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Ultralinear Amp


 
12/10/1997 3:30 PM
Bob K.
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Ultralinear Amp
As I was planning out the steps required to add a pentode/triode switch to the Fender amp I am modifying (two-channel Fender Concert), I noticed something that didn't jive with the 'source' schematic I am using. The B+ voltage source is connected to pin 6 of one 6L6 (no problem). That pin 6 is jumpered to pin 6 of the other 6L6 (again no problem). BUT...pin 6 of the second 6l6 is jumpered over a lug on a terminal strip. Connected to this lug is also wire from the filter supply AND a wire which goes into a small transformer. The other wire from this transformer is connected to the center tap of the ouput transformer. Does this mean that this amp is running "Ultralinear"?  
 
If so.......can I change this? Can I remove the smaller transformer and rewire the circuit to the conventional ouput configuration reusing the existing output transformer without frying the myself, the amp, and burning the house in the process?  
 
Help please.
 
12/10/1997 3:48 PM
Bob K.
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Re: Ultralinear Amp - Sorry!
When in doubt.....do more research!  
 
Sorry for the post.....the small transformer in the filter choke. I don't kow what I was thinking. (It's been a long day.)
 
12/10/1997 4:49 PM
Rusty Bretz
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Re: Ultralinear Amp
Is your Concert a Rivera (sp?) design ?  
If so, please let me know what difference in tone you get running in triode mode. I have an '83 Concert I would like to mellow-out a little. I tried triode mode on my Crate VC3112 but it seems to have "dulled" the sound.  
Thanks,  
Rusty
 
12/11/1997 5:19 AM
Bob K.
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Rusty:  
 
I believe that the amp is the Rivera design. From left to right the controls are: Volume, Treble, Bass, Volume, Gain, Master, Treble, Middle, Bass, Reverb, Presence. The back of the amp has jacks for the effect loop with separate input and output controls.  
 
What I've done so far:  
 
Pretty much gutted the thing. I rebuilt the clean channel and drive circuit to AB763 specs and the overdrive channel to Torres "Super Texan" specs. I rebuilt the presence control circuit to old Bassman specs as I felt that the original circuit was too extreme for my tastes. The Bassman circuit is a lot more subtle. I am still debating whether to leave the effects loop in or reuse its tube for adding vibrato to the amp instead. I've used vibrato in the past much more than I've ever used an effects loop. I do intend to add a Pentode/Triode switch and will let you know how it works out.  
 
The one problem I have yet to conquer is the Concert's over abundance of bass response. I am not sure where it's coming from but I suspect it may be from the massive amount of filtering in its power supply. Just a guess at this point though. I am going to completely rebuild the power supply. The Concert uses a bridge rectifier circuit. I am going to convert it to a full-wave rectifier and drop the B+ voltage down from 475 to about 450 volts. This is the B+ of my reissue Twin.  
 
This was/is my first major amp project. I am aiming to make this Concert sound like my Twin but with half the power. So far so good but I still have a ways to go.  
 
By the way........when I finish the contols will be (left to right): Volume (Clean), Volume, Master(OD),Treble, Middle, Bass (Clean) Treble, Middle, Bass (OD), Reverb, Presence.  
 
I like the Torres mod on the OD channel better than the original OD channel voicing. I am curious about his "Texas Boogie" mod which is still higher gain and am toying with the idea or sending away for the specs on that mod and trying it out before I close this thing up.  
 
Will keep you updated.  
 
Bob
 
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