Another PP GU50

[matejsirk]

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.

[ErikdeBest]

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.

[Wavebourn]

Good luck! :-)

Please come back when you get some results, and I will share my Pyramid experiences. And probably PCBs. :-)

[ilimzn]

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...

[matejsirk]

Originally Posted by ilimzn

IRFP240 as followers at a few mA? Surely not - there are far better suited MOSFETs, IRFP240 has WAY too much capacitance.

Yes I know that. This is testing schematic in LTSpice. I am still looking for better MOSFET. Which one do you prefer?

[matejsirk]

Originally Posted by ErikdeBest

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?

[Kay Pirinha]

I think your design lacks the possibility of adjusting the GU50's bias, doesn't it?

Best regards!

[matejsirk]

Originally Posted by Kay Pirinha

I think your design lacks the possibility of adjusting the GU50's bias, doesn't it?

Best regards!

As I wrote before - this is working schematic for LTSpice. I always do bias like this:

[Funker]

Originally Posted by matejsirk

As I wrote before - this is working schematic for LTSpice. I always do bias like this:

Hi,
it will be more comfortable to use two potmeters , one for the bias balance and another to set the idle current, like this example:


73
Wolfgang

[FLT]

Hi,

I prefer this circuit:

http://www.diyaudio.com/forums/attac...s-kt88_amp.png

No matter which trimpot fails the bias voltage will go full negative and
switch-off the tubes.

No, I am not kruesi...but i use the same bias circuit for my amps.

regards