Hunts 0.05uf 1000v cap off HT

[AndyD]

Question, could someone please explain to me the merit or otherwise of the rusty coloured Hunts cap that comes off the HT fuse in Plexi era amps? I rather hastily removed mine because I was concerned that if the cap shorted for whatever reason, the amp chassis would become live. Have I been too hasty? Does it have a merit in the circuit? If it shorted, would the HT fuse blow and save the amp from collateral damage? Please enlighten me @Pete Farrington and anyone else with the knowhow. The picture below is not my amp but illustrates the capacitor in question.

 

[Jon Snell]

That is a decoupling capacitor that is surposed to stop mains noise entering the HT supply.
Not a common component in these amps.

HT and mains are different.
If the capacitor goes short circuit (very unlikely) it will blow the HT fuse, not make the chassis 'live'.
Maybe you are confusing the 'Death Cap' issue on 110volt amplifiers and never used on 240volt supplies, for obviouse reasons.

 

[AndyD]

That is a decoupling capacitor that is surposed to stop mains noise entering the HT supply.
Not a common component in these amps.

HT and mains are different.
If the capacitor goes short circuit (very unlikely) it will blow the HT fuse, not make the chassis 'live'.
Maybe you are confusing the 'Death Cap' issue on 110volt amplifiers and never used on 240volt supplies, for obviouse reasons.

Hi Jon, thanks for your reply. This cap goes to ground, so it is not a decoupling capacitor. I think it acts as a "pre filter" to the filter capacitors that follow. I just wondered if it was worth keeping connected or a potential for trouble.

 

[Jon Snell]

Hi Jon, thanks for your reply. This cap goes to ground, so it is not a decoupling capacitor. I think it acts as a "pre filter" to the filter capacitors that follow. I just wondered if it was worth keeping connected or a potential for trouble.

Its function is to decouple the HF noise to the chassis/ground. The main smoothing capacitors return to chassis or ground as does all of the inputs and outputs. This small capacitor sits between the HT (before the main smoothing or directly across the main smoothing) and ground/chassis.
Therefore it's a HF decoupling capacitor.
Lots of books about decoupling capacitors and its function.

 

[AndyD]

Its function is to decouple the HF noise to the chassis/ground. The main smoothing capacitors return to chassis or ground as does all of the inputs and outputs. This small capacitor sits between the HT (before the main smoothing or directly across the main smoothing) and ground/chassis.
Therefore it's a HF decoupling capacitor.
Lots of books about decoupling capacitors and its function.

Aha! Thanks for the clarification.
Btw, I had a oscillation problem with this amp before adding output grid resistors. I wonder if the absence of this cap contributed to the oscillation.

 

[Jon Snell]

Aha! Thanks for the clarification.
Btw, I had a oscillation problem with this amp before adding output grid resistors. I wonder if the absence of this cap contributed to the oscillation.

I very much doubt it. All it does is try to stop mains borne interference (not very successfully!)
Nice amplifiers.

 

[AndyD]

I very much doubt it. All it does is try to stop mains borne interference (not very successfully!)
Nice amplifiers.

Cheers!

 

[neikeel]

I leave them in as long as they have not split/cracked open or test bad.

 

[Pete Farrington]

They seem to be a leftover from the 5F6A https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.gif

https://el34world.com/charts/Schematics/files/Marshall/Marshall_50w.pdf

My guess is that their originally intended purpose was to reduce stress on the valve rectifier when the hot switching standby is flipped.
And perhaps nobody dared to question their relevance when Marshall moved to solid state rectification