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Old 13th August 2004, 01:04 AM   #11
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Hi,

Quote:
I'm getting a bit of hum and I wonder if it's caused by not using a cathode bypass capacitor?
The only correct answer is NO as Franz G stated correctly already.

It is a myth that bypassing a cathode REDUCES HUM for the simple reason that you're comparing apples to oranges:

What the unbypassed cathode resistor represents is a different circuit compared to the one with the bypassed cathode resistor.
As you don't bypass the cathode resistor, the cathode, as an input node, becomes more sensitive to picking up AC signal; logical as it's not being shunted to ground as it would be with a bypass across the cathode.

Since you already have the AC heater shunted at the virtual CT, the heater induced hum itself should already be greatly reduced.
However, circuit layout, twisting of the heater wires, ripple riding on the B+ etc., can still inject hum into the circuit.
A simple test would be to temporarily run the heater from a well filtered DC source or a battery, for instance.

In some cases such as phonopreamps or high-ish gain linestages the easiest way out is to resort to DC heating.
Some tubes are also more sensitive than others, so hum may vary from one brand to another depending on construction.

Last but not least, heater to cathode insulation should be kept well within the manufacturer's specs as disrespecting it will effectively couple both electrodes with the possible risk of a heater getting stuck to a cathode. (BANG)

Coupling (other than just capacitive) between the both of them can possibly start already with a potential difference of a mere 30V.
Which is why Philips (among others) recommends having heater and cathode at the same potential where no secondary emission can possibly occur.
This can simply be achieved by biasing the heater up to cathode potential. Or at least high enough to be within stated maxima.

Sorry if I repeated some of what other fellow members had stated already...Just thought it would be useful to condense it in a single post.


Hope this helps,
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Old 13th August 2004, 01:35 AM   #12
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Quote:
Originally posted by fdegrove
The only correct answer is NO as Franz G stated correctly already.

What the unbypassed cathode resistor represents is a different circuit compared to the one with the bypassed cathode resistor.
Oh, you're taking that route. Rather roundabout way of applying logic, if you can call it such. The really interesting thing about your post is that it starts by saying no then in a hundred words, proves the opposite.

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Old 13th August 2004, 01:41 AM   #13
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Hi,

Quote:
The really interesting thing about your post is that it starts by saying no then in a hundred words, proves the opposite
One word: NO.

Cheers,
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Old 13th August 2004, 07:44 AM   #14
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Thanks very much for your replies. Firstly, I agree with those who say that several sources of hum are possible. As to understanding good practice, I think I do understand but that doesn't mean I that did it properly. I will have to track down the hum as best I can.

Secondly, I suddenly realize that one aspect of my circuit that may be unwise is that I have placed the passive Baxandall tone controls, followed by the volume control, before the first (6AU6 triode) stage. This means that the signal is firstly attenuated heavily (about 12dB, I think) before it is amplified. So the noise due to the 6AU6 triode is amplified together with the weakened signal. I did it this way because I didn't want to risk overloading the 6AU6 connected using the screen grid as the anode - apparently, this mode of connection does not work well with large signals. However, the risk of a poor signal-to-noise ratio

Maybe it would be better to place the volume control at the beginning, followed by the 6AU6 in convdentional triode mode (plate, grid 3 and grid 2 strapped together as the anode) to give a lower Rp, followed by the tone controls. This should attenuate the both the noise from the 6AU6 and the signal by an equal amount, giving a better signal-to-noise ratio. In that case, I guess I should bypass the cathode resistor, to achieve low Rp for feeding the tone stack. What do you think?
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Old 13th August 2004, 06:45 PM   #15
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Uh, yeah. 99% of tone networks work best with a low impedance source (triode, 12AU7 or better; 6AU6 should be okay) and high impedance load (CF).

Tim
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Old 15th August 2004, 04:28 PM   #16
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Quote:
Originally posted by fdegrove
Hi,



The only correct answer is NO as Franz G stated correctly already.

It is a myth that bypassing a cathode REDUCES HUM for the simple reason that you're comparing apples to oranges:

[QUOTE]Originally posted by fdegrove
[B]Hi,

Frank,

The answer should be yes

Bypassing the cathode reduces hum for the simple reason that the internal impedance lowers, hence the PSRR ratio increases as the external anode load remains equal.

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Old 15th August 2004, 08:56 PM   #17
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Hi,

Guido,

I'm well aware of what you're saying but I'm stuck with the fact that bypassing Rk will still give a circuit different than the one that was designed from the outset.

That's what stated in post #1:

Quote:
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.
Quote:
I'm getting a bit of hum and I wonder if it's caused by not using a cathode bypass capacitor?
To which I think the answer is no, it not CAUSED by the fact that Rk is not bypass.

Since the stage is part from an integrated amp, bypassing Rk will change the circuit parameters enough to be a potential source of other problems. The cathode could be a node for NFB input for instance, etc.

Other than that I feel hum is best tackled differently than by just bypassing Rk which is not always a practical solution as outlined above and which to my mind is a bandaid, not a solution as it intrinsically modifies the circuit parameters enough to turn it into a new one.


That, however is just my opinion.

Cheers,
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Old 16th August 2004, 09:52 AM   #18
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Quote:
Originally posted by fdegrove
Hi,

Guido,

I'm well aware of what you're saying but I'm stuck with the fact that bypassing Rk will still give a circuit different than the one that was designed from the outset.

That's what stated in post #1:

To which I think the answer is no, it not CAUSED by the fact that Rk is not bypass.

Since the stage is part from an integrated amp, bypassing Rk will change the circuit parameters enough to be a potential source of other problems. The cathode could be a node for NFB input for instance, etc.

Other than that I feel hum is best tackled differently than by just bypassing Rk which is not always a practical solution as outlined above and which to my mind is a bandaid, not a solution as it intrinsically modifies the circuit parameters enough to turn it into a new one.


That, however is just my opinion.

Cheers,

Frank,

Keep it simple, and forget about the rest of the circuit.

After all, one should look at where the hum stems from. If that is the common cathode stage, it will reduce once the cathode is bypassed.

If not, it won't affect the circuit and the hum will stay

I am trying by the way to finish an article on how to tackle hum, and how to decently layout vtube circuits.

best regards
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Old 20th August 2004, 03:09 AM   #19
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Default Just discovered a reason for a lot of the hum

I feel a bit stupid having to admit this! I have just discovered that the multi-output extension lead I've been using has only a 2-core cable. The ground pin is not connected to anything. However, my signal source is a video-CD player connected to a TV, which has a line output that is connected to the amplifier input. The VCD player and the TV are both run from the same extension lead as the amp, so they also are not grounded.

The only grounding comes from a Cable TV connection to the TV! This crazy arrangement works OK, with very little hum. However, it's a different story if I disconnect the TV-to-amp lead, which I often do for testing. Then, because there is no grounding at all, the amp picks up considerable hum very easily.

2-core extension leads are very common in Asia (where life is cheap and profit is everything). The only way I can be sure of a 3-core extension is to make one myself, which I will have to do.

I still intend to reorganize the preamp so that the volume control precedes it and the tone stack follows it, leading directly into the differential phase splitter of the main amp. I think this would be a better topology: the volume control will be at the input, where it belongs, the preamp stage will be a conventionally-wired 6AU6 in triode mode (with a low output impedance), and it will be easy to arrange a tone-defeat by simply switching the main amp input directly to the volume control instead of the tone stack output.

Anyway, thanks very nmuch for all your responses.
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Old 20th August 2004, 07:34 AM   #20
Franz G is offline Franz G  Switzerland
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Guido

Imagine a cathode follower with an cathode bypass! Does not work.

Do cathode followers produce hum, because there is no bypass? Absolutely not!

To use a cathode bypass cap is a question of local current feedback, influences the gain, impedance and distortion.

It is definitely not a question of hum or not.

Franz
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