Conference napalm::guitar

256.0. "A Master Volume Installation." by VIKING::BUSENBARK () Wed May 27 1987 10:17

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		Installing a Master Volume in a Fender Amp


	A standard stock master volume in a Fender amp is located right after 
the preamp stage(7025)before the driver tube(12at7)and power stage(6L6 or 6v6).
Putting in a dual ganged pot inbetween the driver stage or the phase inverter
and the power tubes you limit power drain to this tube which end's up being 
overdriven by your preamp stage's (channel1/normal and or channel2/vibrato). 
	The way to do this in a Twin Reverb(Post CBS)is to replace the 47k 1/2
watt resistor tied to pin 1 of the 12at7 and the 47k 1/2 watt resistor
tied to pin 6 of the 12at7 with a 50k dual ganged variable pot. In my PreCBS
Showman they were 86k and 100k 1/2 watt's which I replaced with a 100k dual
ganged pot. Wire the first two pins of the (dual)pot(the terminals are in back
of each otch other) to where the 47k's meet and wire the middle terminal of 
the first pot to where the 47k resistor was which was connected to pin 1 of 
the 12at7 and wire the middle terminal of the second pot to where the other 
47k resistor was which was connected to pin 6 of the 12at7. Wire the 2 wire's
that went to the intensity pot together and insulate with heat shrink or
electricians tape.
	Depending on which side of the pot you use will depend whether your 
amp is being overdriven at 1 or at 10. I wired mine on the right side (facing
the amp)and it overdrives at a low volume(2).What this means is the wire which 
connected to both the front pot and the back pot determines your knob number. 
I used a linear taper pot but plan to replace it with an audio taper or try
adding 1/4 of the resistance added in parallel when I have time or when I can 
get a audio taper.
	Adjust the channel volume and the new master volume to get the
overdrive effect you want. 
	I modified my Showman by eliminating my tremelo intensity pot and 
replacing it with the dual ganged master volume pot. In order to still have 
the intensity I connected the two wires which go to this pot and removed it
I got the maximum intensity, but I am still able to control the speed and 
avoided drilling a hole in the front.
	Since then I have decided to eliminate the tremelo circuit altogether 
and use the extra tube slot to use as an extra preamp to be overdriven by 
the normal channel and as an effect's send/return.
	There are certain common sense safety rule's to observe and some solder-
ing experiance would be helpful,but essentially it could be as easy as changing
a tire on a car. This can also be applied to other Tube amplifiers.
	I have seen one other master volume modifications where you can replace 
two bias resistor's that feed your power tubes with a 100 to 250k ohm dual gang
ed pot, but have never tried it as it could definately decrease tube life and
increase noise. The above mentioned modification I have described worked for me
and I found no reason explore any other.
	I priced a dual ganged pot at a electronics shop and it was $12 to $15 
approximately. The above work should take no more than an hour from start to
finish.
	Anyone else got some amp modifications? I'm playing with preamp gain at
this point and will post info as I discover it.

							Rick
	

    Wow.  I have a pre-cbs fender showman-amp that I would like to do
    the same to, including the extra gain stage and effects loop.
    
    If I didn't trust myself to do this, can you recommend a place that
    would do this kind of work ?
    

	It's hard for me to reccomend a music store in the NH/Mass area
which is capable of making these kind of modification's. I'm sure there
are many in Boston who could even though I personally couldn't rec-
comend them for even pick's.The master volume article I wrote is fairly 
simple to do and more of a challenge to write so that just about anyone can 
do the work more than anything else.  Most reputable repair shops charge in 
the $30 to $40 an hour dollar range. The amount of time it would take to do 
these mod's(which aren't final) is around 5 hrs at a reasonable and somewhat
relaxed speed. Not saying it couldn't be done in 2hr's. 
	My biggest problem is downtime of the amp as I was using it constantly 
and when I wasn't there always was something else to do. I will eventually get 
to writing the same kind of process sheet as I did for the master volume and 
post it in the notesfile for those daring and willing. 
	Hopefully it will be as simple as the Master volume for someone to do.
Keep in mind that these changes/mod's may lessen the value of your "vinatage"
amp to someone who might want to buy it someday. I knew I was not going to sell
my amp so this doesn't matter to me.

							Rick