THD Hot Plate Upgrade Modified -16dB Rheostat

This is a replacement rheostat for all models of our Hot Plate power attenuators. It has been hand-modified to give a smoother action and longer life than the original stock version.

This is a perfect drop-in replacement part for all stand-alone THD Hot Plates, 2 ohm, 2.7 ohm, 4 ohm, 8 ohm and 16 ohm. It is stock on all Hot Plates manufactured or repaired after 2020.

It is also ideal for the Gibson-branded attenuators that we made, which were sold as the Gibson Power Stealth.

This is NOT for the Hot Plate attenuators that are built into our UniValve and BiValve-30 amplifiers.

It starts life just like the ones originally put into our Hot Plates, a top-quality CTS 5-watt wire-wound rheostat. Then, like all the ones in our Hot Plates, the shaft is cut shorter, as the stock shaft length is too long for the Hot Plate. We then de-burr the end of the shaft with a fine file, so that the knob fits easily back on. Then the internal upgrades begin. We take the back off, remove the stator, and burnish the wire where the stator contacts it on its rotation path. This assures a smoother turning, and less “crackle” as the knob is being turned. We then turn our attention to the stator itself. It’s contact points are also burnished, and the stator arms are re-bent to an improved position, giving both more contact pressure, and a wider contact area. This improves the life of the rheostat, as more turns are touched at once, improves the contact quality, and further reduces the “crackle” sometimes experienced when turning the -16db knob. We then reassemble the unit, give it a proper test, and sign it, to show that it has been modified. 

To replace the rheostat, complete the following steps: 

1. Remove the metal wrap-around on the Hot Plate chassis. This is best accomplished by unscrewing the four Philips-head screws on the side of the attenuator (two on each side). 

2. Slide the inner chassis out of the wrap-around. This is usually easiest done by putting it on the edge of a table with the front two feet hanging over the front edge of the table, thus keeping the wrap-around from moving. You then hold the outside of the wrap-around, and press the inner chassis away from you using your thumbs. Once you get it started moving, it gets easier. Some come out very easily, others take some effort.

3. Use a small blade screwdriver to loosen the set screw holding the black knob on the shaft, and remove the knob itself.

4. Use a ½” wrench or nut driver to loosen the nut holding the rheostat to the chassis.

5. Take a photo or make a drawing of which wire goes to which terminal on the rheostat.

6. Unsolder the 3 wires at the rheostat end (not the circuit board end) from the rheostat.

7. Remove the rheostat from the chassis, saving the star washer (or star washers) that are between the bushing and the chassis.

8. Place the star washer/washers on the new rheostat, and put the bushing through the hole in the chassis, fastening it with the nut on the outside, tightening it just a bit more than finger tight. You don’t need to use much force.

9. Solder the wires back onto the solder tabs on the new rheostat, attaching them to the same positions that they were previously in.

10. Put the old knob back on the shaft, making sure it’s oriented properly, and tighten the set screw again, only finger tight.

11. Put the wrap-around back on the chassis, and fasten it with the 4 screws you removed earlier, taking care not to cross-thread the chassis in the process. They should go in easily. Sometimes a tiny bit of oil or grease on the threads helps reduce the friction, but they should go in easily.

You are done. Enjoy your new, better rheostat!