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Thread: negitave feedback loop

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    negitave feedback loop

    Hey guys, I'm building a 5e3 Deluxe style amp (my first home built p-p amp) and I would like to get your advice on my idea to implement a switchable negative FB loop.

    I plan to delete one of the input jacks, and use it's hole for my NFL switch. It will be spdt. I'll remove the cathode bypass cap from the board and mount it between the switch and one of the input jack grounds, leaving the lead from the tube socket to the cathode resistor in place. I'll add a lead (with resistor) from the speaker jack to the other end of the switch, and another from the center lug of the switch back to where the lead from tube socket joins to the cathode resistor. If I've got this figured out right, the cathode resistor is always in circuit, and with the switch in the normal position there is a path through the cathode bypass cap to ground. With the switch in the other position the cap is out, and a path is created from the speaker jack to the cathode. The 2 extra leads will be routed under the board.

    Does all this sound okay, or would I be creating problems trying to do it this way?

    Thanks

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  2. #2
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by dmartn149 View Post
    ...I plan to delete one of the input jacks, and use it's hole for my NFL switch....Does all this sound okay, or would I be creating problems trying to do it this way?...
    In general you want to avoid running signal lines from the output section back to the area of the chassis near the pre-amp. Therefore, a NFL switch near the input jacks would be a bad idea.

    However, your description sounds like you are talking about a switchable cathode bypass circuit. Is that what you really mean? The 5E3 Deluxe does not use negative feedback from the output at all. Are you planning to add it?

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    Right. the idea is to switch in a NFL and at the same time switch out the cathode BP cap. Another idea is to delete the standby and put my NFL switch there.

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    Lifetime Member Enzo's Avatar
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    Have you actually tried both these things individually, and then in the combinations you propose?


    You need to add a resistor. The cap ground end is to be switched? That means there will be a charge on it when you switch it in, so there will be a POP every time you do it. Put a high value resistor across that switch - 470k, 1Meg, whatever - so when the switch is open, the resistor keeps the cap charged. Closing the switch shorts the resistor.

    Maybe i am the only one confused, but the 5E3 has three bypassed cathodes. Which one is getting switched? The one on the second tube? And if that is the cathode you want to wire your NFB to, then what will your NFB circuit look like? I hope not just a wire?

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    Have you actually tried both these things individually, and then in the combinations you propose?
    Haha! no I'm just starting to build the amp. I got the idea from Robrob's website. I figured it would be easier to do it now rather than later.
    https://robrobinette.com/5e3_Modifications.htm#Switched_Negative_Feedback
    but his switch is on the back. I thought it would be better on the front. Also his has a three way switch that gives NF with the bypass cap still in circuit.

    On mine the bypass cap would be between the switch and ground. The NFL would be going to the cathode of the first triode of the second tube, and it's bypass cap would get switched out at the same time.
    So the switch would provide a path either through the cap to ground or through a resistor to the speaker jack (negative feedback)

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    Old Timer Tom Phillips's Avatar
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    Best to post a schematic of exactly what you plan to do. A schematic is worth 10,000 words of discussion. And I still recommend that you do not run the feedback through a switch near the input jacks.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Tom Phillips View Post
    And I still recommend that you do not run the feedback through a switch near the input jacks.
    And vice versa, don't run input/first pre stage circuitry near the amp's output or power supply. Way too likely to pick up noise or start an unexpected feedback loop with squeeks, howls, honks, rumbles, not to mention ultrasonic junk that can not only make your amp sound awful but wreck output tubes & transformers. Just because "you read it somewhere" doesn't make it a great thing to do. Did HE successfully accomplish this mod, or just dream it up and put it on his webpage?

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    And I still recommend that you do not run the feedback through a switch near the input jacks.
    That's why I'm asking. I suspected that may be a problem. I'm 99% sure that the circuit would work as I expect, but I thought that routing the NFL to the front panel might cause hum, and also what about moving the cathode cap off the board and mounting it between the switch and ground?
    I'll see what I can do about a schem.

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    Click image for larger version. 

Name:	5e3mod.jpg 
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  10. #10
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by dmartn149 View Post
    ...what about moving the cathode cap off the board and mounting it between the switch and ground?
    That's usually workable but all ground points in an amp are not equal so you may need to use a terminal strip and run the ground wire to a suitable point such as the original place where the cap was grounded.


    Quote Originally Posted by dmartn149 View Post
    ...I'll see what I can do about a schem.
    One easy approach is to print a copy of a 5E3 schematic, hand mark it up and take a photo to post. It doesn't need to be fancy to support the discussion.


    General comment for your consideration: It may be best to keep the mod possibilities in mind when you build but don't permanently install them in the first round. Then patch them in one at a time and listen to the effect to determine if you like them. Everyone has different tastes and playing styles. I have found many times that the owner of an amp ends up using only one of the installed switchable mods all the time and never looks back. Just something to think about. Also consider that people who like the 5E3 like it because of its unique qualities and quirks. If you feel that you want mods such as adding a feedback loop then maybe you would be better off to start with a different base amp than the 5E3 Deluxe.

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  11. #11
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    One easy approach is to print a copy of a 5E3 schematic, hand mark it up and take a photo to post. It doesn't need to be fancy to support the discussion.
    I guess that's what I was doing while you were typing. Check the post just above yours.

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    Lifetime Member Enzo's Avatar
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    Routing the NFB near the input is not going to cause hum, but what it likely WILL do is cause instability.

    And you may find that one of those choices or in fact the combination, may not be to your liking, and then why build it in?

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    Quote Originally Posted by Enzo View Post
    Routing the NFB near the input is not going to cause hum, but what it likely WILL do is cause instability.

    And you may find that one of those choices or in fact the combination, may not be to your liking, and then why build it in?

    The option of a little more clean volume sounds like a good thing to me, and if I'm going to do it, (it's only a switch, resistor and a couple bits of wire) why not do it right from the start.

    What about deleting the stand by switch, and routing the NFB to that end of the chassis?

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    Old Timer Tom Phillips's Avatar
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    Dan,

    The mod you posted looks like it will work. I recommend that you put the switch in the external speaker jack hold on the bottom of the chassis. Yep, not as convenient as on the control panel but safer operation and wiring layout. It will make a significant difference in amp performance so I still suggest that you temporarily try it first per the reasoning in my post #10.

    You may also want to check out the feedback circuit topology used in the Princeton Model AA964. Reference schematic attached.

    Cheers,
    Tom
    Click image for larger version. 

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by dmartn149 View Post
    ...What about deleting the stand by switch, and routing the NFB to that end of the chassis?
    That's OK. The original didn't have a SB switch and copies don't need one anyway.

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    Thanks Tom. I probably will mount the switch in the ex sp hole, just to keep everything close, short leads, etc.
    That princeton NFB is interesting. Does the cathode bypass cap have an effect on the nfb at certain freq. or is that a dead end for the negative feedback due to the polarity of the cap?

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    Lifetime Member Enzo's Avatar
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    Doesn't work against the NFB. The cap keeps the voltage stable across the 1500 ohm cathode resistor. The 2700 ohm and 47 ohm resistors form a voltage divider. They sample off that small portion of the voltage signal at the speaker, and use it as the floor for that cathode resistor with cap. So the NFB signal acts as a moving ground point or reference point for that cathode circuit.

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    That Princeton NFB would be just a little more complicated than the circuit I was planning. What would be the sonic difference between them?

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    I put the Robinette NFB switch on my 5E3. I'm a clean-oriented player so it's a help: It lowers the noise floor and provides more clean headroom.

    Putting it in the standby hole makes sense. Drilling into stainless is a pain the neck -- I burned out several bits making the extra hole on mine, but once you get started you might as well finish!


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  20. #20
    Old Timer Tom Phillips's Avatar
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    Nice build photo Sam.

    Quote Originally Posted by S a m View Post
    ...Drilling into stainless is a pain the neck -- I burned out several bits making the extra hole on mine, but once you get started you might as well finish!
    That's one reason that I recommend using the external speaker jack hole for the added switch. It's in a good location to allow the wiring placement and we don't find people using an extension speaker with a 5E3 Deluxe anyway. That also makes it a reversible mod with no leftover hole.
    Cheers,
    Tom

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    Thanks for the input guys. I don't know yet if I'll actually use the amp in the NFB mode, but I doubt if I would ever reverse the mod. The stock circuit is always available at the switch, and this amp is not meant to be a "clone" anyway.

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