First amp design almost finished - help and thoughts?

[thelonious]

Hi all,

I'd be grateful for any suggestions you can provide on my first amp design/build. I relied heavily on Merlin's preamp book for most of it.

The design also incorporates a bypass cap attenuation switch, Da Geezer's version, which works marvelously: http://www.el34world.com/Forum/index.php?topic=4437.0

The tone stack is the Judge Roy Bean and His Bear stack from this AX84 thread: http://www.ax84.com/index.php/bbs/dm.php?thread=394074 It is nontraditional and slightly confusing, but it is very flexible.

The chassis and output transformer are from a Bell 5630 PA amp, but everything else has been reworked. It sounds good enough that I've even dared to gig with it a few times now.

I hope the schematics make sense; there are about a million things I haven't figured out how to do in LTspice yet, so there are no input jacks, and my transformer looks kind of ridiculous. What do you guys use to draw schematics?

Some questions to start with -
 -Do the voltages look normal?
 -Any major mistakes you see in the design?
 -What is your experience with using LEDs for biasing, like this does in the input stage? Good/bad for guitar?
 -It seems like the values for my NFB resistors are weird, but for some reason the real world results didn't line up with my math. I arrived at the values I have now by putting the amp on a scope and looking at the actual reduction in dB… but I don't know why these values are working. Any ideas?

Thanks much!

[PRR]

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[thelonious]

Ha! Thanks, PRR. It wouldn't have made much noise if I had actually wired it like I drew that first schem. I had also forgotten about the grid stoppers I had added to the PI and the 5881s. Updated versions attached. Tomorrow I'll also try changing the NFB resistor values as shown and see how it turns out.

Any thoughts on the effects loop? I forget why I made R18 47k... and now I'm wondering if I just misread the color bands on the resistor I grabbed and it was actually supposed to be 470k. Wouldn't that make for better input impedance?

[tubenit]

What do you guys use to draw schematics?

Usually ExpressSCH, or J-schem if you have a Mac.   SCH is VERY user friendly and of course is editable.

http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm

IF you would like to download ExpressSCH, then you can easily alter the amp design I built for my son into your amp.  

They have quite a few similarities to them.  However, this one uses a VVR instead of Geezer's cathode attenuation.

With respect, Tubenit

[thelonious]

Excellent, thanks! I will definitely check out J-schem. LTSpice has not been too friendly to this user. ;)

Thanks also for the design---I'm looking forward to figuring out some of the tone-shaping you're doing there!

[John]

Another nice thing about J-schem - you can import the SCH file and edit it. However, it usually won't let me save it back to an SCH file that I can open with the SCH program. But still, very nice feature if you're a Mac user.