replacing wax paper line filter with Y2 ; advice needed!

Hi Guys!

I'm recapping (& de-mouldseal-ing!) a lovely old 'Bird Golden Eagle' valve guitar amplifier from about '64.

Things are goin well so far; New poly films,and some of the electros are done.Sounding good.

However ,I'd like some help/opinions on this :

There are a couple of Hunts wax paper tubular caps acting as a line to ground filter.  They are both 10nF 350v AC ( 750v DC) rated . The two of these are in series, between the incoming Live line(at the mains switch), and chassis/ground.

If I understand correctly, this is effectively a 5nF capacitance with a 700v AC (1500v dc)  max voltage rating.

I need to replace this with a modern Y2 cap , and this is where I'm stuck... I can find a 5nF (Y2) cap ,  but they only seem to be rated at 275v , or 300v AC.

Can I just use one of these ?  I've been reading up on x & y type caps, and how they are designed to withstand spikes much higher than their nominal value , but I'd appreciate an experienced opinion on this.

I'm so used to replacing caps with the same, or higher, voltage ratings , that I'm hesitating on this one..

Cheers!

nEOn

> I'm hesitating on this one..

Wise but....

First, do you even want a line-to-chassis cap??? I dunno how you did it in GB, but in the USA we did it because we had no grounded wall-outlets, and buzz was a problem. With a typ 0.005uFd cap from chassis to one line prong, we could flip the plug to find the more-groundy side of the wall outlet and minimize buzz.

This was a Bad Idea, but the best we could do at the time.

This is the 21st Century already. We do (should!) have proper grounded/earthed wall-outlets everywhere we play, and we DO use proper modern 3-wire power cords with the ground/earth conductor run to a dedicated chassis screw as soon as the cable comes in.

And throw-away that death-cap. Correctly-grounded guitar amps don't need them.

> (Y2) cap ,  but they only seem to be rated at 275v , or 300v AC.

On 115VAC we used "standard" 400V caps and they failed. A lot. Often failed SHORT. While a chassis at 115V is not always fatal, it's a strong shock. Combined with beer on the stage and coke up the nose, it is way too close to a heart-stopper. Your 230V more so.

Since standard caps fail badly, and a to-line cap is a bad place to fail, the Safety Agencies invented a new Standard specifically for to-line caps. You need a nominal rating greater than your highest nominal wall-voltage. Those 275V/300V caps appear to be aimed at your nominal 230V-240V walls. The actual withstanding spec is much higher: 5,000V peak, 1,500V for a whole minute, 1,000V for 1,000 hours. There's more specs, and testing is more rigorous than ordinary caps.

> replacing caps with the same, or higher, voltage ratings

Caps for INside an amp may have 1% or 20% safety margin. A 450V electrolytic will run hot at 500V and fail in a month. 400V paper cap may stand 1,000V or 401V depending how good the paper was the day they made it: as long as it stands 400V you can't complain.

Electronic parts in general use safety margins which would be scandalous in any other engineering. Voltages inside an amp are known, rarely spike, usually won't give a shock or start a fire or drop people in a river.

1,500VDC cap for nominal 230VAC wall seems generous. Like this designer was actually thinking about safety, and doing the best possible with low-cost parts. Still, a to-line cap ought to be designed and tested For The Purpose, not some standard blocking cap just up-speced.

These Y2 caps have a built-in safety margin more like house structures, or buildings and bridges in general. A stick of wood breaks at 3,000psi, but the design tables assume 1,000psi, 3:1 safety margin. Good steel will stand 50kpsi but standard design is done with 16kpsi.

These caps can take 1,000V for a month and we use them below 340V peak. They were MADE to sit directly across a wall-outlet, and "Y" means that the rating took acount of potentially fatal shocks. (X is for situations where shock is unlikely.)

I do not think your guitar amp needs or wants a line-cap, assuming you can provide a proper ground/earth outlet and cord. But if you must, those Y2 caps are made for the purpose and risk.

Hey PRR!  Wow, brilliant post..many thanks.

That's interesting background info on how this approach came about .  We definitely had 3 pin earthed mains outlets, over here, when this amp was made in '63/'64 -ish. Maybe the amp just had a twin mains flex,originally...it always intrigues me how so much valve gear , Ham Radio/test gear/hifi ,in metal cabinets came with twinflex and no earth.

There is already a 3 core mains flex installed, with the earth to chassis. Perhaps a later addition....Its all rather clumsy inside tho,and I need to neaten it up with new cable and a better earth stub.

..And those line to chassis caps will be removed immediately..I promise!  I can understand how they shouldn't be necessary in this case, and are really just a timebomb...

Much apprieciated!

Cheers

nEon.