Metal film cap ok in power supply ?

Almach · November 2017

I'm tidying up a recently acquired Prosonic. It needs a 4.7uF 500V electrolytic ( C203 ) but I can't find anything close to that. I've ordered a 4.7 uF 630V polyprop off ebay will this be OK ? It looks like the ESR , ripple current etc are better than than the old wet technology.

RiftAmps · November 2017

Noooooooo.

I have one in stock, DM your address and I'll send it out FoC.

EDIT: Is it axial, or radial?

ICBM · November 2017

Are you sure that's not a typo on the schematic for 47uF? That would be a much more normal value for a power supply cap there. Check the existing cap (if it's original). I haven't been inside a Prosonic for a long time so I can't remember…

If it is a 4.7uF and a non-electrolytic cap can be found in the right value and physical size with the same or better voltage rating then it will be fine to use.

ecc83 · November 2017

I think typo as well IC. Where is it in the circuit. MIGHT be a downstream decoupler in the pre amp circuit but if so pretty mean designer! Always used to be 8mfd but 10 is the minimum these days.

Dave.

ICBM · November 2017

ecc83 said:

I think typo as well IC. Where is it in the circuit.

According to the schematic it's the node 2 filter cap, immediately after the choke. Node 1 is 50uF (stacked 100s) and node 3 is a 47.

RiftAmps · November 2017

ICBM said:

If it is a 4.7uF and a non-electrolytic cap can be found in the right value and physical size with the same or better voltage rating then it will be fine to use.

In theory, yes.

The reason I screamed 'Noooooo' is that I've had two Prosonics across my bench in the past year, both with burnt out chokes and a shorted 4.7uf Poly in that position. Interestingly, both were recapped by the same tech. I now stock that value EC for this exact reason.

Can't explain it, but a 4.7uf 500v elec is cheaper than a choke replacement

ICBM · November 2017

RiftAmps said:

In theory, yes.

The reason I screamed 'Noooooo' is that I've had two Prosonics across my bench in the past year, both with burnt out chokes and a shorted 4.7uf Poly in that position. Interestingly, both were recapped by the same tech. I now stock that value EC for this exact reason.

So is that actually the stock value? And if so is it meant to be?! It wouldn't surprise me completely if it was a factory mistake, it seems like such an odd value to use there. Or the other tech was just going from the schematic.

I've only ever worked on a couple of Prosonics and unfortunately my memory is going a bit as to what was in them.

RiftAmps said:

Can't explain it, but a 4.7uf 500v elec is cheaper than a choke replacement

That is also why all amps should have an HT fuse!

Almach · November 2017

47uF ? Could be , the outline on the board suggest that. OTOH a GZ34 is only rated for 60uF cap load.

anyhoo, on to the pictures ...

Almach · November 2017

DJH83004 · November 2017

No the original design is def a 4.7uF axial, on your amp the main caps have also been replaced using radials with an extension on one leg, which I don't personally like but you do see quite often (mainly by people who haven't got the right bits to do the job

).

Image: https://irp-cdn.multiscreensite.com/87ee640a/dms3rep/multi/prosonic.PNG

RiftAmps · November 2017

ICBM said:

RiftAmps said:

In theory, yes.

The reason I screamed 'Noooooo' is that I've had two Prosonics across my bench in the past year, both with burnt out chokes and a shorted 4.7uf Poly in that position. Interestingly, both were recapped by the same tech. I now stock that value EC for this exact reason.

So is that actually the stock value? And if so is it meant to be?! It wouldn't surprise me completely if it was a factory mistake, it seems like such an odd value to use there. Or the other tech was just going from the schematic.

I've only ever worked on a couple of Prosonics and unfortunately my memory is going a bit as to what was in them.

RiftAmps said:

Can't explain it, but a 4.7uf 500v elec is cheaper than a choke replacement

That is also why all amps should have an HT fuse!

Thinking about it, I suppose 4.7uf could be a typo that was carried through to production. There's certainly no harm in upping the value to 22uf, or even 47uf.

crunchman · November 2017

RiftAmps said:

Thinking about it, I suppose 4.7uf could be a typo that was carried through to production. There's certainly no harm in upping the value to 22uf, or even 47uf.

Presumably that might make the amp marginally quieter as well?

jpfamps · November 2017

RiftAmps said:

ICBM said:

If it is a 4.7uF and a non-electrolytic cap can be found in the right value and physical size with the same or better voltage rating then it will be fine to use.

In theory, yes.

The reason I screamed 'Noooooo' is that I've had two Prosonics across my bench in the past year, both with burnt out chokes and a shorted 4.7uf Poly in that position. Interestingly, both were recapped by the same tech. I now stock that value EC for this exact reason.

Can't explain it, but a 4.7uf 500v elec is cheaper than a choke replacement

Many film caps aren't rated for continuous AC operation.

ICBM · November 2017

RiftAmps said:

Thinking about it, I suppose 4.7uf could be a typo that was carried through to production. There's certainly no harm in upping the value to 22uf, or even 47uf.

It would be interesting to find out what, if any, difference it makes to the sound.

If none I think it can be assumed to be a production error caused by a typo. If it does change the sound, then it's a deliberate choice.

jpfamps said:

Many film caps aren't rated for continuous AC operation.

There shouldn't be much AC on it in that position after the choke though.

I don't know what the ripple voltage will be, but I would guess less than the signal voltage in something like a phase inverter coupling cap position.

ecc83 · November 2017

Ref the two radial caps in series (?) I suspect they are 10mfd at well under 500V and therefore should have had equalizing resistors across them. I agree ALL amps should have HT fuses and HT bleed resistors are very, very desirable.

Re foil V electrolytic? One place where you DON'T want to replace electros' with foil is a speaker crossover network. Even if the value is spot one the frequency response goes to ***T!

Dave.

Almach · November 2017

Apparently Carr use metal film throughout. Mallory 150 in the signal chain and Solen in the PS.

I've got a 22uF@500V that is going in, my cloth ears won't notice the difference.

A recent sticker inside the amp suggests if you live in essex amp repair standards can be variable. I would be mighty ticked off if somebody did a hack job like this on my amp !

Modulus_Amps · November 2017

Poly Film caps are used in lots of amps power supplies and a number of high gain amps in parallel with the electrolytics, there is perhaps something in this amp that causes them to blow though.

I think I have some 4.7uf F&T caps around, you can certainly get them easy enough to avoid going the poly route.

@Almach If that was my amp I would pull the board and do the job properly

jpfamps · November 2017

ICBM said:

RiftAmps said:

Thinking about it, I suppose 4.7uf could be a typo that was carried through to production. There's certainly no harm in upping the value to 22uf, or even 47uf.

It would be interesting to find out what, if any, difference it makes to the sound.

If none I think it can be assumed to be a production error caused by a typo. If it does change the sound, then it's a deliberate choice.

jpfamps said:

Many film caps aren't rated for continuous AC operation.

There shouldn't be much AC on it in that position after the choke though.

I don't know what the ripple voltage will be, but I would guess less than the signal voltage in something like a phase inverter coupling cap position.

There will be some ripple current though.

Alternatively they could be fake caps!

ICBM · November 2017

True!

Almach · November 2017

OK caps done, my 25W Antex struggled with those ground planes !

First thing I did was checked the bias. Switch to SS rectification , 474V on the CT and err ... 474V on both anodes. OT primaries measure around 55 Ohms each. Bias pot is roughly central and there's -51V on the grid.
Looking closer someone has added two 1.5K resistors in series with the grids of the output valves, Those 220Ks are fed from the bias pot via the brown wire.

ICBM · November 2017

Those resistors should be at the valve socket ends of the green and purple wires not the PCB end.

Does the amp sound any different with the 22uF cap?

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Metal film cap ok in power supply ?

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