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Old 3rd October 2012, 04:36 AM   #7351
Ejam is online now Ejam  Australia
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Default GU50 Does Not Fit Nicely

Wavebourn

Your suggestion of "the GU50 fitting nicely" needs some qualification. Direct substitution is not an option as from the GU50 curves at Vpk = 15V, Vpk = 380V and Vg2 =250V, the GU50 draws 200mA hence is dissipating 76W, way more than its rated 40W maximum. You might want to elaborate on your definition of "fits nicely" with a more appropriate operating point.
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Old 3rd October 2012, 06:09 AM   #7352
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Quote:
Originally Posted by Ejam View Post
Wavebourn

Your suggestion of "the GU50 fitting nicely" needs some qualification. Direct substitution is not an option as from the GU50 curves at Vpk = 15V, Vpk = 380V and Vg2 =250V, the GU50 draws 200mA hence is dissipating 76W, way more than its rated 40W maximum. You might want to elaborate on your definition of "fits nicely" with a more appropriate operating point.
01N100 has wide spread of parameters, as well as tubes, so anyway you need to select proper value for the resistor in drain; no way 820 Ohm will fit any random mosfet and tube combination. Select it for 100V on cathode, you will get 100 mA idle current as the result. That means 35W dissipation. For an average Gu-50 in such mode you will have about -30 V on G1 in respect to cathode. But if you set idle current 70 mA as drawn on schematic you will have about -35V on g1 in respect to cathode and even lower anode power dissipation.
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Old 3rd October 2012, 06:29 AM   #7353
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Originally Posted by Wavebourn View Post
This one is better: no transistors that add to emitter current base current dependent on beta (non-linear). Schematic drawn by Alexander from diyaudio.ru forum.
Yeah, only toobes, with their sticky, syrupy, THDing and IMing, full of transformer garble sound.

Fortunately, in about an hour, I'm off to Budapest and will be back on Friday evening, when the wrath of the righteous has calmed down, as per the old Latin saying: Ira furor brevis est (Their wrath is short lived).

Don't bother gathering a posse, Wave, even if you already have a rope ready.

I shall report on the quality of the goulash and the Szerbo cake.
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Old 3rd October 2012, 09:06 AM   #7354
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GU 50/5881 ?

I found the original . I did have a go to build it . Start with a cathode current sink I would say . I think some call these Frankenstein amps . That makes me want to build them . I did find using this type of shunt feedback to the power valve gives exact triode distortion spectrum with a nice gain bonus . About 30% typical in RH88 version . It is a cheap lunch . I must say when I use valves the Frankenstein comes over me . I have to see the anodes glow even though I shouldn't . The only transistor I knew to tolerate that was a BD131 . Smoke came out yet it continued to work when the fault corrected !

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Old 3rd October 2012, 09:46 AM   #7355
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DVV , you neatly bring me full circle . This is an amplifier that can have no existence without global feedback . The output transformer-less type . Philips I seem to remember used it in pentode form for TV's . If a special speaker is made it can have special advantages ( 32 ohm ) . Having heard such an amp I was very surprised , lightning fast . It might have been the distortion spectrum fooling my ears .
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Old 3rd October 2012, 04:00 PM   #7356
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Originally Posted by nigel pearson View Post
Start with a cathode current sink I would say .
No way. Use resistor in cathode, because MOSFET can't keep stable voltage on the control grid.

Use 1k trimpot for R2 in source of MOSFET. Rotate it on minimum. Power on. Slowly rotate the trmpot to set desired idle current measuring voltage across cathode resistor. Listen and enjoy.
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Old 3rd October 2012, 09:40 PM   #7357
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I was thinking just to get some indications . Myself , I used a high value resistor and reduced it . Something went wrong and it got a bit wild for a minute. I think I calculated >100 W dissipated . The valve was fine . I wouldn't advocate a current sink for permanent use as it is at best too complicated . Many times I wish I had used one in the initial stages of a design . I have a current setting on my bench power supply , it has save the day so many times . I must say DC coupling an amp like this seems a bridge too far .
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Old 3rd October 2012, 11:27 PM   #7358
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Suppose the MOSFET draws too low current. That means too high voltage on grid, hence on cathode. Can your current sink stay alive? Gu-50 can dissipate more than 100W easily. It survives even orange glow (Nickel alloy anode). What about that tiny peace of silicone that you use for CCS?
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Old 3rd October 2012, 11:31 PM   #7359
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Yeah, only toobes, with their sticky, syrupy, THDing and IMing, full of transformer garble sound.
Are you ready to eat your hat, бэби?
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Old 4th October 2012, 09:22 AM   #7360
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Hi Wave I am new to valves and learning , it is about having knowledge rather than loving them . I prefer diesel to steam locomotives so there is no hope I guess ( forget electrics that's class D ) . It has to be said steam comes very close and valves too . The Brits call old radios " steam radios " !?

I found by happy accident the right way to do it and the one you recommend . I had a nasty moment when my head went into reverse gear applied the opposite bias that I intended . As you say it survived . The heat haze coming off of the cathode resistor let me know first .
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