In response to the people who E-mailed me, here is a simple mod to
increase/adjust the gain of the V-72. It is based on the V-78 w/ the addition
of a DC blocking capacitor. One caveat is that if you are not familiar with the
type of voltages present then PLEASE be careful or better yet find a friendly
techie who can help you. I have used one or two valve circuitry abbreviations
in the hope that it will dissuade non-technical persons from possibly harming
themselves or for that matter the V-72 itself:
1/ Remove RK1 (1.25K).
2/ Remove small jumper on opposite side of board that connects end
of feedback loop to RK1 tie lug. (jumper is approx 3/4" long & is
at right-angles to & between the .05 & .025 caps.
3/ Solder one end of a 100 ohm 1/2 watt resistor to the ground end
of where the 1.25 RK was located. (use something of good quality
that is non-inductive)
4/ Remove the 2K - 7.5K resistor (usually 5K) from the F.B. loop.
5/ Solder the two small value F.B. caps across just the 80K FBr.
(or better still, replace with high quality 90pF polystryrene)
6/ Solder a (small as you can find) 2uF/250v+ electrolytic where
the 2K - 7.5K R was located. Orient the +tve end towards 80K R
7/ Strip a piece of twin conductor/shielded cable. Attach tails to
screens ala Neve & cover w/ shrink tubing.
7/ Solder one end of cable to V-72 as follows:
a) tail lead = other end of 100 ohm resistor, cover w/ shrink-tube
b) +tve lead = -tve end of 2uF/250V.
c) -tve lead = RK1 tie lug.
8/ Get yourself a 1K pot. (anti-log is best for correct taper but
since these are next to impossible to buy in small quantities
a linear will work fine with a slightly less than optimum taper)
9/ Solder other end of cable to pot as follows:
a) tail lead = top of pot
b) +tve lead = wiper of pot
c) -tve lead = bottom of pot.
(note: orientation of top & bottom are opposite from normal volume-pot type
wiring. If you get it wrong, gain adjustment will
happen in reverse).
10/ Plug in & Stand Back.
The above modification is the easiest one I know of & doesn't upset the
workings too much to boot. The pot could be mounted in the light socket if it
is small enough or brought out the side & mounted on a panel. Ambitious
tweakers could replace
the pot with a stepped attenuator in the style of the V-76 if they wish but the
basic version works fine for me.
The gain swing is not drastic but will take you from approx 33dB to +/-50dB.
There will be a small amount of cathode voltage present on the pot which will
cause slight noise as you turn but whaddya want. I would also include a pad on
the front & if need be a volume on the out.
If you wish to have a schematic for this modification I will be happy to send
one to you along with data on the EF-804S for a nominal charge of $5. I also
have copies of the 25+ page operating manual for the EMI REDD-37 console which
used versions of the V-72 as the active circuitry. Thats it, thanks for reading
& 'Happy Tweaking',
David Hinson.
PAD MOD
A pair of matched 900 ohm resistors in series
with the +tve & -tve inputs along with a 200 ohm resistor shunted between the +
& - ends closest to the V-72 should do the trick. This will present about the
same load to the mic as the V-72 did (2K) & will also be a suitable source
impedance for the amplifier (200 ohms) which means it shouldn't change the
sound too much. Oh yeh almost forgot, the above values will give you about 15dB
of attenuation. If you need more dB just wire two of them together, one after
the other, or raise the value of the series resistors but don't go too high as
that might make the mic sound a little 'zippy'.
Lastly, it is a perfectly ok match impedance wise to run the output of one
unit into the in of another. Altho I would build up the 'internal output
impedance' from 35 ohms to 200 ohms with some series resistors.
