? - Paul Ruby Zener Diode mod

[RudeBoy]

Regarding the Ruby mod in cathode biased amps, and the increase in cathode voltage that occurs with large drive signals shifting the cutoff point - how about connecting the bottom of the zener network to the corresponding cathode rather than ground?

That would eliminate one or two high watt zeners. Though you couldn't take advantage of SC's best-of-both worlds approach.

-- bradley

[hasserl]

How are you measuring that? Is that the rms grid voltage?

DMM (true rms) set to AC volts, one probe on chassis ground the other on the circuit just prior to the 1k5 grid resistors (post coupling cap, so no DC).

[Chuck H]

It seems to me that it would just increase cathode voltage relative to grid voltage effectively 'cooling' the bias and increasing the possibility of crossover distortion. Not sure though.

Keep in mind that the whole point of the PR mod is to reduce the time constant of the grid circuit to nill during that tubes cutoff cycle giving the grid circuit a chance to unload and avoid grid conduction.

I don't fully understand this because in my mind if the grid voltage goes up that would heat the bias of that tube, not cool it, right? So how does this result in the crossover notch shifting into the positive cycle of the tube?

The PR mod diodes aren't high watters. The diode across the cathode R the way I do it is. Though it may not have to be. I only measured one watt across that diode using a current sense resistor with the amp at full tilt. Most of the work is still being done by the cathode R. I'm using a 5 watt diode in several 20 watt amps without any failures. The 5 watters can be had in the standard DO package so theres no funky mounting or heat sinks.

Chuck

[Bruce / Mission Amps]

Just curious.... because I happen to have one on my bench today for a cap job but... I wonder how this zener diode thing would work across the grid load resistors of a cathode biased amp using a paraphase splitter/inverter?
The one in question is the one with each power tube grid load being, say, 220k-270k and one of those sitting on a 6K to 15K "signal divider" to drive the grid on the out of phase tube.
To be more specific like that GA-6 I had a few weeks ago...

[Chuck H]

It seems to me that there would be an audible clip. The LTP PI is cathode coupled so each half of the PI is independant at the plate. But with the paraphase PI the negative half of the input triodes plate AC is what becomes the positive half of the other side of the PI. And that is where the PR mod diodes conduct, on the negative half of the signal. I could be wrong. Sometimes Steve Connor, Enzo or yourself are good enough to chime in and correct my misconceptions.

Chuck

[Bruce / Mission Amps]

It seems to me that there would be an audible clip. The LTP PI is cathode coupled so each half of the PI is independant at the plate. But with the paraphase PI the negative half of the input triodes plate AC is what becomes the positive half of the other side of the PI. And that is where the PR mod diodes conduct, on the negative half of the signal. I could be wrong. Sometimes Steve Connor, Enzo or yourself are good enough to chime in and correct my misconceptions.

Chuck

I don't stock or use very many small 1/2w or 1 watt zeners at all. Maybe in the old two radio biz days, but that is ancient history for me now.
I do have a moderate selection of 5 watt 7v, 9v, 10v, 12v, 15, and 18v zener diodes, used separately or in combinations to trim B+ voltages down on vintage amps.

I'm not convinced an extremely low current circuit like these grid load resistors, would fully cascade a few of the 5 watt zeners if used in series.... you know, to add up to a few volts over bias voltage.
So, I can't really experiment much (well, in a timely fashion).... since I'd have to order up some.
I've never tried stacking them on the cathode biasing resistor though.
Chuck, did you say that works as well or did I miss read that?

[Chuck H]

I've never tried stacking them on the cathode biasing resistor though.Chuck, did you say that works as well or did I miss read that?

I dunno. I may have represented myself poorly... It happens.

I do use a zener across the cathode R in combination with the PR mod in one of my best designs. It allows me to set the PR mod and cathode bias function at the onset of clipping. I value the PR mod zeners a volt or two above the zener I place across the cathode R (to guarantee no audible clipping from the diodes). The zener across the cathode R is chosen by using a scope to find the onset of power tube clipping. When the the amp is OD'ing the power tubes it's fixed bias with the PR mod squelching grid conduction. When playing clean it's just another cathode biased amp. I like the combination of attributes and so far so do the people who own these amps. There is no glitchy audible effect that I can see on the scope or hear with my ears at the clean/clip crossover. If I did hear it (or if you do) you could use small value resistors in series with the zeners to smooth the effect. Is that what you were asking about?

Chuck

[Steve Conner]

Yes, that's a good point. If you clip the "first" stage of a paraphase inverter, you clip the signal feeding the second one. I can't really picture what that would do to the tone, though, and there's probably a similar interaction between outputs in a LTPI anyway, just not as strong. Maybe it just means that with a paraphase PI, you only need one zener and can save $0.10.

I always thought Chuck H's "Paul Ruby mod + cathode zener" was particularly clever. The two mods complement each other in getting rid of the grid blocking effect: the grid zeners stop the grid coupling caps from charging up and blocking the signal, and the cathode zener stops the cathode bypass cap from charging up, which if you think about it, causes blocking distortion in just the same way as the grid caps. (the tube is activated by the voltage difference between grid and cathode)

My only misgiving would be that maybe some players like a bit of that crossover distortion. It could add another layer of complexity to the tone. (I called it blocking distortion, but because it's happening in two push-pull stages at once, it becomes symmetrical and turns into crossover.)

[Chuck H]

My only misgiving would be that maybe some players like a bit of that crossover distortion. It could add another layer of complexity to the tone. (I called it blocking distortion, but because it's happening in two push-pull stages at once, it becomes symmetrical and turns into crossover.)

I agree. And this design isn't devoid of crossover distortion. I'm pounding on the grids pretty hard and I do allow some cathode voltage shift. There is still a little crossover. Certainly a lot less than most amps. But only a fraction of "swirl" that some players like. But with el84's "swirl" sounds more like "bwizzz". I've tried a couple of experiments to get a more musical swirl with no luck as yet. So for these amps I'm having better luck just eliminating most of the crossover notch.

I don't think we would want to get square wave clipping TOO symmetrical, otherwise we may as well be using transistors, right?

Looking on a scope, the PI plate diodes DO effect each other a little even with the LTP PI, but only in the range where the power tubes are in cutoff. The PI traces sure do look funny everywhere but their positive swing.

Chuck

[Dani]

Dear All
This is Dani, a new member. I recently build a Marshall clone 18W with EL84's (Ampmaker PP18 kit). I would like to apply Chuck's smart zener-based fixed/cathode bias method in combination with the Ruby Fix. As I am not that expert, would you be so kind to review what I did:

-dummy load of 8ohm in the 8ohm speaker output
-amp on, stand-by off
-no input signal
-volume and tone at 0
-voltage on top of cathode R: 11.85V at idle
-input signal of 20mV, 1000Hz (perfect sinus wave)
-tone at 12o'clock position
-increased the volume until I could see onset to clipping on the oscilloscope
-left the volume pot in that position
-measured voltage on top of cathode R: 15VDC

If the above is correct, I plan:
-to put a zener of 15V or 16V (5W) in parallel with the cathode R and C. No 1N4007 needed here, right?
-to put for the Ruby Fix: zeners of 17V or 18V together with the diodes 1N4007 to reduce grid conduction according to Ruby schematic.

Any remark?
Already thanks for your reply!

[lowell]

Am I trippin or are the zener and regular diode drawn backwards?? Isn't it the UP-SWINGING half of the AC grid signal that we are concerned about?

[Dave H]

You're trippin The upswing is clipped by the EL84 grids. The idea is to clip the downswing at roughly the same voltage as the upswing so that there is no bias shift in the negative direction which would increase crossover distortion.

[Chuck H]

You're trippin The upswing is clipped by the EL84 grids. The idea is to clip the downswing at roughly the same voltage as the upswing so that there is no bias shift in the negative direction which would increase crossover distortion.

To elaborate on what Dave said... The extra negative swing beyond what's necessary to drive the tube into cutoff is being eliminated to reduce coupling cap loading due to grid conduction and the resulting grid bias shift.

Since the saturation portion of the wave form is the audible "tube clipping" you probably don't want to clip that with diodes. Even if the diode were such that the clipping occurred post saturation because it could still influence the shape. And then instead of "real tube clipping" you'd have "real diode clipping". If such things bother you

[lowell]

Yes that's exactly what was confusing me. I was interpreting the Ruby mod to be "limiting" the UPSWING so as to never allow grid conduction. I was wondering why no one was reporting the sound of diode clipping. . So really, with the mod, the grid still conducts but the blocking is REDUCED because the negative cycle is no longer compounding the problem. Correct?

[Dave H]

So really, with the mod, the grid still conducts but the blocking is REDUCED because the negative cycle is no longer compounding the problem. Correct?

Yes.