Valve amps are dangerous

[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.

[shakalnokturn]

That actually does kind of look like a RF power triode setup.

[trobbins]

It really should be illegal for a tech to service equipment without a schematic and adequate checking of the circuitry and assessment of hazards. 

Audiorepair, I don't get the impression you have acknowledged a lack of safety awareness and seem to want to give the impression that you were as careful as you could be, which is a bit concerning as you are noting that the main filter capacitor(s) could not be directly probed and did not have bleed resistors.

[bob91343]

I have never like Marshall amplifiers, for reasons I won't bring up here.  But they are not built by those who go the extra mile.

[KE5FX]

No kidding.  That thing looks like what the lowest-cost Chinese assembly people took apart to learn from.

[trobbins]

No kidding, that looks like a typical UK or US designed amplifier sent off-shore for manufacture by a well-renowned company that imports the equipment back in to sell, warrant and service under their well-known and respected name.

[TMM]

My question here would be why is the power switch between the DC smoothing caps and the tubes? If you switch the AC mains you don't get this problem as the tubes themselves will bleed it down to at the very most a few tens of volts, probably down to only a few volts if you give it enough time.

[med6753]

Tube audio gear is dangerous only if you are careless. I do a lot of work on tube Tek gear and I typically wear rubber gloves and observe the "other hand behind your back rule". If you don't it can kill you dead.

And do me a favor. Your test gear is also part of your safety protocol and should be kept in top condition. That Fluke looks like it was dragged through the mud. Why don't you treat it to a good cleaning?

[shakalnokturn]

That Fluke looks like it was dragged through the mud. Why don't you treat it to a good cleaning?

The coating is somewhere between ESD safe and HV risky 

[wraper]

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.

It's not a design mistake since there is no need to discharge capacitors and constantly waste power. And it's not any different from many SMPS where capacitors remain charged for a long time too.

[Gyro]

Personally, I think standby switches are pretty dangerous. They allow the HT caps to charge to full unloaded rail voltage (are they rated 450V or 500V?), way above their normal operating voltage, for no good reason. I'd lay odds that that standby switch isn't rated for 460V DC.

[PKTKS]

Use gloves to service tubes.. !!!!

Got zapped countless times before doing so.

By that time parts were a lot more expensive..
saving one or two here and there was a must do..

Today is no longer like that..
But 400V and above is really something to not care less about

Paul

[CaptDon]

I agree with Bob, Marshalls were always designed with as few parts and quality
in mind. I have seen over 550vdc in standby on 450vdc rated capacitors. I have
had more than one fried power transformer where the line fuse didn't blow until
the transformer was completely nuked. Guitar players seem to have a loyalty to
the brand but personally I think Marshalls suck. A far over-rated piece of yesteryear
gear. My go-to amp for the last 25 years is a TelRay Syncordia (the same folks who
make Fender amps) 'SuperNova' twin 12 knockoff of a Fender twin 12 solid state.
Mine was equipped with the Morley Oil Can 'rotating sound' as well as tremelo and
reverb. I rebuilt the output section with two 100wrms solid state bricks and added
a mono output jack and stereo return jack for external effects. The jacks are normalled
so the amp will play without external effects plugged in. Almost no one has ever seen
a SuperNova amp and few know what a Morley Oil Can is, but all agree this amp
is a killer combo amp! I loaded it with Peavey Scorpions and this thing can play clean
acoustic or killer electric overtones. Granted it doesn't have the warm even harmonic
distortion of a tube amp however.

[3roomlab]

now i am curious, what is the electrical std called, that apply to this, such that it be made safe? IEC 1234? UL 5678?

[TimFox]

Re:  standby switches on tube gear.
Unfortunately, most tube gear uses a multiple-secondary power transformer so switching the primary kills both the HV and the heater voltages.
If one has a separate heater transformer, then switching the primary winding of the HV transformer allows the tubes to discharge the filter capacitors (somewhat, at least) in standby mode.

[HB9EVI]

I agree with the opinion about Marshall; far too overpriced for no real adequate quality; though Fender did some design errors too, like my Pro Juniors which require quite some mods to reach an acceptable noise level (buzz, hum, hiss).

never forget the iron rule of the valve sphere: always one hand in the pocket

[Audiorepair]

When I got an HT shock for the first time many years ago now, I adopted the one hand in back pocket procedure.  Nobody wants that to happen twice.
And until now it hasn't.

The issue here is that after testing the pre-amp section with the output valves out, I pulled out the mains plug as I always do, before flipping the amp over to put the output valves back in again.
It was then that my thumb caught the spade terminal of the Standby switch.

I will now adopt the additional policy of "always check the Standby Switch" before flipping amps.
I suggest others do the same.


There are issues with the amps design that could have easily been made to prevent this, I feel.
The spade terminals on the Standby switch are not insulated, and the live wire is uppermost where it can easily be touched.
Reversing the wires and using insulated Spades would make this event pretty unlikely.

I don't think the extra cost in doing this would have broken the bank, and you could still have saved a couple of cents by not having a bleed resistor.

[vk6zgo]

My question here would be why is the power switch between the DC smoothing caps and the tubes? If you switch the AC mains you don't get this problem as the tubes themselves will bleed it down to at the very most a few tens of volts, probably down to only a few volts if you give it enough time.

Exactly!
Thousands of tube radios & amplifiers were constructed over many years without a separate HT switch.
Such things were normally reserved for transmitters.

[vk6zgo]

Use gloves to service tubes.. !!!!

Got zapped countless times before doing so.

By that time parts were a lot more expensive..
saving one or two here and there was a must do..

Today is no longer like that..
But 400V and above is really something to not care less about

Paul

Techs repaired tube stuff for decades without needing gloves, or other nonsense.
What they did have, was a functional brain, something that seems scarce these days.

[med6753]

Use gloves to service tubes.. !!!!

Got zapped countless times before doing so.

By that time parts were a lot more expensive..
saving one or two here and there was a must do..

Today is no longer like that..
But 400V and above is really something to not care less about

Paul

Techs repaired tube stuff for decades without needing gloves, or other nonsense.
What they did have, was a functional brain, something that seems scarce these days.

Agreed. But my hands aren't as steady as they used to be so as an extra measure of safety I wear the rubber gloves.

[Audiorepair]

My question here would be why is the power switch between the DC smoothing caps and the tubes? If you switch the AC mains you don't get this problem as the tubes themselves will bleed it down to at the very most a few tens of volts, probably down to only a few volts if you give it enough time.

Exactly!
Thousands of tube radios & amplifiers were constructed over many years without a separate HT switch.
Such things were normally reserved for transmitters.

I believe the Standby switch was invented so that you could mute a  noisy amp when you weren't playing it, and quickly unmute it when you wanted to.

It otherwise serves little purpose.


Edit:
In fact, I have read on several occasions it is a popular myth that  the Standby Switch is there to prevent "cathode stripping" on warmup, but the reality is that only very high voltage transmitter tubes suffer from this.

Is this a thing?