? - Paul Ruby Zener Diode mod

[hasserl]

You may have seen this before: 18-Watter Buzz Info

Re the value of the zener, he writes: "The trick is to chose a zener value to clip the grid signal somewhere beyond the tube cutoff grid voltage." Which he dtermined in his amp at 7volts.

My question is, How do you determine the grid voltage at cutoff? Is it as simple as scoping the output, slowly increasing the Volume until the tube goes into cutoff, and measure the ac voltage at the grids? I don't know how he got those scope shots of the grids showing the clipping on the positive swing and the deep negative swing.

[Steve Conner]

As a first approximation, I'd just make the zener voltage the same as, or a couple of volts more than, whatever cathode voltage you measure on the EL84s at idle. Or if you were doing it to a fixed-bias amp, a little more than twice the bias voltage.

The scope shots would have been done on a dual-trace scope, with one probe connected to each EL84 grid. They don't actually show where the tube cuts off, because they don't show plate or cathode current, and I don't quite see how Ruby deduced anything from them.

But assuming the tube to be more or less linear, the distance from idle to grid clipping (which is also roughly equal to the cathode voltage in a cathode biased amp, and the bias voltage in a fixed bias one) should be about the same as the distance from cutoff to idle. The latter is a little more, because the part of the tube charateristic near cutoff isn't linear: it's got a knee like a hockey stick. And that's the argument my approximation is based on.

[hasserl]

Thank you Steve, I was thinking that the cathode (bias) voltage was likely a key component.

[Chuck H]

You simply need a ZV that is above the bias voltage. In the case of a cathode biased amp you need to go generously above because the bias voltage shifts. Since the positive voltage on the cathode is the bias instead of the negative voltage on the grid, and rises, you need to go above the rise to insure no clipping of the waveform outside of the cutoff portion. Paul admits in the article that 7 ZV is too low but that's all he had on hand at the time. For a fixed bias amp you only need to exceed the fixed bias voltage. The way I do it with cathode biased EL84's is to measure the cathode voltage, then choose a THIRD zener (5W) that is somewhat above that voltage to parallel the cathode resistor. This effectively "fixes" the bias whenever the voltage rises to that point. I choose the ZV for that zener by using the scope to determine when clipping starts. That way I have an amp that is cathode biased when playing clean, but fixed bias when over driving which, IMHO is more ideal. But that's JMO. Then you can choose your Paul Ruby Mod ZV based on the parallel zener ZV. If you chose, for example, a ZV of 13 for the parallel zener you could then go with 15 ZV for the Paul Ruby Mod and be sure the bias won't shift beyond that point.

HTH

Chuck

[hasserl]

Good stuff Chuck. I've seen you post on the topic before, but now that I've got something like this on the bench it's more relative and makes more sense.

[Chuck H]

Yeah, this little trick to avoid grid conduction really came through for me in a pinch and I sort of massaged it from there.

FWIW I would pay attention to Steve's info about the fixed bias knee being non linear. I haven't had to deal with that yet. I'm biased pretty close to class A so it's less of an issue. But if your biased say, cooler with higher plate volts, as many Mesa and Traynor amps are, I think you would need to give more margin to the ZV than 2V as I have.

Chuck

[Merlinb]

But assuming the tube to be more or less linear, the distance from idle to grid clipping (which is also roughly equal to the cathode voltage in a cathode biased amp, and the bias voltage in a fixed bias one) should be about the same as the distance from cutoff to idle.

I think you're thinking of a roughly centre-biased or Class A amp. In a class AB amp the distance to cut-off is likely to be less than the distance to grid-conduction, so it should be quite safe to use a zener with approximately the same voltage as the bias. (Actually most SE guitar amps are so close to cut-off that you can apply the same rule there too)

[hasserl]

I noticed that the cathode voltage rises quite a bit from quescient to full volume, at least 2X. If you use a zener of the same value, or slightly higher, as the quescient voltage, won't that limit amplification at the power tubes? Or what effect wil it have to limit the voltage of the grids below that of the cathode? Will that result in a loss of power tube distortion?

[Merlinb]

I noticed that the cathode voltage rises quite a bit from quescient to full volume, at least 2X. If you use a zener of the same value, or slightly higher, as the quescient voltage, won't that limit amplification at the power tubes?

If the bias increases naturally then it pushes the tubes further towards class B, so it takes even less voltage to drive them to cut-off. Therefore the zener becomes even less likely to prematurely clip the signal. In other words, it is not a worry- quite the opposite!

[Chuck H]

Looks like I conceptualized that backwards too. I still like the circuit I'm using, but now I understand better why it works.

Chuck

[RudeBoy]

If the bias increases naturally then it pushes the tubes further towards class B, so it takes even less voltage to drive them to cut-off. Therefore the zener becomes even less likely to prematurely clip the signal. In other words, it is not a worry- quite the opposite!

Can a zener be used across the cathode resistance to limit this effect, set up, say a volt or so higher than the idle voltage?

If so then what would be the effect on crossover distortion and other characteristics? (If not - never mind )

[Steve Conner]

Sure it can, and people here have tried it and reported that it works well. Isn't it the Chuck H mod? :-)

[RudeBoy]

Sure it can, and people here have tried it and reported that it works well. Isn't it the Chuck H mod? :-)

Thanks. I did some googling, found this link where Chuck H discusses it.

[hasserl]

Just to follow up on this thread, I had 12.7 volts on the cathode at idle, so I found some 14v zeners at a local supply house and installed those per the Ruby mod. This definitely reduced the "fizziness" of the amp, allowing good full overdrive of the amp.

Before mod voltage check showed the grid voltage at the power tubes would climb above 30vac w/ 1k 250vac sine wave at the input. Post mod voltage check shows the grid voltage will not climb above ~20vac. I thought it would be limited to the 14v of the zeners, but it's not, it does climb up to around 20.

[Merlinb]

Post mod voltage check shows the grid voltage will not climb above ~20vac. I thought it would be limited to the 14v of the zeners, but it's not, it does climb up to around 20.

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