Poly caps in place of electrolitcs?

[AndrewT]

I have recommended the addition of a poly cap across the electrolytic in my amps since day one. The electrolytic provides the bulk energy storage and the poly cap's low ESR provides for a low impedance power supply at the higher frequencies.

What is a good value cap would you recommend for this? Or would any small value polypro cap work?

Wrong location for a supply rail decoupling capacitor.
Wrong type of capacitor for a supply rail decoupling capacitor.

The decoupling capacitor should be one with an inherent esr that is high enough to damp ringing on the rail if there is a step response/demand on the rail.
The capacitor that supplies the current to meet that step demand should be next to the device that demands that step change in current.

 

[AndrewT]

I don't think that ALL motor start capacitors are polarised electrolytics.
You would need to read the datasheet to confirm that they are, or aren't, electrolytic.

 

[Triodethom]

Are these the Clarity Cap TC 300uF/600V (U$88) at Parts Conn? or something else entirely?

More than you need to spend smaller motor run caps say 40 to 50 mfd at voltage for about 1/4 the cost or use 4 of them more than enough mfd and lower esr which measure very low to the point of 3 inch (75 mm) has more esr than the cap at 100k . The Clarity caps look great have not measured them yet. They are more than large enough for power amp so I would not say no to them without measuring them and listening to them.

 

[Tubelab_com]

I don't think that ALL motor start capacitors are polarised electrolytics.

No, they aren't. In many industrial and other critical applications ALL capacitors connected directly to the AC line are poly, or other dielectric material rated for line operation.

The average start caps found on Ebay are however electrolytic. These are the type that provide a temporary phase shift for a small AC motor like a pool or well pump for starting, and then are switched off of the circuit by mechanical contacts once the motor is up to speed. They only see a few seconds of use at motor start up, yet are the reason for most motor failures. The cap goes open and the motor doesn't spin. It cooks itself until the thermal breaker pops. The cycle repeats until the operator notices the failure or the motor fries. These are NOT wanted in tube amps, and typically spill their guts (through the vent or worse) when subjected to constant DC voltage.

You would need to read the datasheet to confirm that they are, or aren't, electrolytic.

If there is one available, yes. Ebay caps don't always have a data sheet, but often the picture tells the story....If the seller is honest and actually understands the difference. If not, go for the caps in a metal can. They are usually poly. Note: many are poly in oil. The older ones may be paper in oil...that may contain PCB's, avoid caps older than 20 years.

The capacitor that supplies the current to meet that step demand should be next to the device that demands that step change in current.

In a simple unregulated tube amp supply it is usually acceptable to use a low ESR cap right at the B+ feed to the OPT's. All other loads are decoupled from this point.

Granted this is a simplistic approach, but it works well in the SE Tubelab amps, and the SPP. Yes, each case should be analyzed for signs of ringing on transients and overload recovery, but not every builder has a good scope.

A single large (around 100 uF) poly cap or a single decent quality electrolytic (in the 100 uF range) in parallel with a GOOD poly cap in the 20 to 100 uF range for the output cap, along with a single electrolytic with some ESR for the input cap, is a good "blanket solution" for these amps. It may not be the optimum solution for all amps without testing.

The typical audiophile solution of several different types and sizes of caps in parallel, is an invitation for instability. So is applying a low ESR cap across the output of a regulated power supply. Many regulators can not deal with a zero at DC.

 

[Triodethom]

No, they aren't. In many industrial and other critical applications ALL capacitors connected directly to the AC line are poly, or other dielectric material rated for line operation.

The average start caps found on Ebay are however electrolytic. These are the type that provide a temporary phase shift for a small AC motor like a pool or well pump for starting, and then are switched off of the circuit by mechanical contacts once the motor is up to speed. They only see a few seconds of use at motor start up, yet are the reason for most motor failures. The cap goes open and the motor doesn't spin. It cooks itself until the thermal breaker pops. The cycle repeats until the operator notices the failure or the motor fries. These are NOT wanted in tube amps, and typically spill their guts (through the vent or worse) when subjected to constant DC voltage.



If there is one available, yes. Ebay caps don't always have a data sheet, but often the picture tells the story....If the seller is honest and actually understands the difference. If not, go for the caps in a metal can. They are usually poly. Note: many are poly in oil. The older ones may be paper in oil...that may contain PCB's, avoid caps older than 20 years.



In a simple unregulated tube amp supply it is usually acceptable to use a low ESR cap right at the B+ feed to the OPT's. All other loads are decoupled from this point.

Granted this is a simplistic approach, but it works well in the SE Tubelab amps, and the SPP. Yes, each case should be analyzed for signs of ringing on transients and overload recovery, but not every builder has a good scope.

A single large (around 100 uF) poly cap or a single decent quality electrolytic (in the 100 uF range) in parallel with a GOOD poly cap in the 20 to 100 uF range for the output cap, along with a single electrolytic with some ESR for the input cap, is a good "blanket solution" for these amps. It may not be the optimum solution for all amps without testing.

The typical audiophile solution of several different types and sizes of caps in parallel, is an invitation for instability. So is applying a low ESR cap across the output of a regulated power supply. Many regulators can not deal with a zero at DC.

George you covered all the important points . How the house coming ?

 

[Tubelab_com]

The shell is built, interior sheetrock done, painting and cabinet work in progress. Expected completion date, Mar 1.

 

[bigfishe]

Any of you guys have any mods you think I should try on these amps?

 

[Triodethom]

Any of you guys have any mods you think I should try on these amps?

Schematic and current stage of build will help answer your question.

 

[bigfishe]

 

[bigfishe]

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[Triodethom]

I would run simulation on the phase splitter take the 12bh7 over au7 any day. You will find that the resistor need to be offset not equal to get the same freq. response in each half . bias supply should be full wave with first cap a muse or silmic II second after 10k resistor a polyprop. polyprop power supply cap ( motor run ) close to the output transformer. That is what I would do for a start . then play with input load resistor . I take it you have a supply of 8417 ?

 

[bigfishe]

Just a few!