CCS bias fixes the wrong thing. Fixed bias sets the quiescent current, and lets the average current vary as it will (generally upwards). CCS bias sets the average current, and lets the quiescent current vary as it will (generally downward if bypassed). Cathode resistor bias is somewhere between these two extremes; the average current rises with signal, but the quiescent current falls so you have to start with the valve biased too hot.
The reason for the change in current with signal is the second-order nonlinearity present in all devices. It is quiescent current you set from the datasheet load lines, but you measure average current with a meter. The two are only the same with no signal present.
The reason for the change in current with signal is the second-order nonlinearity present in all devices. It is quiescent current you set from the datasheet load lines, but you measure average current with a meter. The two are only the same with no signal present.
Also interesting observations.
Perhaps this parasitic and unwanted effects was stronly reduced through Tim de Paravicini's topology used by the EAR integrated Amplifier "V20". 10 pairs of small signal tubes works parallel for driving the loudspeaker output transformer. Read more about follow URL's:
EAR V20 integrated amplifier | Stereophile.com
http://www.diyaudio.com/forums/tubes-valves/94403-ear-yoshino-v20-amplifier.html
..:: EAR Yoshino, professional audio equipment,Cambridgeshire,UK ::..
V20 Vollverstärker (German language)
Paralleling a million ECC83's would not stop them having second-order distortion into a load which is necessarily going to be small for an output stage, so the problem remains. Of course, he may have used a clever bias method but I didn't see any mention of it in a quick scan.
Hello,
I have just bought a pair of custom made mono 300b SE amps(no schematic). Also, bought a new pair of 300b tubes for these amps, but did not know how to bias/adjust plate current and/or voltage! Your assistance and instruction on how to bias the 300b tube amps are greatly appreciated.
Best Regards,
Sb
I have just bought a pair of custom made mono 300b SE amps(no schematic). Also, bought a new pair of 300b tubes for these amps, but did not know how to bias/adjust plate current and/or voltage! Your assistance and instruction on how to bias the 300b tube amps are greatly appreciated.
Best Regards,
Sb
Some information on who made these and what sort of provisions there are for adjusting bias would be helpful. For example if autobias you just plug them in, no adjustment is required or possible as there will be no provision for adjustment - if fixed then adjustment is required, and pots and test points for doing so should be provided. Sometimes contacting the original designer would be most appropriate in such instances... Should sufficient information be known about the particular design any of us ought to be able to advise.
Other solution is micro controller variable autobias. It don't have side effect of Servo type autobias. It can keep 100% sound charateristic of fixed bias but easy to use. More safety to tubes & amplifiers. This topology can combine with power tube protection circuit to protection your amplifiers. You can use A/D converter or comparator to detect plate current and adjust it automatic by micro controller. It can adjust to very precise & easily. I design & sell to some tube amplifiers mfy already. It only need microcontroller + A/D converter or comparators + Digital potentialmeters.
I wish I could remember where I saw it but I remember reading an old article from an electronics magazine where they experimented with fixed bias and resistive cathode bias and found that in a dynamic sense the cathode bias provided greater output. As I recall the experiment was done to try to find out why cathode bias amps subjectively seemed to be able to play louder. IIRC the test amp was a PP EL84.
Anyone seen this and remember what the source was.
Anyone seen this and remember what the source was.
Interesting, and completely counter to my own experience.. 😀
I wonder what the story is...
Maybe the low-loading article in Audio, 1959. I think it was by Dalzell.
edit: yes, Dalzell. Audio, December 1959, p 19. It may be reprinted in one of the AX "Audio Anthology" volumes.
edit: yes, Dalzell. Audio, December 1959, p 19. It may be reprinted in one of the AX "Audio Anthology" volumes.
Sounds like an interesting article however I did find the article I was thinking about.
http://www.mmiworks.net/sprc/SP17-Ravenswood.pdf
I found it interesting if not definitive.
http://www.mmiworks.net/sprc/SP17-Ravenswood.pdf
I found it interesting if not definitive.
The article assumes output stage clipping is caused by the onset of grid current. This might not always be true, although negative feedback will tend to push a circuit in this direction.
I guess it depends on what is meant by "greater output". Higher measured power, just before clipping? Or maximum noise, when seriously overloaded?
I guess it depends on what is meant by "greater output". Higher measured power, just before clipping? Or maximum noise, when seriously overloaded?
Fixed bias , is useful on high-power tubes (amplifiers) to avoid the wasting of power (and heat) on big cathode resistors (KT88 , EL34 , 6L6 ,etc) BUT for low power tubes like 6BQ5/EL84 , 6V6 , ECL82 , I always prefer cathode bias . Simple as that !!!
I have always wondered about the phenomenon of reaching a clipping point then having to back way down below the clipping point to get things back into line.
While having a big cathode resistor in a high power amp is a bit annoying, it does provide some helpful resistance to runaway.
While having a big cathode resistor in a high power amp is a bit annoying, it does provide some helpful resistance to runaway.
if you value safety more then go cathode resistor bias.....
there is a third option though, Patrick Turner designed a 300w power amp using a combination if cathode resistor and fixed biasing scheme...300w-6+7-protection
there is a third option though, Patrick Turner designed a 300w power amp using a combination if cathode resistor and fixed biasing scheme...300w-6+7-protection
I found this thread interesting, because it was unknown to me that fixed bias could på problematic. In the start of the thread, it seems that some preferred cathode bias because of subjective matters and the measuring types stated that it was oppesite to measurements...then the article came up with an explanation, but then the thread died...
I am hoping for some further discussion 🙂
I am hoping for some further discussion 🙂