Author Topic: B+ to cathode feed  (Read 437 times)

Offline El Sid

B+ to cathode feed
« on: September 16, 2019, 11:33:56 AM »
In an interesting discussion on Facebook's Vacuum tube amplifier group, the purpose of the 200k resistor from B+ to the cathode of the 12AX7 in the circuit shown below is discussed. The OP's question is "Hi All. Need help understanding the preamp biasing. Typical 470R on the cathode to ground, but also 220K from the B+ tied there as well. On a Magnatone M13 time machine, never worked on. Thanks!"

Soon dismissed are a voltage divider for bias, and local feedback. The winning answer seems to be feeding B+ noise antiphase to the cathode for cancellation - looks like a good idea for sensitive ccts like mic or phono preamps? I may play around in LTSpice to see.

« Last Edit: September 16, 2019, 11:36:03 AM by El Sid »

Offline charles

Re: B+ to cathode feed
« Reply #1 on: September 16, 2019, 02:03:14 PM »
Very old guitar amplifier

Offline Ampdog

Re: B+ to cathode feed
« Reply #2 on: September 16, 2019, 04:11:36 PM »
El Sid,

Sorry, the right answer is indeed helping to find the right bias without making the cathode resistor too high, resulting in local current feedback and an increase a.o. in the rp of the valve, affecting the fed tone circuit.

Investigation will reveal that feeding a signal/disturbance from B+ to cathode is an in-phase action referred to the output, increasing the effect of said disturbance on the anode output (the PSRR of a triode circuit being relatively poor). Where does the supposed 'anti-phase' occur? This in fact how an r.f. oscillator is created with suitable capacitors. At low frequencies oscillation is unlikely to be excited because of impedance ratios - the input impedance of a cathode circuit being quite low.

Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Steerpike

Re: B+ to cathode feed
« Reply #3 on: September 16, 2019, 09:03:58 PM »
Sorry, the right answer is indeed helping to find the right bias without making the cathode resistor too high, resulting in local current feedback and an increase a.o. in the rp of the valve, affecting the fed tone circuit.

Why is this method "better" than including a capacitor in the cathode resistor segment? Just cheaper?

Offline Ampdog

Re: B+ to cathode feed
« Reply #4 on: September 17, 2019, 12:05:01 AM »
Steerpike,

Mmmmm . . . . The resistor topology is aperiodic - which might sound a mite academic, but when one takes an RIAA circuit which will have consequences down to just about subsonic, might make a difference. Then again as said, more of a reply to you than a practical matter. Economy - cost of a resistor vis--vis that of a capacitor plus perhaps in some quarters the aversion regarding capacitors 'in the signal circuit' and all that jazz . . . .

[Expounding somewhat: In feedback RIAA circuits (my preference) a bypass capacitor can definitely play a role in the subsonic regions. But that could/should then also be used to advantage in creating a sharp cut-off below say 30 Hz, so as to get rid of low frequency rubbish, including possible turntable rumble.
     (MY TURNTABKLE HAS NO RUMBLE!
          Oh really?
               Just asking . . . .  :EGrin: )]   
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

Re: B+ to cathode feed
« Reply #5 on: September 17, 2019, 03:08:03 AM »
     (MY TURNTABKLE HAS NO RUMBLE!
          Oh really?
               Just asking . . . .  :EGrin: )]   

... and what phonostage and turntable do you use Ampdog ?  ;D  >:D


Steerpike,

Mmmmm . . . . The resistor topology is aperiodic - which might sound a mite academic, but when one takes an RIAA circuit which will have consequences down to just about subsonic, might make a difference. Then again as said, more of a reply to you than a practical matter. Economy - cost of a resistor vis--vis that of a capacitor plus perhaps in some quarters the aversion regarding capacitors 'in the signal circuit' and all that jazz . . . .

I thought the 220k may lower the rp of the tube with regards to the next stage .


Expounding somewhat: In feedback RIAA circuits (my preference) a bypass capacitor can definitely play a role in the subsonic regions. But that could/should then also be used to advantage in creating a sharp cut-off below say 30 Hz, so as to get rid of low frequency rubbish, including possible turntable rumble. 

Will a bypass cap over cathode resistor not form a type of low-pass filter combined with the grid resistor ? ( in regards to your sharp cut-off statement @Ampdog ) I'm also assuming that you're not talking about the 33pF cap .
« Last Edit: September 17, 2019, 03:24:35 AM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

Re: B+ to cathode feed
« Reply #6 on: September 17, 2019, 08:41:20 AM »
... and what phonostage and turntable do you use Ampdog ?  ;D  >:D

 ;D ;D ;D   ;) 

Fair enough!

My turntable is a Thorens TD135 - sort of middle-of-the-state. The cartridge is an Ortophon 320 - again middle-of-the-state I believe.
The phono stage is of my own design, part of my integrated 'Thalia 1' 80W/channel transistor design amplifier. The phono stage is a feedback type with one or two special features, which I will discuss at some future time as promised. (It has also been built as a separate unit for some who appeared to be quite satisfied with the result. It is a 3-stage RIAA feedback unit with BC550 low noise input, BC456 amplifier stage and BC456 emitter follower output stage. Thus transistors, but the principles remain the same.  But later!)

Quote
I thought the 220k may lower the rp of the tube with regards to the next stage .

?? -not quite, as explained the 220K only passes extra current through the 470E cathode resistor to get the bias up without a higher cathode resistor - - Okay: in as much as the lower cathode resistor allows less current feedback (which would increase the valve rp), the equivalent rp would then be lower although not as low as with a completely bypassed cathode resistor - if I understand your remark correctly.

Quote
Will a bypass cap over cathode resistor not form a type of low-pass filter combined with the grid resistor ? ( in regards to your sharp cut-off statement @Ampdog ) I'm also assuming that you're not talking about the 33pF cap .

No-o-o-o (think I do not quite understand) - not in combination with the grid resistor - the latter plays no role in the cathode bypass mix affair.  A too low cathode bypass cap will provide a high-pass feature in the sense that cathode current feedback will resume at a low enough frequency. But that is not significant unless the resultant phase shift is combined with others in a feedback circuit with other caps, so as to give a resultant sharp low-cut characteristic (high Q high-pass 'filter', not to get too technical.  Again later!)

Regarding the 33pF capacitor, that only comes in at supersonic frequencies, mainly to obviate r.f. 'slipping' in via an input lead. It is a common feature in many phono input circuits. (We are not in a high r.f. intensity country; perhaps close to transmitting towers. Abroad it could be much worse and can such a capacitor become mandatory to keep r.f. out.) 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

Re: B+ to cathode feed
« Reply #7 on: September 17, 2019, 12:11:34 PM »
 ;D Thanks for indulging us re your setup Ampdog . I didn't really know the TD135 , it looks pretty decent and quite substantial , closely related to the TD124 it appears . Not too shabby at all ! And I'm sure you keep it in tip-top working condition .

Beautiful ! I want one !  ;D



And I'd certainly be interested in learning about your transistor phonostage .

?? -not quite, as explained the 220K only passes extra current through the 470E cathode resistor to get the bias up without a higher cathode resistor - - Okay: in as much as the lower cathode resistor allows less current feedback (which would increase the valve rp), the equivalent rp would then be lower although not as low as with a completely bypassed cathode resistor - if I understand your remark correctly.

Yes , you understand me correctly , and from your answer I gather that the cap by-passing the cathode resistor is a better option with regards to Rp .

No-o-o-o (think I do not quite understand) - not in combination with the grid resistor - the latter plays no role in the cathode bypass mix affair.  A too low cathode bypass cap will provide a high-pass feature in the sense that cathode current feedback will resume at a low enough frequency. But that is not significant unless the resultant phase shift is combined with others in a feedback circuit with other caps, so as to give a resultant sharp low-cut characteristic (high Q high-pass 'filter', not to get too technical.  Again later!)

Regarding the 33pF capacitor, that only comes in at supersonic frequencies, mainly to obviate r.f. 'slipping' in via an input lead. It is a common feature in many phono input circuits. (We are not in a high r.f. intensity country; perhaps close to transmitting towers. Abroad it could be much worse and can such a capacitor become mandatory to keep r.f. out.) 

I have much to learn regarding the various filters formed in even simple audio circuits . Thank you for indulging me , I really appreciate the enlightenment .
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

Re: B+ to cathode feed
« Reply #8 on: September 18, 2019, 01:42:48 AM »
Yah;  oops Fredeb!

Got a bit carried away. All you probably wanted to know was which make phono stage I use. But having at first been too pecuniary embarrassed to obtain anything falling under the ill-defined description of 'classy', then daring enough to think I know enough to make things myself and still p.e., then knowledgeable/inquisitive enough to achieve some modest success despite being p.e., and (varsity in between) finally still p.e. but by then knowing enough to bugger 'class' and design things the way they really should be regardless - gasp for breath   :sweat:- After all that I hardly ever went the brand distinction/credit card way. [Stuff purchased were loudspeakers (the drivers), turntable-and-cartridge, and tuner. (Oh! somewhere a CD player . . .)]

End of biography - technology left at that. Coming up with the promised graph of what I consider a proper phono stage performance to be (just for the record, considering the thread content).
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

Re: B+ to cathode feed
« Reply #9 on: September 18, 2019, 06:44:22 AM »
Yah;  oops Fredeb!

Got a bit carried away. All you probably wanted to know was which make phono stage I use. But having at first been too pecuniary embarrassed to obtain anything falling under the ill-defined description of 'classy', then daring enough to think I know enough to make things myself and still p.e., then knowledgeable/inquisitive enough to achieve some modest success despite being p.e., and (varsity in between) finally still p.e. but by then knowing enough to bugger 'class' and design things the way they really should be regardless - gasp for breath   :sweat:- After all that I hardly ever went the brand distinction/credit card way. [Stuff purchased were loudspeakers (the drivers), turntable-and-cartridge, and tuner. (Oh! somewhere a CD player . . .)]

End of biography - technology left at that. Coming up with the promised graph of what I consider a proper phono stage performance to be (just for the record, considering the thread content).

 ;D Thank you Ampdog ! Looking forward to your dissertation re  "proper phono stage performance" .  :rubhands:
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

Re: B+ to cathode feed
« Reply #10 on: September 18, 2019, 09:16:43 AM »
Fredeb  :)  :)  :)  :),

Beware of an anti-climax!  As said before, not to stop anybody from progressing with what you are already busy with. I can unfortunately not provide an immediate circuit re my ideas.  Lost it somewhere and must 're-invent'. (And I would like to see a picture of that 'massive' power transformer you had made!)  For now reminder of what others hinted: Be mindful of ECC83s (12AX7s) or generally low-gm valves in the wrong places. At RIAA input:  Mmmmm  :nono:  (Current required! ECC88, 6922 or such.)


Meanwhile back to the thread title: I have tried to access the earlier given reference by El Sid to the Facebook Amplifier Group regarding others' ideas  re that 220K resistor, without success - I get thrown off although I am registered. Perhaps you or El Sid can review; I would like to read about the alleged feedback effect or whatever others said there was.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline vleisman

Re: B+ to cathode feed
« Reply #11 on: September 18, 2019, 09:18:20 AM »
I can't find B+ or the 200k resistor in those diagrams. Is there some secret code or are these trick questions to catch the ignorant (me in this case)?

Offline Ampdog

Re: B+ to cathode feed
« Reply #12 on: September 18, 2019, 09:39:22 AM »
Darn - my reply just got thrown off! (perhaps said Zondo Commission is spying here . . .)

I replied to your post: "No no no. we will leave such matters (i.e. secret codes) to the Zondo Commission and . . . never mind.
"The relevant 220K resistor appears in the opening diagram, just to the left of the 100K anode resistor of the first (left) triode. Subsequent posts will explain the ideas of some that the rest of us cannot understand."

(Now let us see whether this post also gets dumped . . . Ah! It went through.)
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

Re: B+ to cathode feed
« Reply #13 on: September 18, 2019, 09:44:02 AM »
Can you please add a link to the facebook article El Sid ? Thank you kindly :)

Sent from my SM-G935F using Tapatalk

...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline El Sid

Re: B+ to cathode feed
« Reply #14 on: September 18, 2019, 09:46:06 AM »
Is there some secret code or are these trick questions to catch the ignorant (me in this case)?

A bit of both really  ;)

200k should be 220k, so finger trouble on my part while typing. "B+" is code for the supply voltage in valve circuits. I should have called it "I" as it is on the diagram.

But please stop derailing this phono amp thread by referring to the original topic.   :ROFLMAO: