How to get more volume from a Davis 106 PA

[hogwylde09]

I found a working Davis 106 PA in a storage building. The only way I could get a "buzz" was to unsolder one of the 1/8 type jacks on the back of the unit. I believe it was the booster out jack. The mic input on the front panel will not make any type of sound when touching the center terminal. I added a short piece of shielded wiring from the booster out and just added a 1/4" guitar input jack (it is now just loose and hanging, not mounted) and is the plastic pc type jack. The amp will work just not much volume. I think you can actually hear a small amount of breakup from the 6eu7, but the 6bq5's are not lit up bright. I know from other amps, these tubes should be cooking. Not redplating, but on the verge. I know this is a weird Self-split design. I am confused about why the mic input does not work, but mainly what resisters do I need to experiment with to get these tubes to get to their more desirable output dissipation.

[Phil_S]

If actually labeled 6BQ5 and not EL84, chances are the tubes are quite old and possibly used up. Try some new tubes. For a PA, you can buy a pair of Sovtek EL84 for cheap. You might take some voltage readings on the 6BQ5's. A little math will tell you how many mA of current are flowing at idle and this could be a good hint on the health or lack thereof of the tubes. If you don't know how to do the math, post readings for plate, screen, grid, and cathode. If cathode biased, the value of the bias resistor.