Harley Benton GA-15

[JiM]

I've seen some interest here for this newer bargain tube amp from Thomann.de ...
I've found the schematic here :
http://www.freeinfosociety.com/media.php?id=1216
and here are gutshots, schematics, some mods, and scope-shots :
http://picasaweb.google.fr/leptitmanu

I don't have one, and actually never seen one in person -- i only have his little brother the GA-5 -- so i can't answer much questions. This amp is said to require quite a lot of mods (more than the single-ended version) to be really great sounding :
- lower the B+, which is really too high and will eat power tubes
- DC heaters for no more hum
- add 1M pull-down at input
- gain control (between first and second triode stages)
- re-bias the PI
- move the line out to OT secondary (à la firezog ?) maybe with cab-sim

One could also want to remove opamp-based FX loop for all-tube mojo, add a presence control, change the tubes (2x 12AX7 and 2x EL84) and the 10" speaker ... and of course add a power attenuator to enable crunch at neighbour-compatible volume.

It should be a great basis to build some Marshall 18W clone, as enclosure and irons look really good.