unclej Posted December 15, 2004 Report Share Posted December 15, 2004 while i've been waiting for all the parts to arrive for the ampeg and alamo amps that i've been working on i've taken the opportunity to study them..tracing wires, testing components, etc. and i've come up with a question...with the neg. wire of my multi-meter clipped to the chasis and the red being used to probe i've discovered that at least one terminal on each of the pin sockets on the alamo 110w bass amp goes directly to ground..on the ampeg geminii II the only one that goes directly to ground is the tube controling the reverb. all of the others go through caps or resistors that then go to ground..any particular reason why? Quote Link to comment Share on other sites More sharing options...
lovekraft Posted December 15, 2004 Report Share Posted December 15, 2004 It could be the heater filament is ground referenced, or it could be that the tube isn't cathode biased - what tube(s), and what pin(s) are we talking about? Quote Link to comment Share on other sites More sharing options...
unclej Posted December 15, 2004 Author Report Share Posted December 15, 2004 the ampeg is sporting a set of 7591's, a 7199, a set of 12ax7's, a 6gc7 and another 12ax7. the only one that goes directly to ground is the 6gc7. on the alamo it has the 5u4gb, two 5881's, a 7199 and a pair of 12ax7a's all of which go to ground. Quote Link to comment Share on other sites More sharing options...
Saber Posted December 16, 2004 Report Share Posted December 16, 2004 (edited) Unless I'm missing something, according to this schematic of the Ampeg Gemini II from http://www.schematicheaven.com the only tubes that have pins connected to ground are the power (output) tubes. Edited December 16, 2004 by Saber Quote Link to comment Share on other sites More sharing options...
lovekraft Posted December 16, 2004 Report Share Posted December 16, 2004 Yep, I'm with Saber - that reverb tube shouldn't have any pins that go to ground. Even the heaters should read about 40-50 ohms, unless your hum balance pot is turned all the way right or left. Like he said, the power tube cathodes (pin 8) are the only pins connected directly to ground. Quote Link to comment Share on other sites More sharing options...
unclej Posted December 16, 2004 Author Report Share Posted December 16, 2004 the schematic on the schematic heaven site is a bit different than the one in my amp. where the jumper between the #8 pins go to ground on that schematic is where the hum balance pot goes on mine so that should solve that. i just received the pot this morning so i'll install it and check again. by the way saber, that's a great site that i didn't have before. it's in my "favorites" now and i appreciate the lead. as for the reverb tube the number #9 pin goes to a small bus bar. the input and output lines and the footswitch for the reverb are wired to it and the number 9 pin is wired to the lug that also has the ground wire for the input line. Quote Link to comment Share on other sites More sharing options...
lovekraft Posted December 16, 2004 Report Share Posted December 16, 2004 Pin 9 on the 6CG7 is the internal shield, and should be wired to ground for quietest operation - sorry, I tend to forget about those unusual dual triodes (unusual only meaning I don't see them very often). Quote Link to comment Share on other sites More sharing options...
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