Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on September 30, 2020, 05:37:10 pm
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I have a spare ground switch left over from my Princeton kit. I'm debating replacing the ext speaker jack with the unused switch to break the NFB. Two questions....
1. Is this a correct way to defeat the NFB connection (see schematic)
2. I assume it would be best to try this first to see if I like it. Can I just unsolder the NFB connection at the Ext Jack for tone sampling?
I did the raw control in the ground switch hole. I like the raw EQ, but I think I am fighting the fixed bias nature of the princeton. It's got a stiffness to it I don't care for when I am going for those tweed tones. I presume a mixture of no NFB and raw EQ bypass will get me more into tweed land.
Note: I am aware the ground switch I have is a SPDT. Not an issue though.
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1. Is this a correct way to defeat the NFB connection (see schematic) yes
2. I assume it would be best to try this first to see if I like it. Can I just unsolder the NFB connection at the Ext Jack for tone sampling? yes
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What I have done on BF 1164 Princetons (and PRs) is take out the #2 input jack and install a mini 250R or 500R pot in its place and wire it inbetween the bass pot left side terminal (as viewed from the rear) and the 5.6K resistor to ground. On Champs and VCs I also like to install the feedback resistor disconnect switch. With these two controls you can dial in some pretty nice tones.. Change out the input jack 68K grid stopper resistor with a 33k when using only one input jack. You can use any push-on knob that you like. The little pot is called a RAW pot. Note - when you partially or fully bypass the tone controls on a PR the amp can get quite loud for a given volume setting compared to stock. The total tone stack bypass is an acquired taste, and to me it's a little to much so I prefer the RAW pot.
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fwiw;
stole this NFB "option" n used it on my last build, also mostly stolen :icon_biggrin:
I'm not a NFB guy, gotta say, i like the subtle changes, but mostly it's off
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looks like this
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looks like this
Yes, I did a 100K raw pot in the ground switch. In series with the “mid resistor“ on the bass pot and ground, as you said. I like it well enough. I tested many values with the Duncan tonestack calculator. It’s gets the fully raw eq curve right but without the massive gain jump. Will probably replace it with a push pull at some point to be able to completely bypass tonestack altogether.
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Or you could short the 47R with a switch - same result
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Yes you could do that. But what jordan suggests is just so simple that it becomes elegant in it's simplicity. You have two wires on the jack. Just unsolder them, pull the jack, insert switch, solder wires to switch. Done. No wires to run.
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Yes you could do that. But what jordan suggests is just so simple that it becomes elegant in it's simplicity. You have two wires on the jack. Just unsolder them, pull the jack, insert switch, solder wires to switch. Done. No wires to run.
Pleasingly ingenious and simple is what elegance in the scientific means. Problem I have is I read how to mod stuff just like Jordan86. Most of my amps at one time had elegance, now they have MODS, scratched up and torn grill cloths. It seems once I build a new one looking for that perfect tone, I MOD (hack ups the inside) of my previous.
Fender amps, every one I have ever had I simply remove the NFB wire. I prefer the tone as it seems more attack responsive and I can get good cleans as well. 2 channel preferred. Then I have more holes and I do own a drill if I need more. I did play an original 1955 PRO a few weeks ago. Yes, I must say the ratty old thing costs more than a $1000 hooker, but just like the gal, worth every penny. I've got a 100,000 dollars in pennies, not a nickel to my name.
Marshall and EL34's are much different. Tweakiing the feedback on these to get the perfect Crrrraaaaaannnnnnggggg!
Story time!
I have a real live audience attended show to play on October 18th. Every scheduled event has been filled with hopes and then cancelled. The organizers of this have said we doing this come hell and high water. Now everyone knows chicks dig guitar players and drummers. Hell, that's why we learned to play, isn't it?
So, the dude who plays Bass. You know bass players are generally very laid back and as a guitar player I remind them I can play faster, better and cooler than you and if you screw with me I will remove everything except full volume and full treble. The bass player says whatever dude, ya wanna spark one? I ain't got time for this nonsense, well not until I get setup anyway.
So he comes back and says, "we gonna have to wear masks"? Dude sings too! So, come hell and high water Mule (bass player) will simply plug into his SWR and turn it on. Said singing through a mask would be like a windscreen on a Mic.
Now what can you say to that?
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I think I'm going to install a nfb switch in the ext spk jack to mine. :icon_biggrin:
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I don’t often have my ideas referred to as elegant. But coming from Sluckey, I take that as a very nice compliment.
I think the elegance was simply a byproduct of wanting it to be as non invasive as possible. You do have to sacrifice the ext jack, but I haven’t found myself using that much at all. Plus there are other ways around that to run multiple cabs. My buddy from another forum makes these. Simple but elegant splitter box. Jacks wired in parallel or series. http://procablesnsound.com/items/speaker-y-cables/HAPCAB-detail.htm
I hope anyone who tries the switch finds it to be simple and useful.
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I've had a nfb switch and the raw pot installed in a Champ AA764 head I built a while back, and I really like the different tones I can get using both.