[HAM] changing values of C2= 39pf (also an E100 mod if ya want it)

[Kon Zissis]

Hi David and everyone.
I am currently  working on the AO28 of my 1962 C3. I have replaced the
power filter capacitors and the plate resistors ,and  the cathode
resistors of l the 6AU6 , the 12AX7 and the 12BH7 valves and also and
the screen grid  resistors of the V1 and V2 6AU6 valves  with new metal
film 1 watt or higher rate resistors.
I am also adding a few switches on the AO28 so that I can get more tonal
variations  by changing the wiring of the feedback resistors and
feedback capacitors  on the  V1 and V2 6AU6 valves and the V4 A   valve.

 
Here are the modifications:
I have wired up a  2 pole / 2 position "centre off " switch on the V1
valve ( non vibrato channel )  and the  V2 valve  ( vibrato channel ) .
One section  of this switch is  wired in series with the R5  10 mega
ohms feedback resistor on the V1 6AU6 valve  and the other section of
this switch  is wired in series with  the   R 14 10 mega ohms  feedback
resistor  on the  V2  6AU6 so that I can :
1)  produce  the stock sound by switching the R5 and R14 feedback
resistors in circuit as per normal ,  ( switch position 1 )  
2) produce  a bass boost by switching in  39 pf capacitors in series
with the  R5 and R14  10 mega ohms feedback resistors , ( switch
position 3 )  
3) produce a deeper bass boost and lower midrange boost by switching
the R5 and R14  10 mega ohms feedback resistors out of circuit. (
"centre off "switch  position 2 ). This produces a fatter , more
''ballsy'' sounding organ tone  and growl .
 
I have  wired up  another   2 pole/ 2 position "centre off " switch ,
and one  section of this switch  is wired in parallel with the  R 4  1.8
mega ohms  resistor that is wired in series  with the C2   39 pf
feedback  capacitor  on the V1  and the other section of this switch  is
wired in parallel  with the  R 13   4.7 mega ohms resistor  that is
wired in series with  the  C7  24 pf feedback capacitor   so that I can
:
1)   produce  the normal stock sound  ( "centre off"  switch position 2
)   
2)  produce  a slight  treble cut / mellow  effect  by switching in a 22
pf capacitor  in parallel with the R4  1.8 mega ohms feedback resistor
that is wired in  series  with the C2  39 pf capacitor on the V1 and  by
switching in a  22 pf capacitor in parallel with the  R 13  4.7 mega
ohms  feedback  resistor  that is wired in series with  the C 7 24 pf
feedback capacitor on the V2 ( switch position 1 )  
3) produce  a  more  noticeable  treble cut / mellow  effect  by fully
short circuiting the R4 1.8 mega ohms  feedback resistor  on the V1 and
by  fully short circuiting  the R13 4.7 mega ohms resistor  on the V2 .
This produces a mellower  , more ''bluesy'' or more ''vintage aged ''
organ tone  and it removes any shrillness from the sound.
 
The above two switching modifications  affect the main organ  tone but
they do not affect the harmonic percussion  effect.
 
I am going  to wire up a 2.2 mega ohms pot in series with the C15  51 pf
feedback resistor  that is wired on the V4 A  valve  so that I can get
the stock sound  when the  2 mega ohms pot is short circuited  and  I
will be able to produce  an upper treble boost  when I turn the pot up
to  around the 1 mega ohms position. This will produce a stronger ,
spittier key click and a brighter upper  top  end but without excessive
upper midrange shrillness. This affects the main organ  sound as well as
the harmonic percussion effect. This upper treble  boost  and the
spittier key click effect  sounds  quite nice for  jazz organ  with the
typical Jimmy Smith  style  settings.
 
The B3 , C3, ,A100 RT3 , D-152 etc organs with the AO28  preamp and the
M1 , M2 , M3 , and M100  series  spinets  have a 10 mega ohms  feedback
resistor  on the V1 and V2  input valves  but  the L-100 series and the
Porta B organs have  a  4.7 mega ohms ( R203 )  feedback resistor
instead of a 10 mega ohms resistor on the V 1 12AX7 input valve  and
this 4.7 mega ohms feedback  resistor  thins out the sound. I do not
know why the people who designed the L-100 amplifier  decided to use a
4.7 mega ohms resistor with the resulting thinner sound. Frederick
Sommerville recommends to  replace this  4.7 mega ohms resistor with a
10 mega ohms  in order to produce a fatter  .
 On my own1963  L-102  organ I am going to replace  the 4.7 mega ohms
resistor with a 10 mega ohms resistor  and I will also wire up a 2 pole
/ 2 position "centre off" switch  to be able to switch this 10 mega ohms
resistor in or out of circuit  in the same way as described further
above for my 1962 C3  so that  I can get the switchable bass boost and
the bass / lower midrange boost  effect. The 39 pf feedback resistor ( C
203 ) does not have a resistor in series with it so  this  causes the
L-100 and the Porta B organs to sound mellower with less key click than
the M3 and the M100 spinet organs  . I will also put in another 2 pole /
2 position " centre off"  switch  that will bring in a 1.8 mega ohms or
a  4.7 mega ohms resistor in series with the 39 pf feedback resistor  so
that I can get a more M3 or M100 like treble response  with more key
click.
 
If you completely remove the  feedback capacitor  from V1 and V2  6AU6
valves and the V4 A  12AX7  valves in the AO28 preamp and the M series
spinet preamps  and the V1  12AX7 valve in the L-100 series and the
Porta B organs  you will get a much more shrill sound  and  if you
replace the stock pf value capacitor with  a  lower pico farad  value
capacitor you will get more upper midrange and treble  response  and if
you  use a higher  pf value feedback  capacitor you will  get a mellower
treble  but also a decreased mid range  because a greater frequency
range is passed through the capacitor  .
All the best.
Kon
 
All the best.
Kon
 
David Wells wrote :
>Changing values of this cap will change the tone yes??
>Frederick somerville suggests that in a L100 you can experiment with
this 
>section of the amp,
>I did all of his suggested variations and settled on the latter mod (I
cant 
>remember exactly what it was now, it was 2 years back)
>Yes removing this cap altogether made it as bright as buggery,(too
shrill in 
>fact).
 
>I have removed these caps altogether( in both vibrato and non vibrato 
>channels) on my E100,(C102 and C112) I really dig it, lotsa crosstalk
and 
>not to shrill for my taste.
 
>I notice that in an M3 these caps,( C6 and C1) are 30 and 20 pf.
 
>So whats the dealey, if I lower the value of these caps I'll get more
treble 
>in the form of; key-click,crosstalk and overall TG characteristic??
 
>Other than the obvious tonal differences is there any averse effect in 
>changing these values??
>Thanks again lads,