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21
Amplifier Discussion / Re: Gallien Kruger GT200
« Last 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?
22
Amplifier Discussion / Re: Gallien Kruger GT200
« Last 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.
23
Amplifier Discussion / Re: Gallien Kruger GT200
« Last 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.
24
Amplifier Discussion / Peavey Classic VT
« Last post by Enzo on December 02, 2019, 09:16:10 PM »
Heree is schematic

25
Amplifier Discussion / Re: Gallien Kruger GT200
« Last 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?
26
Amplifier Discussion / Re: Gallien Kruger GT200
« Last 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?   
27
Preamps and Effects / Re: So you need a preamp - here are your options
« Last post by flester on November 29, 2019, 05:44:46 AM »
OK I built up a buffer with MPF102. Definitely brightens the sound compared to going directly into my Kemo 40w power amp but less so in front of the TDA2003 or the LM386. This is using a 6" speaker, and fairly short guitar cables. Now to try it via my 1x12 and longer cables.

Sent from my SM-A505FN using Tapatalk

28
Amplifier Discussion / Re: Ibanez TSA30 - buzzing
« Last post by theaterguy on November 25, 2019, 02:58:06 PM »
For those it may concern; before anything, just try a short cable between send-return jacks.
Cheap&easy but some curiosity was left...
29
Amplifier Discussion / Re: Ibanez TSA30 - buzzing
« Last post by theaterguy on November 15, 2019, 02:24:52 PM »
Thanks, I try that.
30
Schematics and Layouts / Re: Schematic sites for different brands of amps
« Last post by Jazz P Bass on November 14, 2019, 11:48:19 AM »
Have you attempted to contact Rocktron?

http://www.rocktron.com/contact.html
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