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Old 21st June 2012, 07:43 PM   #21
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I have seen problems with the charging pulses into smoothing capacitors causing hum.
If the grounds arent orgranised properly this hum can be very big especially in relation to small signal circuits.
I had to give up with one pcb and put the power supply seperate on a piece of vero board ! I previously had the power supply grounds upsetting the small signal grounds.
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Old 21st June 2012, 07:44 PM   #22
DF96 is offline DF96  England
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Sorry if this seems obvious, but if a valve is amplifying it then it exists between the grid and cathode circuit yet both should be referenced to a common point. There are really only two options: they are not referenced to the same point, or there is a loop which a magnetic field is coupled to.
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Old 21st June 2012, 08:09 PM   #23
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nigel - totally. the PSU and caps are on the other side of the chassis. transformer is mounted on the outside.

DF96 - right? currently i've got the grid ground and the cathode ground running individual lines to their star point. perhaps the small wire resistance running to the star ground is causing a small voltage noise... or, a loop, sure, although i don't see any trace of that possible. i'll look and consider it again.
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Old 21st June 2012, 08:14 PM   #24
6L6 is offline 6L6  United States
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Try a ground breaker of some sort - the 10 Ohm resistor you mention, a couple of diodes, or better yet, a bridge.

Here's a great summary by John Broskie, of Tubecad.com fame --

Click the image to open in full size.
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Old 21st June 2012, 09:12 PM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by themagicmanmdt
Noise reduced when I changed power cord ground to chassis from close to the PT to next to (but not the same connection as) the B+ CT / first filter cap ground point.
Please confirm that you are not using the chassis as part of the signal ground. This is asking for trouble. Please confirm that you are keeping PSU cap charging pulses in their own loop well away from the signal circuitry.

Quote:
currently i've got the grid ground and the cathode ground running individual lines to their star point.
Please confirm that you are not using more than one star point. Multiple star points (e.g. one per stage) is an RF technique quite unsuitable for low-level audio.
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Old 22nd June 2012, 02:06 AM   #26
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6L6 = will try tonight after further local stage experimenting

DF96 = here's a description of my grounds. i believe that i've got it satisfactory, yet whistleblow if i'm not thinking right:

it's a two channel preamp. i'm grouping channel 1 and 2 grounds of these similar stages:

v1 & v2 = 6sj7 (channels 1 / 2)
v3 & v4 = ef86 (channels 1 / 2)
v5 = 12au7 (CF for each channel)

recap of individual runs of signal ground star:

v1 & 2
cathodes, screen bypass and grids

v3 & 4
grid (volume), cathodes, screen bypass

v5
grid (volume), cathode

this signal ground star run via 12ga wire to chassis ground lug


PSU:
first, second and third filter caps tied together
this point is run to chassis ground lug via ~4" 12ga wire

B+ CT and heater CT tied directly to chassis ground lug

safety ground located 2" away from chassis ground lug, connected via 2" 12ga solid core wire




comment: running the preamp as only one channel (without v2 or v4) doesn't change noise performance other than B+ levels and filter (loss of current draw). also, removal of volume pots in favor of fixed resistor yields no change.

no negative feedback used other than CF bootstrapping.
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Old 22nd June 2012, 02:34 AM   #27
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Just to allay any doubt, do you get any non-zero oscilloscope response with the probe ground and tip both connected to the same point on the amp chassis, or any other ground reference point?
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Old 22nd June 2012, 02:39 AM   #28
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Quote:
Originally Posted by trobbins View Post
Just to allay any doubt, do you get any non-zero oscilloscope response with the probe ground and tip both connected to the same point on the amp chassis, or any other ground reference point?
yes. the same, identical ground noise. attaching the probe ground clears it up a smidge, but doesn't reduce what i see.

but not on other gear. just this project. finished a from scratch tube power amp of my own design (non-star ground, using chassis as buss ground) and had no problems. dead quiet with tons of gain - oscope looked fine. this is my first high voltage gain mic preamp, and first time i've needed to work with a star ground since chassis buss ground doesn't work.




here - i've got a scanner - hopefully this helps. the lower left hand corner sums it up the best i can.

since i've terminated the inputs and outputs, this is all being tested on the bench without interconnects to another piece of gear. everything is seeing a happy load and such and such...
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Last edited by themagicmanmdt; 22nd June 2012 at 02:42 AM.
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Old 22nd June 2012, 08:19 AM   #29
12E1 is offline 12E1  United Kingdom
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A picture helps a lot! Personally, I would never connect the center-tap from the HT winding of the transformer direct to the chassis. That will be introducing noise from the charge current pulses that feed the capacitors.

The center-tap should go to the negative of the first filter capacitor only. Do not connect to the chassis at this point.
The negative of the first filter capacitor should go to the negative of the second. Do not connect to the chassis at this point
The negative of the second filter capacitor should go to the negative of the third. You can then take negative of the third filter capacitor directly to the star point. By the time you are on the third capacitor the current ripple is going to be very small.

That arrangement will keep the current pulses from the transformer and first and second filter stages out of the ground. But they must be wired in the right order as described.

Last edited by 12E1; 22nd June 2012 at 08:35 AM.
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Old 22nd June 2012, 10:21 AM   #30
DF96 is offline DF96  England
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Yes, folow 12E1's advice. Directly grounding the secondary CT is a very common mistake. However, the fact that you still see hum with the scope tip and ground at the same point suggests magnetic field induction as the main problem. Ensure that every grounded point in the system has one and only one path to ground. Ensure that forward and return currents, whether signal or PSU, are routed together and preferably twisted (unless using coax).
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