Feedback tap?

[Orphin]

I look at an amp schematic and see what seems to be a separate tap from the OT called "feedback"?
Looks to be connected to the cathode of the phase inverter.

Please tell me what is, what it does and why it's there?
Thanks.

[Orphin]

Neil? Any one?

[Randy Bass]

Could it be the negative feedback loop? Maybe some if the following text from aikenamps.com will help clear it up:

"There is an output stage topology that is kind of in between, called "ultralinear" operation. This uses local negative feedback to the screen grids of the output stage by means of a tapped output transformer primary. This increases the damping factor and makes the amp a bit tighter without the use of a global negative feedback loop (you can use global negative feedback with ultralinear output stages, but you may not like the tone as much). The Dr. Z Route 66 amplifier uses an ultralinear output stage. There is also a triode output stage, which has even higher damping factor than ultralinear, but some players feel that it sounds too "compressed" and midrangey, while others like it. Part of the reason for the midrange emphasis is the increased input capacitance of triode mode over pentode mode because of the Miller effect, which in effect, multiplies the grid to plate capacitance by the gain of the tube. This increased capacitance rolls off the high frequencies."

[OrangePaul]

I wondered what this was too on the schematic for my OR120. If you look though you should see its connected to the presence control as it is on the circuit for my Marshall 9005. How exactly it works to adjust the presence though i'm not entirely sure, but yes very odd when you first notice it. Me personally thought i'd much rather boost the top end using the presence on my OR120 than using the treble control.

[jontheid]

It forms a negative feedback loop between the output of the amplifier and a point earlier on in the signal path.
On Orange amps that use a cathodyne (split-load) phase inverter this point is at the top of the cathode resistor of the tube before the phase inverter. All the vintage Orange amps use this type of phase inverter as far as I am aware.
On some later Orange amps the 'long tail pair' type of phase inverter is used, and the feedback goes to the cathode side of the phase inverter tube. Are you looking at a schematic for a later Orange?

Negative feedback is a wonderful thing in my opinion. What is does is makes the output of the amplifier more like the signal going into it. Transformers, by nature of their design, don't pass bass frequencies as effectively as higher frequencies. So imagine we are passing a lower frequency into the amp - less of this appears at the output, so less is taken away from the input, so it is attenuated less. Now lets use a frequency that the output stage passes more easily - a greater amount of this appears at the output, so a greater amount gets fed back and taken away from the input, so this gets attenuated more.

So the upshot is - frequencies that the output stage pass easily are reduced in amplitude, those that the output stage cannot pass so easily are not reduced in amplitude. What this does is levels out the freqency response of the output stage and extends the bandwidth. This doesn't come for free though, the cost we have to pay is that the gain of the output stage is reduced.

There's more bonuses to using negative feedback as well. Let's imagine that we have no input to the amplifier and that the output tubes are picking up some mains hum. This hum signal is now appearing at the output. The negative feedback loop feeds a fraction of this hum signal, inverted, back to the input of the output stage.
Now we said that there was no input to the amplifier, so the only signal we have now going into the output stage is the hum signal, out of phase. This will cancel out the mains hum being picked up by the output tubes to a certain degree.

The type of negative feedback used in Orange amps is not ultra-linear or triode mode - these types are usually only used in hi-fi amps and are very rarely used in instrument amplifiers.

There are other advantages and disadvantages of using negative feedback that I won't go into, and to be honest I don't fully understand.

Not all instrument amplifiers use negative feedback on the output stage. I think AC30s don't.
If I disconnect the negative feedback on my Orange head it becomes really noisy, hums and has way too much gain, so I leave it in place.

I hope this makes sense, if it doesn't let me know.
Ta
Jon

[OrangePaul]

That goes along with what i was taught for solid state amplifiers yes....
Improved Bandwidth
Reduced Gain
But better gain stability, preventing 'thermal runaway' i.e. if the gain of the output stage increases, the negative feedback level is increased thus reducing the input to compensate

[Orphin]

It forms a negative feedback loop between the output of the amplifier and a point earlier on in the signal path.
On Orange amps that use a cathodyne (split-load) phase inverter this point is at the top of the cathode resistor of the tube before the phase inverter. All the vintage Orange amps use this type of phase inverter as far as I am aware.
On some later Orange amps the 'long tail pair' type of phase inverter is used, and the feedback goes to the cathode side of the phase inverter tube. Are you looking at a schematic for a later Orange?

Negative feedback is a wonderful thing in my opinion. What is does is makes the output of the amplifier more like the signal going into it. Transformers, by nature of their design, don't pass bass frequencies as effectively as higher frequencies. So imagine we are passing a lower frequency into the amp - less of this appears at the output, so less is taken away from the input, so it is attenuated less. Now lets use a frequency that the output stage passes more easily - a greater amount of this appears at the output, so a greater amount gets fed back and taken away from the input, so this gets attenuated more.

So the upshot is - frequencies that the output stage pass easily are reduced in amplitude, those that the output stage cannot pass so easily are not reduced in amplitude. What this does is levels out the freqency response of the output stage and extends the bandwidth. This doesn't come for free though, the cost we have to pay is that the gain of the output stage is reduced.

There's more bonuses to using negative feedback as well. Let's imagine that we have no input to the amplifier and that the output tubes are picking up some mains hum. This hum signal is now appearing at the output. The negative feedback loop feeds a fraction of this hum signal, inverted, back to the input of the output stage.
Now we said that there was no input to the amplifier, so the only signal we have now going into the output stage is the hum signal, out of phase. This will cancel out the mains hum being picked up by the output tubes to a certain degree.

The type of negative feedback used in Orange amps is not ultra-linear or triode mode - these types are usually only used in hi-fi amps and are very rarely used in instrument amplifiers.

There are other advantages and disadvantages of using negative feedback that I won't go into, and to be honest I don't fully understand.

Not all instrument amplifiers use negative feedback on the output stage. I think AC30s don't.
If I disconnect the negative feedback on my Orange head it becomes really noisy, hums and has way too much gain, so I leave it in place.

I hope this makes sense, if it doesn't let me know.
Ta
Jon

Sorry for the delay. I've been away for a couple of days.
Great post by the way. I hope you stick around. We could need your expertise in the future.

Yes, I was looking at a schematic for the Orange Graphic.
But how come I don't see this feedback tap on any other schematic of different brands?
Sounds like a great thing for a bass amp don't you think? More bass, less noise, less distortion.
I've gotten this crazy idea lately of building a bass amplifier. I'm not even a bass player, but I figure that there are so many guitar amps out there, but I haven't yet played a bass amp that I really loved (including the SVT and AD200), so why not make a bass amp that will have the tone I have in my head?
But I don't know, it's just an idea so far. Maybe I never get to it.