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PostPosted: 30 Jul 2016, 06:38 
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I found the answer to my last question by looking at past post, and yes the screen wires need changing as well. For some reason my amplifier sounds better without NFB. With NFB there is some harshness in the upper ranges. Also there is some oscillation on one channel at the extremes of the volume control that does diminish as the amp warms up. With the NFB the sound stage does appear to be better but there is definitely something happening at the upper frequency. The tone of vocals also changes. Without the NFB the sound is great. I am "almost positive" I have the correct values of the resistors and capacitor and they are hooked up correctly. I am going to do some more checking today. As I have said it sounds fantastic without the feedback but I want to understand exactly why this is happening. Any clues are appreciated.

Update: After looking at Bruce's homepage I read that his commercial version has the option of using NFB or not. I am now satisfied using this amp without the NFB but it does bother me why the NFB deteriorates the quality of the sound when it should make it better.

I changed out the coupling caps from the Cornell DME to some Russian PIO ones I have and I cannot tell that much difference. Both sound outstanding when there is no NFB.


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PostPosted: 30 Jul 2016, 10:22 
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Hi Wdecho,

Sorry I didn't follow the whole thread. Did you use the same output transformer, Edcor model# EMO750? If you didn't use that specific output transformer then you may have to adjust the 470pF feedback compensation cap to one more suited for the transformer you used.

Other than that maybe the lead dress is causing unwanted parasitic capacitance causing excessive phase shifting.


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PostPosted: 30 Jul 2016, 11:19 
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Yes I used the transformers as a kit that Bruce designed. I am thinking that I need to hook up the NFB components closer to the junction of the 100uf cap and 680R. I agree that the problem may be leads being too long and causing the problem. I have the 2 parts of the NFB on a terminal strip with wires connecting the NFB parts to the cathode junction. I am going change this first and see if it corrects the problem. I pulled both the caps and resistors and tested them and they are the correct values but with excess wires the capacitor being just pf that the capacitance is changing. I will see.


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PostPosted: 30 Jul 2016, 12:35 
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One thing I have had happen to me with Edcor transformers that created all sorts of instability when engaging feedback loops was they marked the screen taps wrongly.

For instance you would have to use the blue with the brown/white for one side and brown and blue/white for the other. You can easily confirm this by checking resistance with a multimeter. Place one probe on a plate tap and measure to it's corresponding screen tap, then measure to the other screen tap and see which has the lowest resistance.

Long plate and grid leads spaghetti wiring will give you unwanted phase shift at higher frequencies me thinks. Also always put the grid stop resistors right on the tube socket pins.


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PostPosted: 30 Jul 2016, 12:41 
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Another thing I noticed when looking at the schematic is that there should probably be a small value resistor on the cathode output of the "SRPP" stage serial to the .33uF cap. Try 47R-100R


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PostPosted: 30 Jul 2016, 17:37 
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Hi, Your problems are most curious. None of the amps here have those problems, additionally there are many of the commercial ones out there and they don't either. My suspicion is there is an issue with grounding or layout. The circuit is not particularly sensitive to it, but is worth looking into. One thing that you mentioned that can cause some problems is lead length and placement of the primary side wires going to the output transformer. Way back in a similar amp I ran into some instability because the wires were long and too close to other circuitry. As I recall it was the nfb components, but I may have that mis-remembered (it was about 7 years ago). The only other sort of instability that occurs is because of incorrect transformer phase connections. I am surprised that you got defective transformers. I (and Oddwatt) have ordered 100's and there have been exceptionally few with any sort of problem. The few that had problems FWIW had leads that were not secured tightly enough and came loose. On the commercial side we had two power trannies with open windings...but that is it. Anyhow if the trannie is not right...let them know as they want to take care of any problems. I would not add a resistor to the 0.33 cap. There is quite sufficient impedance in that circuit already. However the 220R resistors do need to be right at the tube sockets to work properly . If your amp still misbehaves PM me and I'll make arrangements for hi-res photos and see if I can spot the problem. These amps excell in clean high frequency response and have never presented a stability issue. BTW I always use the NFB on my amps. Yes there is a slight difference in sound with it defeated, but not of the magnitude you indicate.

Good listening
Bruce

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PostPosted: 30 Jul 2016, 19:52 
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Thanks for the info Bruce. I spent a few hours today rearranging the NFB components closer to the cathode connection and then checked all the wiring to be absolutely sure everything is hooked correctly with the right values. With the NFB connected the sound is not good but it is outstanding without the feedback. I have played around with positive current feedback in a SS F6 amp and had somewhat of the same problem that was solved by repositioning some wires. I also am wondering if there is some problem with my choice of tubes, the 6P15P's. One channel with the NFB is somewhat unstable for a few minutes and then settles down without problems but the sound from both channels sound as if the high frequencies are cut off and there is some edginess on the high notes. I am sure it is an easy fix. The problem is finding the problem if I want to use the NFB in the amp. From a builder's perspective I would like to know what the problem is but the amp is great without the feedback.

For now I am just going to enjoy the amp as is and not worry about what or how to fix the amp with NFB. With my speakers I am probably never using more than 2 watts so the distortion without feedback is going to be so low as to be a non issue. As I have said this amp will hold it's own against any of the amps I own and I am like Bruce and do not know how many amps are laying around the house. Most all of them are Class A amps.


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PostPosted: 30 Jul 2016, 20:27 
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Hi, if the amp sounds OK without the NFB but edgy with it then it is likely that the output is phased backwards. The amps will work that way, but the sound is poorer. If the sound level goes down when the NFB is active and up when it is off then it is correct and there is some other problem. However if it goes up with the NFB active the phase is backwards. When you use NFB the sound can't get worse really, It could alter the frequency response, but not cause instability in the small amount used. The distortion without NFB is typically between 1-2% (2% at full power). The NFB cuts those values in about half. The response measures the same out to about 30KHZ with NFB and will (depending on the build and components) go past 50K without. That incidentally is the reason for the NFB....just enough to ensure stability above the audio band. I have not seen it happen, but in theory it is possible to excite the resonance of the output transformer which in this circuit (with the components I used) is at about 65K.

Good listening
Bruce

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PostPosted: 31 Jul 2016, 09:48 
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I noticed that I had not ground the common output wire but after doing this the amp oscillated horrible. I then did some more testing with jumper wires and found that if I used the yellow OPT wire as common and ground from there and used the white as 8 ohm the amp worked as it is supposed to with NFB. I think I should probably swap the plate connections, blue and brown wires along with the screen wires again for it to follow the wiring colors and schematic and then ground the white common wire and use the yellow wire for 8 ohm and I believe Bruce has addressed this before. I hope no other builder is going to throw stones at me for my error.

Please let me know if I need to rearrange the plates and screens and correct the outputs to follow the color of the wires. For now it sound great wired as is.


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PostPosted: 31 Jul 2016, 11:13 
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Let me get this straight here.

Upon firing the amp up for the first time you had the output phasing wrong and disconnected the feedback effectively eliminating the positive feedback loop.

You then reversed the primary plate AND screen taps and the feedback still didn't work?

You reversed the output and the feedback works?

I re-read the posts and I am guessing that you never reversed the primary taps and instead rearranged the feedback wiring which then lead us to think you had an instability issue.

Reversing the output which I suggested in the email is the same thing as reversing the primary, but DO NOT DO BOTH, either one or the other cures the problem. You reversed the secondary so leave it as is you are all done. :)

Enjoy the new toy! :D


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