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Old 12th June 2012, 08:24 PM   #1
Magz is offline Magz  United States
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Default Question re: Signal Ground versus HT ground

Hello all,

I'm planning out the umbilical cable to connect my 2-box 26 preamp chassis to its power supply chassis and I'd like to separate the grounds/returns as follows (I have 8 pins on the connectors I'm using):

HT
HT return
Left DC filament
Left filament return
Right DC filament
Right filament return
Signal reference ground
Safety ground

My question is...what should I call "signal reference ground" in the preamp box versus HT ground? I'm fairly certain about the final filter/decoupling cap (HT ground), gas regulator tubes (HT ground) input jacks (signal ground), grid leak (signal ground). How should I classify the 26 cathode grounds (signal or HT?), volume control ground (volume control is on the output of the preamp after the output transformers), output transformer shield, and output jack grounds (is signal signal, or is output signal different from input signal)?

I've been playing the various options over and over in my head and frankly it's starting to spin. Any help is greatly appreciated!

Last edited by Magz; 12th June 2012 at 08:39 PM.
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Old 12th June 2012, 09:11 PM   #2
DF96 is offline DF96  England
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Think about where the currents go. For directly heated valves the filament circuit is unavoidably part of the signal circuit. For all valves HT ground is usually part of signal ground, although a separation of these could be possible. I would regard the cables in the umbilical as being merely extensions of the PSU, so things need to be grounded in the preamp and not in the PSU (apart from safety ground).
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Old 12th June 2012, 09:44 PM   #3
Magz is offline Magz  United States
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The idea is to make the single ground to chassis for everything in the PSU, at the star ground point where cap grounds and transformer CT meet.

I agree about the intermingling of the filament and signal in a DHT, another complication/puzzlement on the path to "perfect grounding".

I usually just lump the HT and signal together into one return, but a day of reading about grounding on the net and I come up with lots of creative (if not necessarily feasible) ideas; separating the noisier HT return, containing the ripple filtered out by the gas tubes, from the signal seems like a logical way to quiet things down.

In a completely opposite thought, I was also considering making the HT return the safety earth connection between the chassis as well. After all, the safety earth is only there in case of fault and doesn't do anything for the circuit until then, why not have it do double duty as the HT return and save a wire in the umbilical? My Supratek preamp does it this way. In case of a fault the fault current will still be sent directly to the chassis at the CT ground point of the PSU, so no harm there, right?
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Old 12th June 2012, 09:53 PM   #4
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Remember that safety ground is common to all other pieces of electronics that have a safety ground, so forms (a) ground loop(s) with signal grounds. There are no elegant answers that don't involve transformers (or balanced lines with large common mode ranges).

All good fortune,
Chris
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Old 12th June 2012, 09:54 PM   #5
DF96 is offline DF96  England
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Don't confuse the signal ground with the connection from signal ground to safety ground. If you use a star point then this should be in the preamp not the PSU. The transformer CT should not go to the star point even if they are in the same box. CT to cap -ve, then cap -ve to star point, is the right way to do it. If there are two caps (reservoir C1 and smoother C2) then -ves go CT-C1-C2-star. The CT-C1 connection must be in the PSU, with as small a loop as possible between this and the +ve connections from the rectifier. C2 could be in the preamp - up to you.

Keep signal connections out of the PSU. Don't send signals down the umbilical - it should just be an extension of the PSU.
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Old 12th June 2012, 10:17 PM   #6
MelB is offline MelB  Canada
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Ground potential difference between points is raised by the amount of resistance between those points so whatever scheme you choose use as many of the conductors as you have available "free" for the ground part. Does that make sense?
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Old 12th June 2012, 10:25 PM   #7
Magz is offline Magz  United States
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So DF, are you saying that the CT should NOT still be grounded to the PSU chassis? I've seen PSUs where the CT ground and all the individual PSU cap grounds come together at a star point in the PSU connected to the PSU chassis, signal and HT grounds come together at a star point in the preamp connected to the preamp chassis, and the two are connected by the umbilical.

Last edited by Magz; 12th June 2012 at 10:32 PM.
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Old 12th June 2012, 10:39 PM   #8
DF96 is offline DF96  England
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The CT should be kept away from any star point, even in the PSU, as it carries charging pulses. As I said: CT-C1-C2-star is the way to do it. People often send charging pulses through their star point then wonder why they have buzz. You should have a connection to safety ground in the PSU but this should come from the 'clean' end of the PSU not the dirty end.

Having an umbilical makes things more complicated, as you have discovered.
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Old 12th June 2012, 11:36 PM   #9
Magz is offline Magz  United States
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So many folks ground the CT to chassis that I just figured it was standard practice...I'll have to try your scheme and see. It makes a lot of sense.
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Old 13th June 2012, 02:10 AM   #10
Magz is offline Magz  United States
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Well what do you know! I just rewired the power supply for my existing preamp so that the CT is no longer connected to chassis. Instead it is connected to the first cap (-) in a CLCLC, the first cap is connected to the second cap (-) and the second cap (-) is grounded to the chassis.

The preamp was quiet before, but I could still hear a faint buzz/hum with my ear to the speaker - NOT ANYMORE! It's dead silent! Maybe it's my imagination but the music sounds better, too!

Thanks DF96!

P.S: The 26 preamp I'm building will have an LCLC PS, I suppose CT will still go to the (-) of the first cap even though it's choke input, correct?
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