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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: 21 May 2020, 21:21 
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Hi, A number of folks are using the 240 volt transformers on 220. The loss of B+ voltage is not significant and the output power will be essentially the same. In cases where the ac mains drops much more that can be an issue. At the 200 volt level it might matter and I would find a different transformer. Changing the resistors will not make much difference and a choke would would be expected to drop the B+ significantly more as the internal resistance is likely much higher than the 50 ohm resistor. The driver stage can actually work quite well down to about 150 volts.

Good listening
Bruce

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PostPosted: 22 May 2020, 01:22 
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I'm more concerned on the heater voltages, usually theres not much head room especially 5 or 6.3v so a 240v on 220v will nearly likey mean you may have less than recomended heater voltage


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PostPosted: 27 May 2020, 02:02 
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This is a good point. I may have to do convert the heater AC to DC to maintain the voltage.


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PostPosted: 27 May 2020, 03:59 
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You can always use a buck boost converter, one cheap from aliexpress, pay expensive shipping if you don't wanna wait 1-3months.
Which converts higher or lower input voltages to a fixed one


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PostPosted: 27 May 2020, 10:35 
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The drop in heater voltage is unlikley to cause any issues. The EL84s run fine from as low as 5.7 volts to about 6.5. So a 10% drop from the nominal 6.3 from the transfromers is still going to be fine. If you are using a variation with DC heaters that is regulated there should be no issue. and if DC unregulated changing a series resistor if present ought to work. A Boost converter will also work..just watch for the power load and check to see if you get switching noise that will need to be cleaned up.

Good listening
Bruce

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PostPosted: 27 May 2020, 19:18 
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My main concern is that below 6V heaters the drop in sound quality, also with arround 5-6V AC after regulated, there isn't enough voltage to either buck or boost the voltage, normally requirer diffrencee voltage of 2-3V to work well, so I recomend the buck boost.


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PostPosted: 03 Jun 2020, 18:36 
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Hi all, I'm a complete newbie here. I have a fair bit of digital experience, some analog experience, a tiny bit of Spice experience, but I'm a total newbie when it comes to vacuum tubes.

I am trying to figure out how the Poddwatt circuitry works before I attempt to build one. I found the post earlier in this thread showing the 5 different variants of the Poddwatt circuitry as it has evolved. To make it easier for anyone reading this to find all those circuit diagrams, here is a link to the exact page: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=1232&start=670

I started off by trying to make a Spice model of the Poddwatt type "E" (the most modern version). It went surprisingly well (aside from Spice being Spice), and I now have what appears to be a working Spice model of the Type E circuit.

While looking around at the signals during operation, the first thing that came up was that I don't see how the subwoofer output signal works. It was introduced in Type C, where the A/C coupled input signal gets divided down by the subwoofer divider resistors to create the output signal. The subwoofer output signal is referenced to GND due to the .33 uF AC signal coupling cap and the subwoofer voltage divider, and everything looks fine. In the type D and E circuitry, a positive bias voltage got added to the grid inputs, apparently to make the LM317 perform better. However, the new bias voltage will also bias the subwoofer voltage divider, and that seems like a potentially bad thing for whatever subwoofer amp might be downstream. A spice run clearly shows that the subwoofer output gets shifted entirely into the positive voltage domain. I tried to attach a screenshot of the Spice run showing that the subwoofer output for this particular test exists entirely in the positive voltage range, but the system is throwing an error when I try to attach the JPG.

In any case, please let me know if I am not thinking about this correctly.


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PostPosted: 12 Jun 2020, 12:26 
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Bruce will likely chime in here, but I suspect it was an oversight that the sub output wasn't modified when the positive grid bias was added. The grid bias is about +10V and with the 200k / 4.7k divider on the sub output that would give about 0.23V on the sub.

I'm not that familiar with sub amps but if yours sub amp input is not AC-coupled it would probably be good to add a series cap on the sub output (keeping in mind that you want to pass quite low frequencies). I just looked at a few sub amp schematics and depending on the design they tend to have 0.22uF to 10uF caps at the inputs but I see a few dicey-looking designs out there which are DC-coupled... that would be trouble.

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PostPosted: 12 Jun 2020, 21:34 
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Hi, Eric is correct. Somewhere in the umpteen posts is a description of that. Yes if you have a powered sub woofer that does not have input capacitors you should add one. The actual value will depend on the input impedance of the sub line stage. Most seem fairly high (over 10K) so something in the 1uf range would likely be fine. I would use a poly, but a non-polar electrolytic ought to do as well. With so little DC on it a polar electrolytic would probably not function correctly.

The design was done without Spice at the time of the original amps. I didn't have a copy of it that would deal with the tubes correctly. Since then the later evolutions are based on how things sound first and then verify that factors such as dissipation and voltages were OK. Testing is always done with two dual trace DSOs, several meters and an HP distortion analyzer. The latter is invaluable for audio design. I have found the hard way that the math and models don't always do what you expect as components especially tubes are never exactly what the specs say they are. My thoughts on design are model and crunch the numbers first, then refine by testing and listening. The latter is most important. I have listened to many pieces of gear with great specs and test data that were not great sounding. I attribute that to designers that want the lowest distortion, highest output power and widest bandwidth. Nice....how about sound quality as none of these will guarantee that.

Good listening
Bruce

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PostPosted: 13 Jun 2020, 14:20 
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Thanks for the replies! For sure, I can't listen to the output of a simulation. OK, I probably could arrange for that, but it would probably take a CPU month to generate data for a minute of music. And it wouldn't tell me anything anyway because the output would still just be data, not sound waves.

I am finding Spice to be very helpful as a learning tool to let me probe the circuit to see what is going on here or there, or to see what would happen if I were to change this component or that. For example, I can see that any input signal voltage at the grid of the upper EL84 power tube will also appear as a voltage on the input of the LM317 constant current sink, but at 1/2 the original amplitude. Today's epiphany was that the LM317 will see this 1/2 amplitude input signal in the form of power supply ripple that it must contend with. And because Spice is just a simulation, I can trivially swap out the LM317 for a "perfect" constant current sink with infinite ripple rejection to look for differences in behavior. I can't possibly listen to the difference, but it was a fun learning experience. Thanks for sharing your designs!


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