Negative Feedback Question

[glpg80]

I just want to point out the obvious that NFB circuits can have DC and AC both on them from the long tailed pair phase inverter. The whole reason fender put a 4.7k to ground across the presence was to provide a DC path to ground to remove scratchy presence pots. So when Marshall copied it they did so verbatim.

Just know what you’re adding and where, and understand depending on your amp or circuit you’ll need DC blocking caps for your depth circuit. If you do need one go to at least 0.02uF if not 0.047uF.

 

[Rdodson]

When I said I like less, I didn't really mean zero NFB. You get into Tweed territory, and I don't like my lower end to sound that swampy. That being said, what I do like, is that infinite transition from clean to dirty and the unreal dynamics. Way less to hide behind for sure, but then if I want more of "that" I step on a fuzz and get all square wavey.

Now, this is for sure as far from chugga/metal etc. as it gets but it really sounds great in a more sparse context.

 

[Racerxrated]

What I’ve noticed so far is, looser definitely, but improved clarity/rawness….which makes it damn near perfect when I hit my SD1.

 

[Cap217]

Your stock NFB resistor is 47K, right? If you want to experiment with a lower value, you'll need to remove it entirely. If you just want to know what higher values sound like, just lift one end of the stock resistor (the end that's connected to the hardwired depth circuit at the junction of the 220K/4700pF).

1. Lift that end of the stock NFB resistor and attach the pot between it and where the resistor was previously attached. Or...
2. Remove the stock NFB entirely and place the pot and new 33K in its place.

To wire up the pot as in #1: One wire to the pot's lug 1. Solder the pot's middle lug and lug 3 together. Then attach another wire to middle lug. Alligator clips on the other end of the wires. Attach one clip to the turret and the other clip to the free end of the 47K resistor. Now your NFB is variable from 47K to 97K.

For #2: Attach the 33K resistor to lug 1. Solder lug 2 and 3 together. Attach one wire to lug 2 and another wire to the free end of the resistor. Alligator clips on wires to attach to turret board where the stock resistor used to be. Now your NFB is variable from 33K to 83K.

You can also experiment with moving the NFB from the 8ohm to the 4ohm or 16ohm taps.

Agreed, but I would suggest he actually solder in the parts. Nothing against him, but he seems pretty green and I’d hate to see something bad happen to him or the amp.

My plan was to unsolder 1 leg of the 47k as instructed above. Then allagator clip on a pot and measure. Then/Or allagator clip in different values and types of caps WHILE THE AMP WAS ON. Is this not the right way? Do I need to power down and drain the caps for this? I dont think so but I should ask.

And what will I notice from the cap differences? I bought:

mallory 150
sozo
synergy yellow
cheap ceramic disc
silver mica
orange drops

Will be interesting to see if I notice a difference. I know a couple people said the cheap ceramics are a good cap for this.

 

[FourT6and2]

My plan was to unsolder 1 leg of the 47k as instructed above. Then allagator clip on a pot and measure. Then/Or allagator clip in different values and types of caps WHILE THE AMP WAS ON. Is this not the right way? Do I need to power down and drain the caps for this? I dont think so but I should ask.

And what will I notice from the cap differences? I bought:

mallory 150
sozo
synergy yellow
cheap ceramic disc
silver mica
orange drops

Will be interesting to see if I notice a difference. I know a couple people said the cheap ceramics are a good cap for this.

You're talking about the depth cap I assume?

You can just put the amp into standby if you're temporarily swapping out depth caps and NFB resistors. But only if you're comfortable doing it. Use insulated pliers with one hand to hold the component. And keep your other hand in your back pocket. People are probably going to disagree with me. But it's your life and just be attentive about it.

What I usually do in this case is clean the turrets and desolder. Then mesure/cut each component leg so it'll just sit down in the turret opening. Makes it all plug/play like a lego and you don't need to solder if it's just to test.

 

[glpg80]

Always put into standby detaching or attaching anything from NFB path since DC is on it. If you don’t you’re going to get a loud thump that could damage your cabinet.

 

[Cap217]

Always put into standby detaching or attaching anything from NFB path since DC is on it. If you don’t you’re going to get a loud thump that could damage your cabinet.

Is it safe to turn the pot? I assume I’d have to hold the pot with my left hand and turn the knob with my right. This is the pot I’m attaching to find cap value. I’d be touching all metal there with both hands.

Or do I need to standby to even do this?

 

[RedPlated]

You can turn the pot while the amp is on. Use alligator clips that are long enough for the pot tp sit on top of the cabinet, outside of the amp.

 

[FourT6and2]

Is it safe to turn the pot? I assume I’d have to hold the pot with my left hand and turn the knob with my right. This is the pot I’m attaching to find cap value. I’d be touching all metal there with both hands.

Or do I need to standby to even do this?

You won't be finding cap value with the pot. The pot is for NFB resistor. And yes, you can turnt he pot while playing. Just don't touch the lugs. I usually mount a pot like this on a little box or third hand stand and let it sit on my bench next to the amp chassis.

 

[Cap217]

I moved the depth pot off to the 4ohm tap. Made a big difference. It made the depth knob useable. Before I couldn’t move it past 2-3 without adding so much bass and mud. Now I can turn it up to 6-7 and it’s good.

Then I used alligator clips and clipped onto the .0047 cap on the depth pot. I added in a .0022 and played back and forth. I think with the .0022 added it made the bass deeper and removed some mids but I also could have been hearing things. I went back and forth many times but decided to just leave it alone.


Question. When draining filter caps, what is safe? It never goes to 0 but it will be like .0025 or .035 which is basically 0 I assume, right?

And i check all the caps but can we assume that if 1 is discharged, they all are? Or never make that assumption?

 

[glpg80]

I moved the depth pot off to the 4ohm tap. Made a big difference. It made the depth knob useable. Before I couldn’t move it past 2-3 without adding so much bass and mud. Now I can turn it up to 6-7 and it’s good.

Then I used alligator clips and clipped onto the .0047 cap on the depth pot. I added in a .0022 and played back and forth. I think with the .0022 added it made the bass deeper and removed some mids but I also could have been hearing things. I went back and forth many times but decided to just leave it alone.


Question. When draining filter caps, what is safe? It never goes to 0 but it will be like .0025 or .035 which is basically 0 I assume, right?

And i check all the caps but can we assume that if 1 is discharged, they all are? Or never make that assumption?

Rule of thumb is anything over 35V DC assume protective measures. Anything below 20V and you’re safe to touch no worries. Between them depends on skin, water body content, etc.

Once closely drained, Just take a gator clip and clip it from the caps to the chassis somewhere while you work on the amp if you’re really concerned. I recommend not doing this if you’re hot swapping parts though as you will definitely forget it’s there and cause all kinds of damage.

I use less than 0.0047. I like what it does to the mids. I have a 5150 II and I’ve kept the 0.0068 stock value. Every amp is different.

I use the 4 ohm tap as well because that’s stock JCM800 for EL34s, although 6550s are supposed to have 8 ohm tap. I don’t follow that routine - I like my amp just fine off of the 4 ohm tap.

Not all caps are connected together so you cannot assume that just because some are discharged, they all are - anything connected together at that branch or node which rise and fall together.

 

[FourT6and2]

I moved the depth pot off to the 4ohm tap. Made a big difference. It made the depth knob useable. Before I couldn’t move it past 2-3 without adding so much bass and mud. Now I can turn it up to 6-7 and it’s good.

Then I used alligator clips and clipped onto the .0047 cap on the depth pot. I added in a .0022 and played back and forth. I think with the .0022 added it made the bass deeper and removed some mids but I also could have been hearing things. I went back and forth many times but decided to just leave it alone.


Question. When draining filter caps, what is safe? It never goes to 0 but it will be like .0025 or .035 which is basically 0 I assume, right?

And i check all the caps but can we assume that if 1 is discharged, they all are? Or never make that assumption?

I'm confused. What amp do you actually have? Not the one in the layout you posted in your OP?

Also, clipping caps in parallel yields the sum of both caps' values. So you're kinda going in the wrong direction if you put a 2200pF in parallel with 4700pF. That shifts the depth frequencies lower. So it makes sense that it removed mids and made the bass deeper. If you want a more punchy, aggressive sound, you need to lower the depth cap value, not increase it. Scott's suggestion of 2200pF earlier in the thread is spot on. I'd try that. Or split the difference with a 3300pF.

Ultimately, it will depend on the rest of the amp. So what amp do you have here?

 

[scottosan]

I moved the depth pot off to the 4ohm tap. Made a big difference. It made the depth knob useable. Before I couldn’t move it past 2-3 without adding so much bass and mud. Now I can turn it up to 6-7 and it’s good.

Then I used alligator clips and clipped onto the .0047 cap on the depth pot. I added in a .0022 and played back and forth. I think with the .0022 added it made the bass deeper and removed some mids but I also could have been hearing things. I went back and forth many times but decided to just leave it alone.


Question. When draining filter caps, what is safe? It never goes to 0 but it will be like .0025 or .035 which is basically 0 I assume, right?

And i check all the caps but can we assume that if 1 is discharged, they all are? Or never make that assumption?

In your amp usually powering it down without turning it on standby will drain the caps. But test to make sure you’re comfortable if that’s the case

 

[LightsOfSorrow86]

I'm not sure what you mean by, " the wiper."

More NFB will cancel out frequencies in the power amp section. In simple terms, more NFB sounds darker and tighter. Less sounds brighter and more open or "raw."

The impedance selector is simply a rotary switch that selects either the 4. 8 or 16 ohm secondary taps of the OT and routes the selected tap to the speaker jack. Depending on which tap you route the NFB wire to, you'll have a greater or lesser amount of NFB. 4 ohm will have the least amount of NFB, 8 in the middle and 16 the most. You can change the NFB resistor so that moving from one tap to another is almost the same. If you have say a 47k NFB resistor on the 16 ohm tap, you'll need around a 33k NFB resistor on the 8 ohm tap for similar overall NFB. Multiply the resistor x 1.4 with each move from 4 to 8 to 16 to keep NFB approximately the same. A larger NFB resistor creates less NFB as it's blocking the signal.

A JTM45 is dark and tight feeling. It's a 27k NFB resistor on a 16 ohm tap. A TON of NFB. A JCM800 is 4 or 8 ohm tap with a 100k NFB resistor. Way less NFB (because of the tap being moved and much higher value resistor blocking the NFB signal) and is much brighter and more aggressive.

A depth pot is not just another NFB control. It's a resistor/ capacitor in parallel (with each other, and in series with NFB resistor) that removes certain frequencies from the NFB so that they aren't canceled out, but rather make it to the speaker jack. Commonly, a 1meg pot with a .0047 cap is used here. The capacitor determines the frequencies that are blocked and the pot adjusts how much of that frequency is being blocked, and therefore reproduced at the speaker. A pot is just a variable resistor...so, you can replace the depth pot with a resistor and set the depth to a fixed value. A common place to set a 1 meg audio pot is around 1 o clock. That's roughly 220k on an audio pot (this would be higher on a liner taper), so you can substitute the pot with a 220k resistor, or any value resistor that you choose, depending on how much fixed depth you want.

In that Ceriatone diagram, the 220k/.0047 combo is the fixed depth circuit that I just described above. The 47k is the NFB resistor. The circuit is: NFB signal from presence control to 47k NFB resistor to 220k/.0047 depth to 4 ohm tap on impedance selector.

I wanted to try to move my NF wire on the 4 ohm tap. A friend of mine has an older version of the same amp. His amp has a 100pF cap with a 22.3k reistor in parallel on the 4 ohm tap, mine has a 100pF with a 22k resistor in parallel and also an additional 100nF cap + a 68k resistor on the 8 ohm tap. What should I expect in terms of sound changes moving it on the 4 ohm tap? Thanks

 

[valve]

A JTM45 is dark and tight feeling. It's a 27k NFB resistor on a 16 ohm tap.

Bad example! That thing is f*** loose and saggy!
(Well not because of the negative feedback but because of the low Filtering and low current tube rectification)