Electronics > Repair

Valve amps are dangerous

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No matter how careful you are with a valve amp, you can still get caught out.
Happened to me today.

Here is a Marshall DSL 201, unplugged from the mains.  HT voltages on the valve bases were measured insignificant, I checked that.

Yet the main HT cap doesn't have a bleed resistor, and the full HT voltage remains on the standby switch.
Normally I check directly on the PSU caps themselves, but this has PCB mounted radial types so you can't access them directly.

Ouch, that hurt when I grabbed the amp to turn it over and my thumb hit the standby switch with my other hand on the chassis.

Youch! There's a lot of noobs in these old companies making design mistakes. They're assuming the O/P tubes are always in and will discharge things I guess. The factory test fixture for the board... big liability if Manufacturing pulls a board off the test fixture and gets zapped. I've seen that happen.
On home-brew gear I always put in a LED, resistor, diode indicator just to remind anyone to engage brain before going in. But I have bleeder resistors always.

I design my own valve amps and always have LED in series with a resistor across power supply capacitor.
If its red dont touch.

I remember my very first valve project.
It wasnt working so turned it off and grabbed the pcb and got a shock.
Tutor laughed and said that I needed to discharge the cap.
So next time I discharged the cap and grabbed the pcb and got a shock again.
Forgot to turn it off !
Amazingly that was 42 years ago and I am still alive.

It really should be illegal NOT to have bleed resistors IMHO, I have seen the same on other modern/reissue amps.

This amp is a 1999 manufacture date.

My thumb has a burn hole, my heart survived though.
Newbies' mileage may vary.

Every tech should have an A0 sized copy of this pinned to the wall above their bench, and look at it every ten minutes.


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