For my SE 6V6 stereo amp I use OTs from an old Japan hifi amp. This OT has not originally been connected to ground at the speaker output so I didn't connect it either. But I've been trying to add some feedback line and getting no results even with only 220ohm resistor. Any ideas why?
I did try doing feedback from next stage tube but that didn't sound good to me so want to try it from the speaker which seem more common anyway.
I did try doing feedback from next stage tube but that didn't sound good to me so want to try it from the speaker which seem more common anyway.
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The no ground connected secondary has no reference with respect to the main circuit, therefor neither does the feedback signal. You will have to ground one side of the speaker connection and take the feedback signal from the other side.
Best to run the two wire together (twisted is good) and make the ground connection at the closes 0V point to where you are applying the feedback.
If it oscillates then the feedback is the wrong phase, swap the wires such that you are grounding the other end of the speaker output.
Cheers,
Ian
Best to run the two wire together (twisted is good) and make the ground connection at the closes 0V point to where you are applying the feedback.
If it oscillates then the feedback is the wrong phase, swap the wires such that you are grounding the other end of the speaker output.
Cheers,
Ian
You will have to connect one side of the secondary to your amp's common 0v rail, to get GNFB to work unless you have a specific winding on the OPT just for negative feedback
Edit: Ian beat me to it 🙂
Edit: Ian beat me to it 🙂
Thanks. I'll give the grounding a go and see how it goes. I was little worried grounding it since it wasn't grounded in the first place for what ever reason.
It wasn't grounded because the original circuit didn't use it for feedback (possibly not too interested in sound quality) and wasn't interested in user safety.
The reason you have to ground it for feedback is that there is no such thing as 'a voltage'.
The reason you have to ground it for feedback is that there is no such thing as 'a voltage'.
Yes. Important to get the signal input and feedback to have the same 'ground' reference.gingertube said:
True, I was wondering what would happen if the OT went short not being grounded.
Anyway, I've grounded it and FB now "does something". I did it temporarily on cathode, had to use 2k to get something that is noticeable. Sound gets brighter but brighter it gets, the more bass it looses. I used only one channel on the amp so it should be phasing effect. Top end is good but the loss of bass I don't like. What em I doing wrong?
Anyway, I've grounded it and FB now "does something". I did it temporarily on cathode, had to use 2k to get something that is noticeable. Sound gets brighter but brighter it gets, the more bass it looses. I used only one channel on the amp so it should be phasing effect. Top end is good but the loss of bass I don't like. What em I doing wrong?
The 'loss of bass' is almost certainly a sign that the feedback is doing one of the things it is supposed to do: lowering output impedance. This adds damping to the speaker bass resonance; you are now hearing the sound more as it is supposed to be, rather than with lots of 'one-note' bass. Another explanation is that the feedback is reducing bass distortion so you are hearing cleaner bass; bass distortion can mimic extra bass.
If you have been used to hearing distorted bass-heavy sound then something a bit more like hi-fi may sound strange to your ears.
If you have been used to hearing distorted bass-heavy sound then something a bit more like hi-fi may sound strange to your ears.
Yeah, I always listen to tube amps without feedback 🙂 I like that big soft sound. But for this amp I thought I'll try to put one in, maybe with a switch in case I dont' want it.
So I guess if I put a pot on the feed back I'd be able to tune the sound to the tone balance I like or is there a way to calculate the parentage I should be using so it's done properly?
So I guess if I put a pot on the feed back I'd be able to tune the sound to the tone balance I like or is there a way to calculate the parentage I should be using so it's done properly?
You can't calculate what is for you a matter of taste. A circuit which has variable feedback will almost always be sub-optimal, because ideally other things in the circuit need to change with the amount of feedback (e.g. open loop gain, frequency compensation). The result is that people who try variable feedback usually decide to switch it off, and then proclaim that feedback is bad for sound.
If your aim was sound reproduction fidelity then feedback could be designed to deliver the required low distortion, low output impedance and wide frequency response. Your aim is something different, so best to use a bit of variable feedback and twiddle the knob until you like the sound.
If your aim was sound reproduction fidelity then feedback could be designed to deliver the required low distortion, low output impedance and wide frequency response. Your aim is something different, so best to use a bit of variable feedback and twiddle the knob until you like the sound.
I feel compelled to say this every time a floating secondary is mentioned: Everyone, please don't do this. It's foolishly dangerous. I don't care about your skunky butt, but children and pets deserve the best you can do about safety. Get right with the gods of electricity!
Rant off, and thanks,
Chris
Rant off, and thanks,
Chris
Well I've done few hours of listening and adjusting the NFB. I find some benefit to it as there is less distortion when driven louder or when parts in music peak high. Not sure if it is the NFB doing it or just because it cuts the bass a bit, having less bottom end makes the tubes distort less. It's interesting but I need to do more listening to see how I like it.
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Hi Chris,
I've often wondered about this, but I don't understand the mechanism by a which a floating secondary becomes hazardous. Is it an electrostatic thing, or am I way off? If you have a minute to explain or post a link, I (and presumably others) would appreciate it. Thanks!
-- Jim, whose butt is currently fresh as a daisy.
I've often wondered about this, but I don't understand the mechanism by a which a floating secondary becomes hazardous. Is it an electrostatic thing, or am I way off? If you have a minute to explain or post a link, I (and presumably others) would appreciate it. Thanks!
-- Jim, whose butt is currently fresh as a daisy.
Yes the broken insulation makes sense, specially with old trafos. What I wonder is why would they make it floating. I've came across some tube amp making website where they boast that they make floating OTs and that they are hard to make. So why even bother when there is potential safety issue?
Sorry, I don't understand. The secondary winding is always 'floating' in the sense that it is insulated from the primary and the core. The amp circuit should then ground one side of the secondary.
Maybe the "floating" not correct term. It is an OT that has positive and negative wires to speaker from the secondary. The negative was not grounded for some reason I don't understand, it would have been simple to do with no extra cost to the manufacturer of the amp.
Not sure if I'm on the right track but I'm still fiddling with the NFB. One thing I've noticed when I put only little on it, the music doesn't change any more but some songs that have excessive bass seem to level out more. Apart from controlling distortion, is this also the desired effect?
Only you know what the desired effect is. The actual effect of feedback, as I said, is to reduce output impedance and distortion and gain. Also to widen bandwidth.
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