Cathode bypass capacitor

RE: Cathode bypass capacitor, posted on July 26, 2014 at 08:20:44
tubular.well Dealer Posts: 223 Joined: March 4, 2013	It depends on your meaning of "better".

No., posted on July 26, 2014 at 08:28:18

Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012

All a cathode bypass cap does is eliminates the cathode feedback. Once you are below 5 hz or so, using a larger cap is counter productive. You do not want more of a 1 hz signal in your amp than you have to have. The quality of the cap is important as well. A 1K cathode resistor bypassed with a 50uf cap will have a cutoff frequency of 3.2 hz. With a 150 ohm cathode resistor, you are looking at a 500uf cap. Larger is not better!

RE: No., posted on July 26, 2014 at 09:23:16
Tubenstein Audiophile Posts: 251 Joined: August 5, 2011	The resistor value is 270R, so the cap value....??

RE: No., posted on July 26, 2014 at 10:42:45

Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000

Without checking Chip's calculated values, it would be around 250uF, for the 5 Hz cut-off he chose. The simplest way to look at it is that the values of Rk and Ck multiplied together should be a constant for a given desired cut-off frequency. Note that 50uF*1000R is roughly equal to 500uF*150R (the numbers Chip chose; obviously you could use a 450uF or 400uF cap with the 150R resistor for a result closer to that which you get with Rk = 1000R, or you could use a 100R resistor with 500uF).

RE: No., posted on July 26, 2014 at 10:53:02

Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006

The value of the cathode bypass cap is determined by more than just frequency response. Things like the class of operation, topology, and even the designer's philosophy regarding issues such as overload recovery must be considered. That aside, typical values to bypass a 270 ohm cathode resistor in a AB1 PP amplifier would be 47-100uF. The exact value isn't critical, as you will see if you research schematics of commercial equipment online. Note that while the cathode should have sufficient bypassing so as not to create significant degeneration or phase shift within the audio passband, frequency response is usually determined during the design phase primarily at the grid.

What Triode said... what I gleaned from Radiotron handbook 3rd/4th edition~nT, posted on July 26, 2014 at 14:15:45
Cleantimestream Audiophile Posts: 7551 Location: Kentucky Joined: June 30, 2005	~! The Mind has No Firewall~ U.S. Army War College.

RE: No., posted on July 26, 2014 at 23:49:07
danlaudionut Audiophile Posts: 5480 Location: Schenectady Joined: June 6, 2002	TK Andy has a input 6GK5 CCS loaded with a 270R cathode resistor. That makes a 120uFd for 5Hz F3. DanL

RE: Cathode bypass capacitor, posted on July 26, 2014 at 23:59:10

danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002

This film cap will sound better than ANY electrolytic.

DanL

http://www.ebay.com/itm/100uF-100V-POLYPROPYLENE-FILM-MKT-AR

I Try to Avoid Bypass Caps if possible, posted on July 27, 2014 at 03:29:52
Maxamillion Audiophile Posts: 856 Location: New Jersey Joined: May 26, 2006	For small signal applications, cathode bias using diodes (such as Cree SiC Schottky types) sounds far better to me than a resistor and cap. For larger bias values I find fixed bias to be much cleaner sounding.

You can avoid it with filament bias, posted on July 27, 2014 at 06:49:42
andy evans Audiophile Posts: 4382 Joined: October 20, 2000	If you use appropriate DHTs you can avoid the cathode bias cap by running the filament through the cathode resistor. With the extra current you end up with a small cathode resistor (like 10 to 50 ohms) with no need to bypass. Tubes often used with filament bias are 4P1L, 26, 01A, 10Y.

RE: Cathode bypass capacitor, posted on July 27, 2014 at 10:09:49

Thorsten
Manufacturer

Posts: 4209
Location: Somewhere nice on planet dirt
Joined: September 25, 1999

Hi,

I like "infinite value" capacitors for cathode bypass.

Okay, a piece of wire.

Just use a primary battery of suitable voltage in series with the grid. I normally put a 10M resistor and a 10n Silver Mica cap in parallel with the battery (some variation there willnot be the end of the world). Using the right battery you get "shelf-life" for the battery, so change every few years...

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

RE: No., posted on July 27, 2014 at 11:04:36

Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009

The mention of overload recovery above deserves some attention. In the context of guitar amps that are frequently driven into distortion, a too-large cathode bypass will result in very unpleasant sounds during the time it takes for the output stage (for example) to return to normal operating points following a momentary overload. It has to do with the time constant of the resistor and capacitor. If the cathode bypass is smaller, the recovery may be barely noticeable.

Of course, we don't want HiFi amps to be driven into distortion, but a very large cathode bypass can make the amp more fussy about transient overloads. Some bandwidth limiting at the lower end could also make an amp less susceptible to subsonic signals like record warps.

It's usually not that difficult to experiment with different cathode bypass values to determine which one sounds best. Use your ears :-)

I'll second that!, posted on July 27, 2014 at 11:07:38
wheezer Audiophile Posts: 4309 Joined: January 24, 2001	For signal tubes, in amps and preamps, diodes on the cathode here as well. I've found the same sonic benefits as you have.

RE: No., posted on July 27, 2014 at 11:47:45
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	A class AB1 push-pull amplifier should not have a cathode resistor at all beyond something for current sensing.

RE: I Try to Avoid Bypass Caps if possible, posted on July 27, 2014 at 11:48:58
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	Fixed bias still has a big cap in the signal current loop, it has just moved to a different location.

RE: You can avoid it with filament bias, posted on July 27, 2014 at 11:49:26
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	So you don't have a capacitor across the last node of your filament bias power supply?

RE: I Try to Avoid Bypass Caps if possible, posted on July 27, 2014 at 14:20:36
Maxamillion Audiophile Posts: 856 Location: New Jersey Joined: May 26, 2006	Not if you put a CCS-fed VR tube regulator between that cap and the tube! That's what I do in my preamp, then ground the cathode and put a 9V battery feeding bias voltage to the grid. Very clean.

It is often done with a current source as the last element, posted on July 27, 2014 at 14:22:29
Chip647 Audiophile Posts: 2653 Location: The South Joined: December 24, 2012	The last thing you do is out a current regulator like a TO3 317 or the LT1085 as the last thing, no cap afterward. On the other hand, having only a 10 to 50 ohm resistor does not generate hardly any feedback to cancel out with a cap.

RE: I Try to Avoid Bypass Caps if possible, posted on July 27, 2014 at 15:44:10
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	I was referring to the power supply cap in the negative voltage supply for fixed bias. If you have a 9V battery on the grid, then you get to add a coupling cap at the input of the stage. There is no free lunch.

Rod Coleman regs., posted on July 27, 2014 at 15:53:25
andy evans Audiophile Posts: 4382 Joined: October 20, 2000	The filament raw supply with choke input and large cap feeds a Rod Coleman regulator stage.

Nope, no coupling cap with the battery bias, posted on July 27, 2014 at 17:41:05
Maxamillion Audiophile Posts: 856 Location: New Jersey Joined: May 26, 2006	The battery feeds through an input transformer (LL1676), so no need for a coupling cap. See schematic. The battery is bypassed with a 10nF Multicap - the other cap and resistor shown are an RC network across the transformer secondary to snub any ringing. Made a big difference in clarity versus cap-coupled, which I tried first.

RE: Nope, no coupling cap with the battery bias, posted on July 27, 2014 at 18:50:55
Triode_Kingdom Audiophile Posts: 10049 Location: Central Texas Joined: September 24, 2006	"Made a big difference in clarity versus cap-coupled" Capacitor coupling suffers from a much longer recovery time following a transient overload than does IT or direct coupling. If it wasn't so difficult (and expensive), I'd design every stage for A2 and be done with it.

Naz' idea to use a Schottky for cathode bias,..., posted on July 27, 2014 at 20:50:50
Lew Audiophile Posts: 10912 Location: Bethesda, Maryland Joined: December 11, 2000	and I am doing that in a phono stage with great success.

"I find fixed bias to be much cleaner sounding" Interesting... I find it more like ... Solid scrape soundingnT, posted on July 28, 2014 at 02:29:31
Cleantimestream Audiophile Posts: 7551 Location: Kentucky Joined: June 30, 2005	~! The Mind has No Firewall~ U.S. Army War College.

RE: "I find fixed bias to be much cleaner sounding" Interesting... I find it more like ... Solid scrape soundingnT, posted on July 28, 2014 at 05:58:06
Maxamillion Audiophile Posts: 856 Location: New Jersey Joined: May 26, 2006	We're all different, I guess.

If it wasn't so difficult (and expensive), I'd design every stage for A2 and be done with it" Hear here~nT, posted on July 28, 2014 at 08:41:44
Cleantimestream Audiophile Posts: 7551 Location: Kentucky Joined: June 30, 2005	~! The Mind has No Firewall~ U.S. Army War College.

The formula:, posted on July 28, 2014 at 09:39:24

Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002

F = 1,000,000/R x C x 2PI

where

F is the Frequency of cutoff (-3 db point) in Hz.
R is the resistor in ohms
C is the cap in *microfarads*
2PI is 6.28

Normally the formula is shown with a 1 instead of the 1 million but this way solves for actual values so its a little easier to use. I would look at the timing constants in the rest of the amp- the cathode bypass should be about the same or a little lower then the timing constant in the succeeding stage of gain. If you are dealing with an output transformer I would set it at the same value as the cutoff spec of the output transformer (IOW if specc'ed do 5 Hz set the cutoff at 5Hz).

In a push-pull output section, I would not use a bypass cap- the differential effect of the cathode resistor will help with the push-pull operation.

RE: The formula:, posted on July 28, 2014 at 09:52:58

danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002

Ralph

Sorry it is not that simple ...
You need to take into account the impedance
of the tube section that is in parallel
with the cathode resistor.

From the linked website -

It can be shown that the resistance seen looking into the cathode (Rk unbypassed) is:

Rk' = (Rp+ra)/(mu+1)

Therefore, the total cathode resistance, or the output impedance if you take the signal off the cathode, is the parallel combination of the cathode resistance, Rk', and the cathode resistor, Rk, as below:
R = Rk' || Rk

DanL

http://www.aikenamps.com/index.php/designing-common-cathode-

RE: Naz' idea to use a Schottky for cathode bias,..., posted on July 28, 2014 at 10:21:01
wheezer Audiophile Posts: 4309 Joined: January 24, 2001	I've not looked back.

RE: The formula:, posted on July 28, 2014 at 10:46:20

Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002

"Sorry it is not that simple ...
You need to take into account the impedance
of the tube section that is in parallel
with the cathode resistor."

Dan, you're right!

The formula is, bypass cap value in uf = 1000000/ 6.28 * F3 * r

r = (RK * rk) / (RK + rk) This is what is being bypassed, not just the cathode resistor value. "r" represents the impedance of the cathode itself in parallel with the value of the cathode resistor.

rk = (RL + RP) / (mu + 1)
RK = value of cathode resistor
RP = plate resistance of tube
RL = value of plate resistor (or load that the plate is seeing)
Mu = in circuit Mu of tube
F3 = the 3db down point of the filter

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

Oops! Thanks for pointing that out nt, posted on July 28, 2014 at 11:35:53
Ralph Manufacturer Posts: 4778 Location: Minnesota Joined: April 24, 2002	-

To be fair it's not just Naz' idea ..., posted on July 28, 2014 at 23:51:08

Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005

but thanks. I know Maxamillion is another guy who has been singing the praises of Schottkies and SiC in particular.

I've tried batteries (in the cathode and on the grid), caps, TT, Rs of all kinds and various types of diodes over the years but overall Schottky is hard to beat up to a few volts, SiC providing higher voltage drop.

I guess we can argue all day long about the relative technical merits but some things are hard to fully justify technically, they just work and sound "right".

True that caps are already in the PSU etc and there's been a ton of argument over the fact that they are still in series with the signal. But we have to look at the quantum of destruction and generally, I find that the closer to the signal source a cap lies the greater the impact.

Caps directly in series with small signals are the worst and therefore best avoided. It's easier to get (make) good small value caps at reasonable prices so designing largish values out of the direct signal chain is advisable.

Even then, it's not as simple as it seems and far too often global statements of truth are made that are really only relative. Case in point, a cathode bypass cap in a circuit with a relatively high impedance in the plate circuit provides very little bypass benefit but correspondingly, also has little sonic effect. It's all about ratios.

Naz

RE: You can avoid it with filament bias, posted on July 29, 2014 at 09:51:57
Al Noakes	The filament is buffered one side with a gyrator and the other side a ring of two CCS . I think that puts the last cap well clear of the signal path . Lots of SS though , but the things sound good :) Al

I guess I should more accurately have said..., posted on July 29, 2014 at 10:53:10
Lew Audiophile Posts: 10912 Location: Bethesda, Maryland Joined: December 11, 2000	that you gave me the idea. Thanks.

RE: I guess I should more accurately have said..., posted on July 29, 2014 at 11:14:47
wheezer Audiophile Posts: 4309 Joined: January 24, 2001	that you gave me the idea. From a thread that started roughly three years ago. ....Again thank you Naz!

RE: Nope, no coupling cap with the battery bias, posted on July 29, 2014 at 14:32:23
mqracing Manufacturer Posts: 4323 Joined: June 29, 2000	Hi tk. Just put off curiosity have you ever heard an Altec 1570? Or comments on other a2 amps you've listened to? Builder of MagneQuest & Peerless transformers since 1989

RE: Nope, no coupling cap with the battery bias, posted on July 30, 2014 at 07:37:23

Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006

Hi Mikey, long time no see. Hope you're doing well. If I'm not mistaken, the 1570 uses zero-bias triodes in Class B. I'm not a big fan of this topology, but I admit its been many years since I've heard them. As I'm sure you know, Class A2 is very different. In an A2 amplifier, operating conditions at most power levels (including "the first watt") are essentially Class A. The signal may not be exactly centered in the curve, but it's usually close. This means nothing untoward happens (grid current) until power levels are very high, well into the area where nuance and detail are usually masked anyway by the higher dynamics of the sound. More important, being IT or direct coupled, the A2 input circuit creates minimal recovery issues. The design is enhanced from this perspective if the cathode is also connected to ground with a low impedance and no bypass cap. The fact that an A2 design can not only resolve the recovery issue, but also eliminates the audible liability of grid and cathode caps, makes it my favorite topology. ;)

RE: Cathode bypass capacitor, posted on July 30, 2014 at 08:57:00
Stuben Audiophile Posts: 669 Location: Guber Ohio Joined: December 30, 2005	Hey Tubenstein, Simply stated.. Bypassing is a fence walk between the valves distortion levels it's ability to drive the next stage. Bypass and increase drive, decrease Cathode feedback which increases distortion. It's about implementation and needs of the next stage. Ultimately, I end up designing the bypass out...on most of my builds with time ..but not always:>) Stuben

RE: Nope, no coupling cap with the battery bias, posted on July 30, 2014 at 16:49:29
Naz Audiophile Posts: 2184 Location: Sydney Joined: September 2, 2005	Hey TK, should just re-iterate that you are talking about A1 designed to transition seamlessly into A2. FWIW, I've been building amps that way for many years and would never go back to cap coupling for output stages and even gain stages where possible, though for slightly different reasons. Naz

Class A2, posted on July 30, 2014 at 18:25:19

Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006

"you are talking about A1 designed to transition seamlessly into A2."

I consider that to be more nearly the classic definition of A2, essentially a single-ended version of AB2, the latter used to similarly describe PP amps that don't draw grid current when idling. OTOH, I tend to think of the positively-biased designs (which draw grid current regardless of signal level) as a more contemporary topology, and as such, being the class subject to a need for supplemental notation. I'll try to remember to be clearer on this point in the future. :)

What are the "slightly different" reasons you're referring to?

A previous 'class of operation' discussion

RE: Class A2, posted on July 30, 2014 at 19:26:22

Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005

I wasn't having a go at you TK ... more for the sake of those unfamiliar with these classes. I tend to agree that there should be a class specifically describing the operation we are discussing for the sake of clarity.

To answer your question, for output stages I seek to ensure that my driver will provide sufficient drive to ensure that the OP tube transitions smoothly into grid current up to the tube's saturation level without a significant increase in distortion.

For gain stages the grid is generally sufficiently negative that it will never become positive with respect to the cathode even at the highest signal levels it will ever experience. Therefore designing to drive the following stage into grid current under overload conditions is normally irrelevant.

However, I do see value in direct coupling and whilst this element of the design may normally be common to driving into A2 (normally being the operative word) I do it to take the coupling cap directly out of the signal path wherever it makes sense. Where it doesn't (and this is relevant to the original question in this thread) I try to use a high quality coupling cap, generally as low a value as I can get away with. More often than not I use Schottky diodes for bias because I like their sound or lack of it, eliminating the need for a bypass cap without the performance loss associated with an unbypassed cathode R (gain, internal resistance, OP impedance etc).

Hope this covers it.

Naz

RE: Class A2, posted on July 31, 2014 at 08:23:31
Triode_Kingdom Audiophile Posts: 10049 Location: Central Texas Joined: September 24, 2006	I think that's a worthwhile design philosophy. I've been thinking lately about various ways to implement this at all stages within an amplifier. As you say, the idea is to eliminate as many coupling caps as possible, and simultaneously - where possible - to install power drivers capable of pushing grid current. This will probably be my goal for all future designs.

RE: "I find fixed bias to be much cleaner sounding" Interesting... I find it more like ... Solid scrape soundingnT, posted on July 31, 2014 at 12:58:15
Retsel Audiophile Posts: 1239 Joined: April 17, 2000	If your fixed bias supply is a solid scrape-type design, then it would sound like that... ...but if it is designed well, this works very nicely!!! Retsel

Tube DIY Asylum

Cathode bypass capacitor

218.254.72.144