With the foil side of the Ampeg SVT-VR Tube Board exposed, we can now check for any resistor problems on the board.

The solder points for the plate and screen-grid resistors are identified in this photo.

The yellow lines indicate 5-ohm plate resistors and the red lines indicate 220-ohm screen grid resistors.

Measure the resistances between the pairs of solder points to see if they're correct.

Tube Board Schematic

Full set of schematics

We found that two of the plate resistors and two of the screen-grid resistors were in fact open circuits and so we continued our journey toward the resistors themselves.

The Tube Board is attached to the metal cover plate by the twelve tube-socket screws.  These screws also hold on the tube retainer springs.

Use a 5/64" hex key (Allen wrench) to extract the twelve screws, as shown here.

Now you have separated the Tube Board from the metal plate and can see the component side of the board.

In this photo, the four resistors we found to be open are marked and circled.

From the foil side of the board, we de-soldered and removed the bad resistors, then soldered in new ones.

Here, the process is underway on three of the resistors.

Here you see those three newly installed resistors.  The large one is a plate resistor and the two smaller ones are screen-grid resistors.

Notice that this board can accommodate two diodes (D3 & D6), one in parallel with each screen-grid resistor.  However, this Ampeg model doesn't employ those diodes.

The Ampeg SVT-VR plate resistors are
5Ω, 3W, 5% parts.

The SVT-VR screen-grid resistors are
220Ω, ½W, 5% parts.

You can purchase them using the above links.

After reassembling the Ampeg SVT-VR, we connected the speaker output to a dummy load and let the amp warm up for 20 minutes or more.

Then we injected a 40Hz sine wave and raised the volume control until the voltage across the load reached 25VAC.

Under these conditions, we successfully adjusted the Balance trimmer to light the yellow Balance LED (lower left).