[Harp-L] VHT Special 6 mods- revised

[Jim Rossen <jimjimdr@xxxxxxxxx>]

A recent session with my modified amp lead to some more investigations
involving the Duncan tone stack calculator and a long stare at the
schematic.  I also reread the Frugal Guitarist web article and
concluded there was an error in the article regarding the effect of
changing tone control resistor R12-  The article says increase R12 for
less treble, but this will actually increase the treble.  Article is
quoted below.

link to article
http://www.frugalguitarist.com/Frugal.aspx?Issue=19Page=Terry%20Buddingh%20Interview

begin quote-
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
end quote-

The schematic is at-
http://www.vhtamp.com/avsp16.html

All this lead to some further changes, the total of which is listed
below, with changes since my first set of posted mods being
underlined.

TMB values-
R10- 100K
R12- 1.5K
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
C12- .01 mfd Mallory 150

Input grid resistor
R7- 5 meg


B+ dropping resistor
R5- 68K   Reduces 12AX7 plate voltages from about 150V to about 125V.
(somewhat difficult because of position on board)

The major effect of the new round is further boost of lows and reduced
highs.

All the above is a lot to do, but I think most of the improvement is
accomplished by three changes that involve easy to work on spots.
These all involve revoicing the tone circuit.  I think someone with
modest soldering skills could easily do the changes, which are-

R10- 100K
R12- 1.5K
C11- cut or unsolder one leg of the cap from the pot terminal

I am happy to post pics showing the location of these resistors on the
circuit board.

I hope others will fiddle with their Spec 6 amps and share their
observations.

I continue to be impressed that the high/low power and clean/boost
controls provide four distinct and useful sounds.


Jim R