Electronics > Repair

Valve amps are dangerous

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--- Quote from: trobbins on October 20, 2021, 06:06:16 am ---
CH2 on page 1 appears to be associated with a 'metal washer' safety critical marking, and CH could mean a chassis connection, as CH1 and CH3 are also identified for circuit sections that make sense for local chassis grounding.  Not being the service agent for Marshall with access to training and other documentation, and just relying on a schematic, is a risk to any unauthorised repair work.  Companies do just prepare schematics for in-house use, so being a critical outside observer is a bit rich.  It's lucky that schematics have made it in to the public domain, as that is not the case for some manufacturers (eg. TEK and Picotest are two instruments I have that I would love to get schematics of) and a schematic may only become available via the effort of a diyer doing some reverse-engineering.

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It's not rocket science---it's just a valve type audio amplifier, the like of which were manufactured in their 100s of thousands back in the day.(mostly vastly superior ones, too!)

Comparing that to Tektronics equipment (all the older analog stuff not only provided schematics, but detailed repair manuals) is like comparing a duck poo to a diamond!

Amp returned to customer.  Customer happy.

After the myriad of other stuff to sort, turned out 470k bleed was open circuit, replaced with 1 Watt.

Heatshrinked all the Standby Switch connectors, so you would really have to try hard to get zapped now.

I didn't feel the need to modify the bias further.

Clean is clean.

They need to identify hazards with stickers, its the best way, because eventually you will run into some hidden electrolytic behind a wiring harness or some shit like that.

You need to dig through it real slow like an archeologist and use nonconductive hooks to look past movable obstacles. Lifting a panel to have a board mated with something else without being able to see the other side is downright frustrating though. Ideally you would use a boroscope but no one is gonna do that. You can always bath it so long its not potted.

The worst I have seen I think is in a hughes amp, if the bleed resistor was not there, it would be very dangerous, since there was a cable harness infront of a hidden screw stud capacitor, if I recall correctly. So if you reach near the cables to maybe yank on one, attach a sensor (magnetic field), or reseat, you could brush your fingers on a hidden electrolytic that is fairly substantial. However, they did have a socket crimp leg power bleed resistor over the cap that drained it in a hurry.. I would have however appreciated a screw on shield or clip on shield between the cap and the cable harness, to increase the safety factor. Ever notice that with tek scopes too, the early 90's one, they have cable harnesses just running about possibly making direct contact with the chassis too? I managed to nick one when I was closing a box. God forbid they put some clamps for that kind of stuff.

Yeah this is how I felt with that capacitor, at two minutes, complete with the overkill breaker switch.

I just say this, hey dumbass, the raptors capacitors belong in a pen.

But, I know why it happens. Its not the electrical designers fault because you can easily lose your lungs and your job screaming at desperate mechanical integration engineers that make life so unpleasant.

This is about putting profit over quality. DSL-201 did retail at $600 "Made in England" and here we have a R133  470k 1/2W sitting at ~450VDC... pretty much the max. ratings.
I think Jim Marshall would've preferred hiring designers that can do Ohm's Law and not cater to the executive trying to save $0.10 towards his new Jag lol.
Modern Vox amps no better, the PCB arcs and burns up because the designers bungled the HV spacings  :palm:

For SMPS primary caps, I did make an LED+resistor in a old ballpoint pen for checking HV and slow discharging. Something to make it easy to add this as a habit in your workflow.
It needs an update to using a PTC or larger 5W bleeder resistor, or maybe an SCR would be best.


--- Quote from: Audiorepair on October 16, 2021, 05:53:45 pm ---
--- Quote from: vk6zgo on October 16, 2021, 03:25:12 pm ---
--- Quote from: TMM on October 16, 2021, 06:03:21 am ---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.

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

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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.

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?

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I think this guy covers all of the pros and cons of standby switches on vacuum tube guitar amplifiers. The explanation that resonates most with me is "Fender was a cheap-ass and Marshall was a copy-cat".



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