Altec 1567A buzz.....ideas?

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emrr

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Gets weirder. The dedicated AC filament winding for the 6CG7 died in one. I scabbed in a new Hammond filament transformer. It only feeds the one tube. No buzz.....

I do note chasing other resistor noise, there's no place I can ground a grid without huge buzz.
 

Winston OBoogie

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Gets weirder. The dedicated AC filament winding for the 6CG7 died in one. I scabbed in a new Hammond filament transformer. It only feeds the one tube. No buzz.....

I do note chasing other resistor noise, there's no place I can ground a grid without huge buzz.

So, crud from the DC heater circuit is coupling through the power transformer to the 6CG7's 6V3 winding?

Will "scabbing in" a separate 6V3 transformer fix the other 3 units too?

Will the customer cover the cost of adding an extra heater transformer per?

If yes to the above, do it and get them the f**k outa your life.


P.S. Never heard the term "scabbed in", I like it.

P.P.S. I re-listened to a bunch of stuff done at Norman Petty's place through his Altec 1567's and, honestly, putting aside the historical aspect and the artists and songs etc., I'm not sure I still stand by my earlier statement that the stuff sounded good for the era.
Same time frame and the recordings done at RCA Nashville kinda kill it sonically.

I'll be going to Hell for saying that no doubt :oops:
 

emrr

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So, crud from the DC heater circuit is coupling through the power transformer to the 6CG7's 6V3 winding?

Will "scabbing in" a separate 6V3 transformer fix the other 3 units too?

Never heard the term "scabbed in", I like it.

I don't think I made that term up....gets used in construction over here, maybe?

The 6CG7 isn't what's polluted, and it makes the least sense that it's winding would have any effect, unless there's some sort of reflection of a reflection thing going on. Turn the master all the way down on any, they're quiet.

Taking some load off the PT?

None of this makes any sense.

Maybe I should convert one to fully AC heaters and see what happens. The leads are already twisted the whole path. Maybe I should add DC filament switcher? : )
 

solkatten

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I don't think I made that term up....gets used in construction over here, maybe?

The 6CG7 isn't what's polluted, and it makes the least sense that it's winding would have any effect, unless there's some sort of reflection of a reflection thing going on. Turn the master all the way down on any, they're quiet.

Taking some load off the PT?

None of this makes any sense.

Maybe I should convert one to fully AC heaters and see what happens. The leads are already twisted the whole path. Maybe I should add DC filament switcher? : )
The problems are definitly more predictable with ac heathers

When I had the dc buzz problem I ended up shielding the PT because it was radiating the buzz....
 

Winston OBoogie

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I don't think I made that term up....gets used in construction over here, maybe?

It's getting used right here too from now on.




The 6CG7 isn't what's polluted, and it makes the least sense that it's winding would have any effect, unless there's some sort of reflection of a reflection thing going on. Turn the master all the way down on any, they're quiet.

I don't think it's the 6CG7 winding that's the issue. But I thought there could be a chance that the DC winding is coupling into the AC winding because there's probably no electrostatic shield between them. Maybe they're both wound right on top of each other? Dunno.
Having the 6CG7 on a different transformer now puts the electrostatic shield (between the primary and secondaries) of the Altec's transformer between the two sets of heaters. Helps get rid of any coupling.

Just tossing stuff out there. It worked on one. Should work on the others right?
F**cking Hell, end of the day, I just don't know dude.



Taking some load off the PT?
Maybe that too. Although I'd think that if you're not dealing with a regulator that drops out because there isn't enough in/out voltage margin, that wouldn't matter.


None of this makes any sense.

No not really. Seems like a bunch of stuff that conspired to make it all a bit of a clusterfuck.


Maybe I should convert one to fully AC heaters and see what happens. The leads are already twisted the whole path. Maybe I should add DC filament switcher? : )

Yeah sure. A steady low 60Hz hum would beat all that 120 + H2, H3, & all those partials, any day.


Edit: Now that you know how to create a unit without buzz, what about trying a cap across the AC of both heater windings. A cap would add C (obviously... der!) to the whole inductance and capacitance in that resonant circuit, push the offending frequencies down and out of your audio.

Try 1uF across both 6V3 AC coils.
 
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Cranehazard

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The only way I get the heaters quiet enough is by using a voltage doubler into an lm317 (you need double the current on your filament winding) WITH A MASSIVE HEATSINK! the lm 317 will thermal protect instantly without it. Turn it down to 6.3v and it's perfectly quiet.
 

emrr

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Now that you know how to create a unit without buzz, what about trying a cap across the AC of both heater windings. A cap would add C (obviously... der!) to the whole inductance and capacitance in that resonant circuit, push the offending frequencies down and out of your audio.

Try 1uF across both 6V3 AC coils.

no change, either a stock or extra filament transformer unit.
 

emrr

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For reference, the internal filament winding for the 12VDC is 11-0-11 VAC.

Mindless data filing part 397, master at 28 (10 o'clock) and all mix knobs off, only mix channel 3 mix resistor connected to C6:

External 12VAC to V1-3 is louder at all harmonics, mostly follows the shape of the internal 12VDC harmonics, except 60 is much hotter and 120 is lower.

Turn off the filament heat to V1-3 after they are warmed up, 60 drops 6dB, 120 drops 3dB, 180 and up drop minimum 20dB.

Back to no mix resistors connected to C6, 120 is actually 6dB higher, but everything else is significantly lower. Compared to no heaters on V1-3, 120/240/480 are what's most noticeably higher. For the most part, no heaters is much lower.


First one, master off.

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Winston OBoogie

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Damn! That's a whole mess of fucked-up-ness and noise dude. Really, really sorry none of my suggestions helped.
I don't know what to say right now other than "You're a better man than me, I'd have drop-kicked 'em long ago".

I should think very carefully about what I suggest from now on, you've wasted enough time on this crap.

Sorry.
 

emrr

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The open question is whether the premise is wrong; this repair is to make these 'quieter as they should be' - is that a thing? There are examples that are quieter. What does it mean? Handwiring; is this the tolerances we get, between that and age? I have crushed the crap out of carbon resistor noise, but it's meaningless in the face of this buzz.
 

Cranehazard

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Maybe the scabbed in filament transformer has enough current then it's only a small board with diodes and a lm314 run 3 wires to it and bolt it to the chassis. Chassis could be a massive heatsink. You only really need it this clean on the input tubes also. Just a few ideas.
 

skipwave

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Oh man, how painfully frustrating.

I can’t offer any fix ideas, only my limited experience having repaired one of these. It was intermittently cutting out due to some previous tech’s sloppy solder bridge. Once I fixed that goober the thing was quiet enough for loud singing and louder screaming into a SM-57 with no noticeable buzz.

These old rack space hogs are favorites in lots of studios for hardcore screaming vocals, which to a degree surely masks any buzz, but the 1567 I worked on was no more noisy than the shitty starved plate toob preamps I had back around way back then to compare it to.
 

emrr

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I just got a report of a hifi guy who keeps the master dimed, and uses the mix faders to set source levels. That would be impossible with these.
 

emrr

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Maybe the scabbed in filament transformer has enough current then it's only a small board with diodes and a lm314 run 3 wires to it and bolt it to the chassis. Chassis could be a massive heatsink. You only really need it this clean on the input tubes also. Just a few ideas.

The smallest Meanwell 12V switcher would just barely fit, $8. May try one to see.
 

Winston OBoogie

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Just trying to make any sense at all of this - I can't tell if it's shown in your plots but, where does the noise sit when you tried an external 6V3 transformer to feed V1-V3 with DC? On the units where that winding isn't blown, but on the one that is and you said had no buzz too I guess.
 

emrr

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No plot of that, actually I did use a meanwell switcher that normally powers a 1932 RCA thing, it was quieter but not by enough. So I’ve tried that, I shouldn’t again.
 

Winston OBoogie

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Another good post by Bill in the other "buzz" thread that might apply here too?



This is sounding like the power transformer is running in magnetic saturation! Contrary to popular belief, saturation (and the resulting lamination buzz and huge radiated magnetic field) is NOT caused by over-loading it's output, but by over-voltage on its primary side. Is your AC line voltage over 120 V? A common move to save money on power transformers is to operate them very close to magnetic saturation - sometimes a primary voltage increase to just 123 or 125 will put them into saturation (and they'll get hot even if there's no load on the secondary at all!). Overheating due to too much loading of a transformer secondary is caused only by too much load current and the DC resistances of the windings. It amazes many people when I explain that the strength of the magnetic field in a power transformer is the same, whether it has no load or a full load (it actually decreases very slightly under heavy loading). The field strength is determined only by the primary voltage and frequency. Saturation is also commonly caused by operating a 60 Hz transformer on 50 Hz!

When a power transformer saturates magnetically, the magnetic field literally squirts out of the transformer and it has a very "pulsey" character that often magnetically induces a "buzz" voltage in every wire near it. A good way to test for saturation is to power the unit from a Variac. If everything calms down if you reduce line voltage to say, 105 VAC, then you've got a saturation issue! I hope this little mini-tutorial helps!
 
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