cathode caps

brianqq · 16th September 2011, 06:43 PM

I have a question about preamp tube cathode caps on the stage before a cathode follower.
Should these caps be electrolytic? I have seen a lot of schematics with no indication, and others that show them as not electrolytic.
What do you think?

artosalo · 16th September 2011, 07:03 PM

If a cathode by-pass capacitor is used, it should be big, some 100 uF or more.
Then it is natural to use electrolytics.
In most cases at preamps, no cathode by-pass capacitors are needed at all.

DF96 · 16th September 2011, 08:08 PM

If a cathode bypass is needed, it may need to be big. If it needs to be big, then it probably will need to be an electrolytic and then it must be big because you should not use an electrolytic to set the LF rolloff.

brianqq · 16th September 2011, 08:33 PM

I have attached the schematic I am working on, it's a mix of the Altec 1566 and a tone buffer from anglefire. Ignore the power supply, this schematic has been modified since I printed it.

I have looked at many schematics and they almost all have caps on the cathodes. the one I am concerned about is C10 on V1B.
I would assume that it would be electrolytic due to it's size, and you stated the big ones are electrolytic.

Let me know what you think

DF96 · 16th September 2011, 08:49 PM

Yes, if present it needs to be 10's of uF so an electrolytic is needed.

Alternatively, you could try omitting it. As this stage provides the loop gain for a Baxandall active tone control you might get away with less. It is followed by a CF so the anode load resistance will be high enough. So, electrolytic or miss it out.

brianqq · 16th September 2011, 08:53 PM

The good thing is, it is easy to swop in and out for experimentation

Thanks
Brian

artosalo · 17th September 2011, 06:51 AM

You can also omit C19 and get better linearity.
R28 should be some 100...120 kohms.

ruffrecords · 17th September 2011, 09:13 AM

Quote:

Originally Posted by brianqq

I have looked at many schematics and they almost all have caps on the cathodes. the one I am concerned about is C10 on V1B.
Let me know what you think

An electrolytic there would be normal.

Cheers

Ian

DualTriode · 17th September 2011, 02:22 PM

Hello All,

If you are torn about using a cathode bypass capacitor or not try this thing that I sometimes do.
Place a 10 ohm resistor in series with and between the tube cathode and the capacitor, this will add to the ESR of the capacitor and will reduce the amount of bypass slightly. The resistor is the great equalizer among capacitors. The resistor swamps the variable ESR behavior among electrolytic capacitors.

Get fancy, adjust the value of the series resistor to adjust the gain and adjust the degree of feedback at the tube cathode.

Get technical, add Fast Fourier Transform to adjust the THD and the ratio of H2 to H3 harmonics.

DT

DF96 · 17th September 2011, 02:33 PM

C19 boosts signals above 60kHz. Are you sure 5nF is the correct value?

16th September 2011, 06:43 PM	#1
brianqq diyAudio Member Join Date: Apr 2011	cathode caps I have a question about preamp tube cathode caps on the stage before a cathode follower. Should these caps be electrolytic? I have seen a lot of schematics with no indication, and others that show them as not electrolytic. What do you think?

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16th September 2011, 07:03 PM	#2
artosalo diyAudio Member Join Date: Feb 2010	If a cathode by-pass capacitor is used, it should be big, some 100 uF or more. Then it is natural to use electrolytics. In most cases at preamps, no cathode by-pass capacitors are needed at all.

16th September 2011, 08:08 PM	#3
DF96 diyAudio Member Join Date: May 2007	If a cathode bypass is needed, it may need to be big. If it needs to be big, then it probably will need to be an electrolytic and then it must be big because you should not use an electrolytic to set the LF rolloff.

16th September 2011, 08:49 PM	#5
DF96 diyAudio Member Join Date: May 2007	Yes, if present it needs to be 10's of uF so an electrolytic is needed. Alternatively, you could try omitting it. As this stage provides the loop gain for a Baxandall active tone control you might get away with less. It is followed by a CF so the anode load resistance will be high enough. So, electrolytic or miss it out.

16th September 2011, 08:53 PM	#6
brianqq diyAudio Member Join Date: Apr 2011	The good thing is, it is easy to swop in and out for experimentation Thanks Brian

17th September 2011, 06:51 AM	#7
artosalo diyAudio Member Join Date: Feb 2010	You can also omit C19 and get better linearity. R28 should be some 100...120 kohms.

17th September 2011, 02:22 PM	#9
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello All, If you are torn about using a cathode bypass capacitor or not try this thing that I sometimes do. Place a 10 ohm resistor in series with and between the tube cathode and the capacitor, this will add to the ESR of the capacitor and will reduce the amount of bypass slightly. The resistor is the great equalizer among capacitors. The resistor swamps the variable ESR behavior among electrolytic capacitors. Get fancy, adjust the value of the series resistor to adjust the gain and adjust the degree of feedback at the tube cathode. Get technical, add Fast Fourier Transform to adjust the THD and the ratio of H2 to H3 harmonics. DT

17th September 2011, 02:33 PM	#10
DF96 diyAudio Member Join Date: May 2007	C19 boosts signals above 60kHz. Are you sure 5nF is the correct value?

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