What is the effect of too small a decoupling cap?

[Tre' Perry]

I just changed things in my 6sn7 pre-amp and am wondering if the
decoupler capacitors (not cathode resistor bypass caps, but Cd, just
before the top of the plate resistor) might be too small.
thanks Tre'

[Patrick Turner]

Tre' Perry wrote:

> I just changed things in my 6sn7 pre-amp and am wondering if the
> decoupler capacitors (not cathode resistor bypass caps, but Cd, just
> before the top of the plate resistor) might be too small.
> thanks Tre'

Do you mean the electro caps from rail supply to ground?

These should never be changed to a lower value because
it may cause the amp to oscillate at low frequency,
because the amp becomes a phase shift oscillator.
When the oscillations are fast enough, it sounds like a
motorboat, hence the term motorboating.

If changing these caps, aleways go to a higher value.

The caps which are being charged straight off the power supply
rectifiers should not be changed to a higher value if there is a tube
rectifier unless the rise in value is still below the
safe value limit for the tube rectifier used.

Be careful when changing coupling caps between stages in a preamp,
since a change can lead to motorboating.

Patrick Turner.

[Paul D. Spiegel]

I'll assume that you are asking about a power supply cap and that
it's not the first cap after the rectifier.

If there is any AC 'ripple' at the plate resistor in a single
ended circuit then this can be heard as hum. A larger cap could
eliminate this.

A larger cap may also improve the voltage regulation of the power
supply. This could be audible as improved bass response and a
general improvement in linearity.

As with all cases, the effect is dependent on the application and
the overall circuit. If you want to experiment, you could always
increase the size of the cap value and determine if you can hear a
difference.

Have fun!

[Fred Nachbaur]

With that much filtering, and with each stage decoupled, I doubt that
the capacitor value would be very critical.

If you get oscillation or other signs of instability, increase the
value. Observing a square-wave test on an oscilloscope can sometimes
reveal such instabilities, for example "ringing" on the leading edge, or
other artifacts.

Cheers,
Fred

Tre' Perry wrote:
> right now the supply is 10H>80mf>10H>250mf>10H>530mf
> then from that B+ point I have a resistor to each of 6 stages
> each resistor is followed by the "decoupling cap"
> it's this decoupling cap that I'm asking about
> Thanks Tre'
>
> "Paul D. Spiegel" wrote:
>
>
>> I'll assume that you are asking about a power supply cap and that
>> it's not the first cap after the rectifier.
>>
>> If there is any AC 'ripple' at the plate resistor in a single
>> ended circuit then this can be heard as hum. A larger cap could
>> eliminate this.
>>
>> A larger cap may also improve the voltage regulation of the power
>> supply. This could be audible as improved bass response and a
>> general improvement in linearity.
>>
>> As with all cases, the effect is dependent on the application and
>> the overall circuit. If you want to experiment, you could always
>> increase the size of the cap value and determine if you can hear a
>> difference.
>>
>> Have fun!
>
>

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