Adding a bypass capacitor... effect on the NFB resistor?

[tubedood]

If I add a small bypass capacitor to my 5F2-A projects second gain stage cathode (.15uf) for my simple switchable "Brilliance" treble type boost will it affect the usefulness of the 33k NFB resistor I have going to the output transformer 8 ohm tap?

Dale

[Enzo]

A cap and a clip wire and you can answer the question yourself and report back to us.

[Dave H]

You've added a 'Presence' control. The NFB resistor will still be effective at low frequency.

[Chuck H]

Right ^^^^^^^^^^^^^^^^^^

In this regard it's a double brilliance control. Listening tests are ultimately in order to make sure the amp doesn't become uselessly bright then.?. Oh, and here's the schematic so that other readers know what we're talking about.

[tubedood]

Thanks again guys and thanks for adding the pic Chuck. I am eagerly getting things together to power up and try out these ideas and give 'em a listen! Maybe if it sounds too bright when the .15 capacitor is switched in I could always have the switch also open the connection to the nfb resistor (33k in my case with the 8ohm tap). There's so much to learn in all of this, you all make this look easy.

[Dave H]

You don't want a switch across the feedback resistor. It will upset the bias of the second stage. The 1k5 cathode resistor will be shorted to ground through the OT secondary.

[tubedood]

Oh Wow Just caught that Dave thanks! Here's what I was thinking

Dale

[Chuck H]

If you change the circuit a little you can have your cake and eat it too. This would keep the NFB lift and the brilliance circuits independent of each other.

[tubedood]

Thanks Chuck H I'm liking it! I was curious on that NFB resistor and what altering it can do. I dug up some info on NFB (seems like a whole science is dedicated to this alone!) and it appears it works out to 33K in my case of using the 8ohm tap (I guess the schematic is reflecting the old 4 ohm transformer?) Would your 1500 stay in that ballpark value using the 8 ohm tap?

I'm about to research the use of your 100 ohm to ground I have seen that a lot on the bottom of tone stacks etc. Not quite sure what he's there for but I am curious!

Dale

[Chuck H]

Thanks Chuck H I'm liking it! I was curious on that NFB resistor and what altering it can do. I dug up some info on NFB (seems like a whole science is dedicated to this alone!) and it appears it works out to 33K in my case of using the 8ohm tap (I guess the schematic is reflecting the old 4 ohm transformer?) Would your 1500 stay in that ballpark value using the 8 ohm tap?

I'm about to research the use of your 100 ohm to ground I have seen that a lot on the bottom of tone stacks etc. Not quite sure what he's there for but I am curious!

Dale

Right, 8 ohm tap, I was working from the stock circuit. In that case the 1.5k resistor in series with the 22k should be changed to 2.2k (I will edit).

The 100R resistor at the bottom of the second stage cathode would now be the feedback circuit shunt resistor. The old circuit simply used that cathode resistor as the FB shunt resistor. I'm sure there are some small differences in FB ratio with the change, but I don't think it's significant and the 100R at the bottom of the cathode resistor allows you to use your "brilliant" circuit as intended without making it a simultaneous "presence" switch. No harm in trying it anyway.

[tubedood]

Trying to figure out the way to mount this on my turret I made already... I can add a few more posts if needed. 22K or the 33K main feedback resistor? I am using the 8 ohm tap (I have both 33K and 22K in my box of stuff)… and the NFB branches off from the intersection of the 1.5K and the new 100r reference to ground? Just makin' sure

Dale

[Enzo]

We are dancing around it here, but try not to think of THE feedback resistor and what IT does. That resistor is part of a whole circuit. it is all about the circuits, not the parts.

You are taking the signal voltage at the speaker, and sending a portion back into the amp circuits. That feedback resistor works with the other resistors to form a voltage divider.

Not sure I get the 100 ohm resistor. Originally we had the 22k over the 1500 ohm. Now we have it over 100 ohms, so you cut the NFB by a factor of 10. If you shunt the 22k and leave a 2.2k, then in my view, you now have a 2200/100 divider, which is the same 20 to one we had before. But now we have the NFB coming in at 10% of the cathode resistor instead of all of it.

[tubedood]

Understood Enzo... The feedback resistor some folks love to play around with, it looks like it can be whatever sounds good. I will try tonight to research more into how that works I can already see there's a lot of thought to the whole NFB area. What I'm hoping to accomplish is to add this .15uf capacitor to my lil 12ax7 second half gain stage ( I researched a bit and it seems the sites I saw are under the impression that a "bright" type bypass capacitor takes a better effect on later gain stages.) as I only have 2 preamp stages to work with in this tweed Princeton. A lot of people that have made this style amplifier seem to say Mr. Fender got the NFB value pretty spot on, which in my case using an 8ohm transformer works out to about 33k. I'm totally on board so far. Can I add that .15 bypass capacitor to that second 1/2 AX7 stage without impacting the usefulness of the NFB loop?
I would really like this amp to sound as close as possible to the true circuit with the push/pull features off. I have nowhere near the technical skills of you all. I am trying hard to figure this out.

Dale

[Enzo]

I said it in post #2, get a clip wire and a cap and find out. it surely won't hurt anything.

[Chuck H]

Absolutely make yourself some insulated alligator clip leads. A whole bunch of them from 6" to 3'. IME you can't have enough if you tinker.

And Enzo is right. The "proper" value for this circuit is somewhat arbitrary. The FB circuit is still going to be a FB circuit and you should probably experiment a little to find what performs best by your criteria with the design. In that light there is no correct value until you have tried going up, going down and then settled on what you like (a potentiometer works well for this).

As to the 100R resistor... Enzo, TD wanted to use a "briilliance" circuit that is supposed to be a cathode bypass cap. With the FB going straight to the top of the cathode that would be bypassed he would also be implementing a pseudo presence control set full up. That's likely to be too much instant gratification and brilliance. TD also expressed the desire to futz with removing the FB circuit. In my design above I left the 22k in place and used it as the switch shunt just to keep it quiet. A feedback ratio of 220:1 might just as well be no NFB. So the 2.2k would be the median starting point value that is close to stock. Remember that TD doesn't have a head full of useful reference notes on this stuff. The 100R resistor as a cathode "tail" and FB shunt allows the switching of a bypass cap across that cathode resistor to remain an independent circuit from the NFB loop and makes manipulation of the NFB circuit independent of the cathode bypass cap switching circuit. That was my goal.