- Cosmic Ampworks
®
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
© 2020 -
5
Dedicated to the Advancement of Stellar Tone!
2.3 Where to check for remaining high voltages?
The legs of the 3 HT filter caps themselves are inaccessible, as they are mounted flush with the board.
However, in the amp schematic we can see that some components directly connected to those legs: the HT
fuse F3 (Figure 4) and both legs of R38 (Figure 5), for instance.
So, again, attach the black (negative) multimeter probe to the chassis, keep one hand in your pocket, hold the
uncovered tip of the red multimeter probe carefully against the HT fuse F3 (Figure 4) and check the voltage.
Then do the same by holding it against the legs of R38 (Figure 5).
Figure 5 R38 (the voltage dropping resistor for the preamp tubes-
DON’T TOUCH I
T!
) is a good place to check for remaining high voltages
after switching off and disconnecting the amp. Hang the black (negative) multimeter probe in one of the cage nuts (as described in the
text), so that you only need to use one hand. With that hand, hold the red multimeter probe carefully against the
resistor’s legs
and check
the voltages.
Not measuring any remaining voltages at F3 or R38, shortly after switching off the amp, is a reason for
suspicion! Put down the red probe, slightly wiggle the black probe in the cage nut, put it down and measure
again with the red one.
If there’s still no voltage, select a more sensitive range on the potmeter –
some low
voltages (< 1V) should still be measurable. With tube amps you never know, so by all means take a few
additional measurements in other places to double check! In any case, wait until the voltages are below some
20V. From experience: the GA15 filter caps will discharge quickly enough by keeping the multi (voltage)meter
connected for a minute or two. Don
’t short them to chassis ground!
Also, after having taken a break, check again for high voltages: capacitors can self-recharge due to dielectric
memory.
Cosmic Ampworks does not guarantee you
that the GA15 is safe to work on if you don’t measure any
remaining high voltages at R38 or HT fuse F3.
Always also rely on your own insights and measure and
doublecheck everything.
Some further essential safety advice at this excellent site:
https://robrobinette.com/Tube_Amp_Safety.htm
).
R38
