Negative Feedback Question

[dude]

I've added NF (series R 47K) and Presence pot(5K lin./.1uf) to a 6V6 plexi like Ampeg J20. Works great but when I turn off the negative Feedback, I lose my Presence control. Looking at the schematic I used (similar to Mark Huss Plexi), I can see that turning off the NF, you lose the presence control.


Is there a way to have the Presence control to work both with NF on and off?


al

[pompeiisneaks]

Not that I'm aware of, the idea of presence is that it adjusts the frequency range of the NFB itself, so that the NFB attenuates a more bass range allowing more mid/treble through in it's original form.  If you have no NFB, it can't work.

[HotBluePlates]

Is there a way to have the Presence control to work both with NF on and off?

You might explore bright switches, and whether adding brightness/trimming bass away from the feedback loop will give the result you want.

[dude]

Is there a way to have the Presence control to work both with NF on and off?

You might explore bright switches, and whether adding brightness/trimming bass away from the feedback loop will give the result you want.

Yes, a top cut pot would cut the top, like on the Vox. I have a very bright 18 watt lite II with a top cut pot, works great. Much better than fooling with the tone stack.


I guess a pretty dumb question on the NB switch....  Anyway, I like to turn off the NF to get a little mid-boost, kind of like a fat switch.


I've wired the switch in series between the OT's 8 ohm secondary and the series resistor (27K). I get a pop when switching, what size R would I use to ground, (1 meg?) Which side of the that loop would the R go? to sec. or to the resistor, then ground?


al   

[HotBluePlates]

... I like to turn off the NF to get a little mid-boost, kind of like a fat switch.

I've wired the switch in series between the OT's 8 ohm secondary and the series resistor (27K). I get a pop when switching ...

Let me get it straight:  You have a switch to break the connection from OT 8Ω tap to the 27kΩ resistor.  And now you get popping when you open/close the switch, right?

Try soldering a 1MΩ resistor across the switch.  Now when the switch is closed, the 1MΩ is shorted out and the 8Ω tap connects to the 27kΩ resistor.  When you open the switch, the 1MΩ is in between the two.

It should function the same (feedback, no feedback), but you're going from "high resistance to low resistance" instead of "no connection to connection".

[PRR]

The 27K goes to a cathode?

Then one way the cathode is biased with (say) 1.5K, the other way with 27K||1.5K= 1.42K. Enough for a tenth-volt shift of DC at cathode, so many-volts shift at plate. "Pop!"

Put a cap in series with the 27K. Wants to be more than 0.1uFd. The 0.68uFd cap that Marshall was fond of would be suitable. Then add HBP's 1Meg on the switch, or cap-to-ground, or somewhere, to hold the cap's change when switch is open. (Can't really talk without a diagram.)

[dude]

Still pop's with 1meg across switch, yes HBP wired switch as you mentioned, between the 27K feed R and 8 ohm tap.


Here's a schematic, replace C10 and R16 with 5K pot and .1uf.  Connected .1uf from sweeper to ground. 


Something else might be wrong.  Now I'm getting static without a signal plugged in, worst when neg Feed Back is off, amp plays but constant static. I think disconnecting C10 and R16 with FB off I have no connection to ground since I don't have R16 (1K5) and the 5K pot in place. Did I wire NF wrong? Or I can't have an on/off switch on the NF circuit...?


I'm going to disconnect the on/off switch and presence control put the 1K5 (R16) back and see if static is gone. Also, check that I didn't lose a wire under the board when soldering or possible a loose connection like the ground..?


al   

[dude]

I got the amp's negative feedback working, the problem was two things, the static was solder had dripped down the trem tube socket's grid making a bad connection (I notice that the tube wouldn't go all the way down). Had to replace the socket, tried to get the solder to drip down out of the pin but that wasn't happening. Learned to be more carefull with the heat.


THe second thing was I had the on/off switch to the NFB wired improperly. I was cutting off the circuits path to ground with the switch off. I wired the on/off like the November amp, just grounded the Presence control and that left left a path to ground for the circuit to work.  Works great, I'm glad I figured this out on my own. I'm not an electrical engineer, have no training in electronics except what I read here and on the net. I am familiar with household electric, and custom woodworking is my business. I know I should be further along at this point but I put this stuff on the shelf and forget simple basics.


Breaking the series path from the  OT secondary is not the best way for an on/off switch, as that disconnects the feedback but leaves no ground path for rest of the circuit.


Thanks for the responses,
al 

[sluckey]

Look at this schematic to see how I switched the NFB on and off. I don't recall there being any pops.

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

[PRR]

> I'm glad I figured this out on my own.

So are we. The schematic you posted did not show ANY switch, so don't know what to think or say.

[dude]

> I'm glad I figured this out on my own.

So are we. The schematic you posted did not show ANY switch, so don't know what to think or say.

No Presence control either, I added both but posted the basic schematic. I ended up getting a half inch hole carling switch for the NFB, works great, wired as the schematic Slucky posted on this thread. But still a big pop. I tried a 1 meg across the simple on/off, still a pop.


I'm sure there's a way to stop the pop, I'll search around and start reading. I put a mini toggle, on-on-on in one of the inputs, and now can switch V1 to either one half or the other or both. I have one channel set up bright and one set a little darker, switched in the middle (both channels) the amps sound pretty good. There is a slight pop switching that mini toggle but not all the time, not sure what's causing that either but could be the same reason I get a big pop all the time on the NFB on/off switch...? Perhaps I need to change that ground on the feedback switch, right now it's between the power ground, and preamp ground. The amp was wired that way from Ampeg. Now I'm thinking, even though the amp is dead quiet, those two grounds should be separated and have no connection to each other...? Star ground for the power section and another separate preamp ground down the board for inputs, I might try grounding the NFB switch to the preamp ground...?


Thanks for all your help, al

[PRR]

> Now I'm thinking

But not putting up a sketch of what you did. You can probably be helped, but you have to help us help you. There's way too many variant ways to do what you think you done to cover all cases.

As there wants to be DC bias here, and a bias-change is a "pop", it can't be "silent". But it can be worse than it needs to be. Sluckey's November NFB switch "pops" but both sides of the push-pull near-equal, and may be totally unobtrusive. (Maybe less than the mechanical snap.)

[dude]

> Now I'm thinking

But not putting up a sketch of what you did. You can probably be helped, but you have to help us help you. There's way too many variant ways to do what you think you done to cover all cases.

As there wants to be DC bias here, and a bias-change is a "pop", it can't be "silent". But it can be worse than it needs to be. Sluckey's November NFB switch "pops" but both sides of the push-pull near-equal, and may be totally unobtrusive. (Maybe less than the mechanical snap.)

I understand, here is a schematic of the amp with the changes I made, click on the sticky notes for the changes in detail on the schematic.


I have 11 vdc on the Presence pot, and it's a bit scratchy. I took voltage reading at R16 and R19 (where the wire goes to Presense pot and 11vdc there. I did change those two caps, C11 and C12 thought they may be leaking. Aren't those caps suppose to block dc...? Since I changed them with new, "could something be causing them to pass dc..?"  I get the pop very loud now and I'm thinking it's the voltage on that Presence pot...?


The sticky notes don't open, all I did was take out the normal input, replaced with a on-on-on mini toggle to get normal, bright or both in parallel. Wired the switch from each 470K (voltage divider) with center going to volume, added the bright cap in parallel to bright channel. Separated the cathode on V1A and B, for a bright and darker channel. Connected the grids together w/68K (V1 A&B) Added a presence pot (5KL, with.1uf) pulled C10 and R16 ran wire from bottom of R16 & R19 to Presence pot. That's it. I have voltage 11vdc, from the take off wire from the junction of R19 and R16, so it's on the Presence pot...? Big pop.   


NFB off, no voltage at Presence or at R19 and R16.  Is voltage suppose to be there with Presence on...? 11vdc
took NFB and switch out: now 3.5volts, with C10 .1uf and R16 1K5 back in place of presence pot...?

[PRR]

I have to go back to the eye-doctor. I still do not see any NFB switch.

Ah, hidden in the notes. Crayon is better.

What if you *break* the 47K instead of shorting the 5K?

[dude]

OK, I'll try that. At one time in the beginning of installing the Presence  I had no pop....?


One question, is low voltage "normal" at that point in the PI? Seems to be less without the 5K pot and cap around 3.5V (original schematic) and a 11v when a Presence pot is installed, wired like Huss's Plexi.


Just want to know if that voltage is normal, that's all. Junction of the 10K and Presence pot. No use in fixing something that isn't broken...


Thanks,
al

[sluckey]

One question, is low voltage "normal" at that point in the PI?

yes

[dude]

Thank you very much Sluckey, I thought it looked that way from the schematic. That answers the "pop" with the NFB on/off, it's the low voltage on the switch. I'll put the switch between the NFB resistor the secondary, like PRR suggested, there's no voltage at that point.


I was just going by Huss's schematic that you posted, has the pot getting grounded..? But like you said yours didn't pop.


The strong tremolo intensity was lowered with the 1Meg vs the 220K feed from the plate , thanks for that too.


Take care,
al


 

[sluckey]

But like you said yours didn't pop.

Actually I said I don't "recall" any pops. 

I built that amp over 10 years ago and I tried a lot of gee whiz stuff like switchable NFB, Cathode/Fixed bias, Preamp out/power amp in jacks, and some other stuff I also don't recall. My memory ain't as sharp as it used to be.

[dude]

I think I need to get out of the "gee whiz stuff" and build a reliable known circuit. I have a few chassis's lying around, time to make something of them.


I fixed the NFB problem, it was so simple. A pic of the guts probably would have lead someone here to the answer. I said in one post "I had it not popping at one point", big clue. It was "lead dress". I started chop sticking wires and immediately found the problem, I had run that feed from the PI to the Presence pot under the board, thought I'd make things neat. I was picking up all kinds of noise. Once I changed to a shielded wire all the noise, static sounds and the pop was gone. I occasionally hear a weak pop turning off NFB, perhaps a 1 Meg across the switch would cure that now.


Thanks for all the help and patience,
al