Harley Benton GA 15 Ultimate Mod Kit Manual page 5

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- Cosmic Ampworks

- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127

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).

R38

Figure 5 R38 (the voltage dropping resistor for the preamp tubes-

DON'T TOUCH

IT!) 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).

www.cosmicampworks.com

Dedicated to the Advancement of Stellar Tone!