How to use KT88 and KT120 tubes in the ProLogue One, Four, Six & DiaLogue One, Four, Six
Adjusting the Adaptive AutoBias™ Board
Before you begin: Please note that most customers will NEVER need to refer to this page. Making adjustments to the Adaptive AutoBias board when none are needed may have an adverse effect on the sonics of your amp. Also, if you make a mistake and turn the AutoBias setting too high, you may cause severe damage to your tubes or even the amplifier, and such an event would void your warranty.
Consult your local PrimaLuna Partner or contact PrimaLuna USA directly before attempting the procedure outlined below.
You will need: A multimeter that measures milliVolts (mV) and a philips screwdriver. A good, inexpensive choice is on Amazon called the Inova 3320. If you have multimeter leads that clip on, that's much safer. If you don't, be wary as you can easily look away and leads can touch, causing damage and even injury or death.
1. Lay your PrimaLuna upside-down on a soft cloth. Remove the bottom cover. Keep all tubes installed in the amplifier. It's easier to keep the amp level if you have the tube cage on.
2. Plug in the PrimaLuna and turn it on. Give it a few minutes to warm up. Take special care, as you are dealing with high voltages!
3. Touch the black lead on your multimeter to the wire bridge located near the smaller tubes (see picture below).
4. Each of the power tubes sockets has a gray lead that goes to the Adaptive AutoBias board. That is your test point on the tube socket.
(see picture below).
5. Measure each power tube. For the ProLogue One, Four, Six, or DiaLogue One, make note of which tube measures lowest. On the ProLogue Two, Five, Seven, or DiaLogue Two, Five and Seven, make note of which tube measures highest.
6a. For PrimaLuna amps that come stock with EL-34s, touch the red lead to the tube that has the lowest test measurement. Adjust the small white pot located on the end of the AutoBias board by turning it with a philips screwdriver until the multimeter reads 350 mV (.350 v). Refer to the picture below. If the lowest tube already measures at 350 mV no adjustment is needed.
6b. For PrimaLuna amps that come stock with KT88s, touch the red lead to the tube that has the highest test measurement. Adjust the small white pot located on the end of the AutoBias board by turning it with a philips screwdriver until the multimeter reads 400 mV (.400 v). Refer to the picture below. If the highest tube already measures at 400 mV no adjustment is needed.
You will notice the reading may fluctuate slightly up and down as you measure. This is completely normal. Do not stress if you can't get your amp to the exact number indicated above. Just get as close as you can.
7. Once you have finished making the adjustments, turn the amplifier off, and put the bottom cover back on.