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Author Topic: Gallien Kruger GT200  (Read 1165 times)

Jazz P Bass

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Re: Gallien Kruger GT200
« Reply #15 on: December 02, 2019, 11:06:49 PM »
According to the schematic, there are two pots that need to be adjusted.

gbono

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Re: Gallien Kruger GT200
« Reply #16 on: December 03, 2019, 04:27:00 PM »
No bias adjust just two trim pots that adjust the voltage across emitter resistors to get around .05V...I noticed D1 & D2 are across the emitter base of the driver transistors Q7,8,9,10. Are they there to supply the 1.2 V need for the darrlington's VBE? I assumed that the new output devices had a lower turn on VBE but it looks like that may not be the case.

Enzo

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Re: Gallien Kruger GT200
« Reply #17 on: December 03, 2019, 08:42:14 PM »
Those sound like bias controls to me.

On your schematic from early on, I see D1,2, but they do not connect to Q9,10, only to the bases of Q7,8.  They do connect to other transistors, but not 9,10.  They do go to the collectors of Q3,4, which are the bias transistors.

Unlike the common complementary SS output stages, this is two single sides stages in push pull.  In other words, the bottom half is identical to the top, just drawn upside down.  SO usually we see one bias transistor between the positive and negative sides, we have two positive sides, so we need two bias controls.

The adjustment is for 1.65v on the base of the drivers - Q7,8.  Qs3,4 draw current through those diodes to set that voltage.  The bias is adjustable exactly because transistors vary.  Whenever outputs are changed, there is no assumption the old adjustment will still be correct.

What did you use for PMD10K80?

g1

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Re: Gallien Kruger GT200
« Reply #18 on: December 04, 2019, 02:11:52 PM »
.05V across .33 ohms would be 150mA current through those output devices.
That sounds pretty hot.  I would not expect to see crossover distortion with that kind of idle current.

gbono

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Re: Gallien Kruger GT200
« Reply #19 on: December 04, 2019, 02:54:12 PM »
I substituted 2N6284 for the PMD1603. The trim pots have very little effect over the crossover distortion. I will look at this adjustment again. Also will look at voltages at base of Q7/8 and cathode end of D1/2.
« Last Edit: December 04, 2019, 03:34:27 PM by gbono »

 

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