Direct Coupling with Zener Diodes

[R.G.]

Because "they" isolated me when I first started writing about using fixed bias in preamps going back about 6 years ago. This is the same kind of ground I've already covered... Why should I help them ?

One could logically ask, why do you bother tossing in your little bits of non-helpful bile here at all, if you're so unappreciated?

Have a nice day.

[R.G.]

this has got to be the worst thread I've ever read.... I'm either laughing my ^ss off with some of these comments, or just being taken to new lows in the world of electrical theroy... I sure hope some of you don't have any plans on quitting your "day jobs"...

Don't know how I missed this one. 8-)

So, Gary; electrical theory. Let's talk about that one, shall we? Don't I remember some ancient quip about people who live in glass houses ought to have an aversion to throwing stones? How 'bout them fuses, fella?

[J M Fahey]

para·noia
noun \ˌpar-ə-ˈnȯi-ə\
Definition of PARANOIA
1
: a psychosis characterized by systematized delusions of persecution or grandeur usually without hallucinations
2
: a tendency on the part of an individual or group toward excessive or irrational suspiciousness and distrustfulness of others

1. A psychotic disorder characterized by delusions of persecution with or without grandeur, often strenuously defended with apparent logic and reason.
2. Extreme, irrational distrust of others.

In its mild form paranoia may consist in the well-marked crotchetiness exhibited in persons commonly called "cranks."

Paranoia is a symptom in which an individual feels as if the world is "out to get" him or her. When people are paranoid, they feel as if others are always talking about them behind their backs. Paranoia causes intense feelings of distrust, and can sometimes lead to overt or covert hostility.

An individual suffering from paranoia feels suspicious, and has a sense that other people want to do him or her harm. As a result, the paranoid individual changes his or her actions in response to a world that is perceived as personally threatening. Objective observers may be quite clear on the fact that no one's words or actions are actually threatening the paranoid individual.

Paranoia is a very human feeling. Nearly everyone has experienced it at some or another time, to varying degrees. Paranoia exists on a continuum, ranging from a feeling of distrust due to an occasional misinterpretation of cues that can be appropriately dealt with and reinterpreted, to an overarching pattern of actual paranoia that affects every interpersonal interaction.

# chronically suspicious that people are lying or cheating him or her in some way
# cannot confide in others for fear of being betrayed
# misinterprets benign comments or events as being personally threatening
# harbors long-term grudges against others who are perceived as having been threatening or insulting in some way
# sees others' actions and/or words attacking him or her in some way, and therefore goes on the counterattack

Because of the inherent mistrust felt by paranoid individuals, they often must be coerced into entering treatment. As unwilling participants, their recovery may be hampered by efforts to sabotage treatment (for example, not taking medication or not being forthcoming with a therapist), a lack of insight into their condition, or the belief that the therapist is plotting against them.

Resources
American Psychiatric Association. 1400 K Street NW, Washington, DC 20005. (888) 357-7924. American Psychiatric Association.
American Psychological Association (APA). 750 First St. NE, Washington, DC 20002-4242. (202) 336-5700. American Psychological Association (APA).
National Alliance for the Mentally Ill (NAMI). Colonial Place Three, 2107 Wilson Blvd., Ste. 300, Arlington, VA 22201-3042. (800) 950-6264. NAMI: National Alliance on Mental Illness - Mental Health Support, Education and Advocacy.
National Institute of Mental Health. Mental Health Public Inquiries, 5600 Fishers Lane, Room 15C-05, Rockville, MD 20857. (888) 826-9438. NIMH · Home.

[R.G.]

Tee hee...

Also look up "narcissistic personality disorder". It's an eye opener, and accounts for some of the "people" (note the use of quotes!) I've run into over the years.

[imaradiostar]

Because "they" isolated me when I first started writing about using fixed bias in preamps going back about 6 years ago. This is the same kind of ground I've already covered... Why should I help them ?

-g

That's not why you're isolated. I think it's fair to say you may have brought a certain amount of criticism upon yourself in the past.

This argument is sad. It's very simply proven and we're stupid to not have done so. I believe everyone here would agree if there was scientific evidence of this exact circumstance. If I had the time I'd have already posted pictures and numbers. To be fair this info is already out there but I'm trying to be open minded about proving it anew.

jamie

[Steve Conner]

So, anyway, the results are in.

I built the gain stage + DC coupled CF unit as it appears in the Fender 5F6A schematic. (gain stage has 820 ohm cathode resistor and 100k plate resistor, CF has 100k cathode resistor)

I powered it from a 330V DC supply as per the 5F6A schematic voltages. The plate voltage ended up approximately 180 as per the schematic.

Then, I disconnected the CF stage and tested the gain stage alone, overdriving it heavily with a signal generator at 400Hz. The oscilloscope shows the grid voltage at 5V per division, and the plate voltage with a 100x probe, so at 100V per division.

Note how the plate voltage goes all the way up to 330V (shown by the green line)

Then I reconnected the CF and scoped the CF output.

It doesn't go all the way up to B+ any more! Why could that be?

I connected a 10k resistor in between the gain stage plate and CF grid, and hooked it up to a battery-powered scope with isolated inputs.

This shows grid current at 500uA per division. (Note the trace is upside down)

Lastly I reduced the input voltage until there was no longer any obvious distortion:


And here's grid current at 50uA per division. (again upside down, oops)


I think this proves that the CF does draw grid current even with the plate and cathode resistors the same.

[imaradiostar]

Steve, did it draw any current with no input?

jamie

[Steve Conner]

No, it only draws current on positive signal peaks, as the CF triode starts to run out of plate voltage, even while it's being asked to pass more current.

[kg]

thanks for documenting this, steve.

by my eye, with the CF pulling grid current, the max Vp was reduced by about 40vpk, would you agree?

considering a 100k Rp, that's about 0.4mA, which jives with your inline R current measurement.

it would seem to me that this Ig is partially contingent on the cathode follower's Rk, with higher resistance netting lower grid current (at least up to a point), since the Rk of the CF is defining its total cathode current.

may explain why i like the sound of lower value Rk in this role... lots of Ig and lots of positive peak flattening.

ken

[jmaf]

Steve, thanks for taking the time. It seems to me like the current you measured is AC, right? Did you have any DC current at all in evidence?

What is the impedance of that oscilloscope by the way? The 2nd grid impedance is much higher than the scope, once you connect it to the circuit that may be the cause of that drop, not DC current.

Does the CF rectify the positive pulses like I said it would? Does that scenario ever happen in any Marshall or Fender in existance?

Thanks again.

[jmaf]

No, it only draws current on positive signal peaks, as the CF triode starts to run out of plate voltage, even while it's being asked to pass more current.

Did you have gid 2 made positive DC and just let a microampere ammeter in series with the grid? I think the current you have is the grid to cathode miller capacitance grounding that AC signal. Which would imply rectification of the signal and not amplification.... If you could please post the output of the CF when the grid goes positive, we can see if I'm right or wrong.

The question I have is : does that positive peak get amplified at all? Thanks.

Edit: I ran these tests too last weekend. Reached different conclusions from Steve Conner. I also had the probe at the CF grid and the probe ground to CF cathode...it's easy to see then how the signal is rectified and not amplified, that grid only goes positive when the amplifier is far into cutoff. I was unable to replicate some statements made in this thread in any Earthly amplifier setting.

Of course, for the sake of forum politics, let's do one thing: you folks keep your grids positive and I keep mine negative. I can live with that.

[Steve Conner]

Jmaf: the grid current scope measurement is DC coupled, zero is the centre of the screen. Therefore the asymmetrical waveform implies a net DC grid current.

I am quite sure it's not just current in the Miller capacitance. The Miller capacitance is linear, so the current would be a neat sine wave. But we have a nasty peaky or square wave, even when the plate voltage is an undistorted sinusoid. It is real grid conduction.

[jmaf]

The output of the CF does not have any of the positive pulse, it was never amplified, you didn't see that question for some reason.

Miller capacitance is not linear, it is constant(the stage gain is constant).

No Marshall or Fender, or any other human-made amplifier that actually produces music, has that setup(by the time it clips negative to get that plate to +B, the positive phase is long long gone...).

Positive grids on 12ax7's do not amplify, the signal is rectified.

Don't eat the yellow snow. Thanks very much. Over - and out of this discussion.

Edit: no need to mention politics. Out.

[imaradiostar]

I feel like I'm taking crazy pills!

You can see it in the scope traces- it continues to amplify the signal on the second grid, only that signal is diminished as grid current is drawn and the first plate lacks the ability to push the grid further positive.

jamie

[Steve Conner]

Jmaf, the DC coupled CF doesn't work the way you think it does, get over it.

More details on the grid current measurement: it was done with a battery-powered Tektronix 222A. The scope ground clip was connected to the plate of the first stage, and the input was connected to the CF grid, at the other end of the 10k "current shunt" resistor. I used a 1x probe, so a 1M input impedance, but it is shunted by the 10k.

I checked that adding the floating scope and current shunt didn't alter the waveforms significantly.

By turning the input signal right down, and the scope sensitivity up, I could see a sinusoidal current that may well have been in capacitances. But the grid conduction current was bigger and obviously distorted.

It most certainly does continue to amplify after the onset of grid current: I showed that grid current starts even with about 6dB of headroom remaining. The amplification only stops, and the stage hits a brick wall when the grid is eating all of the current that the first stage's plate load resistor can supply.

If you got different results, let's see your scope traces.