[HAM] Simple and useful bass boost and treble boost preamp mods and percussion mods for the AO28 preamps.

[Kon Zissis]

Hi everyone .
Last week I found out that a  few  reasonably simple  bass boost and
treble boost or treble  cut modifications can be done to  the V1 and V2
6AU6  input valves of the normal channel and the vibrato channel of  the
AO28 preamps of the B3 / C3 / A100 / D-152 etc organs and I expect that
these modifications can also be done to the  corresponding V1 and V2
6SJ7  input valves  of the AO10 preamps of the B2 / C2 / RT2 etc organs
and it should also be able to be applied  to  the L-100 , M2 , M3 ,
M-100 ,  Porta - B  etc organs.  
These simple  bass boost , treble boost and treble cut   modifications
are reasonably simple and they  can  produce  very useful extra tonal
control  without you needing to add an external bass boost or an
external EQ circuit or this can be an alternative to  TG recalibration
and the treble boost can be used as an alternative to recapping a TG if
you do not want to do this..
 
The bass boost is created   by removing the R5  10 mega ohms feedback
resistor on the V1 6AU6  input valve of the normal channel and the R14
10 mega ohms feedback resistor  from the V2 6AU6 input  valve of the
vibrato channel . I am trying this out now on my 1962 C3 and I quite
like the  noticeably fattened bass and lower midrange  response.
 I am going to wire  in a  2 pole / 2 position ''centre off''  switch so
that I can :
1 )  produce  the normal  stock  sound with the 10 mega ohms feedback
resistors in place .
2 )  really fatten up the bass and the  lower midrange sound with the
two 10 mega ohms resistors being switched out of circuit. This can help
to solve the problem of an anemic or thin sounding organ.
3 )  switch  two 39 pico farad capacitors in series with the two 10 K
ohms feedback resistors  so that  the bass  is  boosted but the lower
midrange is not affected  as much. This is useful if you find the full
bass / midrange boost to be too strong sounding   for your organ.
 
You can also  produce a treble boost effect in the  V1 6AU6 valve of the
normal channel and the  V2 6AU6  valve  of the vibrato channel of the
AO28  and the corresponding V1 and V2 6SJ7  valves  of the  AO10 preamps
by wiring a 2 pole / 2 position  '' centre off '' switch  to increase
the value  of the feedback  resistors in series with the 39 pf  feedback
capacitor in the normal channel  and the 25 pf feedback capacitor  in
the  vibrato channel , so that you can  switch between  either   the
stock resistor values  or two higher resistor values . 
Alternatively , if you want a treble cut effect as well as a treble
boost  effect , you can wire the switch to :
1 ) produce the normal stock sound be leaving the R4  1.8 mega ohms
feedback resistor of the normal channel  V1 6AU6 valve and  the  R13
4.7 mega ohms feedback resistor the normal channel V2 6AU6  valve .
2 ) create a treble  cut effect by shorting  out the  R4  1.8 mega ohms
feedback resistor of the normal channel  V1 6AU6 valve and  the  R13
4.7 mega ohms feedback resistor the normal channel V2 6AU6  valve .
3 ) create a treble boost  by switching  in  two additional  resistors
in series with the R4 1.8 M ohms and the R13 4.7 M ohms feedback
resistor.
I have not yet experimented enough to find out  suitable ohms  values
for these two resistors.
 
These  bass boost, treble boost and treble cut  mods  on the V1 6AU6
and the V2 6AU6  valves only  affect the main organ signal  but they  do
not affect the percussion signals s because the percussion signal is
introduced into the signal path after the V1 and V2 stages.  
One advantage of this  is that you can retain the stock percussion sound
whilst adding  bass boost ,treble cut and treble boost effect to the
main organ signal , for example you might want a mellower  organ sound
but a brighter  percussion effect so therefore the mods to the V1 and V2
will allow you to 
 
If you want the treble boost effect to also affect  the percussion
signal as well as the main organ signal,  you can experiment with
switching in a  feedback resistor in series with  the C15  51 pico farad
feedback resistor that is wired across the V4 12AX7 valve  in the AO28
( C15  75 pf feedback capacitor  across the V4 6SC7valve  in the AO10
preamp ) . Apart from temporarily  unsoldering one end of the C15  51 pf
capacitor in my AO28  and getting an excessive signal boost ,  I have
not yet experimented enough  to  find out a suitable  resistor  ohms
value  to get a useful treble boost  in the  V4 but you can try out  a
resistor ohms value such as 1 M ohms , 1.8 M ohms , 2.7 M ohms  , 4.7 M
ohms etc or perhaps a 2 mega ohms potentiometer wired as a rheostat in
series with the C15  feedback capacitor can work well here  as an
adjustable treble boost  . 
 
If you want the percussion to have  more a more spitty  key click or a
cutting percussion sound , you can remove the  C32   390 pf capacitor
that is wired across the R62 100 k ohms resistor , and if you want a
fatter ,  more woody sounding thunk with more lower midrange  frequency
content on the percussion signal heard  on the lower octaves of the
upper manual  , you can wire a 150 pf or a 220 pf bypass capacitor
between positions ''P'' and ''D'' as shown on the AO28 schematic.  
 
You can create a variable volume control for the percussion  signal  by
wiring a 100 K ohms potentiometer  as a rheostat between the position
''P' and the ground.  Depending on the year of production , some AO28
preamps had a  R 61   33 K ohms or a 39 K ohms resistor between the
position'' P'' and the ground. This resistor sets the volume  level of
the percussion . Interestingly , the AO28 -2 preamp of my friend's 1959
C3  and the H-AO2 8-2  preamp of my 1962 C3  do not have the R61
resistor . If your AO28 has the R61  33 k or 39 k resistor , you will
need to remove it if you want to add the 100 K  percussion volume  level
control.
 
The fat sounding  bass  / lower midrange boost of the main organ signal
combined with the fattened  up and spittier sounding percussion signal
that is set at the right sounding volume level balance with the main
organ signal sounds very  nice indeed  in my 1962 C3 , both with the
clean sound and the overdriven sound . 
 
Depending on the signal levels  of the TG output curve of your organ ,
you might need to replace the  V4 12AX7  valve with a 12AU7  if  you get
unwanted distortion whilst boosting the bass or the treble with these
modifications because the 12AU7 has lower  gain levels than the 12AX7.
My own 1962 C3 has a louder than stock TG output curve  and when I
switched in  the bass / lower midrange boost  there was some distortion
when the expression pedal was turned up so I took out the V4 12AX7 and I
replaced it with a 12AU7 and this cleaned up the sound. 
All the best.
Kon