Hi Guys
Where I leave out Ck in cathode biased push-pull stages to retain dynamic balance and a more "cathode bias tone", others generally do not. I'm also not trying to get maximum power in cathode-biased mode. It is certainly the majority to have Ck in MI amps, and I find even in hifi it is a majority.
All the matched whatever stuff I find to be sort of nonsensical in tube amps as the vaguaries of the tubes themselves swamp much of the passive tolerance spreads. Of course, it is nice to tame whatever circuit aspects we can if even for aesthetic reasons.
My main point about diode biasing is that it is really fixed biasing, but with a difference. In a preamp the currents are low so power expended in the diode is inconsequential. However, the increments of voltage control are large as we can only make them as fine as diode turn-on thresholds. This might lead to a compromise in the plate operating voltage resulting in asymmetry (maybe only of clipping).
Personally I prefer to keep tube preamp circuitry fairly traditional in the means and methods to make it work. I believe this results in a more pure tube tone. Hybridising with active bias, or with jeft cascoding on the cathode end or mosfet cascoding/buffering at the plate end tends to wring out all the tube character. I assume most people choose tubes because of what the tube is going to add to the sound even if you nominally want a "clean" sound.
Cathode bypass values become noncritical in the context of flat fullrange gain. The same applies in many ways to coupling caps: they may impose a roll off but it should generally be low enough to block unwanted rumble, or as shunt filters be high enough to filter out-of-band high-f noise without knocking off treble.
Everyone has their own agenda and aesthetic when it comes to designing each piece of equipment. We can change our priorities from project to project just to keep it interesting.
Have fun
Kevin O'Connor
londonpower.com
Where I leave out Ck in cathode biased push-pull stages to retain dynamic balance and a more "cathode bias tone", others generally do not. I'm also not trying to get maximum power in cathode-biased mode. It is certainly the majority to have Ck in MI amps, and I find even in hifi it is a majority.
All the matched whatever stuff I find to be sort of nonsensical in tube amps as the vaguaries of the tubes themselves swamp much of the passive tolerance spreads. Of course, it is nice to tame whatever circuit aspects we can if even for aesthetic reasons.
My main point about diode biasing is that it is really fixed biasing, but with a difference. In a preamp the currents are low so power expended in the diode is inconsequential. However, the increments of voltage control are large as we can only make them as fine as diode turn-on thresholds. This might lead to a compromise in the plate operating voltage resulting in asymmetry (maybe only of clipping).
Personally I prefer to keep tube preamp circuitry fairly traditional in the means and methods to make it work. I believe this results in a more pure tube tone. Hybridising with active bias, or with jeft cascoding on the cathode end or mosfet cascoding/buffering at the plate end tends to wring out all the tube character. I assume most people choose tubes because of what the tube is going to add to the sound even if you nominally want a "clean" sound.
Cathode bypass values become noncritical in the context of flat fullrange gain. The same applies in many ways to coupling caps: they may impose a roll off but it should generally be low enough to block unwanted rumble, or as shunt filters be high enough to filter out-of-band high-f noise without knocking off treble.
Everyone has their own agenda and aesthetic when it comes to designing each piece of equipment. We can change our priorities from project to project just to keep it interesting.
Have fun
Kevin O'Connor
londonpower.com
No. Tube character is minimum of high order non-linear distortions, decrease of level and of order of distortions when sounds fade out, and much lower phase intermodulations than in solid state amps. Loading tubes dynamically on higher impedance, be it transformer, choke, resistor and higher voltage power supply, any kind of active dynamic load be it vacuum tube mu-follower, solid state constant current source or gyrator, the result is the same: more "tube" character, cleaner and softer the sound.
Leaving out Ck provides less stage gain and lowers the distortion due to the local negative feedback effect.
If there is a need for higher gain, connect the cathode directly to ground and use fixed bias on the grid. This results in a more complex circuit, but eliminates any argument about the effects of Ck. The fixed bias is easily tweaked using a trimpot for best anode operating point and lowest distorition.
If there is a need for higher gain, connect the cathode directly to ground and use fixed bias on the grid. This results in a more complex circuit, but eliminates any argument about the effects of Ck. The fixed bias is easily tweaked using a trimpot for best anode operating point and lowest distorition.
I wasn't quoting anyone in particular, corroborating SY's comments and since the thread is titled " Capacitors in the cathode of preamp tubes" one could be forgiven for thinking that this was the topic of the thread..
Please also note we are not discussing MI amps, there is another forum for that, let's stick to discussing cathode bias and its alternatives in something loosely related to the thread title.
My amp has a 12BH7 driver with ccs loading and I removed Ck a while ago, doesn't seem like it was doing anything, gain is about the same, the difference in sound quality is small enough I don't notice it.
I wasn't expecting that, because adding a film cap bypass to my power tube's Ck made a rather large improvement in sound quality.
Slightly OT... I had a tube amp professional who owns a repair shop, sells tubes and has a line of Hi Fi amps tell me to get rid of Ck on my power tube's cathode entirely if I don't need the gain, what do you guys think?
I wasn't expecting that, because adding a film cap bypass to my power tube's Ck made a rather large improvement in sound quality.
Slightly OT... I had a tube amp professional who owns a repair shop, sells tubes and has a line of Hi Fi amps tell me to get rid of Ck on my power tube's cathode entirely if I don't need the gain, what do you guys think?
Well,
I would try a couple of these before you ditch the caps they are cheap and might make you think again..
Elna Cerafine and Elna Silmic Capacitors homepage
I have no connection with above link.
Regards
M. Gregg
I would try a couple of these before you ditch the caps they are cheap and might make you think again..
Elna Cerafine and Elna Silmic Capacitors homepage
I have no connection with above link.
Regards
M. Gregg
It's actually adding more of a solid state character. There's nothing about solid state electronics per se in a bias network that have the capability of isolating 'tube character' and then removing it. It primarily adds its own character, although any measurable performance improvement the solid state circuit adds may influence the circuit's overall sonic signature to a certain extent.
I think this is a major reason that most solid state circuits that attempt to simulate 'tube sound' don't completely succeed to a greater or lesser extent.
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And is also apparently the one who took the thread entirely off topic as well.. (see post #6)
Assuming this is a triode SE removing Ck will substantially raise rp which will result in a rather premature LF rolloff and considerably more LF distortion as well. Consider fixed bias instead, allows you to easily fine tune operating point and eliminates the need for Ck altogether. (The red light district provides another viable option as well..)
Probably should try to get back on topic we aren't really discussing power stages in this thread, although we are. My bad..
It think if it is SE amp it would add negative feedback by current increasing output resistance and decreasing bass response due to limited inductance of transformer.
Assuming this is a triode SE removing Ck will substantially raise rp which will result in a rather premature LF rolloff and considerably more LF distortion as well. Consider fixed bias instead, allows you to easily fine tune operating point and eliminates the need for Ck altogether. (The red light district provides another viable option as well..)
Probably should try to get back on topic we aren't really discussing power stages in this thread, although we are. My bad..
Thanks, I was thinking removing the cap would increase AC resistance and maybe not drive the OPTs as well as possible.... I may try led bias too.
I was pretty surprised that removing Ck from the driver stage seemed to make little to no difference.... (on topic
)
Oops... I did not see you already answered that, sorry.
Lol, I missed all those posts, you guys are too quick!
What?
What²?
Hum couples from AC heater and heater wiring to cathode wire, and the is amplified by the tube as it appears as a voltage difference between grid and cathode.
Are you serious? He asked about output stage. If heater is capable to cause audible hum through output tube the same heater already makes wild hum through preamp tubes.
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