My experience with the Spec 6 is similar- fabulous harp amp for the price.
Rick D- I am interested in the circuit mods, particularly done in conjunction with a master harp amp builder like Bruce. Can you post the specifics, perhaps referring to the published schematic, which is at-. http://www.vhtamp.com/avsp16.html
I have done some mods myself and reported my observations on the Weber harp board. I reproduce my post about this below. Since that post, I made additional mods of change the tone control R12 to .01 mfd and replace the original speaker with an Eminence blue frame Bassman RI speaker.
Jim
Prior Weber board post:
I can't keep a soldering iron away from an eyelet board, so I had at my Spec 6. I used the amp at a low volume gig last night and thought it needed more low-mids. This could probably be accomplished with a speaker swap, but I decided to make some circuit changes. I was guided by in part by an excerpt from an interview with the designer where he encouraged mods. He said-
http://www.frugalguitarist.com/Frugal.aspx?Issue=19&Page=Terry%20Buddingh%20Interview
If you want to re-voice the clean mode’s sound (non-boost mode), resistors R10 and R12 form the clean mode’s “Treble pot;” decrease R10 for more treble, increase R12 for less treble. R13 is the “Bass pot;” increase R13 for more bass, decrease it for less bass. R15 is the “Middle control;” increase R15 for more mids, decrease it for less mids. R22 controls the amount of boost you get in boost mode; increase it for more boost, decrease in for less boost. For a cleaner tone, change R3 and R4 to 100k ohms
Schem is at- http://www.vhtamp.com/avsp16.html
My read of the schem is that the amp has a Fender TMB tone stack with fixed values for the three pots, plus an adjustable tweed Princeton type tone control. The changes described above are changing the fixed values for the TMB pots. My changes include this and other typical harp stuff, as below, referring to the schem. I listened after each change. One of the last changes was the slope resistor, which had the biggest benefit. One might try this first, but effect is uncertain in the absence of the other changes.
TMB values- R10- 1.5K R12- 100K R13- 100K
Slope resistor R9- 56K
Cathode bypass caps C8, C13- 25 mf
Coupling cap C14- .047 mfd Mallory 150
Princeton tone control C11- lift
Input grid resistor R7- 5 meg
As expected from the eyelet board construction, the changes were pretty easy to do. Some of the hookup wire lengths were short enough to make insertion into the eyelets a little tricky.
The changes did bring up the low mids, and preserved the feedback resistant quality of the amp. It also reduced the strength of the pull boost control, a good thing, I think.
Your mileage may vary........ I am not an amp tech.
Jim