Paul Ruby mod in Vox Night Train

[Silvio55]

Iīm now trying the Paul Ruby mod on my night train, to get rid of the buzz when you crank the MV. As the mod consist on the 1N4007 and the zener parallel to the grid resistor, that on most amps goes after the coupling cap, in this amp you have one cap before the resistor and one cap after the resistor (never knew why 2 caps in series with the resistor in betwen), if I try this mod should I remove and bypass the cap that goes after the grid resistor? if I do that should I lower the value of the remainig cap?
Thanks in advance!!

[Chuck H]

I actually don't think the PR mod is a good idea with a PPIMV.

For one, if the buzz is crossover distortion you would hear the effect get worse as the power tubes are driven harder. But with the NT the problem becomes less audible as the power tubes are driven harder.

Also, the PR mod clips the negative swing of each PI output. This MUST happen when the power tubes are driven into cutoff or the diode clipping will be heard. Using a PPIMV will cause that to happen.

[Silvio55]

Chuck, the fizzy note dacay is not what Iīm trying to fix with the PR mod, when I crank the MV I hear that buzz of the crossover distortion wich sounds like a weird fuzz and a phaser effect.
Now I didnīt knew that the PPIMV would be a problem with the PR mod, now I think it youīre right (DAMN!!!)
Is it possible to do if I mod the MV and make it a pre PIMV?

[Chuck H]

Is it possible to do if I mod the MV and make it a pre PIMV?

Yes. But you won't have the same level of distortion at low MV settings. Not nearly in fact.

The PR mod should be tailored to the amp it's being used in. There are no generic values for the zeners. This might go long but hang in there.

You need to measure your cathode voltage when the amp is at full clip. The reason is that when an amp is cathode biased it is "self biased". The more current the tubes try to pass the more the bias automatically cools in order to accept the input signal. This bias cooling is the negative relationship between the grid and cathode and presents as a rise in cathode voltage. The more positive the cathode is, the more negative the grid is WRT the cathode. EL84's seem especially prone to this phenomenon. The cathode voltage can rise to double or more what it is at idle when the amp is clipping. It's this cold bias that causes the crossover distortion.

The cathode voltage determines how far the signal can swing before the tube saturates or cuts off. Since the PR mod zeners must conduct ONLY when the tube is in cutoff to stay out of the signal chain, the zener voltage must exceed the maximum cathode voltage. So, if you were to measure your cathode voltage at full clip and find it's at, say, 20 volts, you would choose a zener voltage of 22volts. This won't eliminate all of the crossover distortion because the bias is still allowed to go very cold due to the self biasing effect of cathode bias. The primary purpose of the PR mod is to allow the PI coupling capacitors more time to discharge. This reduces "grid loading"

Grid loading also cools the bias. When the power tubes draw current all those electrons must first pass the grid. Called the "control grid" for good reason. Because of the huge amount of electrons passing through the grid circuit, some voltage builds up on the grid. The grid must unload this voltage in order to prevent bias shift. The trouble is that the "time constant" of the PI coupling caps and the grid load resistors (how fast this circuit can unload voltage as dictated by it's impedance) is too slow to prevent grid loading. The grid circuit simply can't unload voltage as fast as it's building up. That standing voltage on the grid presents as yet another bias modifier. The purpose of the PR mod is to allow a low impedance circuit for the grid circuit to unload while the tube is in cutoff. This way the zener effect is never part of the actual signal. It just lends a hand as another bias modifier that reduces the grid loading effect.

So if you were to use a zener voltage of 22 volts the bias would already be substantially cold even without the grid loading. So there will likely still be some crossover hash. But there's another bias modifier circuit that can help with that too.

What I do is add another zener parallel to the cathode bias resistor. I set this zener voltage just above the point of clipping. That way the amp still has the cathode bias mojo when running clean, but once it begins to clip the bias essentially fixes at the zener voltage. So it's like having a cathode biased amp clean and a fixed bias amp when clipping. The other advantage to the added circuit is that you will now know what the maximum voltage on the cathode will be because it cant rise above that zener voltage. So you can select the PR mod zener voltage a volt or two above the cathode resistor zener and be sure that the PR mod zeners won't conduct unless the tubes are in cutoff. This new zener also allows for a lower peak cathode voltage. So the bias cooling effect is minimized AND there is more time for the grid circuit to unload minimizing the grid loading effect as well.

The end result is almost no crossover distortion. For better or worse. I say that because crossover distortion is one component of "swirl". That "wha" effect that some amps seem to have. IMHE that effect is lost on EL84 tubes and doesn't sound so much like "wha" as "bwizz". So I just squash it with the above described circuit.

So... You need a way to measure your cathode voltage at the onset of clipping. Then choose a cathode resistor parallel zener (5W DO package) at a voltage just above the clipping point. Then use PR mod zeners (1/2 watt or better) just above the cathode zener voltage. That's the circuit I use with EL84's and it's a little too good for me to share it so freely.

EDIT: The "onset of clipping" measurement should be taken with the MV set full up or removed.

[Silvio55]

Chuck, remeber that the Night Train has a cathodyne PI, I dont know if much part of the distortion cames from the PI itself.
I read on another thread that the voltage I needed to measure was the cathode voltage at idle, and then choose a zener a couple of volts more, maybe itīs just an aproximation but itīs the only method I think I can use, not having access to a scope.

[Chuck H]

Cathodyne, right. I did forget.

Ideally choosing a PR mod zener a couple of volts above the cathode voltage will work. I was a little dramatic above. In truth, if the cathode voltage is rising then the amp should be conducting in AB. So there shouldn't be any audible zener clamping. But to be sure I like to use the zener parallel to the cathode resistor as well and then size the PR mod zeners a volt higher. You don't need to be able to pinpoint clipping to do it. I just do it to idealize. If you read 10V on the cathodes a few volts higher for the cathode resistor zener should get you into safely into the clipping area. So a cathode resistor zener voltage of 14V should work to clamp the voltage rise on the cathode. Then your PR zener voltage would be 15V.

You do still need to move the master volume control for this circuit to work correctly. And FWIW I've never used the PR mod on a cathodyne but I don't percieve any reason it shouldn't work.

[Silvio55]

Ok, I measured the grid voltage and got 9.5 volts, the only 1/2 watt zeners I could get were 12 volts, wich I think itīs ok. I donīt have right now a pot to do the MV mod or 5watts zeners. What would be the ideal value for a pre PIMV? 1M? I have to order the pot and the 5W zeners I donīt think I could get them until next month.

[Chuck H]

Approximating on my own experience I would make the cathode zener 13 volts and the PR zeners 15 volts (or 14 if you can get them).

1M is the right value for the MV. If you look at the schem, just replace the 1M load resistor with a 1MA pot and put the PI on the wiper.

[Silvio55]

Just asking... Are EL84 fixed bias amp less prone to generate crossover distortion? if so, is it possible to convert the Night Train to fixed bias without changing the PT?

[Chuck H]

Are EL84 fixed bias amp less prone to generate crossover distortion?

Yes. You know how the voltage on the cathode resistor rises as the amp draws current? That voltage IS the bias. So if the positive voltage on the cathode of a cathode biased amp goes higher it's the same as in a fixed bias amp if the negative grid voltage were to go higher. Either way the grid becomes more negative with respect to the cathode. And that is a "cooling" of the bias. A cold biased amp is prone to crossover distortion. Cathode biased amps usually idle hotter than fixed biased amps and then the bias cools as current is drawn due to the "self biasing" circuit.

FWIW this is what the extra zener across the cathode resistor is for. It "fixes" the bias at the zener voltage. I like to leave a little room for the amp to operate "self biasing" when clean, but set the zener voltage so that as the tubes begin to clip the bias voltage is fixed. this method IS usually a tad cooler bias than one might choose for a grid biased, fixed bias setup. But it's a good compromise. And combined with the PR mod pretty much squashes crossover distortion. My amps do have a little. But it's just a very small bump at the crossover point instead of a dash.

If you want to try fixed bias you can use this same method but choose a cathode zener value that is the same as your cathode resistor voltage. If the voltage relationship between the grid and cathode is steady, and it would be if you did this, then the bias is "fixed" It's just being done at the cathode instead of the grid.

if so, is it possible to convert the Night Train to fixed bias without changing the PT?

The plate voltage on the NT isn't high. You should be able to convert to a typical "grounded cathode/grid bias" without the need for different transformers. My only caveat would be that the amp would probably increase in power by as much as a few watts. If that causes any current demand problems for the existing transformers I can't say. It shouldn't. The iron should be over rated enough to handle it. But I've never seen one of these amps in person.

[Silvio55]

FWIW this is what the extra zener across the cathode resistor is for. It "fixes" the bias at the zener voltage. I like to leave a little room for the amp to operate "self biasing" when clean, but set the zener voltage so that as the tubes begin to clip the bias voltage is fixed. this method IS usually a tad cooler bias than one might choose for a grid biased, fixed bias setup. But it's a good compromise. And combined with the PR mod pretty much squashes crossover distortion. My amps do have a little. But it's just a very small bump at the crossover point instead of a dash.

If you want to try fixed bias you can use this same method but choose a cathode zener value that is the same as your cathode resistor voltage. If the voltage relationship between the grid and cathode is steady, and it would be if you did this, then the bias is "fixed" It's just being done at the cathode instead of the grid.

Ok, so basically the zener accros the cathode resistor makes it act like a fixed bias, if I put a zener value thatīs the same as the cathode voltage the amp would become fixed bias but it would be biased very hot wouldnīt it? If I just put the zener across the cathode resitor, with a little room for cooling the bias (a zener value of a few more volts) but donīt put the PR mod diodes at all, would it help with the cross over distortion?

[Chuck H]

If I just put the zener across the cathode resitor, with a little room for cooling the bias (a zener value of a few more volts) but donīt put the PR mod diodes at all, would it help with the cross over distortion?

Yes. That's why I tried it in the first place. I liked the secondary affect of it tightening up the distortion as well so it became a permanent feature in my design. I use the PR mod as well.

[frus]

but could you put PR diodes between the wipers of PPIMV pots and the ground?

[Chuck H]

but could you put PR diodes between the wipers of PPIMV pots and the ground?

Back up and read the whole thread.

[Silvio55]

Chuck, thanks so much for your answers, even when I do some stupid questions, you know when I first posted in this forum I knew NOTHING about tubes and electronics, itīs been a long way thru learning this few months thanks to this forum, real cool people around here!!!
Back to the thread, I really donīt want to mod the MV for now, so doing the PR mod is not feasible, but just putting a zener across the cathode resitor looks pretty easy, I used the weber bias calculator and got 13 watts dissipation at idle (at 10.3 volts of cathode voltage and 318 volts plate voltage) wich is hot, I know cathode biased amps idle in the hot side but is it normal, to excede the tubeīs rated max dissipation?
13 Volts is the most recomended value for the cathode zener? How much does the bias cool down at 13 volts?