ST-70 Up but...Dirty mains?

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ST-70 Up but...Dirty mains?

Postby DoubleAA » Fri Aug 07, 2009 12:07 am

Hello,
As debugged in another thread, my ST-70 is once again running good. Music sounds great but...I have a couple questions. I'm getting big vibration/hum out of the power transformer (so it seems). This (like everything else in the amp) is a brand new (triode electronics). Using a shorting cable diminished it a bit.
The buzz is also out of the speakers - quite loud when nothing is playing. I also am getting RF - I hear something (TV or radio) in both speakers as well as static. This was most pronounced late at night. I think it's contributing to the buzz problem.

Which brings me to the dirty mains...maybe. My house has odd wiring (seriously - 24V low voltage system throughout and flourescents (never on when listening)), When we moved in a new circuit box was installed. I also ran a dedicated line from my listening room to the breaker box and a dedicated 20A breaker. Despite this, if there is other significant electrical activity in the area (kitchen disposal or even turning on some lights) I notice my system 'pause' momentarily. This is most evident on vinyl as the turntable slows, but the tubes dim ad brighten up again.

So my question: How do I get the dimming tubes 'surge' to stop and what can I do about the RF (once that's fixed, I'll focus on the buzz should it still remain)?

Thanks!
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Postby Geek » Fri Aug 07, 2009 12:58 am

Sounds like your circuit may be oscillating and acting like a direct-conversion radio receiver.

The buzzing transformer would be from the huge power drain of an oscillating PA.

Cheers!
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Postby DoubleAA » Fri Aug 07, 2009 7:26 am

What can I do to verify that - or fix it?

Thanks!
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Postby TomMcNally » Fri Aug 07, 2009 7:43 am

It sounds like you have a couple of things going on here ...

1) you shouldn't run the amp with nothing connected to the inputs,
any stray noise will be amplified

2) is there a radio or TV tower close to your home (within 2 miles) ?
if so, you could be picking up some serious RF which can be
easily filtered with small value capacitors to ground. A solid
ground from the amplifier to cold water pipe ground or a
ground rod may help

3) if your power is so bad that things blink and stop, you may have
a bad neutral coming from the pole, or you live at the end of
the line (are you in the US ?) That needs to be fixed.
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Postby battradio » Fri Aug 07, 2009 9:51 am

It sounds like you need an electrician , you most likely have a bad eletrical connection , it sounds like the line is backfeeding through another circuit and needs to be corrected qwickly .

Mark
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Postby Geek » Fri Aug 07, 2009 3:25 pm

DoubleAA wrote:What can I do to verify that - or fix it?


A couple of things...

Simplest is check the cathose bias. If oscillating they will be drawing lots of current.

If you have an oscilloscope, put it on it's least sensitive range and use a X10 probe to check the plates. If there's oscillation, it'll appear as a solid "carrier" wave.

No scope? A RF probe an meter will work.


However if it's as simple as the other fellow's suggestion of RF pickup, a 1000pF cap from pretty much every input or output terminal to ground and metal shielding over the driver board (aluminum window screen on a frame grounded to the chassis is an idea) will likely cure it.

Good luck!
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Postby davygrvy » Fri Aug 07, 2009 3:33 pm

battradio wrote:It sounds like you need an electrician , you most likely have a bad eletrical connection , it sounds like the line is backfeeding through another circuit and needs to be corrected qwickly .

Mark


Ditto. A floating neutral is a dangerous situation (neutral connection to the service transformer intermittent).
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Postby Dynacophil » Fri Aug 07, 2009 5:20 pm

that killed my dads laptop (connecte to the plug) when I wanted to connect the hdmi-cable from tv :)
"Iss dein Schnitzel, sonst kriegst du keinen Nachtisch
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Postby DoubleAA » Fri Aug 07, 2009 5:50 pm

Many thanks for all the replies. Here's what I do know:

Bias current does not appear to be excessive - in fact I've been actively monitoring both sides when music is not playing and after about 45 minutes it pretty much settles to about 46mA (I'm running KT77s).

I am not aware of any radio/tv towers near my home - at least not that close. I had previously installed two small value caps on the AC into the amp - I do not recall what the values are (1000pF doesn't sound familiar...I'll try to check it this weekend). I incidentally DO have a metal shield cage for the driver, but not this driver board (with 3 tubes). I'll see what I might be able to do...Unfortunately, no scope here (and I keep finding reasons to have one around...sigh).

I think though, the bad neutral may be the issue here. My electrician when we bought the house fixed a number of violations, but I don't recall if he did any work outside of the house in case it's something going up the pole. I may give him a call to come and check things out if nothing else seems to be the issue. The wiring from the socket is solid - dedicated line to the breaker, though I do not have a ground hooked up on the ST-70 (I have an IEC socket installed, but have no connected the ground line as I've read about ground loop issues with this amp).

Right now the amp has been running for the last couple hours. Bias is solid at 47mA. The PT is warm to the touch (this is a new triode electronics one) and right now has a slight vibration - not as bad as I noticed yesterday. With no music playing (so inputs connected to my preamp) I get a buzz - though noticeable only right up close. I just swapped out a 12ax7 that was marginal when I tested though I'm not convinced this is this issue.

Again, thanks for the suggestions and I'll let you know what I find!

Aaron
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