ST-70 & PAS AC switch impulse noise

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ST-70 & PAS AC switch impulse noise

Postby joeztech » Sun Nov 09, 2014 12:15 pm

I have been noticing an odd thing with my ST-70 and PAS-3X combination. I can hear AC switching impulse noise rather loudly in the left channel, but not at all in the right channel. This seems odd. I would think that any such impulse noise would appear equally in both channels. I have another PAS, a PAS-2 that I have restored recently. I plan to substitute it and listen a while to see if there is any difference. I hear this noise if I turn on or off an overhead light in the same room as the equipment or if the washing machine cycles in the laundry room (on a different AC breaker & circuit). I do note that in the ST-70 the two channels have separate filament windings from the power transformer. Each filament winding has 1/2 of a double .02mFd capacitor bypassing its center tap to ground. The ground point is not the same as the star-signal ground near the electrolytic capacitor. I am wondering if perhaps one side of the double .02mFd disc ceramic capacitor used could be open and cause this problem, or if it might have something to do with the center point of the part that is grounded not being grounded at the same point as the star-signal ground.

Has anyone else noticed such an issue?

Joe
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Re: ST-70 & PAS AC switch impulse noise

Postby joeztech » Sun Nov 09, 2014 12:27 pm

Oops! I checked the schematic and the component layout photographs I have on the ST-70 and the center lead of the double .02mFd disc ceramic capacitor does go to the signal-star grounding lug near the rectifier tube and the electrolytic capacitor. So, it would seem the only explanation involving that part would be if one side was open. I still have some testing to do.

I also noticed recently that the left channel also picks up some RF interference while the right channel does not. There could be a common cause.

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Re: ST-70 & PAS AC switch impulse noise

Postby Shannon Parks » Mon Nov 10, 2014 12:35 pm

Is this a standard ST70 driver with 7199s? Does the problem follow the 7199s if swapped? You could also put some small caps (100pF maybe?) at the ST70 inputs.

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Re: ST-70 & PAS AC switch impulse noise

Postby Geek » Mon Nov 10, 2014 2:23 pm

Something they didn't do back then that helps tremendously with RF interference is grid snubbers as well as Shannon's tip of bypassing the inputs.

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Re: ST-70 & PAS AC switch impulse noise

Postby joeztech » Mon Nov 10, 2014 3:29 pm

Greg and Shannon;

Thanks for the suggestions. I am familiar with adding signal grid snubbers at the tube socket, so that is definitely a good idea. Ditto adding the small value cap on the input. 100pFd is so small that it will have no effect even at the upper end of the audio spectrum. One other possibility that occurred to me is that the channel that exhibits the problem has its left channel speaker wires running near my computer's wireless router which also has a few local LAN wired connections. I might add some ferrites to the speaker leads near the speaker terminals in case RF is getting in that way.

The ST-70 was modified to use 6GH8A tubes, but of course they are quite similar to the 7199s and could respond to RF very easily. They saw use in color TV sets as IF amps, Video and Chroma amps etc., so they definitely can amplify RF signals at least up to 50mHz or so, perhaps higher. One substitute for a 6GH8A was a 6EA8 which is seen in some VHF mixer/oscillator circuits.

I have some suitable silver mica capacitors in that range that I can easily add along with snubber resistors at the signal grids of the pentode gain stages.

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