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Another PP GU50

This is schematic for my new PP with GU50. Max power more than 100W, THD at 80W 0.55%. I plan to build that amp in next few mounths. All voltages other than 700V will be regulated.
Ask for comments.
 

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Hi, I think it would be good to leave some more headroom for the source followers. With headroom I mean a bit more than the +20V (maybe +100V) and possibly more negative voltage as well (more than -100V, and larger resistor). The reason is that the capacitances of mosfets are not very linear when VDS becomes small, so it is better to have at least 30V VDS under all circumstances.

I think your definitive amplifier will have independent voltage sources (pots) for bias adjustments, to compensate for differences between GU50's?

have you looked into the 'bestpentode' connection for the input stage? I do not know if it works together with the local feedback arrangement, but I think it will. It could give a bit more gain, less noise, and possibly better balance in the LTP.
 
IRFP240 as followers at 90 mA? Surely not - there are far better suited MOSFETs, IRFP240 has WAY too much capacitance, and this will be extremely nonlinear as Vgd approaches 8-10V. Even at 100mA per follower (which seems a bit excessive but I am guessing you are trying to make the change in current lower as the G1 of the GU50 goes positive) there are better (smaller) MOSFETs to use, also a bit more voltage on the current 20V supply...
 
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Hi, I think it would be good to leave some more headroom for the source followers. With headroom I mean a bit more than the +20V (maybe +100V) and possibly more negative voltage as well (more than -100V, and larger resistor). The reason is that the capacitances of mosfets are not very linear when VDS becomes small, so it is better to have at least 30V VDS under all circumstances.

I think your definitive amplifier will have independent voltage sources (pots) for bias adjustments, to compensate for differences between GU50's?

have you looked into the 'bestpentode' connection for the input stage? I do not know if it works together with the local feedback arrangement, but I think it will. It could give a bit more gain, less noise, and possibly better balance in the LTP.

Very good idea for bestpentode, I must find the Linear Audio publicitation where is that described. I belive that is in volume 1?
 
Dynaco GU50

40W GU50PP on Dynaco ST70 chassis (which retain only the transformers). Frequency response at 30W -3db @ 10 Hz, 55Khz.
The input & drivers tubes are 5814. It sounded so much better than my stock ST70 EL34. Just a quick fun experiment with Russian GU50 tube.
 

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GU50 PP Dynaco

That's because I tested at 30W, close to full power, where it would tell me how the amp will perform with real music, I always test the amplifier at power level when clipping start. Most manufacturer specified frequency response at 1W , sure it looked much better on paper, but it wont tell you how it will perform when the music is demanding. Also, I used much less negative feedback then the original dynaco, -3db at above 40Khz is fine with me.

Below is frequency response with 1Watt into 8 ohm.

gu50-5.jpg

The frequency response at high end is not OK. It begins to drop above 2 kHz.
With original output transformers the response should be flat up to 50 kHz.
Have you optimized the GNFB loop ?
 
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artosalo

Member
2010-02-16 9:00 am
There is something wrong. Again, have you checked that the feedback loop is OK ?
Especially the capacitor (390 pF at the original schematic) from UL tap to cathode of 7199.
ST70 should give the same frequency response with 30 W and 1 W, like all well designed and built amplifiers.