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Old 22nd March 2012, 05:48 PM   #1
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Default Should I ground my ST 70?

I wired an IEC plug into the back of my ST 70. Should I ground the 3rd prong to the chassis?
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Old 22nd March 2012, 07:07 PM   #2
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Yes......and add bleeder R's across the PS caps if it hasn't been done already.
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Old 22nd March 2012, 07:14 PM   #3
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I'm using a Classic Valve power supply board, so the bleeders may not be an issue.?
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Old 22nd March 2012, 07:43 PM   #4
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a bleeder is a bleeder. If there is a componen of equivalent function already on the board you can omit adding one obviously. If there is not, a bleeder resistor is a must for safety issues.
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Old 22nd March 2012, 07:48 PM   #5
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I don't ground my ST70. It is in the corner on a wood floor no one can touch it. That way I can do an actual star ground with the preamp as the hub, connected to the 3rd pin of the outlet strip. The preamp has no 3rd pin either. I ran a separate wire from the screw where the turntable green wire connects on the preamp, to a separate plug on the power strip for the safety ground. The actual "ground" on the ST70 is RCA jack outside to RCA jack outside on the preamp. Not a UL registered 3 amp 120 VAC conductor.
However, at the end of the record - - - Ahhh- silence.
Pro PA amps with a grounded case, have a differential op amp hookup on the input to reduce the resulting hum. The ST70 doesn't have differential anything. Not enough active elements in 1960 with tubes at 2 hours pay plus 6 hours pay for the other components around each tube. The ST120 transistor amp doesn't have differential inputs, either. I think that double transistor input all the later discrete amps have has something to do with hum rejection,
I did safety ground the keyboard chassis of my Hammond organ, that I touch quite frequently.
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Last edited by indianajo; 22nd March 2012 at 08:08 PM.
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Old 22nd March 2012, 08:46 PM   #6
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I am about to ground an old Fender that no longer has the original zipcord.
Was thinking to put a silicon diode bridge in series/parallel, such that 1.4V
drop either direction would block low voltage ground loop hum. Such diodes
need to be rated higher current than the fuse, to insure the fuse will always
blow well before the safety diodes from mains ground to chassis ground.

The chassis ground is also signal ground, so I am a little worried what will
happen with hum, once this new safety ground is added.

If this is not a safe plan, or does nothing for hum, I need to know soon.
This is probably gonna happen Saturday, with an electrolytic recap too..

And what is with these guys? The original fuse is located AFTER the big
metal switch you would surely grab with your hand.

Last edited by kenpeter; 22nd March 2012 at 09:13 PM.
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Old 22nd March 2012, 08:58 PM   #7
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Quote:
Originally Posted by richamor View Post
I'm using a Classic Valve power supply board, so the bleeders may not be an issue.?
Have you confirmed that there are bleeders across the PS caps? They are probably a couple hundred Kohms or so, maybe 2-3W size.

I've only spent about an hour with an ST-70.......are the RCA inputs isolated from the chassis? I'm assuming that they are....IIRC, the one I was playing with had a 3 prong cord and ground conductor tied to chassis (it had been updated a few times).
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Old 22nd March 2012, 09:05 PM   #8
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The inputs are isolated. As for the bleeders across the caps, I have not a clue.

I think tying this to the house ground is a good thing, I'm just not sure how.
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Old 22nd March 2012, 10:41 PM   #9
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Do you have any crappy speakers? Ground the green power/IEC entry module wire to chassis and let her rip with cheap speakers. You will know if you have a hum problem soon enough......keep one finger on the power button when powering up.

The various safety agencies require a #10 ground stud in the chassis with star toothed lock washers and nuts between each ring lug connected to the stud, so use that as a reference if you can. The lock washers assure good ground contact by biting into the chassis and nuts. The ring lug assures a reasonable ground if the nut comes loose, since the wire cannot fall off the stud. Since you won't be filing for any certifications, a #8 stud will also work, or a convenient transformer mounting screw and a couple of star lockwashers.

As for bleeder R's, look around the PS caps, you should see some large resistors....just confirm that they tie across the cap leads. You may have to follow traces on the PCB to see if they are wired across the cap leads.

Last edited by boywonder; 22nd March 2012 at 10:45 PM.
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Old 22nd March 2012, 10:42 PM   #10
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Quote:
Originally Posted by boywonder View Post
Do you have any crappy speakers? Ground the green power/IEC entry module wire to chassis and let her rip with cheap speakers. You will know if you have a hum problem soon enough......keep one finger on the power button when powering up..
That sounds like a plan!
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