Electronics > Repair

Valve amps are dangerous

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Audiorepair:
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.

floobydust:
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.

nigelwright7557:
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.

Audiorepair:
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.




KevinA:
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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