Adding Easier Access for setting the Bias Trim Pots

[Watson]

Why?
It is still winter here and I'm still bored so I invented another simple project.

I recently swapped out the 6L6 tubes in one of my amps for KT66s and was reminded what a PITA it is to get to the trim pots to adjust the tube bias on the power tubes.  I use a bias setting helper device for convenience but in order to get to the trim pots, the reverb tank still has to be removed and the chassis propped up on wooden blocks in order to have access to the circuit board containing the trim pots. 

So my project was to add two holes in the chassis directly above the trim pots so I can adjust the bias from the tube side of the chassis instead of the business side of the chassis.

Safety
Not a remarkable project, just makes it more convenient to do the deed. It could be argued that it is safer too. After all, who wants to end up like this poor fellow?

http://vintageamps.com/plexiboard/viewtopic.php?f=16&t=106572

[Watson]

STEP 1: Make a template
To aid in locating the position of the chassis holes needed, I just used a piece of paper and pressed it onto the main circuit board such that the trim pots were somewhat in the middle and the 4 power tube sockets were all on the paper.  Pressing down gently popped the pins from the tube sockets through the paper.  This makes the tube positions visible and will help orient the paper template once the circuit board is removed from the chassis.

With the paper still speared onto the socket pins, punch a hole over each of the trim pots. I also labeled the tubes for sanity's sake.

[Watson]

STEP 2: Remove the PCB and mark the chassis
Now remove the printed circuit board from the chassis and set it aside. Line up the template's socket pin holes with the chassis' tube socket cutouts and tape it in place. Then mark the trim pot holes on the underside of the chassis.

Go ahead and drill a 3/8" hole where you marked the trim pots on the chassis.

[Watson]

STEP 3: Add grommets
I have an assortment of grommets for automotive wiring and popped a grommet into each of the 2 access holes.

Since you would be using an insulted tool to make the bias adjustments anyway this step is not really necessary but it looks more finished.

[Watson]

STEP 4: Set the Bias
Put things back together and flip the chassis over tubes up (which is really upside down on this amp). Now since I use a bias setting helper device (Compu-Bias), I can just plug in the socket probes, plug in the tubes and then make the bias adjustments from the chassis side. 

Way easy and safer too!

Nope, not rocket science, but it is easier now and I don't have to prop the chassis up on wooden blocks.