ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: visit Schematic Hell!
 
The sunn still shines online!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  
previous: Sean Kilback hi guys,I just pick... -- 6/29/2000 8:00 AM View Thread

Re: Gibson Discoverer GA-8T thoughts

6/30/2000 6:47 AM
Sean Kilback
Re: Gibson Discoverer GA-8T thoughts
Here's a small update on the GA-8T progress...  
 
Taking pumpstein's advice, I decided to trace part of the circuit out myself. The closest schematic to start with is the one in Weber's book labeled GA-18T with two 6EU7s, two EL84s, and a 6CA4 rectifier. The GA-8T schem from Casey is from an interesting earlier amp which uses 6BM8s.  
 
Here's a link to the GA-18T schematic for ref:  
 
http://members.xoom.com/shuffleswing/Gibson_GA-18T_scan.gif  
 
I compared all components in my amp to this schematic and noticed some differences:  
 
1. The 1M input resistor to ground is AFTER the 68K input resistors instead of Fender style, right on the pot. What's the tonal difference here? The only other amp with this configuration I could find is the Vox AC15.  
 
2. There is a strange flat object that might be a 3-legged cap. It's labelled "Sprague 102C084". This is located like a coupling cap between the volume wiper and grid of stage 2, but pin2 goes to ground? What the heck is this thing?  
 
3. The "monitor" jack actually runs directly from the wiper of the volume pot!  
 
4. There is a .1 uF bypass cap on the 2nd stage cathode of V1.  
 
5. A 2000pF cap across the plates of the output tubes...is this to reduce oscillation? If so, can I remove it if the amp is stable without it?  
 
6. There are two small diodes mounted on the empty 6CA4 socket to do the rectification. I'm planning on removing these and installing the correct tube.  
 
That's all for now! Thanks for reading! All comments or feedback is welcome.  
 
cheers,  
Sean

 
Replies:
MBSetzer Hi Sean:I think you... -- 6/30/2000 7:28 PM