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Old 26th February 2005, 09:50 PM   #1
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Default tubeamp project, weekend timekiller

Hello tubelovers! Do you have any suggestions on this ? Click the image to open in full size.
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Old 26th February 2005, 11:24 PM   #2
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I saw you also posted in this thread.

You've used RC coupling between the input and phase splitter stages, but the grid of the phase splitter will be floating at some potential above ground anyway, so why not use the traditional Williamson input/phase splitter arrangment (common cathode dc coupled to cathodyne phase splitter). It'll require a bit of fiddling with operating points, but it will do away with C7, R13, R10, and an LF rolloff.
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Old 27th February 2005, 07:49 AM   #3
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Draw me a good but still simple schematic please, preferrabli with ECC82 or 83. I noticed the LF rolloff, first the sim shows distortion around 100Hz, then the signal goes weaker for every Hz down in freq. at 20Hz only like 2 watts were left with the same input voltage.
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Old 27th February 2005, 08:13 AM   #4
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It's not that easy for me to just come up with a circuit... I haven't a clue how to use simulators, and the good ol' pen-and-paper is a little slow. So I'll just go through how it's done - that way you can do it for whatever input/driver you like. You really see this circuit everywhere.
[list=1][*]Select the (equal) cathode and anode load resistances for the phase splitter[*]Draw the loadline for the phase splitter, choose Vgk (and hence find Va)[*]Find the voltage across Rk of the phase splitter by subtracting Va from the HT then dividing by two[*]Find the voltage at the grid of the phase splitter by subtracting Vgk from the voltage across Vk. This is also the voltage at the anode of the input valve.[*]Draw a loadline for the input valve, and check that the operating conditions are acceptable. If they are not, you'll have to fiddle around from the beginning until they are - of course this will be easier for someone who knows how to use SPICE [/list=1]

Another possible topology is to build a fully balanced amplifier, by substituting the Williamson style driver with a differential pair (preferably with a constant current source in the tail since it is easy to do nowadays with cheap silicon). The advantage here is that the amp can accept balanced signals, or feedback could be applied to a grid, which makes the calculations easier
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Old 27th February 2005, 08:31 AM   #5
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Quote:
I noticed the LF rolloff, first the sim shows distortion around 100Hz, then the signal goes weaker for every Hz down in freq. at 20Hz only like 2 watts were left with the same input voltage.
Yes, it is as expected as the values on the coupling capacitors you have selected are too small, increase the coupling capacitors betqween phase splitter and power tubes to at least .22uF.

Do as "audiousername" suggested with direct coupling between first 12AU7 and the phase splitter, this make it easier to get feedback to work properly.

Suggestion, use 100k as anode resistors to first 12AU7 and direct couple anode to second 12AU7 grid, use 47k as cathode and anode resistors at the first phase splitter, adjust first tube cathode resistors for maximum undistorded output from phase splitter, try with 1.2k for a start and connect a 100 ohm in series, (connect feedback at that point), use a capacitor in parallell with the 1.2k. You probably also need to decouple B+ to first tube but negative effects of not doing that will not be shown in the simulation. How close your simulation will be to a real circuit depends on the tube models you are using, models available on internet range from quite good to horrible.

Regards Hans
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Old 27th February 2005, 08:53 AM   #6
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You mean something like this ?
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Old 27th February 2005, 08:59 AM   #7
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Yes, something along those lines.
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Old 27th February 2005, 09:20 AM   #8
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It looks OK but you need decoupling of first stage B+ with a series resistor from raw B+ to get down to 300V or what you want to have and a cap to ground. You need to adjust the feedback resistor also and ideally you should add some phase adjusting components also but there is no use to do that in the simulator if you dont have exact models for all components including the output transformer.

Regards Hans
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Old 27th February 2005, 01:34 PM   #9
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I have done some more changes, is it better or worse ?
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Old 27th February 2005, 04:13 PM   #10
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I have done some more changes, is it better or worse ?
Better, but can be improved.

It is better to use higher anode and cathode resistors in the phase splitter as it will give higher possible voltage swing and therefore less distortion, (The total amplifier should ideally be designed so that the distortion in the power tubes dominates, i.e. the distortion in the drivers stage should be as low as possible and not contribute to the total distortion).

I would suggest that you change R3 to ~10k and connect R13 to the connection point of R3 and R1, this will give much better decoupling of the first stage. C4 and C5 can be much higher, I use 220uF as C4 and 100uF as C5 in a similar circuit.

Edit, add a cap in parallell with R12 to increase the total gain, 100uF is enough.

Regards Hans
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