Snubbers? Do I really need them?

[Greg_L]

Possibly? This goes beyond MY pay grade. Maybe inrush charging the caps after the switch caused a problem with some switch in some test. It's possible a different switch was used in the actual amps and so never was a problem. Or that the standby switch location was ignored on other models with snubbers indicated in the schematics. I've often seen design artifacts in amps or their schematics that don't make immediate sense, but don't usually cause any harm. And in this case, as in others, it's not uncommon to see that the actual amps don't always match the factory schematic.

Yes to all that, especially considering every single Marshall made after these things do not use snubbers across the PT secondaries...not that I've ever seen anyway. And as mentioned neither do the reissues which are surprisingly pretty accurate.

I'm gonna use the ultra-fast diodes and modern switches so I'll give it a try sans-snubbers. Then I'll put some in and see if there's any noticeable difference.

[Helmholtz]

If one of these caps fails shorted, the mains fuse will blow and protect the PT.

I think the only caps that must be designed to fail open are class Y types.
https://www.allaboutcircuits.com/tec...-y-capacitors/

[Chuck H]

Then I'll put some in and see if there's any noticeable difference.

That could be interesting. I wouldn't expect any difference but there may be something subtle that's not immediately noticed in the background noise or a particular harmonic quality with heavy clipping or something. The problem with such a test is the circuit being HV. Which makes AB ear testing dubious for switching. Though I've done things like that myself being careful and willing.

[Helmholtz]

Nevertheless the caps should be rated for class X1 or at least X2 because when the HV winding is unloaded (in standby) high voltage mains spikes can appear across the secondary.

[Greg_L]

Nevertheless the caps should be rated for class X1 or at least X2 because when the HV winding is unloaded (in standby) high voltage mains spikes can appear across the secondary.

My PT's HV secondary voltage is supposed to be approx 185v on each half....which is where those caps would go.

[Helmholtz]

My PT's HV secondary voltage is supposed to be approx 185v on each half....which is where those caps would go.

So best use 250V class X1 (or X2) capacitors.

Class X capacitors can stand kV spikes (see link above).

[Greg_L]

So best use 250V class X1 (or X2) capacitors.

Class X capacitors can stand kV spikes (see link above).

Yeah I'm gonna pick some up today. Thanks.

[pdf64]

…
That rectifier configuration doesn't make it easy to fuse at the windings, and I'd suggest that if the B+ feed fuse popped then the OPT and choke would likely get hit with a stressful high voltage spike.

What are better options for HT fusing here?
eg a fuse in series with each pole of the standby switch.

As I see it, provided the fuse blew whilst the valves were hot enough to conduct, they might act to damp the back emf spike from the choke?

[mozz]

Nothing about noise being transmitted back into the mains? I thought some UL testing qualifications required some components on the secondary side at times.

[Helmholtz]

Nothing about noise being transmitted back into the mains?

Diode switching noise propagates in both directions. Mitigation helps both ways.
Yes, EMI regulations might be another reason.

AFAIK, UL only cares about safety. But there might be FCC regulation applying in the US.
In Europe we have EN/IEC standards for EMC.

But assessment would require appropriate lab equipment.

[mozz]

When I worked for a electronics manufacturer, I know it was expensive to have things certified , pass or fail. Maybe they didn't want to have to retest so they were included whether there was a problem or not. Could have been years later they figured they were not needed, changed diodes or power transformer and noise was reduced. I'd say they are not needed if you are building a clone, unless you run into problems.

[g1]

What are better options for HT fusing here?
eg a fuse in series with each pole of the standby switch.

Yep. Or similar.

[Greg_L]

Yep. Or similar.

Ha I've seen that drawing before. That's the Canadian version I believe.

[trobbins]

The schematic in post #27 certainly likes fuses! It may not be obvious what happens if say just one fuse blows - so a thorough assessment may be needed - for example the issue with bias supplies for some designs can provide a reverse path if a fuse blows, leading to other damage. A quick view of that schematic indicates that if say F4 blew, then D4 and D5 would still provide half-wave rectification for B+ supply - hopefully that is noticeable, as it could affect the e-caps.

Wrt the evolution to using no snubber caps, that may have been due to evolution of ss diode type available/used. I can't identify a datasheet for the A10D10, but it is in a 1975 cross-comparison chart as a 1kV 1A part. I had noticed that the datasheet for the venerable 1N400x range had not changed for decades (first introduced 1965), but manufacturing technology moves on, and it seems they now have much lower leakage and much higher PIV than the ratings suggest. In those early days there were likely major changes happening - and that could have related to reverse recovery performance.

[g1]

Ha I've seen that drawing before. That's the Canadian version I believe.

Yes, that's the CSA (Canadian Standards Association) version. It has a separate board that is just fuses.
Also of note is that it does not have a 16 ohm tap, as at that time the standards did not allow for the higher voltage it could develop, being accessible to the user if they hold on to the plug of the speaker cable.