Well, this guy was damaged during shipping. One preamp tube was missing, and although its owner got a replacement there still was no output. Here’s what I found out.

The part of tube socket that hug and hold the pins of the tube were open too wide almost as if the tube had been yanked sideways.

From this:

To this:

Although the solder joints looked fine, they were dressed either way out of precaution. Now there was output but the reverb wasn’t working. The most common issue with this is the driver/recovery chip. I removed the chip and placed a socket to make future repairs easier. Once I tried a different chip there still was no reverb! So, the next obvious place to check was the reverb pan and that’s where the problem was found. A single wire from the input jack had broken off.

Moving on to the adjustable bias modification.

Test points:

The range for the bias wasn’t low enough to bias the amp properly so I ended up changing the value of the resistor next to the bias trimmers(on top):

Now it can be bias colder/hotter as one wishes, but following the chart found on weber’s bias calculator page, with plate voltage of around 350vdc(340vdc in this Blues Jr) then avg% of 21mA per tube(AB EL84/6V6)

Left tube:

Right tube:

Done!

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