Owner's Manual

SVT-VR Bass Guitar Amplifier

Changing the Tubes

Tubes wear out in direct proportion to how often and how hard you use the amplifier. Power tubes

should be checked at least once a year – more frequently if you use the amplifier nearly every day.

When power tubes wear out, the amplifier will begin to grow weak, lack punch, fade up and down, or

lose highs and lows. Power tubes work together in a push/pull configuration and should be replaced

at the same time with matched or balanced tubes. Your dealer can recommend the best replacement

tubes for your amplifier.

Preamp tubes aren’t worked as hard as power tubes and typically last longer. When a preamp tube

wears out, the amplifier may squeal, get noisy, lose gain and sensitivity, or just quit working. A service

center can determine which tube(s) may need replacing.

To get to the power tubes in the SVT-VR, the rear screen must be removed and the tube retainer(s) must

be moved out of the way. Qualified service persons may follow these steps to change the tubes:

 •Turntheampoff,unplugitandletitcoolforatleast5minutes.   

 •Removethescrewswhichholdtheperforatedmetalscreentotherearofthecabinet. 

 •Settheperforatedmetalscreenaside.      

 •Removethetuberetainer(s)byliftingthemoffthetube(s)andmovingthemtooneside. 

 •Graspthetubeatitstopandgentlyworkitoutofitssocketbyrockingitslightlybackandforthas

you lift up on it.

 •Wheninsertingnewoutputtubes,alignthetabinthetube’splasticbasewiththeslotinthesocket

and press the tube gently but firmly into place by pushing down on its top.

 •Replacetheperforatedmetalscreenandscrews.    

 •Poweruptheamplifierandletitsitforatleast20minutes.Biastheamplifierasdirectedinthe 

section below.

Setting Tube Bias and Balance

Tube Bias:

Allow the amplifier to warm up to proper AC line voltage for at least 20 minutes. With no signal

present, adjust each bias control until its adjacent LED illuminates.

The Bias 1 control affects the three power tubes on the left (as viewed from the rear). The Bias 2

control affects the three power tubes on the right (as viewed from the rear).

Balance:

Insert a 40 Hz signal into the amplifier. Adjust the volume control for approximately 25VRMS output.

Slowly adjust the balance control until its adjacent LED illuminates.