Negative Feedback in Two Stage Preamp - diyAudio
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Old 11th February 2013, 03:28 PM   #1
robrob is offline robrob  United States
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Default Negative Feedback in Two Stage Preamp

First let me say that I am new to working with amplifiers but I want to install a switched and variable negative feedback loop in my VHT Special 6 Ultra guitar amp. I want it just as a tone modifier, not to solve any issues with the amp.

The Ultra has two stages of preamp (2 12AX7). My plan is to inject the negative feedback from the 4 ohm speaker output to the second stage tube using only a switch and 100k linear pot. My concern is that the voltage at the injection point is shown as 104 V DC (TP6 marked with "Inject Neg Feedback" in schematic). Will this voltage be a problem? Does this feedback loop look correct?

Thanks in advance for the assistance,

Rob

EDIT: I realized I was looking at the wrong injection point and updated the schematic. When I came back to edit this post JMFahey had already caught my error. You guys are quick, thanks for the help.

Click the image to open in full size.

Last edited by robrob; 11th February 2013 at 04:26 PM. Reason: Realized I was very wrong!
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Old 11th February 2013, 04:16 PM   #2
JMFahey is online now JMFahey  Argentina
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Not TP6 !!!!
Inject NFB in TP10, after removal of C22.
And a pot is not quite realistic.
Use it *once* just to find a value that's low enough to "do something" but not so low to cause instability (I guess it'll be between, say, 8K2 and 47K , choose by best effect) and then just use a fixed resistor and a switch.
You can't apply too much NFB to a tube amp, specially so using a cheap "Guitar" OT.
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Old 11th February 2013, 04:22 PM   #3
robrob is offline robrob  United States
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Thanks for the correction. Just after creating this thread I found out I was looking at the wrong injection point.
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Old 11th February 2013, 04:31 PM   #4
robrob is offline robrob  United States
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While I've got you guys here, here's another question. I want to add treble and bass 250k ohm audio pots to the tone stack. The bass is simple with replacing R15 with a pot, but the treble uses two resistors, R13 and R14. Can I simply replace R14 with a 250k audio pot for treble control or do I need to modify R13's value too?

Again, thanks for helping a helpless noob,

Rob
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Old 11th February 2013, 07:30 PM   #5
robrob is offline robrob  United States
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Quote:
Originally Posted by robrob View Post
While I've got you guys here, here's another question. I want to add treble and bass 250k ohm audio pots to the tone stack. The bass is simple with replacing R15 with a pot, but the treble uses two resistors, R13 and R14. Can I simply replace R14 with a 250k audio pot for treble control or do I need to modify R13's value too?

Again, thanks for helping a helpless noob,

Rob
After more research can you guys confirm that the line out to C18 connects to the treble pot's center terminal and the two outside terminals connect to where R13 and R14 were?
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Old 11th February 2013, 07:56 PM   #6
JMFahey is online now JMFahey  Argentina
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There you have a "fixed"classic tone control, with fixed resistors.
Compare the VHT to a Marshall tone stack and you'll easily find which is which.
The bass pot was replaced by a single resistor because it's always "on 10" ; the treble one is set to some intermediate value.
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Old 11th February 2013, 08:15 PM   #7
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The thing about adding negative feedback is that it brings onto the table the issue of phase margin, which if not happy causes either constant or spurious oscillation. The more NFB you employ, the more likely this will be a problem. If you don't understand this, I'd recommend against it.

I researched, designed and built several guitar amps, none of which have any NFB. It's my opinion that it sounds better without the NFB. It sounds very clean when I want that, or whatever amount of distortion I want with the front end overdrive and master volume control arrangement. See my amps if interested at:
Bob's Website
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Old 11th February 2013, 09:02 PM   #8
robrob is offline robrob  United States
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Quote:
Compare the VHT to a Marshall tone stack and you'll easily find which is which. The bass pot was replaced by a single resistor because it's always "on 10" ; the treble one is set to some intermediate value.
Thanks JM. I found a schematic of a Marshall tone stack and I see how the tone controls should be wired.

Quote:
The thing about adding negative feedback is that it brings onto the table the issue of phase margin, which if not happy causes either constant or spurious oscillation. The more NFB you employ, the more likely this will be a problem. If you don't understand this, I'd recommend against it.
Thanks Bob, but I'm going to give NFB a try. If it doesn't work out I'll either just leave it off or pull out the mod.
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Old 12th February 2013, 09:52 PM   #9
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Robrob,
Ians "rave" FWIW.

Try the tone stack modifications first to see if that gets you what you want.

You could also mess with the capacitor values on the texture switch to see if that gives you what you want (see below).

If you still want to try global feedback around the power amp after that then:

I think switched and variable feedback maybe required because of the the powerscaled output. That is, required feedback may change according to the power level set. It may also change according to where you have the texture switch set. This (texture) switch modifies the impedance vs frequency curve seen by the output valve. Speakers tend to be inductive at higher frequencies. The texture switch will tend to correct that and keep the power amps frequency response "flatter".

Stabilizing Guidance
Should stability be a problem when feedback is applied the correct way to address that would be with a series resistor and capacitor (zobel network) across R25. Set the R to say 22K (approx 10% of the main resistor value) and adjust the capacitor to get it stable, start with 47pF, if that is not enough increase that cap. Don't use big steps in cap value.
That 47pF is calaculated to get a roll off the signal level by half at 15kHz, 100pF would give that roll off at around 7 to 8kHz, if it is still unstable with 220pF then you have another problem and that would need fixing first.

Cheers,
Ian
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Old 13th February 2013, 12:50 AM   #10
robrob is offline robrob  United States
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Thanks for the great info gingertube.
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