120Hz hum in a 50R

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Re: 120Hz hum in a 50R

Postby Unit_1 on Sat Mar 16, 2013 4:55 pm

Papa Dog wrote:
Unit_1 wrote:or you could just pay a couple of bucks for a full wave rectifier, temporarily bypass that part of the circuit and see if that works. if not, then just de-bypass to keep as original as possible.


Except that it should be a bridge rectifier, not a full wave. There IS a difference in that "full wave" uses two diodes and a bridge uses 4. The idea of a bridge being that it should leave a barely noticeable ripple, whereas a full wave has to have good filtering to smooth out the bumps. One of the kewl things about the Sound City circuitry is that it uses the bridge rectifier and still lays on a lot of filtering, and in most cases a choke, so it SHOULD have one of the quietest power supplies on the planet. But we all know that some models are notorious for hum and hiss. Could be the cheap diodes they used, or that sometimes diodes will degrade over time and let things pass that should not. Or it could be the cheap wiring, or electrolytic caps that people sometimes miss. Interesting thing to me is that the design was ahead of its time, but the bean counters in the front office tried to save money and specified lowest cost parts for lots of things. Good thing that they used arguably the best iron available at the time, which makes a rebuild well worth the money. IMHO YMMV etc etc ad nauseum, et al


ummm.... whoops. yeah one of those... lol... i get names wrong *a lot*

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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 4:55 pm

Not to flog the proverbial dead horse, but here is my own experience and why I blieve that what I am saying is correct.
Back c.1980 my 200+ started popping fuses, so I took it to the local "amp guru" who decided that my power transformer or output transformer was shorted "or something".
He did discover a leaky filter cap and did a royal butcher putting in an axial lead cap where a radial lead can is called for, and very dis-similar to the other caps. Seems that parts were hard to find back in the pre-internet daze.

First thing I did was to gather up some decent caps and redid all the big cans. Higher voltage (500v all around instead of 450v and 350v) and since the choke had been lost somewhere along a 30+ year route from one storage locker to the other, a new MM choke. Bigger I think, than they currently offer in the "tone clone" series, but one which, If I can find the specs on I would recomend to ANYONE re-doing a 120+ or 200+. Got that all done and discovered that the amp was still acting "hinky" so I checked the diodes. I can't really explain it, but they just did not "look right" to me, so i got 10 ne 1n4007s and re-did the diodes. Still, when doing voltage and current measures it didn't do right. Then I discovered sumthin that i have found no other reference to. When I changed out the diodes there were/are some thin wires connecting some of the adjacent turrets upon which the diodes rest. Some of those wires, while still in place, were not passing ANYTHING. Re-did those and all at once a beast once declaired "DOA" rose from the ashes one breathed fire. For the first time in over 30 years.

Maybe that is your issue, or maybe it is not, but it is interesting to me that the flux from the old solder could get between those wires and the turret and act as an insulator, making the bridge rectifier not work properly.
Just a public service announcement, FYI, IMHO, YMMV, and etc.
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Re: 120Hz hum in a 50R

Postby tracynorton on Sat Mar 16, 2013 7:06 pm

So, I swapped in a known good Partridge (MM checked it out for me a couple years ago), and a known good A470....still 120Hz ripple problem at speaker.
THE ENTIRE POWER SUPPLY EXCEPT THE PT IS BRAND NEW PARTS now. Brand new match EL34s. Biased at 70% plate dissipation.
The 6Vp-p ripple on power supply is normal (a tad over 1%).....the 4Vp-p 120Hz on the output is not.

BTW: a four diode bridge and a two diode w/ CT are BOTH fullwave rectifiers.......Papa-Dog....I suggest you purchase one of Merlin's books or read thru his website: http://www.valvewizard.co.uk/
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Re: 120Hz hum in a 50R

Postby tracynorton on Sat Mar 16, 2013 7:27 pm

I'm done. Guitar amps are $hit. I've had it. I'll never get those hours of my life back. It wasn't even fun.
I'm buttoning it up and giving it back to owner. No more.
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Re: 120Hz hum in a 50R

Postby Baron Von Machinenmann on Sat Mar 16, 2013 7:31 pm

tracynorton wrote:
BTW: a four diode bridge and a two diode w/ CT are BOTH fullwave rectifiers.......Papa-Dog....I suggest you purchase one of Merlin's books or read thru his website: http://www.valvewizard.co.uk/


Indeed.

There is no merit to the bridge "smoothing" comment as well, ALL dc as rectified before it hits the caps or choke is full of AC ripple. The choke and caps provide the filtering action that removes the remnant 120hz ripple. When caps start failing, the hum you hear is this 120hz AC.

The Jack Darr book has an excellent section on rectification here: http://www.diyguitarist.com/PDF_Files/j_darr3.pdf
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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 7:36 pm

tracynorton wrote:So, I swapped in a known good Partridge (MM checked it out for me a couple years ago), and a known good A470....still 120Hz ripple problem at speaker.
THE ENTIRE POWER SUPPLY EXCEPT THE PT IS BRAND NEW PARTS now. Brand new match EL34s. Biased at 70% plate dissipation.
The 6Vp-p ripple on power supply is normal (a tad over 1%).....the 4Vp-p 120Hz on the output is not.

BTW: a four diode bridge and a two diode w/ CT are BOTH fullwave rectifiers.......Papa-Dog....I suggest you purchase one of Merlin's books or read thru his website: http://www.valvewizard.co.uk/


Tracy, thanks for the link. I figure that one can never have too many resources to call on in understanding.

While you are technicly correct, I was just thinking "old school". My Uncle Harold, who, back in the day, was one of the best techs in the east Texas area, corrected me when I called the bridge a "full wave bridge". In his words (or close enough) we call them bridges to make sure that there is no confusion, as a common full wave recifier has only two diodes, and a bridge has 4." So...while it is technicly a full wave recifier, it is more than that.
At any rate, it is really not worthy of our time and effort to keep arguing about the definitions. WE both know the diode configuration to which we are refering.

I will be going back to the schematics again later today. I am certain that the answer is a simple one, and if the ripple is showing up on the speaker side of the OT, it would seem to be getting to ground somewhere or somehow. This whole deal has been a total head-scratcher for me, and I reckon a hair-puller for you. Yeah, I am prolly worng again, and I know you have put in lots of new parts. The question still remains, have you gone back and rechecked the solder joints on the rectifier? Have you checked the orientation of the filter caps? Not meaning to come off like any kind of "guru" since you obviously have forgotten more about tube amps than I will ever know, but as my Dad used to tell me: "when you fix something and it doesn't work as it should or as you expect, retrace your steps, 'cause ya prolly missed something small that makes a great big difference." There is no shame in re-checking your work, especially if you can find and fix the problem that way.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 7:47 pm

Baron Von Machinenmann wrote:
tracynorton wrote:
BTW: a four diode bridge and a two diode w/ CT are BOTH fullwave rectifiers.......Papa-Dog....I suggest you purchase one of Merlin's books or read thru his website: http://www.valvewizard.co.uk/


Indeed.

There is no merit to the bridge "smoothing" comment as well, ALL dc as rectified before it hits the caps or choke is full of AC ripple. The choke and caps provide the filtering action that removes the remnant 120hz ripple. When caps start failing, the hum you hear is this 120hz AC.

The Jack Darr book has an excellent section on rectification here: http://www.diyguitarist.com/PDF_Files/j_darr3.pdf


Heya Baron, I didn't know you lurked around these parts.

Ya know, i reckon that you are right, in once sense of the wording. The is always some "ripple" before it hits the caps and/or choke, but I would avow that rectified DC is way smoother than is AC, and a bridge produces a far "smoother" output than does a CT dual diode full wave rectifier. But then again, maybe it is in how *I* am seeing things and has no basis in what other perceive as reality (whatever that is). I printed out most of the Darr book. Good information there, for the most part, except that his orientation seems to be all about "clean". Sometimes that way of thinking builds some sterile sounding amps. I found the BBC Handbook on Negative Feedback and Distortion to be much more usefull in learning the "rules" so that I might know where to break them for the effect which I want. Kind of like learning music theory so that you understand that you should follow the rules and only break them when those rules break the music. Following the best practices policy of the BBC for audio and power amps is a good thing to do, except where the "best practices"effect things in undesirable ways.
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Re: 120Hz hum in a 50R

Postby Grendyll on Sat Mar 16, 2013 7:52 pm

you are in way over yer head bro, and i call bs on the bridge comment as well. a bridge is nothing more than four diodes in a convenient package, if you think that these "smooth" the pulsating dc you'd better crack open the books in those links. this isn't a question of semantics, and for the record the us navy trained me back in '80 using your tax dollars.
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Re: 120Hz hum in a 50R

Postby Baron Von Machinenmann on Sat Mar 16, 2013 8:02 pm

Papa Dog wrote:Heya Baron, I didn't know you lurked around these parts.

Ya know, i reckon that you are right, in once sense of the wording. The is always some "ripple" before it hits the caps and/or choke, but I would avow that rectified DC is way smoother than is AC, and a bridge produces a far "smoother" output than does a CT dual diode full wave rectifier. But then again, maybe it is in how *I* am seeing things and has no basis in what other perceive as reality (whatever that is). I printed out most of the Darr book. Good information there, for the most part, except that his orientation seems to be all about "clean". Sometimes that way of thinking builds some sterile sounding amps. I found the BBC Handbook on Negative Feedback and Distortion to be much more usefull in learning the "rules" so that I might know where to break them for the effect which I want. Kind of like learning music theory so that you understand that you should follow the rules and only break them when those rules break the music. Following the best practices policy of the BBC for audio and power amps is a good thing to do, except where the "best practices"effect things in undesirable ways.




Sorry, I cut my teeth on amps and DIY ham radio and I stand by my statement.

The oscilloscope will show you exactly what we are talking about as you probe through the high voltage section.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 8:08 pm

Grendyll wrote:you are in way over yer head bro, and i call bs on the bridge comment as well. a bridge is nothing more than four diodes in a convenient package, if you think that these "smooth" the pulsating dc you'd better crack open the books in those links. this isn't a question of semantics, and for the record the us navy trained me back in '80 using your tax dollars.


Never seen you around the board at all, but I know you must have been here fore some time to have over 300 posts.
Thing is, you do not know me, or what I know, and I am happy that the US Navy trained you in electronics. That is way 2 kewl 4 skewl. Just please, call BS all you want. I can call that right back atcha cause the bridge in most DA era and DMI era Sound City amps are NOT in a "convenient package" at all, but are 4 diodes wired in a ring pattern with AC going in to oposing sides, one end going to ground and the other producing the B+. If you call rectified current less "smooth" than full-on AC, then maybe it is YOU who needs to get back to the books.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 8:16 pm

Baron Von Machinenmann wrote:
Sorry, I cut my teeth on amps and DIY ham radio and I stand by my statement.

The oscilloscope will show you exactly what we are talking about as you probe through the high voltage section.


I understand what you are saying. Maybe I am not using the correct terminology here. Maybe Grendyll is right and I am way over my head.

You have been at this a lot longer than have I, so I will yield to your way of reference. OK....so no rectifier "smooths" AC current. Point taken and conceaded.

Still rather petty of all of us to be arguing semantics over whether or not a bridge rectifier is the same as a "full wave rectifier". I believe that Tracy Norton was right in calling it a "full wave bridge". I was just thinking back some 40 years ago when my Uncle was teaching me about DIY ham radio. Some of the things which he said have kept me from frying in the last 3 years, and yes, I have traced the HT circuits with a O'scope, so I know whatcher tawkin about.
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Re: 120Hz hum in a 50R

Postby Baron Von Machinenmann on Sat Mar 16, 2013 8:37 pm

It is simply a matter of class of rectification which uses standard terminology to define half wave or full wave rectification. The bridge rectifier arrangement allows for full wave rectification without using a center tap, this is a cost savings measure and again as seen/measured on a scope does not produce a superior/smoother/whatever pulsating dc than a center tapped transformer using only two diodes.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Sat Mar 16, 2013 8:43 pm

Baron Von Machinenmann wrote:It is simply a matter of class of rectification which uses standard terminology to define half wave or full wave rectification. The bridge rectifier arrangement allows for full wave rectification without using a center tap, this is a cost savings measure and again as seen/measured on a scope does not produce a superior/smoother/whatever pulsating dc than a center tapped transformer using only two diodes.


OK, you are right, and i am, once again, worng.
Or maybe not. I do not see using two bridges and two secondary windings on the OT of a 200+ to be "cost saving" in any sense of the word. I think that you might actually find that if you re-do your O'scope check with a finer resolution you will notice that the bridge actually cuts some of the crap that gets past a CT dual diode FW rec. Some guy that calls himself the "Valve Wizard" on the internet says it, then it MUST be the absolute truth, 'cause ya know, they can't put anything on the internet that ain't true. :wink:
Last edited by Papa Dog on Sun Mar 17, 2013 3:08 am, edited 1 time in total.
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Re: 120Hz hum in a 50R

Postby tracynorton on Sat Mar 16, 2013 8:44 pm

Baron Von Machinenmann wrote:It is simply a matter of class of rectification which uses standard terminology to define half wave or full wave rectification. The bridge rectifier arrangement allows for full wave rectification without using a center tap, this is a cost savings measure and again as seen/measured on a scope does not produce a superior/smoother/whatever pulsating dc than a center tapped transformer using only two diodes.

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Re: 120Hz hum in a 50R

Postby Unit_1 on Sat Mar 16, 2013 8:52 pm

tracynorton wrote:I'm done. Guitar amps are $hit. I've had it. I'll never get those hours of my life back. It wasn't even fun.
I'm buttoning it up and giving it back to owner. No more.


i could be just fartin' in the wind here, did you try different speakers, different power cable?

not that it's likely to be, but it wouldn't take much to check and it seems you've eliminated everything else...
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