[HAM] hammond Digest, Vol 10, Issue 10

[Dominik ter Meer]

Hi Kon,

thanks for your reply. I will try shorting out the
820k resistor. I might install a switch as you did. 

Thanks again for your tips!

Dominik

Kon Zissis wrote:

Hi Dominik and everyone.
I have experimented with the AO28 wiring.
The R9 820 K resistor is wired in series with the
output of the normal
channel and the high resistance causes the treble to
roll off whilst
allowing the rest of the signal to pass through.

Totally shorting out the R9 820 K resistor makes the
normal channel
 more
or less as bright as the Vib/Chorus channel. There is
no loss of bass
with this mod regardless of whether you bypass the R9
with a capacitor
or whether you completely short out the resistor so
therefore shorting
out the R9  forms an ''all pass filter'' instead of a
high pass filter.
If you want the normal channel and the chorus channel
to sound more or
less identical , then shorting out the R9 is the
easiest way to do
 this.

The feedback circuit of the Vib/chorus channel V2 6AU6
 produces a
brighter sound than the feedback circuit of the normal
channel V1 6AU6
in order to compensate for the losses  created by the
vibrato line box
and the vibrato scanner.

By the way , you can get a bass boost  in the normal
channel and the
 Vib
/ chorus channel by inserting a 39 pf capacitor in
series with the R5
 10
M resistor on the feedback  circuit of the V1 6AU6 of
the normal
 channel
and a 39 pf capacitor in series with the R14 10 M
resistor on the
feedback circuit of the V2 6Au6 Vib/ chorus channel. 
If you want an even stronger bass boost and a lower
midrange boost ,
 you
can achieve this by completely removing  the R5 and
the R10  feedback
resistors. 
I have put switches in my C3 to select these settings.

All the best.
Kon 


there is a chap out there selling an e-book on ebay
concerning mods and
maintenance of console organs (see link to homepage of
this guy). He
states, that there is a mod to make the normal channel
equally as
 bright
sounding, as the Vib/Chorus channel. 

I took a look at the schematic and the only way I
figured this could be
done is by bypassing the 820k series-resistor (R9)
after V1 of the
normal channel with a small capacitor. This would form
a high pass
filter, thus one could get a brighter sound (with the
loss of bass
though).

The other difference between the circuit of the
pentodes of the normal
and chorus-channel (V1 and V2) is the amount of local
negative feedback
(R4,R5 and C2 in case of V1 and R13, R14, C7, R15 on
V2). 




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