120Hz hum in a 50R

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120Hz hum in a 50R

Postby tracynorton on Wed Feb 20, 2013 6:29 pm

I've narrowed this down to the PI and feedback circuit via the opposite input (.15uF cap). Because all the Mains and HT wiring runs willy nilly back and forth across the whole amp.....I just don't know what to do about it. It's appearing at ~140mV on the output....and that's ridiculously loud coming out of a speaker.....
The HT and PI HT voltage is dead flat on the scope...so I think some kind of current ripple is inducing a voltage on the NFB below the PI.....it only appears on the .15uF cap side of the PI....but gets amplified by the PI to the output.
I've recapped the entire amp (large ARS caps on the HT). Both inputs to the PI are shielded (which helped a lot....output was over 200mV before!)
Has anyone encountered this? Is there a solution w/o redesigning the whole amp.
TIA
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Re: 120Hz hum in a 50R

Postby Papa Dog on Wed Feb 20, 2013 7:02 pm

tracynorton wrote:I've narrowed this down to the PI and feedback circuit via the opposite input (.15uF cap). Because all the Mains and HT wiring runs willy nilly back and forth across the whole amp.....I just don't know what to do about it. It's appearing at ~140mV on the output....and that's ridiculously loud coming out of a speaker.....
The HT and PI HT voltage is dead flat on the scope...so I think some kind of current ripple is inducing a voltage on the NFB below the PI.....it only appears on the .15uF cap side of the PI....but gets amplified by the PI to the output.
I've recapped the entire amp (large ARS caps on the HT). Both inputs to the PI are shielded (which helped a lot....output was over 200mV before!)
Has anyone encountered this? Is there a solution w/o redesigning the whole amp.
TIA


Tracy,
I have never worked on a 50+ before, so I have no ready answer.
Give me a couple days to look over the schemo and I *might* be able to point you in the right direction.

In the mean time, I would be forever in your debt if you would be so kind as to take some measurements of the head cab and PM them to me. A reference pic with a ruler included would be nice as well.

Thanks,
John
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Re: 120Hz hum in a 50R

Postby tracynorton on Wed Feb 20, 2013 8:00 pm

Papa Dog wrote:Give me a couple days to look over the schemo and I *might* be able to point you in the right direction.

In the mean time, I would be forever in your debt if you would be so kind as to take some measurements of the head cab and PM them to me. A reference pic with a ruler included would be nice as well.

Thanks,
John


Well, I can tell you right now these things are a cluster#uck as far as layout.....there is no definitive 50R schematic. I made a schematic of this amp...but the layout...holy #ucking chri$t!

If anyone wants a copy...PM me w/ an email address...it's a 4 page pdf.
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Re: 120Hz hum in a 50R

Postby Mr. Foxen on Fri Feb 22, 2013 1:56 am

High res gutshot of the B50 I'm in currently here: http://ampstack.wordpress.com/2013/02/1 ... ey-aor-30/

I took a pic of the sleeve and a tape measure, last gasp of camera battery, might end up sending later.

Edit: mine goes into motorboating oscillation when you go past a certain volume. All caps have been one, pointers would be appreciated.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Fri Feb 22, 2013 5:02 pm

tracynorton wrote:Well, I can tell you right now these things are a cluster#uck as far as layout.....there is no definitive 50R schematic. I made a schematic of this amp...but the layout...holy #ucking chri$t!

If anyone wants a copy...PM me w/ an email address...it's a 4 page pdf.


Tracy, I honestly thought I remembered a 50R schematic on the Sound City Site, but, alas, as you said there is not one there. What is there is a 50+ schem, and a couple of reverb schems that would apply to any Mk4 amp.
A total PITA IMHO. I got your e-mail and printed off your schematic. Very well done, sir. You are a credit to your profession, and seem to be a heck of a nice guy as well. With your permission I will post that on the Sound City Site. I will make certain to give credit to you for your fine work.

I believe that the issue you are having with the 50R on your bench is three things.
(1) there is an issue with the bridge recifier circuit. at least one of the diodes is shorted to ground.
(2) the NFB resistor has drifted off tolerance. it is passing way too much NFB to the PI.
(3) the decoupling cap on the PI is likely bad as well. I would change it, even if it scopes as good.

I hope that helps. Please do keep us updated on your progress.
Thanks again for your help with my project.
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Re: 120Hz hum in a 50R

Postby tracynorton on Fri Feb 22, 2013 5:18 pm

Papa Dog wrote:With your permission I will post that on the Sound City Site. I will make certain to give credit to you for your fine work.

I hope that helps. Please do keep us updated on your progress.
Thanks again for your help with my project.


It would help if anyone else that has a 50R check it against their stock amps and confirm...I could update the sch and eventually you would have a definitive working sch for these things.
Right now, that sch is just an 'as built' for the one I have on the bench.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Fri Feb 22, 2013 5:35 pm

Mr. Foxen wrote:High res gutshot of the B50 I'm in currently here: http://ampstack.wordpress.com/2013/02/1 ... ey-aor-30/

I took a pic of the sleeve and a tape measure, last gasp of camera battery, might end up sending later.

Edit: mine goes into motorboating oscillation when you go past a certain volume. All caps have been one, pointers would be appreciated.


Oli,I got your e-mail, and thanks much. Between an actual picture and Tracy Norton's excelent CAD drawing I should be able to get a copy done in short order.

RE: motorboating...I read something about that on the Marshall forum. IFRC on bass amps too little NFB will cause motorboating. I would wager that the NFB resistor is off spec or has a broken connection. I hope that helps.
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Re: 120Hz hum in a 50R

Postby Mr. Foxen on Sun Feb 24, 2013 7:46 pm

I checked the NFB resistor and it was in spec according to its stripes, but different to the schematic. Looks like the actual problem is the new preamp valves the guy had fitted in the hope of fixing tone issues with the amp were not working properly. Lesson in not trusting anyone else's work for me there.
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Re: 120Hz hum in a 50R

Postby tracynorton on Tue Mar 05, 2013 1:00 am

I recapped this amp w/ brand spankin' new ARS 200uF 500V caps. I'm getting ~1Vrms ripple at B+ at the rectumfryer......that seems like an awful lot compared to other amps I've seen......is this normal? It's about 1.5Vp-p on the scope. I have some F&Ts...but want to rule out anything else before I swap them out. I took the PI out of the equation....the 'hum' is emanating from the power section. The OT is not cancelling it out very well. The EL34 are fairly well matched.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Tue Mar 05, 2013 1:44 am

tracynorton wrote:I recapped this amp w/ brand spankin' new ARS 200uF 500V caps. I'm getting ~1Vrms ripple at B+ at the rectumfryer......that seems like an awful lot compared to other amps I've seen......is this normal? It's about 1.5Vp-p on the scope. I have some F&Ts...but want to rule out anything else before I swap them out. I took the PI out of the equation....the 'hum' is emanating from the power section. The OT is not cancelling it out very well. The EL34 are fairly well matched.


In that case I will return to my "gut instinct" first response. Even if the diodes check out on the meter, I would replace them all. Diodes are cheap, and you won't be out much if that proves to NOT be the cause of the problem.
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Re: 120Hz hum in a 50R

Postby Mr. Foxen on Tue Mar 05, 2013 2:40 am

Does it have snubber caps on the diodes?
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Re: 120Hz hum in a 50R

Postby tracynorton on Tue Mar 05, 2013 4:15 am

brand new UF4007s, still ripple
no snubbers, but this is definitely ripple...not switching transients
I'm swapping out the ARS caps and putting in F&Ts.....
I can put in 330uFs and prob knock it down w/ serious time constant, but that will be if the 200uF F&Ts don't work.....I'm sick of this POS
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Re: 120Hz hum in a 50R

Postby Papa Dog on Tue Mar 05, 2013 9:47 am

tracynorton wrote:brand new UF4007s, still ripple
no snubbers, but this is definitely ripple...not switching transients
I'm swapping out the ARS caps and putting in F&Ts.....
I can put in 330uFs and prob knock it down w/ serious time constant, but that will be if the 200uF F&Ts don't work.....I'm sick of this POS


Now, that makes it a real head scratcher....the thing that puzzels me most about it is that the "ripple" is 120Hz and not 60Hz. That would seem to indicate that only one of the diodes is actually "smoothing", one of them is not working and the other two are making a very crude (but effective) frequency doubler. That is the heart of many of the "octave doubler" effects on the market...so....I would want to check the bridge carefully and make sure that everything is passing current the way that it should....sometimes the little wires that run between the diodes will not actually be passing current/voltage and will look like they are hooked up just fine....other than that? I dunno. I will give it further thought.
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Re: 120Hz hum in a 50R

Postby Papa Dog on Tue Mar 05, 2013 5:10 pm

Tracy,

I have spent the last 6-8 hrs pouring over schematics.
The schematic which you drew of the "as built" circuit seems to have a major flaw.
Comparing that schematic to the 50+ and the reverb circuit on the SoundCitySite shows that there should be 2x 470k resistors between both preamp channels to the reverb circuit input. You schematic shows taps of 470k off the plates of the three EQ triodes. I think that someone has "modded" that amp to run the reverb post EQ instead of pre EQ as it was designed. Whether or not that would cause the issue which you are having is a shot in the dark, at best. Either way, putting it back to spec might not be a bad idea.

If I had that amp on my bench I would first get the 50+ part back on spec and then do the reverb section as per the reverb circuit schem on the SoundCitySite.

I still think that there is an issue with the bridge rectifier, but I could be worng.
Hope that helps some.
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Re: 120Hz hum in a 50R

Postby tracynorton on Fri Mar 15, 2013 6:26 pm

READ CAREFULLY
Everything works dandy...EXCEPT...
there is 6Vp-p 120Hz ripple on the HT caps when standby is switched on. Textbook ripple. Real purty. This translates to 4p-p on the OT secondaries. Loud.
Amp puts out healthy 50W clean (just before clipping).
Amp draws 105mA at idle.
I disengaged the preamp HT from power and screen HT. Ripple is still there. Unsoldered all the OT connections and measured DCR w/ DMM. Looks good, but I also know that doesn't tell the whole story.
I've isolated the EL34s from incoming coupling caps. Ripple still there.
I've moved wires around (and chopsticked)...absolutely no change in ripple.
When I shut off standby (disengages all circuitry from rectified HT)...HT goes flat, very nice ripple free 495V).

Now, I can only surmise that for whatever reason, there is a massive DC loading issue with the OT...and it's not able to cancel it out. But the idle draw doesn't raise a red flag here (or does it?).

EL34s biased at 70% plate dissipation

Before I go out and buy another OT (at my expense) and tack in for test...I need to know if I've missed anything.
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