Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: gbono on September 07, 2019, 06:24:51 PM

Title: Gallien Kruger GT200
Post by: gbono on September 07, 2019, 06:24:51 PM
I'm working on a GT 200 where the preamp board is working great but no level from the output board. I found Q1/2 were damaged but also noted that R1 and C1 are swapped on the output board. I get a strong signal from the preamp board to the PA but almost no signal at base of Q1. Assuming C1 provides DC block for preamp out??? What am I missing?
Title: Re: Gallien Kruger GT200
Post by: Enzo on September 07, 2019, 06:54:59 PM
SO is there signal between the cap and resistor?
Title: Re: Gallien Kruger GT200
Post by: gbono on September 07, 2019, 06:56:55 PM
Yes there is a signal between the cap and resistor but almost nothing at the base of Q1,,
Title: Re: Gallien Kruger GT200
Post by: g1 on September 08, 2019, 12:07:04 PM
It looks like R1 and C1 are in reverse order from what is shown on schematic.
This doesn't matter but makes me suspect C1, or Q1.
Title: Re: Gallien Kruger GT200
Post by: gbono on September 08, 2019, 02:43:18 PM
Well C1 and Q1/2 have been replaced but the bias on Q1 is not right.
VB = +9.5V
VE=+10.4V
VC=+10.6V

node "I" is +12.6V

Q1 is turned off - where does Q1 get base bias from?
Title: Re: Gallien Kruger GT200
Post by: Enzo on September 08, 2019, 03:16:56 PM
Bias is the difference between emitter and base, not just the base voltage.  SO with the base esentially at ground for DC, look at the schematic.  Your emitter then must be negative to turn on the Q1.

Note the top and bottom halves of this amp are the SAME, not opposite polarities as usual.  The upper side acts like a plain single ended amp, then the OUTPUT from the speaker winding becomes the signal for the bottom side.  That signal is inverted from the input signal, so the NFB acts more or less as a phase inverter.

ANyway, over left, R4, R5 are the emitter resistors for Q1,2.  The join, but not at ground, they are wired up and over to the right with the current control and H supply of -8.9V.   H comes from Q26 and ultimately from the -30v rail.

So... do you have voltage H at that control?  At the joint of R4,5?   My first suspect is an open R4, but surely a missing supply there would do it too.

Note by the voltages on the schematic the top and bottom halves mirror one another, so the voltages are all the same.  Use Q2 side to compare.
Title: Re: Gallien Kruger GT200
Post by: gbono on September 09, 2019, 12:42:03 AM
Duh R2 is grounded - the issue was the current lim knob was turned to 0 so voltage at R4 was positive. So there is an amplified signal at the collectors of Q1/2 but still no output level into 8 ohms??? Q7/8 have 1.7 to 2 v on their bases. Wondering if I should test the OT?
Title: Re: Gallien Kruger GT200
Post by: Enzo on September 09, 2019, 12:53:36 AM
I see no reason to suspect the OT yet, but I do have some shop wisdom:

never think up reasons not to check something.

SO go ahead.

But remember, the signal for Q2 comes directly FROM the speaker side of the OT.  Seems like it works then.

Before you test the OT, apply signal to the amp, and see if signal is on the hot pin of the extension speaker jack.

Look at the speaker jacks.  Note the main jack requires the signal to come THROUGH the extension jack contacts, so if they are bent or dirty... no sound.  And if you plug the speaker into the extension jack, there is nothing in the main jack to complete the circuit to ground, and... no sound.

A quick test?  Two speakers, one into each jack.  If that works then we can be fairly certain the extension jack contacts are bad.
Title: Re: Gallien Kruger GT200
Post by: gbono on September 09, 2019, 01:11:51 AM
Plugged in two resistive loads to the main and extension jacks - no output.
Okay simple didn't work so it's time to pull the OT and test it - unless there are any other suggestions?
Title: Re: Gallien Kruger GT200
Post by: g1 on September 09, 2019, 01:32:52 PM
Check voltages at Q26 first.  That is where the DC for the current limit pot comes from.
Seem to recall a similar issue recently and the problem was in the Q26 area.
Title: Re: Gallien Kruger GT200
Post by: gbono on September 09, 2019, 03:01:30 PM
Thanks g1 - I have noticed that the voltage between R4 and 5 comes from the current limit pot and node H. With the pot turned to "10" I get about -8V between the resistors and Q1 conducts. As I turn the pot towards "0" I end with +8 or so volts?! This isn't how I envision the current limit should work.  I'll look at Q26 again.
Title: Re: Gallien Kruger GT200
Post by: mandu on September 10, 2019, 06:31:10 AM
When you replaced Q1 and Q2, probably the transistor pins are not same as the replaced ones, even if the same number. The pin connections some times vary between manufacturers. Regards.
Title: Re: Gallien Kruger GT200
Post by: willpirkle on September 23, 2019, 04:56:02 PM
O/T: hey could you please tell me the P/N for the Accutronics reverb tank in that 200GT? I'm trying to find a replacement for a 1978 GK210G and the schematics appear to be nearly identical, so I am thinking I can use the same tank. The part number should start with "4" TIA.
Title: Re: Gallien Kruger GT200
Post by: gbono on December 02, 2019, 02:53:22 PM
Amp is finally repaired - I had to replace the output transistors, .3 ohm emitter resistors, driver xistors, etc. There is some crossover distortion in the output sine-wave - any ideas on how to minimize this?   
Title: Re: Gallien Kruger GT200
Post by: Enzo on December 02, 2019, 09:10:26 PM
That is what the bias adjustment is for.  May I assume there is one?
Title: Re: Gallien Kruger GT200
Post by: Jazz P Bass on December 02, 2019, 11:06:49 PM
According to the schematic, there are two pots that need to be adjusted.
Title: Re: Gallien Kruger GT200
Post by: gbono 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.
Title: Re: Gallien Kruger GT200
Post by: Enzo 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?
Title: Re: Gallien Kruger GT200
Post by: g1 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.
Title: Re: Gallien Kruger GT200
Post by: gbono 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.