[Harp-L] Re: Happy Thanksgiving, and the amazing VHT Special 6 amp



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




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