capacitor upgrade question

jaskah · December 2019

hello

somewhat of a newbie question.

i wanted to upgrade the capacitors in my amp. one of the capacitors is currently rated at 350 volt 10uF. the capacitor i wanted to upgrade this with is only available in a 500 volt 10uF rating.

my question: will this be a problem for the operation of the amp, using the 500 volt 10uF capacitor instead of a 350 volt 10uF?

thanks for any help!

ecc83 · December 2019

Not in the slightest but you would never go the other way! Only possible issue could be that the 500V component is bigger but not likely, modern caps are way smaller than those of 20 years ago of equivalent value and voltage.

But, you say "upgrade"? If the 10uf has failed fine but, contrary to popular belief, caps last for 20 years and more, depending on location and equipment of course.

Dave.

jaskah · December 2019

thanks for your reply. the 500 V cap will indeed be a bit larger, but i'll have enough room to install this.

the reason for the upgrade: not because the old cap failed but because someone suggested i could reduce some of the dc ripple hum i'm experiencing by switching my current illinois capacitor brand caps out for f&t caps.

jason

ecc83 · December 2019

jaskah said:

thanks for your reply. the 500 V cap will indeed be a bit larger, but i'll have enough room to install this.
the reason for the upgrade: not because the old cap failed but because someone suggested i could reduce some of the dc ripple hum i'm experiencing by switching my current illinois capacitor brand caps out for f&t caps.
jason

I very muc h doubt it but best of!

Dave.

jpfamps · December 2019

jaskah said:

thanks for your reply. the 500 V cap will indeed be a bit larger, but i'll have enough room to install this.
the reason for the upgrade: not because the old cap failed but because someone suggested i could reduce some of the dc ripple hum i'm experiencing by switching my current illinois capacitor brand caps out for f&t caps.
jason

Unlikely there will be much if any effect on the hum.

If you are really concerned about this, then using a higher voltage cap than necessary is counter productive as a higher voltage rated cap will have higher equivalent series resistance (ESR) which is the main parameter that determines the quality of caps with respect to reducing ripple; a higher ESR means less ripple is shunted to ground through the cap.

Regardless, I'd be surprised if you hear than much difference, assuming that the old cap is working correctly.

ecc83 · December 2019

jpfamps said:

jaskah said:

thanks for your reply. the 500 V cap will indeed be a bit larger, but i'll have enough room to install this.
the reason for the upgrade: not because the old cap failed but because someone suggested i could reduce some of the dc ripple hum i'm experiencing by switching my current illinois capacitor brand caps out for f&t caps.
jason

Unlikely there will be much if any effect on the hum.

If you are really concerned about this, then using a higher voltage cap than necessary is counter productive as a higher voltage rated cap will have higher equivalent series resistance (ESR) which is the main parameter that determines the quality of caps with respect to reducing ripple; a higher ESR means less ripple is shunted to ground through the cap.

Regardless, I'd be surprised if you hear than much difference, assuming that the old cap is working correctly.

Too true. Duggy Self has done extensive tests on capacitors, both electrolytic and foil and although the former do produce measurable distortion it is still low by guitar amp standards. There are good and bad foil types as well but we are talking two decimal places for distortion here instead of three!

In an HT filter circuit, forget it.

Dave.

ICBM · December 2019

Don’t bother unless the cap has clearly failed. That said, if you don’t have any means of testing it outside the amp, it’s often easiest to just try a different one.

What position in the circuit is this cap? It’s likely that increasing the value will make far more of a difference than a different brand, if the cap is the problem at all.

jaskah · December 2019

thanks everyone for their input on this.

so, i'm getting the impression from all the replies here that this might be a bad idea.

but just to better understand how this works with reducing dc ripple: the more ripple that's shunted to ground, the less one will hear in the amp? therefore, capacitors with higher equivalent series resistance will result in more audible dc ripple? and for this reason, replacing a 350 volt 10uF cap with a 500 volt 10uF cap would actually result in more dc ripple being audible?

i know someone with the exact same amp i have and they seemed to think that there was an audible difference when they swapped the caps out.

jason

ICBM · December 2019

What amp is it and which cap is it? 10uF is quite a low value for any cap in a HT filter chain - even in most vintage amps the lowest is normally 8uF (more or less the same thing, given cap tolerances), and these were usually chosen for cost reasons when caps were much more expensive relative to their values than they are now. Almost all manufacturers increased the values when prices and sizes came down.

RiftAmps · December 2019

What makes you think that that hum is DC ripple? Have you measured its frequency? Have you scoped the HV-rail?

Are you sure it's not a ground loop and/or crosstalk?

jaskah said:

but just to better understand how this works with reducing dc ripple: the more ripple that's shunted to ground, the less one will hear in the amp? therefore, capacitors with higher equivalent series resistance will result in more audible dc ripple? and for this reason, replacing a 350 volt 10uF cap with a 500 volt 10uF cap would actually result in more dc ripple being audible?

Here is a proper explanation - http://www.valvewizard.co.uk/smoothing.html

jaskah · December 2019

ICBM said:

What amp is it and which cap is it?

the amp is a vintage 47 mini:

http://www.vintage47amps.com/The-Mini/

What makes you think that that hum is DC ripple? Have you measured its frequency? Have you scoped the HV-rail?
Are you sure it's not a ground loop and/or crosstalk?

the only reason i think it's dc ripple is because someone else i know with this amp seemed to think that; and that by changing the capacitors they found the amp to be quieter.

i did have the amp looked at by an amp tech once, but he didn't seem to think there were any issues with ground loop, cross talk or even dc ripple.

the amp sounds good, but turned up at higher volume it becomes in my opinion somewhat noisy. but maybe all 5 watt amps when cranked get this way. this is my first amp of this sort.

thanks.

thermionic · December 2019

Nothing to do with the wattage per se, but single-ended power stages are inherently noisier than ones with push-pull stages.

Keefy · December 2019

I remember reading that single-ended amps can be more prone to hum than push-pull designs but I can’t remember why that was!

Can you tell whether the hum is at 50Hz (possibly heater supply) or 100Hz (suggests HT)?

ICBM · December 2019

jaskah said:

the amp sounds good, but turned up at higher volume it becomes in my opinion somewhat noisy. but maybe all 5 watt amps when cranked get this way. this is my first amp of this sort.

Single-ended amps always hum more than push-pulls because there is no inherent ripple cancellation in the output transformer.

I'm not familiar with that amp but having looked at pics, at a guess it will be something like a 5C2 Princeton circuit, in which case both the first two filter caps would be 8 or 10uF. I would definitely try increasing the value of either the first or both to 16uF or 22uF.

If you don't want to modify the amp until you're sure, you could temporarily attach an extra 10uF cap in parallel with one of them using croc clip leads, or just temporarily wrap the leads of the new cap around the existing one - be very careful to discharge them fully before connecting or disconnecting, the simplest way to do that is pull the rectifier valve and turn the amp back on for a minute or so.

ecc83 · December 2019

jaskah said:

ICBM said:

What amp is it and which cap is it?

the amp is a vintage 47 mini:
http://www.vintage47amps.com/The-Mini/

What makes you think that that hum is DC ripple? Have you measured its frequency? Have you scoped the HV-rail?
Are you sure it's not a ground loop and/or crosstalk?

the only reason i think it's dc ripple is because someone else i know with this amp seemed to think that; and that by changing the capacitors they found the amp to be quieter.

i did have the amp looked at by an amp tech once, but he didn't seem to think there were any issues with ground loop, cross talk or even dc ripple.
the amp sounds good, but turned up at higher volume it becomes in my opinion somewhat noisy. but maybe all 5 watt amps when cranked get this way. this is my first amp of this sort.

thanks.

Ten microfarads is about adequate for smoothing to the input triode but a bit mean. Usually 22mfd or 47mfd is used.

But the usually 'ripple' problem with single ended amps is the output stage and that would be a constant hum regardless of volume setting.

I am not convinced changing that cap' will make any difference at all.

Dave.

jaskah · December 2019

i'm attaching a copy of the circuit diagram.

I'm not familiar with that amp but having looked at pics, at a guess it will be something like a 5C2 Princeton circuit, in which case both the first two filter caps would be 8 or 10uF. I would definitely try increasing the value of either the first or both to 16uF or 22uF.

But the usually 'ripple' problem with single ended amps is the output stage and that would be a constant hum regardless of volume setting.

this is not a constant hum, but a noise in the amp when turned up beyond a certain point. at around 9 o'clock volume the amp is very quiet. at 12 o'clock still usable but noisier. at 3 o'clock noticeably noisy.

Image: http://jasonkahn.net/images/pix/mini_schematic.jpeg

and i believe that the values of the capacitors in my build of this circuit are actually illinois capacitors 22/500, 10/500, 10/350. i wanted to swap these out with f&t caps at the same values except with the 10/350 going to 10/500 (as f&t doesn't offer a cap in the 10/350 value).

Keefy · December 2019

ICBM said:

Single-ended amps always hum more than push-pulls because there is no inherent ripple cancellation in the output transformer.

...

Yes that was it, thanks!

jaskah said:

i'm attaching a copy of the circuit diagram.

Ooh, an old Valco design!

This would have been a student model amp for home use, so there would be no expectation that it would have low background noise when cranked.

Modulus_Amps · December 2019

More filtering on the first filter stage would help, you can go up to 30uf safely with the 5Y3 rectifier (you can fit a 22uf cap in parallel with the existing cap to test.

I effing hate hum in amps, you can kill all power supply hum in this amp by adding another filter stage and dropping resistor between the 5y3 rectifier and the first filter stage.

That is an interesting choice of bias resistor on the power valve, and it is not bypassed???

If the noise increases with the volume control is suggests the 6SL7 is noisy or poor grounding in the amp. can you post gut shots of what the amp looks like inside?

jaskah · December 2019

Ooh, an old Valco design!

This would have been a student model amp for home use, so there would be no expectation that it would have low background noise when cranked.

yes, this might explain the whole situation. but i'd still like to try and optimize the sound, if at all possible without changing the circuitry.

If the noise increases with the volume control is suggests the 6SL7 is noisy or poor grounding in the amp. can you post gut shots of what the amp looks like inside?

the original J&J 6SL7 has been replaced with a sylvania 6SL7WGT. this actually seemed to quiet down the background noise a bit.

for what it's worth, here's a gut shot of the amp.

Image: http://jasonkahn.net/images/pix/PC132074.JPG

i was basically just thinking of replacing the IC caps you see here in the photo with f&t caps of the same values (except for the 10/350, which i can only find in 10/500 from f&t). someone else with this same amp did this cap switch and seemed to think it helped quiet down the amp a bit.

as i said before, the amp sounds great. i'd just like to quiet it down a bit for recording purposes.

and here again is the original circuit diagram:

thanks for any help.

jason

ICBM · December 2019

I notice the filament winding for the 6V6 and 6SL7 appears to have a centre tap (the green/yellow wire) which is not grounded but heatshrinked off - how is the filament loom grounded, via one side of the 6SL7 as the schematic shows? If so it will probably reduce hum to remove that and use the centre tap instead. The original was almost certainly only done like that to save cost on the transformer.

If you're doing that, ideally you should move both it and the HT centre tap (red/yellow) to a grounded tag on one of the transformer bolts, and the mains earth to a separate tag that isn't used for anything else. That won't affect the noise but is more compliant with electrical safety regs.

jaskah · December 2019

ICBM said:

I notice the filament winding for the 6V6 and 6SL7 appears to have a centre tap (the green/yellow wire) which is not grounded but heatshrinked off - how is the filament loom grounded, via one side of the 6SL7 as the schematic shows? If so it will probably reduce hum to remove that and use the centre tap instead. The original was almost certainly only done like that to save cost on the transformer.

If you're doing that, ideally you should move both it and the HT centre tap (red/yellow) to a grounded tag on one of the transformer bolts, and the mains earth to a separate tag that isn't used for anything else. That won't affect the noise but is more compliant with electrical safety regs.

thanks for your input. i'll definitely keep this in mind if i have any more work done on the amp. but for now, i'm mainly interested in just reducing the background noise a bit through this theoretical cap swap.

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