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Old 10th August 2004, 06:39 AM   #1
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Default Does a cathode bypass capacitor reduce hum?

Hi,

I have built a simple preamp, using a 6AU6 triode-connected with grid 2 as the anode and grounding the plate and grid 3. This gives a plate resistance of 18k and mu of 36. The cathode resistor is 1.2k and is unbypassed, which I think raises the effective internal plate resistance to almost 100k. The heater uses 6.3v AC, with a virtual (resistive) center tap set at +50v, with 1uF to ground.

I'm getting a bit of hum and I wonder if it's caused by not using a cathode bypass capacitor?
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Old 10th August 2004, 07:12 AM   #2
Franz G is offline Franz G  Switzerland
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Hi Ray

No no, the "missing" cathode capacitor cannot be the reason for hum.

Can you show us the schematics from the power supply?

And how did you the grounding?

Franz
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Old 10th August 2004, 07:13 AM   #3
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Is the heater grounding cap connected to one leg of the 6.3V or is it centered between them, say with two 100 ohm resistors or a 100~500 ohm potentiometer (aka hum balance)?

Your hum may be coming from elsewhere but otherwise yes, bypassing will reduce hum.

Tim
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Old 10th August 2004, 08:07 AM   #4
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To answer your questions, I can't post a schematic because I don't know how. However, the preamp stage is integrated with the power amp (stereo) and the power supply consists of:

340v 700mA transformer secondary
MUR1400E rectifier diode bridge
100uF smoothing capacitor
2x 6D22S damper diodes in parallel (for soft startup)
5H 40ohm choke
470uF reservoir capacitor
(take-off for EL34 triode PP stages with 6SN7 drivers = 410v)
680 ohm resistor (to drop a bit of voltage)
10H 260 ohm choke
235uF capacitor
(take-off for 6SL7 splitter = 397v)
27k resistor
100uF capacitor
(take-off for 6AU6 preamps = 220v)

The heater circuit uses two 330 ohm resistors in series across the 6.3v winding. The artificial centre tap given by these resistors is connected to a voltage divider across the 235 capacitor after the 1H choke, consisting of 1M above and 200k below. There is a 1uF capacitor connected between the heater centre tap and ground (i.e. across the 200k resistor).

This positions the heaters at about +65v. This is to protect the heater-cathode insulation of the 6SL7 LTP splitter, which has a cathode potential of about 120v because of the 6AU6 pentode acting as a CCS in its tail. It's also supposed to help reduce hum.

There are actually two heater windings but they're both connected up in as described above. I use them to feed the heaters of the two channels; however, the right channel heater winding feeds both of the input 6AU6 triode stages because they are positioned very close together on the right-hand side of the chassis.

The preamp is stereo and I find that if I remove the 6AU6s I get no hum at all. The 6SL7-6SN7-EL34 pp power amps are well balanced and very quiet. There's no doubt the hum is coming from the 6AU6 stages, which are preceded by passive Baxandall tone stack plus volume control. One 6AU6 is somewhat noisier than the other but they both generate hum.

I hope that helps to clarify things. Sorry it had to be so long-winded!
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Old 10th August 2004, 11:24 AM   #5
Franz G is offline Franz G  Switzerland
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Maybe, you try to change this 1uF Heathercap to a 47uF or change the 330 R resistors to a 100R wire poti.

But I think, this must be a grounding problem ore some induction somewhere (input cables, position of the trafos etc).

How does your grounding scheme/system looks like?

Franz
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Old 10th August 2004, 11:55 AM   #6
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Default A few things to check

Take a multimeter and set it to AC volts, place it accross the B+ supply to the plate of the 6AU6 and measure the ripple voltage, if it is greater than 3V, try adding a resistor between the plate resistor of the 6AU6 and add a capacitor on the B+ side of the 6AU6. I would start with 100ohm 5 watt and 100uF cap, adjust accordingly. You should check to assure all grounds go to a single point.
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Old 10th August 2004, 12:06 PM   #7
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Default Re: Does a cathode bypass capacitor reduce hum?

Quote:
Originally posted by ray_moth
Hi,


I'm getting a bit of hum and I wonder if it's caused by not using a cathode bypass capacitor?

Hi

It is not caused, but placing it reduces induced hum from B+ due to lower Rp

cheers
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Old 10th August 2004, 12:15 PM   #8
MIKET is offline MIKET  United States
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Default A few things to check

Take a multimeter and set it to AC volts, place it accross the B+ supply to the plate of the 6AU6 and measure the ripple voltage, if it is greater than 3V, try adding a resistor between the plate resistor of the 6AU6 and add a capacitor on the B+ side of the 6AU6. I would start with 100ohm 5 watt and 100uF cap, adjust accordingly. You should check to assure all grounds go to a single point.
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Old 10th August 2004, 12:29 PM   #9
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Default Re: Does a cathode bypass capacitor reduce hum?

Konnichiwa,

Quote:
Originally posted by ray_moth
I'm getting a bit of hum and I wonder if it's caused by not using a cathode bypass capacitor?
This can be the case, especially if the valve is heated with AC. The cathode is a input node, with a capaictance to the heater. If the cathode is not AC grounded than a highpass is formed between cathode and heater trough which some heater noise can leak into the signal circuit. Also, the valve bceomes more subject to electrostatic pickup with the cathode at high(ish) impedance.

Normall good layout and implementation practice should avoid having hum even in such situations. Are you aware of of what is "best practice"?

Sayonara
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Old 10th August 2004, 04:38 PM   #10
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Default Re: Re: Does a cathode bypass capacitor reduce hum?

Quote:
Originally posted by Guido Tent

It is not caused, but placing it reduces induced hum from B+ due to lower Rp
That too.

Saying that pulling the 6AU6 stops hum because it's the source is like saying your car's engine stopped because you pulled the computer (fuel injection and all is still there, but it ain't gonna be workin). Or something. You've just stopped the LTP from operating, and that's all.

Tim
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