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Gemini I phase inverter distortions

10/13/2005 9:13 PM
Marc
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Gemini I phase inverter distortions
I have a real head scratcher here.  
An Ampeg Gemini came in to my friends shop a while back that has had us puzzled all summer.  
I will give ya'll the symptoms and see if somebody can shed some light into why this amp becomes a crunch monster at 50% volume. We are Stumped!  
This is the PI version we are working with:http://members.aol.com/fyrebottle/ebaypics/g12.jpg> (Thanks to Joe Piazza for the snippet of his schematic)
 
The components all measure with in tolerance.  
The caps aren't leaky.  
The only voltages that are out of whack are the grid of the triode (30V instead of 15V) PI and the Cathode(80V instead of 50V).  
The amp had been serviced before it came to our shop and most noteably has a Hammond organ OT installed in place of the original OT.  
The power supply voltages are within 10%.  
I tried an NOS 7199 with little change.  
I changed the feedback resistor to 100K from 270K and it made a minor difference, like I can get to 55% before the signal gets crunched-but it makes the amp sound contricted too.  
We scoped it today and a sine wave goes in the voltage amplifier as nice as you please but as the volume is turned up it becomes a triangle wave at the plate of the pentode section.  
Looking at the PI output the signal looks like the sinwave gets pinched in the middle looking somewhere between a triangle and a sine wave.  
 
Why would an increase in signal to the input of the voltage amplifier convert it to a triangle wave?  
 
Should I reduce the 1.5M resistor from the voltage divider feeding the PI grid to reduce the grid voltage closer to the schematic value?  
 
 
Any ideas would be greatly appreciated!  
 
Marc

 
Replies:
jbrew73 i had a very similar problem with a... -- 10/14/2005 4:36 PM