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Author Topic: Gallien Kruger GT200  (Read 9191 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 »

gbono

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Re: Gallien Kruger GT200
« Reply #20 on: December 19, 2019, 03:38:49 PM »
Adjusted bias at the two emitter resistor and I think there is another typo on the schematic since I can only adjust to 5mV not 50mV. This adjustment has no effect on the distortion.

Jazz P Bass

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Re: Gallien Kruger GT200
« Reply #21 on: December 20, 2019, 01:06:31 AM »
Can you take a picture of the distortion as it appears on a scope?

If so, upload it.

willpirkle

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Re: Gallien Kruger GT200
« Reply #22 on: December 20, 2019, 09:10:14 PM »
Yes, picture of x-over distortion. Right at 0V or biased?

g1

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Re: Gallien Kruger GT200
« Reply #23 on: December 21, 2019, 03:11:05 PM »
I think there is another typo on the schematic since I can only adjust to 5mV not 50mV. This adjustment has no effect on the distortion.
I doubt that is a typo.  They show it on the schematic and also state it in the biasing notes on the side.
But even if it were a typo, you should be able to adjust to much more than 5mV there.
I think if you find the problem with the biasing you will most likely solve your distortion problem.

willpirkle

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Re: Gallien Kruger GT200
« Reply #24 on: December 21, 2019, 07:02:39 PM »
I have a ‘78 210G which is nearly identical in the power amp, and attached that one (way easier to read). These are just for reference - hope they help someone. The 210G has identical output and bias transistors but emitter resistors are 0.15
« Last Edit: December 21, 2019, 10:01:30 PM by willpirkle »

willpirkle

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Re: Gallien Kruger GT200
« Reply #25 on: December 21, 2019, 09:54:43 PM »
Here’s the preamp (shows letters).

gbono

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Re: Gallien Kruger GT200
« Reply #26 on: December 22, 2019, 04:38:24 PM »
Here is output waveform with 8 ohm load 1000Hz. The distortion is not noticeable at low output levels (less than 2VRMS). The crossover distortion increases with output drive - you an still see it a 25 VRMS output level.

Can someone explain this bias scheme to me?
« Last Edit: December 22, 2019, 06:28:21 PM by gbono »

gbono

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Re: Gallien Kruger GT200
« Reply #27 on: December 25, 2019, 06:05:56 PM »
Merry Christmas and Happy New Year..............

Jazz P Bass

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Re: Gallien Kruger GT200
« Reply #28 on: December 25, 2019, 08:26:43 PM »
Not too sure what you are attempting to fix here.

Yes there is a small x over notch at higher settings.
I really doubt that you could here that.

What I see from the image is a gross mismatch in the top & bottom width of the waveform.
THAT will be noticeable.


gbono

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Re: Gallien Kruger GT200
« Reply #29 on: December 25, 2019, 09:50:55 PM »
At 25RMS this amp is in clipping with the negative half of the waveform into clipping earlier than the positive part of the cycle. Both sides of the PA draw from the positive supply rail if I'm not mistaken ....