Blackface Super Reverb PT issue

[Helmholtz]

Wait,...... you have a JTM50?

Yep, a '66.

[Helmholtz]

If they did so in the JTM50, I would argue it was because they had them on hand, rather than an effort to fail-safe the circuit.

You may be right, here.

Here's a pic of an original JTM 45 using 450V bias caps:



But I've seen 150V types used as well.

Over-rating all components to anticipate circuit failure would be very impractical. Where would you draw the line?

It's a normal approach in industrial product design to "anticipate" component failures.
A product must stay safe with all kinds of single failures.
We tested all new or modified designs against relevant component shorts and opens and documented the results.
That was required to get the safety approval (e.g. VDE mark, UL etc.).

[Greg_L]

Given what we know of the earliest Marshalls I think it's safe to assume they just used what they had on hand. A 1966 JTM50 is still in the era of Marshall just finding their way.

[Helmholtz]

Given what we know of the earliest Marshalls I think it's safe to assume they just used what they had on hand. A 1966 JTM50 is still in the era of Marshall just finding their way.

What does Marshall use in the JTM 45 reissues?
Meanwhile they should have had enough time to find out what voltage rating is sufficient.

[Greg_L]

What does Marshall use in the JTM 45 reissues?
Meanwhile they should have had enough time to find out what voltage rating is sufficient.

Looks like the JTM45 reissue uses 10uf/100 bias caps. And it taps the bias off the high voltage AC secondary. No dedicated bias tap on the reissue PT.

[Helmholtz]

I know the schematic, but it doesn't show the voltage rating.

[Greg_L]

I know the schematic, but it doesn't show the voltage rating.

None of the modern ones do. But from my experience with the Marshall vintage reissues they all get the same 10uf/100v bias caps.

[mozz]

If you sat there and tried to find all possible fault scenarios for single components and multitude failures of multiple components, your product would never get to market. Nobody overbuilds things for the market. I have seen 15v caps on 12v circuits that somehow passed UL inspections and some Bugera atrocities.

[Helmholtz]

If you sat there and tried to find all possible fault scenarios for single components and multitude failures of multiple components, your product would never get to market. Nobody overbuilds things for the market.

Certification procedures are exactly descibed in (European) safety standards. Conformity to European Directives is a legal requirement.
Has nothing to do with overbuilding things. Financial controlling wouldn't allow that.
Takes time, involves manpower and increases development costs.
Can be outsourced to accredited certification institutes ("Notified Bodies") but is expensive
Eventually products do get to market .
I've was involved in these processes for over 26 years and know what I'm talking about.

[g1]

I know the schematic, but it doesn't show the voltage rating.

The triode kit (I posted layout above) and mojotone JTM45 kits both use 150V caps, so I had assumed that is also what the re-issue used. In the Traynor circuits I mentioned earlier, they used a 250V cap there.