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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 18 Oct 2018, 23:39 
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Here are some photos of the innards. It’s pretty tight where the tubes are, as it was built as a 4S Universal initially, and then the cathode follower was added, with not a lot of room available. I don’t think it is power supply related, as the original 4S was pretty much dead silent except for a bit of tube rush at full volume. I’ll post a power supply schematic shortly.


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PostPosted: 18 Oct 2018, 23:58 
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Here is the PSUD II sim for the power supply...


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PostPosted: 19 Oct 2018, 00:07 
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The circuit is exactly as per the one in this thread, with the output capacitor switch added and the optional cathode bypass capacitor of the original 4S Universal deleted. The switch picks between a .22uF and 2.5uf output capacitor. The power supply feeds the plates of the two tubes from the same node. What I am hearing through the speakers is definitely a hiss, and not the 120 HZ hum that would be expected from too much power supply ripple. Also, the hiss is present with both values of the output capacitor.


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PostPosted: 22 Oct 2018, 06:09 
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Certainly packed in there but quality parts regardless.

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PostPosted: 22 Oct 2018, 06:13 
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Matt, I wonder is there any sonic advantage of active load over cathode follower or vice-versa. Both use two triodes. Which has the better harmonic or distortions profile? Active load requires only three resistors per channel and no inter-stage cap just output cap.

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PostPosted: 22 Oct 2018, 08:11 
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mwhouston: I noticed that in addition to the hiss, there is some hum as well with the cathode follower added that wasn’t there with the regular 4S Universal. I checked the power supply circuit on PSUD II, and the additional current draw raises the ripple to a point that is too high with my 104 DB/W/M speakers. I think that the power supply should have a bit more capacitance added to it to reduce this ripple if your speakers are very efficient. I’m going to try adding a 47uF cap to see if it eliminates the hum in my system. The hiss issue remains however. Any strategies or advice that will reduce the hum would be much appreciated.


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PostPosted: 22 Oct 2018, 08:21 
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I checked the power supply on PSUD II with more capacitance. In order to get ripple down to under 1mV, an additional 147uF is required.


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PostPosted: 22 Oct 2018, 16:40 
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tizman wrote:
mwhouston: I noticed that in addition to the hiss, there is some hum as well with the cathode follower added that wasn’t there with the regular 4S Universal. I checked the power supply circuit on PSUD II, and the additional current draw raises the ripple to a point that is too high with my 104 DB/W/M speakers. I think that the power supply should have a bit more capacitance added to it to reduce this ripple if your speakers are very efficient. I’m going to try adding a 47uF cap to see if it eliminates the hum in my system. The hiss issue remains however. Any strategies or advice that will reduce the hum would be much appreciated.


I had similar hum issues which I couldnt fix so remove the cct. brd. and replaced it with a single stage. But what I did find and have found over the years is where you attach the earth or negative of the PS. On one build I attached the earth at the RCAs and in another at the pot. If I switched it around on the respected amp I got a hum. So now if I attach the earth at the RCAs and get a hum I move it to the pot and vice-versa.

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PostPosted: 28 Oct 2018, 04:19 
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I added the capacitance, and the hum is gone. The hiss remains however.


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PostPosted: 28 Oct 2018, 11:36 
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tizman wrote:
I added the capacitance, and the hum is gone. The hiss remains however.
After looking at the pictures, I am not surprised.

Most hum is generated in one of two ways; coupling of magnetic fields between conductors or magnetic elements, or by insufficient filtering of high voltage supplies. The former is eliminated by keeping H fields low (shielding transformers, using twisted pair wiring, etc.) and the latter by adequate filtering (i.e. your addition of capacitance).

"Hiss" (i.e. pink noise, or 1/f noise) is generally due to oscillatory circuit interactions outside of the auditory band with "skirts" that fold back (or extend) into the auditory band. It is important to remember that high frequency oscillatory behavior is significantly affected by parasitic elements within the circuit.

There are some elements in your build which can drive this high frequency behavior. The most important of which is the proximity of lumped circuit elements. Stray capacitances between rolled capacitors in combination with the inductive characteristics of wires, resistors, etc. when added to active circuits can cause high frequency feed back and the low level "hiss" you describe. Unfortunately, the general solution is reduced proximity to minimize unwanted coupling. I am afraid that in the form factor you have chosen, you may be forced to live with the hiss unless you wish to rebuild in a larger form factor.

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