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Old 1st August 2012, 07:20 PM   #1
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Default Ground or amp hum-which?

Hey folks
I have a Pilot 505 stand-alone I have been trying to re-build-complete re-cap and resistor replacement has been done. It has a hum?
It does not increase with volume and is not loud but it is there. It will increase with voltage off the Variac. The amp runs fine and powerfull with great sound.
The hum is even in both channels.
I have moved the MT a little further out-I have done a star ground then lifted the ground.
I have soldered the MT ground wire directly to the 40uF Rectifier ground.?????
I finally broke down and bought a Teck 475A with a DM44 a pair of 8 ohm loads and am looking for a square wave sig gen.
I did short the inputs-still hum?
I shorted the input channels-still hum.
Anything else I can do while I shop for a sig gen.?
Any good suggestions on what to look for in a sig generator? I have never owned one!

here is some pics and a schematic

Thanks for any help and time you can give me with this!
Ignore the treble/balance part of the schematic in the upper right. I run it with a NAD pre.
JamesIMG_3742.jpg

IMG_3743.jpg

Pilot schematic.jpg

Last edited by oldtexasdog; 1st August 2012 at 08:14 PM.
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Old 1st August 2012, 08:47 PM   #2
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Looking carefully at the picture of the wiring, I would say that you have a ground problem for sure. I can see several ground points all around the chassis. The input jacks (bottom), the speaker terminals (top)(actually double grounded), two filter caps right off the 5AR4 tube (right), a bank of three filter caps (center), a can electrolytic (lower center), a small electrolytic (left) and miscellaneous grounds at various tube sockets. You have a multitude of ground loops.

If this were my amplifier, I would make all power supply grounds to one place. Probably at the can-type electrolytic capacitor since it is there mounted on a metal plate. I would run a heavy (#12) copper buss wire over the tube sockets for all signal grounds, and ground that to the can's mounting plate. You could use the center connector of the tube sockets to physically support the buss line. But ground it only at one place, the filter can mounting plate. A second buss line over the output tube sockets, and grounded to the capacitor mounting plate as well.

To be proper, ground the output transformer secondary on the buss close to the capacitor plate ground point. Bring the speaker terminal ground there too. Leave the input jacks alone for now. They may be ok if you do all the above since they are close to the ground point.
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Old 1st August 2012, 08:49 PM   #3
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Quote:
Originally Posted by HollowState View Post
Looking carefully at the picture of the wiring, I would say that you have a ground problem for sure. I can see several ground points all around the chassis. The input jacks (bottom), the speaker terminals (top)(actually double grounded), two filter caps right off the 5AR4 tube (right), a bank of three filter caps (center), a can electrolytic (lower center), a small electrolytic (left) and miscellaneous grounds at various tube sockets. You have a multitude of ground loops.

If this were my amplifier, I would make all power supply grounds to one place. Probably at the can-type electrolytic capacitor since it is there mounted on a metal plate. I would run a heavy (#12) copper buss wire over the tube sockets for all signal grounds, and ground that to the can's mounting plate. You could use the center connector of the tube sockets to physically support the buss line. But ground it only at one place, the filter can mounting plate. A second buss line over the output tube sockets, and grounded to the capacitor mounting plate as well.

To be proper, ground the output transformer secondary on the buss close to the capacitor plate ground point. Bring the speaker terminal ground there too. Leave the input jacks alone for now. They may be ok if you do all the above since they are close to the ground point.
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Old 1st August 2012, 08:50 PM   #4
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Woops!
Thanks will do and see if that helps!
Big thank you for your time!
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Old 1st August 2012, 08:58 PM   #5
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Quote:
Originally Posted by oldtexasdog View Post
Hey folks
I have a Pilot 505 stand-alone I have been trying to re-build-complete re-cap and resistor replacement has been done. It has a hum?
It does not increase with volume and is not loud but it is there. It will increase with voltage off the Variac. The amp runs fine and powerfull with great sound.
The hum is even in both channels.
I have moved the MT a little further out-I have done a star ground then lifted the ground.
I have soldered the MT ground wire directly to the 40uF Rectifier ground.?????
I finally broke down and bought a Teck 475A with a DM44 a pair of 8 ohm loads and am looking for a square wave sig gen.
I did short the inputs-still hum?
I shorted the input channels-still hum.
Anything else I can do while I shop for a sig gen.?
Any good suggestions on what to look for in a sig generator? I have never owned one!

here is some pics and a schematic

Thanks for any help and time you can give me with this!
Ignore the treble/balance part of the schematic in the upper right. I run it with a NAD pre.
JamesAttachment 294156

Attachment 294157

Attachment 294158
Although many people believe that star gounding is the best method to eliminate hum...it's not at all.
Separating the power supply filter ground from the preamp ground will produce much less noise than star grounding.
The preamp ground should be somewhere in the vicinity of the input.
The power supply ground should be at the opposite end of the chassis.
And by doing this, you have introduced a small resistance between the grounds of the preamp and power supply, through the metal chassis.
Believe it, this is a benefit.

And there are several places that hum can originate. Not just the grounding. There are at least 4 other major causes of hum in amplifiers, and I will try to briefly describe each one, and how to correct it.

However at this point I see that your power trans center tap is connected to the three filter capacitors negative terminals, but not to the chassis.
And your first power supply filter caps are grounded to the preamp tube sockets lugs.
You can't do that.

Last edited by soundguruman; 1st August 2012 at 09:09 PM.
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Old 1st August 2012, 09:07 PM   #6
Hi_Q is offline Hi_Q  England
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Is it 60Hz hum or 120Hz? The former will be grounding as detailed very well by HollowState. Incidentally, the heater wiring could be much improved using well twisted wires and kept close to chassis and away from sensitive areas. If the hum is the latter 120Hz then suspect the main smoothing can and other capacitors in the smoothing circuit. Sometimes the old multi-cans have problems with internal leakage across their individual sections which bypass the resistors forming the pi section filters. This really looks like a great project for de-gutting and re-building from the chassis up. It all depends of course on how far you want to go with it but certainly something I would enjoy doing. Otherwise, get those grounds sorted first and see how you go. Hoping you have complete success with it.
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Old 1st August 2012, 09:59 PM   #7
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Quote:
Originally Posted by Hi-Q View Post
Is it 60Hz hum or 120Hz? The former will be grounding as detailed very well by HollowState. Incidentally, the heater wiring could be much improved using well twisted wires and kept close to chassis and away from sensitive areas. If the hum is the latter 120Hz then suspect the main smoothing can and other capacitors in the smoothing circuit. Sometimes the old multi-cans have problems with internal leakage across their individual sections which bypass the resistors forming the pi section filters. This really looks like a great project for de-gutting and re-building from the chassis up. It all depends of course on how far you want to go with it but certainly something I would enjoy doing. Otherwise, get those grounds sorted first and see how you go. Hoping you have complete success with it.
Thanks
It is a 60Hz hum and all the can caps have been replaced with under chassis caps. I just left the one in place on the top as I did not have a plug for it.
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Old 1st August 2012, 10:20 PM   #8
DF96 is offline DF96  England
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I can't really see any heater wiring in the picture - that is no tightly twisted carefully routed AC heater wiring. Correcting this might be difficult, because heater wiring should be the first to go in so it can be right next to the chassis.
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Old 1st August 2012, 10:44 PM   #9
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The center tap of the transformer (red/yellow),
AND the (-) terminal of C33A,
AND the (-) terminal of C 33B,
AND the (-) terminal of C34A,
AND the (-) terminal of C34B,
AND the (-) terminal of C46
should all be connected to the chassis, in one spot, as far away from the preamp ground as possible. (on the PT side of the chassis)
The connection should be bolted to the chassis, the wires soldered to ring terminals, tightly secured with lock washer and nuts.
The capacitors should not be grounded to the lugs on the tube sockets.

Twist the heater wires, and move them as far away from the input jacks as possible.
Untwisted wires will allow AC 60 Hz to enter the audio path, especially when it's near the input jacks.

The preamp will ground to the same spot as the input jacks. Grounding to the tube socket metal frame will produce a poor ground.

When you are testing this amp, short both inputs to ground. If the inputs are not grounded, you can expect lots of hum.

Last edited by soundguruman; 1st August 2012 at 10:51 PM.
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Old 1st August 2012, 10:50 PM   #10
DF96 is offline DF96  England
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No, that will guarantee some buzz from charging pulses.

Connect CT to C33A -ve, then to C33B -ve etc. The final cap -ve should then be connected to the signal ground. This keeps charging pulses away from the ground connections. Always ground a PSU at the quiet end.
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