TubeFex Motorpot Page

created 11/21/00 by JF

Main TubeFex Page

The History: The TubeFex has a motorized slide potentiometer inside. This pot controls the tube preamp's pregain parameter. This signal is between the two tubes and has a very high voltage - around 250V. There aren't any digitally controlled pots that can handle that voltage, and other methods were not considered accurate enough at the time - or not smooth enough. Unfortunately, this component has caused some reliability problems with some units. I've tried to consolidate all the information I could onto this page to help you deal with any problems that may arise from this part.

NOTE:  This pot is in the TubeFex only, not in the TransTubeFex, TransFexPro, or Session 2000.

CAUTION:Don't attempt to do the mods listed here unless you are experienced with electronics and soldering. Some components inside can be damaged by static discharge, so take a seat and discharge yourself before poking around (touch the chassis of a metal piece of gear that is plugged in). Make sure the TubeFex isn't plugged in - just because the switch is off doesn't mean there isn't high voltage in there (like, between the power cord and switch). Even after being unplugged, some of the big cylindrical capacitors near the transformer can still be packing a voltage wallop that you don't want to feel. You get the idea - if you don't know this stuff, find someone who does. Changing the whole motorpot (see below) is the only mod on this page that I would recommend for us to do here. The other stuff is easy enough to do yourself, or with a friend.

Problem #1: Sometimes the volume will fade in and out, for seemingly no reason (only when TB is in the chain).

Solution: Some early units didn't have the body of the pot connected to the chassis. This allowed the ground to float when the chassis ground was lifted for noise reasons, and volume was lost when it floated too far. It can also cause chattering in some cases, so it helps with Problem #2, also. Basically, the pot needs to be chassis grounded in all cases, not just when the player decides to use the "third prong" AC ground.

There is a standoff under the pot (just inside the pot standoff under the motor) that is responsible for getting circuit ground to chassis ground. This is swaged to a rather large ground pad that is present both on the top and bottom layers of the board. If this swage is not tight enough, it is possible for the top ground (the pot ground) to not be at the same potential as everything else, causing chatter and/or volume fade. What we do here is to solder a piece of wire to the back side of the pot body, then hardsolder this wire straight to the top ground pad. Then make sure the screw on the bottom of the unit to the standoff is indeed tight.If this ground is loose, and the AC ground is lifted, the audio output can actually cease intermittently!

See the following picture for the final fix (view is from rear of unit):

Problem #2: Motor will "chatter" for a few seconds when a new preset is recalled.

Solutions: There are 3 possible problems here (4 if you include the grounding fix, above). First a little more history:  around the time of the 1.2 software upgrade, a resistor on the circuit board needed to change to a lower value to aid in the pot control. Versions 1.2 and later will help decrease the chatter with this resistor change (solution #1, below). Upgrading a unit to 1.2 and later without changing the resistor can actually create the problem. Upgrading from, say, 1.5 to 2.0 will NOT create the problem. Other than software and resistor value, the pot can become dirty or corroded. It dirty, cleaning will do the trick (solution #2, below). If corroded, replacement will be necessary (solution #3, below).

Solution #1: This is assuming you have version 1.2 or later. (If you don't have version 2.0, what are you waiting for?) First thing to do is find the resistor. Take the top off and locate the little guy using this pic from Ray Killough's site::


Check the color code of R1431 towards the front of the unit by the motor control IC. It must be a 1.3kohm resistor. If it isn't, then a 4.7k resistor must be paralleled across R1431. A 1.3 Kohm resistor will have a color code of: brown, orange, red with a gold tolerance band indicating 5% (that's what you want to see). A 1.8 Kohm resistor will have: brown, gray, red, then gold (then you need the mod). If there are two resistors already piggybacked together, that should be fine. The top one should be a 4.7 Kohm - yellow, violet, red, gold. The piggyback looks something like this:

Solution #2: spray the pot with a deoxit like Cramolin Dn5 or "Tweak" then run the pot back and fourth a few times via the pregain control. You'll probably have to bend the "straw" so the stream get get into the element - since the pot is mounted upside down in the TubeFex.

I have a 5.0 oz. can of DeoxIT in front of me, so let me do an info dump:

Caig DexoIT DN5 - part # DN5S-6
Contains: DeoxIT D100L, 1,1-Dichloro-1-fluoroethane, Tetrafluoroethane Propellant.

Solution #3: If you have version 1.2 or later, a 1.3 Kohm resistance for R1431, and have grounded and cleaned your pot, hopefully your problems are gone. If you still have chatter, you will probably need the motorpot assembly replaced by our service department.

In this case, contact Repair Assistance here at the factory - 601-483-5365. Ask for Max if you get someone unfamiliar with this problem - he'll know the deal. You can also e-mail at

If you're in Europe, send e-mail to Andy Holland at Peavey UK.

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