bypass caps across HT Standby Switch terminals

[nevetslab]

One of the parts I never did get an identification on when I had the Hiwatt DR201 here in the shop are the two Vishay MKT1813 Series Film Caps that are across the two sets of Standby Switch terminals (switching HT Plate and HT Screen/Preamp Voltages.



The body size looks like around 12mm, so might be 1000V/47nF, or maybe 630V/100nF. I don't see those caps on their lower powered amps Custom 50W or Custom 100W. None of the published older DR201 schematics show those caps. And, their limited technical staff is very slow in providing ANY details on such questions.

Checking to see what I have on hand, I have several 47nF/600V Orange Drop caps. I also have 0.05uF/500V Ceramic Disc, 0.10uF/500V Ceramic Disc as well as 0.01uF/1kV Ceramic Disc caps, though I don't recall ever seeing that type cap used for arc-suppression on toggles switching high DC Voltages.

Any recommendations?

[Chuck H]

Sorry I don't know about those caps. I will say that I've had some trouble with HV spikes and plain ol film caps at rated voltage. I had to go to KV rated caps purpose built for suppression. So I wouldn't even bother trying to copy original for this since it's possible Hiwatt had a go to cap they knew would work regardless of rating. This discovered after some failures I suspect. If they'd had KV rated suppression caps when the amps were made (originally) you can bet they would have used them. Later amps may just be copying the old design as best as possible without knowing any different. I mean, if they short it's just the standby switch, right? Interesting those caps don't appear on any schematic and there's no discussion about them anywhere on line I could find.

JM2C

[J M Fahey]

Agree.
Personally would use 1200/1600V rated caps there.

Value wouldn´t be as important as voltage rating.

again, jm2c also.

[Jon Snell]

Suppressing a DC switch is tricky and not usual, after all, what is the point.
AC power switches must have class Y and class X capacitors if suppression is required but DC is not needed.
Those capacitors shown have been fitted after the build date as HiWatt don't call for them. The Alpha pots show either a modern clone of a DR201 or they have been replaced at some point.
Here is the DR2012 power supply schematic;

[mhuss]

These caps are used by the company that currently owns the Hiwatt name, and were not part of the original Hylight Hiwatts. They're supposed to limit switch arcing (which burns up the contacts over time), but normally in arc suppression there's an RC network across the switch, not just a capacitor. There's a reasonable explanation here: https://testguy.net/content/362-Arc-...ssion-circuits

[trobbins]

I'd suggest reading the Quencharc description rather than the testguy (as that description is ambiguous and imho flawed). The link also shows some practical part info:
https://www.verical.com/datasheet/pa...00-6736828.pdf

[Chuck H]

In the end it's as Juan and I have stated. They are suppression caps. Mallory 150's (which appear to be the caps in use?) are entirely inappropriate. Just use suppression caps. 47n should be fine? The impedance is very low on that circuit but the spikes typically occur at very high frequency. I wouldn't balk at 100n suppression caps there. Still shouldn't impinge on the circuit impedance for delivering DC.

[nevetslab]

I ordered some 10nF/2kV radial lead Polypropylene caps, made by TDK. 22.5mm L/S, 17mm long leads, so the spacing is very close to the switch terminals I have from this Blackstar chassis. 25A DPST toggle switches. Should see them next week. I'll have a read thru with the links provided.

[nevetslab]

Now working on getting the Power and Standby Switches fitted with their wiring, seeing how close the Power Switch is to the Standby Switch, I see I shouldn't have even bothered ordering a potential part. Not enough space between the Power and S/B switch! Oh, well.