is shorted to ground to disable the NFB.

[psychepool]

What kind of noise?

I attached a noise sample file.

Your OT secondary common must be connected to chassis (or same circuit ground as your Presence circuit). I notice you have Cliff jacks for output, so not necessarily grounded to chassis.

The output jack is insulated type.
The output transformer secondary conmon is connected to the chassis ground point of the resorvior cap with the ground of the output jack.

The grounding circuit is roughly configured like this

grounding_01.jpg

Each ground connection is like this.

grounding_02.jpg

What do you mean by shorted?

The feedback should either be applied By the switch , or that feedback circuit should be open.

Is that what you have taking place or no?

Yes. This means that the NFB is connected/disconnected by the switch, and noise occurs when connected.

I've seen a few schemes where the small-value resistor at the bottom of the PI cathode chain is shorted to ground to disable the NFB.

Are there any circuits that can be reference?




When I make an amplifier, I always make the poweramp first and put a jack on the PI input to make the remaining part after the operation test.
This NFB noise also occurred when only the power amplifier made.

[sluckey]

I've seen a few schemes where the small-value resistor at the bottom of the PI cathode chain is shorted to ground to disable the NFB.

Are there any circuits that can be reference?

Here's one...

http://sluckeyamps.com/november/november.pdf

[alkuz1961]

if I hear correctly, it's mostly the frequency of 60hz and some harmonics. Probably there are some problems with the grounding of the filter elements. Perhaps a current loop through the ground points.
I mainly use a ground circuit in which the ground points of the central output of the transformer winding, (or rectifier diodes, when used bridge), the first filter capacitors, the cathodes of power lamps and the speaker connector rings are combined. The common connection point is located next to the power transformer.
The grounding scheme you showed is often found in textbooks, but in practice I have not seen such connections.

grounding_01.jpg

[xtian]

Your OT secondary common must be connected to chassis (or same circuit ground as your Presence circuit). I notice you have Cliff jacks for output, so not necessarily grounded to chassis.

The output jack is insulated type.
The output transformer secondary conmon is connected to the chassis ground point of the resorvior cap with the ground of the output jack.

What you show in your illustration is the OT PRIMARY CT connected to ground. I'm pointing out that the OT SECONDARY common wire needs to be grounded in order for the gNFB to work right. Please confirm, thanks.

[psychepool]

What you show in your illustration is the OT PRIMARY CT connected to ground. I'm pointing out that the OT SECONDARY common wire needs to be grounded in order for the gNFB to work right. Please confirm, thanks.

Wow! Solved!
As you said, the OT 2nd Center common was not connected to the chassis!
Connected it to the ground tip of the output jack only, not the chassis.

The work of connecting this to the chassis was the part that I did at the beginning of build, so I thought it was already done.
There was something wired in the output jack, so I thought it would have been connected to the chassis.
Due to the high density of wiring, it was difficult to check with my eyes.

When the NFB switch was turned on, the expected soft tone was obtained with a noticeable volume reduction. Thank you so much!

There was a hum that was in the 'volume 0' of the clean channel, but it wasn't too big, and I thought I could solve it slowly, but this also completely disappeared.
While using a clean channel, there was a problem that sounds from other channels were mixed somtimes, but this problem also disappeared.

It's very interest thing that there's a sound even though OT 2nd center is not connected to the ground.
There were some strange troubles mentioned, but it was also curious that the sound of the guitar itself was normal.

Anyway, I think I can finish the remaining work comfortably. Thanks for the great help!

[psychepool]

I've seen a few schemes where the small-value resistor at the bottom of the PI cathode chain is shorted to ground to disable the NFB.

Are there any circuits that can be reference?

Here's one...

http://sluckeyamps.com/november/november.pdf

First time to see the nfb on/off circuit like this.
Thanks for you show me a good reference!

I have a question about this NFB on/off circuit.
It seems not only turn off the NFB but also Presence knob. is it right?

if I hear correctly, it's mostly the frequency of 60hz and some harmonics. Probably there are some problems with the grounding of the filter elements. Perhaps a current loop through the ground points.
I mainly use a ground circuit in which the ground points of the central output of the transformer winding, (or rectifier diodes, when used bridge), the first filter capacitors, the cathodes of power lamps and the speaker connector rings are combined. The common connection point is located next to the power transformer.
The grounding scheme you showed is often found in textbooks, but in practice I have not seen such connections.

grounding_01.jpg

This amplifier is circuitly similar to the other amplifiers I made last time.
The chassis size and layout were also the same, so the intention was to handle the ground in the same way as the previous amplifiers that didn't have a problem.

I heard the among amplifiers that mainly use preamplifier distortion, the circuits with FX Loop are the better choice to separate the grounds of the power amplifier and FX Loop, so the grounds are separated rather than gathered in one place.
I think it is a better choice to gather the ground of the bias circuit and the cathode of the outptu tube, which belong to one part of the power amplifier, together with the ground of the first filter cap. I will try later.

Anyway, the problem was solved well by connecting the current OT 2nd center tab to the chassis, and it works well. Thanks for your help!

[sluckey]

I have a question about this NFB on/off circuit.
It seems not only turn off the NFB but also Presence knob. is it right?

That's right. But think about this... Even when you switch the NFB off by opening the circuit, the presence pot no longer has a signal to work with so it effectively becomes useless.