New ReleaSSSes

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glasman
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Re: New ReleaSSSes

Post by glasman »

mhartman wrote:
David Root wrote:I used a Fasel yellow inductor. Measures between 0.6 and 0.7 H on my LCR meter, depending on the frequency used. (120Hz & 1 KHz)
Great idea David. I wonder what the Fasel Red inductor measures? It looks like Cinemag has some inductors that would come close as well:

http://www.cinemag.biz/inductors/inductors.html

Also, they typically can do custom work pretty cheap.
The fasels are around 600mH. If you model the filter you will find the absolute value of the inductor is not as critical as just having a fairly large inductor at that location.

Small bear electronics has WAH inductor kits for the wind it yourself people.

Cinemag is cool stuff, but the prices are pretty steep.
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glasman
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Re: New ReleaSSSes

Post by glasman »

gar13 wrote:I have a few of extra 820mH Wilco inductors, seems like the value might be close. If anyone wants one, they can have it (I'll keep one for myself). PM me.
First time I checked out the Wilco site, nice options.

Found quite a few in the right range. Ordered a few to try out and test.
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passfan
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Re: New ReleaSSSes

Post by passfan »

swt wrote:can someone explain to me, please, the use of those feedback networks...sonically speaking...
Feedback is somewhat an error correction device. In this case I believe we are correcting for distortion in the signal and possibly linearity. By giving up output we inject that back into the circuit, out of phase (someone correct this if not so) with the original signal thereby cancelling out the offending artifacts and allowing the preamp to "boost" the signal even more. It's a balance between how much output and how much correction...like a see saw trading off one for the other. Dumbles use of local feedback (section specific) instead of global feedback may be part of the key to the SSS being so loud and clean. Hifi guys use feedback to correct for linearity and distortion all the time ..... sometimes to correct for poor circuit design due to lazyness.
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swt
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Re: New ReleaSSSes

Post by swt »

thansk for the explanation. will have to try it and listen, but i guess that the cf stage is more important to the sound i'm after. has anybody ever tried a cf after first stage before tone stack?
tictac
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Re: New ReleaSSSes

Post by tictac »

anybody ever tried a cf after first stage before tone stack?
That's actually a mod Dave Funk used to do with Fender amps....

Some people must like it....

TT
oldmacman
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Re: New ReleaSSSes

Post by oldmacman »

passfan wrote:
swt wrote:can someone explain to me, please, the use of those feedback networks...sonically speaking...
Feedback is somewhat an error correction device. In this case I believe we are correcting for distortion in the signal and possibly linearity. By giving up output we inject that back into the circuit, out of phase (someone correct this if not so) with the original signal thereby cancelling out the offending artifacts and allowing the preamp to "boost" the signal even more. It's a balance between how much output and how much correction...like a see saw trading off one for the other. Dumbles use of local feedback (section specific) instead of global feedback may be part of the key to the SSS being so loud and clean. Hifi guys use feedback to correct for linearity and distortion all the time ..... sometimes to correct for poor circuit design due to lazyness.
EE here. Yes and no... negative feedback (NFB) effectively trades gain for reduced distortion and lower output impedance.

NFB does indeed make a stage more linear and "clean". The tradeoff is that the distortion that remains is the higher-order harmonics that people find more musically objectionable, so there's a balance between too little and too much.

NFB reduces the output impedance of a stage. This is useful for a couple of things. When used in the power amp, it increases the damping of the stage, which means that the power amp can more tightly control the motion of the speaker. (This makes the amp feel not as "tube-y" and many guitarists don't like the feel.) When used in the preamp, the lower output impedance means that the stage does a better job of driving capacitive loads like a tonestack or filters without the high frequencies getting rolled off (see feedback to cathode of V1a/V1b). When used in the reverb send a la ODR/SSS (see feedback to grid of V5), it increases the damping so the stage can do a better job of driving the reverb tank.

Finally, you can adjust the overall frequency response of a stage by tuning the NFB. When you adjust the cathode cap, you're actually tuning the NFB of that triode stage. You can see that Dumble did this by adjusting the value of the capacitor in the NFB loops to V1a and V1b (.1u in one and .22u in the other).

So when is it a good idea to use NFB? If a stage is never going to be driven into clipping, and the stage has more gain than you need (e.g. you're going to be dumping some/most of the signal with a voltage divider anyway), and lower distortion / reduced output impedance is something you care about, then it might be a good idea.

When is it not a good idea? I don't usually like too much of it in a stage that's going to be overdriven. The NFB results in a more sudden onset of distortion -- as long as there's gain/headroom to maintain the NFB, it stays clean, but when it gets pushed to the point that the negative feedback signal doesn't have much effect any more, then suddenly you've got clipping. This is why people love Vox AC30's for jangly dirty rhythm -- no global NFB means a seamless transition from OD to clean as the signal decreases. I used to have a Traynor Bass Mate with no global NFB that did that sound better than anything else I've played.
passfan
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Re: New ReleaSSSes

Post by passfan »

And now I have an even better understanding myself, thanks.
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67plexi
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Re: New ReleaSSSes

Post by 67plexi »

This helped me out to get a better understanding, 15-Tone-Compensation-and-Tone-Control.pdf

http://frank.pocnet.net/other/RCA/Radio ... h-Edition/

In fact it’s worth your time to read all the RCA. Designer's Handbook - 4th Edition.
swt
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Re: New ReleaSSSes

Post by swt »

wow guys thanks a lot for your effort, and time. really helpful. thanks!!
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Re: New ReleaSSSes

Post by crbowman »

The inductor L1 has to carry the greater part of the plate current of v1 (several milliamperes in a typical case) and should have a butt-joint or air gap to reduce the effect of the plate current on the inductance.
First off, thanks Plexi for the link to the RDH from which this quote has been lifted.
Not being very familiar with inductors in general can someone elaborate on this a bit?
Do we know if the Fasel inductors have this butt-joint/air gap? Haven't yet found a definitive data-sheet.
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Re: New ReleaSSSes

Post by ER »

An air gap in the laminations reduces the permeability so you will raise the amount of DC current the transformer can handle before reaching magnetic saturation.

Single ended applications need this since the primary sees the full DC (unlike a push pull primary) also since the air gap reduces the permeability you will need a larger core than you would without it.
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glasman
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Re: New ReleaSSSes

Post by glasman »

I had some time this morning and breadboarded the SSS Filter stack. The only liberty I took was to use a 1M pot in lieu of the switched resistor array. The results were interesting. I measured the frequency response of the filter using an Ono Sokki CF-5220 FFT Analyzer. Swept from 1 to 20000 Hz.

Observations :

1. The frequency response does not change at all for inductors from .5H to 1H.
2. You get EXACTLY the same response if you substitute the inductor with a piece of wire.

Conclusion : Inductor is not required. Look at the XL of the inductor it is extremely low at the frequencies we are working with and has no effect.

Here are the response charts I measured for the various switched cap values. All measurements were taken with the pot set to simulate Position 4.
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glasman
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Missing Chart

Post by glasman »

The 150pf Chart
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Colossal
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Re: New ReleaSSSes

Post by Colossal »

glasman wrote: Observations :

1. The frequency response does not change at all for inductors from .5H to 1H.
2. You get EXACTLY the same response if you substitute the inductor with a piece of wire.

Conclusion : Inductor is not required. Look at the XL of the inductor it is extremely low at the frequencies we are working with and has no effect.
That is extremely cool work Gary. Thanks very much for sharing that. I love seeing the results of experiments like these. Can you postulate on why HAD included the inductor if your data show that it has no effect?
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glasman
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Re: New ReleaSSSes

Post by glasman »

Colossal wrote:
glasman wrote: Observations :

1. The frequency response does not change at all for inductors from .5H to 1H.
2. You get EXACTLY the same response if you substitute the inductor with a piece of wire.

Conclusion : Inductor is not required. Look at the XL of the inductor it is extremely low at the frequencies we are working with and has no effect.
That is extremely cool work Gary. Thanks very much for sharing that. I love seeing the results of experiments like these. Can you postulate on why HAD included the inductor if your data show that it has no effect?
I have heard that later SSS's did not have the inductor, this is only street information.

Unknown, he may have been following some cook book recipe for the EQ (radio handbook etc) so not sure. The Radio Handbook, etc. The XL of the inductor is so low as compared to the size of the other parts it is totally swamped value wise.
Located in the St Croix River Valley- Afton, MN
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