Exactly.TriodeLuvr wrote: βMon 01/04/21 3:18 am
The cathodyne is the least only if we don't consider the other half of the tube. If we think about the 12AX7 driver as a whole, then the cathodyne PI will have one half of the tube with a gain of roughly 50 driving the other half (the actual cathodyne stage) with a gain of about unity. The LTP exhibits gain of about half that amount or a little more.
Parasitic oscillation problem untraceable
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Re: Parasitic oscillation problem untraceable
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Re: Parasitic oscillation problem untraceable
Thx for that clarification
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Re: Parasitic oscillation problem untraceable
I'm not sure about "LPT" but for a Long Tail Pair PI ("LTP"), that shared resistor sets the bias and there's nothing to worry about.
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Re: Parasitic oscillation problem untraceable
Unbalancing the PI creates even-order harmonics. That's OK, but it's not related to his problem and it might make troubleshooting more difficult. If he builds the phase inverter exactly according to the original Marshall design, it can be eliminated as the culprit.JMPGuitars wrote: βMon 01/04/21 7:07 am
Yup. The (almost) perfectly balanced PI is more for hifi than guitar.
As an aside, it appears to me that the original 82K/100K design is already unbalanced. The correct values for a balanced PI appear to be 91K/100K. Maybe 100K/100K simply unbalances the PI in the opposite direction.
https://www.ampbooks.com/mobile/amplifi ... iled-pair/
This could be confirmed with a distortion analyzer. I'll try to remember to do that when I get my amp built.
Jack
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Re: Parasitic oscillation problem untraceable
How does a slightly lower amplitude on one side introduce harmonics?
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Re: Parasitic oscillation problem untraceable
The imbalance creates an asymmetrical waveshape. Asymmetry is composed of even-order harmonics.
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Re: Parasitic oscillation problem untraceable
That is correct. I don't think it matters either way for his issue.TriodeLuvr wrote: βMon 01/04/21 12:08 pmUnbalancing the PI creates even-order harmonics. That's OK, but it's not related to his problem and it might make troubleshooting more difficult. If he builds the phase inverter exactly according to the original Marshall design, it can be eliminated as the culprit.
As an aside, it appears to me that the original 82K/100K design is already unbalanced. The correct values for a balanced PI appear to be 91K/100K. Maybe 100K/100K simply unbalances the PI in the opposite direction.
91K is correct, but 82K supposedly became the standard because it was a value more readily available. Honestly it's kind of silly. But also consider that they were using mediocre carbon comps whose values would stray up. So they could have found 82Ks that ran high fairly easily. If it's 10% tolerance, and straying up, then they could be very close to balanced if the 100Ks are accurate enough.
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Re: Parasitic oscillation problem untraceable
LOL, you're absolutely right. It's easy to forget sometimes about the real tolerances with this type of circuitry. I'm old enough to remember when most schematics had a note at the bottom, something like "All voltages are +/- 20%." Modern circuitry with regulated supplies and semiconductor junctions is really OCD in comparison.
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Re: Parasitic oscillation problem untraceable
Well I think there is some progress. There is still fizzy on the OD but it's creamier. Perhaps it's just the nature of the amp?
The changes I made:
PI:
RK 470R + 22k tail
6V6's:
RK 330R + 1000uf cap (91.7%)
1k5 5W screen resistors
All in all sounds pretty good, but I'll have to play some more. It's pretty late now...
Here's the voltage chart.. is the negative grid voltage normal on V1? Nothing's plugged in the input.
Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V
Sound clip:
https://www.dropbox.com/s/er24jamf71c5q ... 4.m4a?dl=0
The changes I made:
PI:
RK 470R + 22k tail
6V6's:
RK 330R + 1000uf cap (91.7%)
1k5 5W screen resistors
All in all sounds pretty good, but I'll have to play some more. It's pretty late now...
Here's the voltage chart.. is the negative grid voltage normal on V1? Nothing's plugged in the input.
Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V
Sound clip:
https://www.dropbox.com/s/er24jamf71c5q ... 4.m4a?dl=0
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Re: Parasitic oscillation problem untraceable
Both V1 grids should be grounded by the input jacks when nothing is plugged in. If you didn't use switched jacks, a small negative voltage might be normal. 1M to ground is probably high enough to allow the grid(s) to develop voltage due to contact potential. IOW, the 1M might be acting as a grid leak resistor, not just a grid resistor (the two terms are frequently misused).
At 30V, the amp is deep into Class B. That's almost certain to be the problem. You're hearing crossover distortion.Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V
Jack
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Re: Parasitic oscillation problem untraceable
It's a single input cliff jack. Grounded when nothing's plugged in...TriodeLuvr wrote: βMon 01/04/21 5:15 pmBoth V1 grids should be grounded by the input jacks when nothing is plugged in. If you didn't use switched jacks, a small negative voltage might be normal. 1M to ground is probably high enough to allow the grid(s) to develop voltage due to contact potential. IOW, the 1M might be acting as a grid leak resistor, not just a grid resistor (the two terms are frequently misused).
At 30V, the amp is deep into Class B. That's almost certain to be the problem. You're hearing crossover distortion.Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V
Jack
PT bias is 92% now... so raising that more???
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Re: Parasitic oscillation problem untraceable
If it's grounded, and the grid is connected to it, the grid can't have standing DC. Check it with an ohmmeter.
No, that won't fix it. The issue is that bias is changing when you apply signal. The only solution is fixed bias, either at the cathode or at the grid. You can test it the way I suggested earlier. Just make sure the batteries are securely connected. You wouldn't want to lose bias once the cathodes are grounded. Set the 100K pot so you have -20V on the grids, and you'll be OK. Once you've confirmed this is the problem, you can decide how to proceed.PT bias is 92% now... so raising that more???
Jack
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Re: Parasitic oscillation problem untraceable
That's not the only solution. It's just the one you prefer.TriodeLuvr wrote: βMon 01/04/21 5:46 pmIf it's grounded, and the grid is connected to it, the grid can't have standing DC. Check it with an ohmmeter.No, that won't fix it. The issue is that bias is changing when you apply signal. The only solution is fixed bias, either at the cathode or at the grid. You can test it the way I suggested earlier. Just make sure the batteries are securely connected. You wouldn't want to lose bias once the cathodes are grounded. Set the 100K pot so you have -20V on the grids, and you'll be OK. Once you've confirmed this is the problem, you can decide how to proceed.PT bias is 92% now... so raising that more???
Jack
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Re: Parasitic oscillation problem untraceable
For whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).
-Dan
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Re: Parasitic oscillation problem untraceable
The PR mod is a good option to try, and Bieworm knows how to do it (though he'll need a higher voltage zener). The Snubber circuit could also help. A couple other possibilities too. There's more than one way to skin a walrus.dbharris wrote: βMon 01/04/21 6:01 pmFor whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).
-Dan
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Re: Parasitic oscillation problem untraceable
That's exactly what I said - it's necessary to stabilize the bias either at the cathode or grid. Installing Zeners would stabilize the bias at the cathode.
Did Marshall use self-bias with 6V6s in PP? You can't just throw any tube type into the mix without re-engineering the design. EL84s run close to Class A in many older amps, and those designs are less prone to this problem.
I'm curious, being as the schematic that Bieworm posted in this thread shows the 6V6 outputs with fixed grid bias. How did the change to a self-biased version come about?
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Re: Parasitic oscillation problem untraceable
Because I had this fizz problem to begin with. I thought the PI gave too much gain causing this fizz.TriodeLuvr wrote: βMon 01/04/21 9:26 pmThat's exactly what I said - it's necessary to stabilize the bias either at the cathode or grid. Installing Zeners would stabilize the bias at the cathode.
Did Marshall use self-bias with 6V6s in PP? You can't just throw any tube type into the mix without re-engineering the design. EL84s run close to Class A in many older amps, and those designs are less prone to this problem.
I'm curious, being as the schematic that Bieworm posted in this thread shows the 6V6 outputs with fixed grid bias. How did the change to a self-biased version come about?
Jack
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Re: Parasitic oscillation problem untraceable
That's what I was thinking too. So I re-added the PR mod that I already tried earlier. I figured the 24V zeners I had should do the job. But to no avail...JMPGuitars wrote: βMon 01/04/21 6:11 pmThe PR mod is a good option to try, and Bieworm knows how to do it (though he'll need a higher voltage zener). The Snubber circuit could also help. A couple other possibilities too. There's more than one way to skin a walrus.dbharris wrote: βMon 01/04/21 6:01 pmFor whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).
-Dan
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Re: Parasitic oscillation problem untraceable
TriodeLuvr wrote: βMon 01/04/21 1:23 pmThe imbalance creates an asymmetrical waveshape. Asymmetry is composed of even-order harmonics.
OK, I am going to postulate something everyone will probably hate here. Asymmetry does not necessarily imply any harmonics at all. I believe a lot of folks have simply taken Merlin's statement (from his discussion of the AC coupled LTP)...
"the inverting output will be 6% higher than the non-inverting output. This could be corrected by making Ra1 6% smaller in value, but in practice it is not necessary; a slightly unbalanced phase inverter is often quite benificial [sic] to guitar tone, due to the additional 2nd harmonic it introduces."
...as gospel without actually thinking about whether it's accurate. All the higher amplitude of one side does is make the height of the tops larger than the depths of the troughs (or vice versa). It merely attenuates one side of the fundamental first order harmonic. That is very different than adding a 2nd, 3rd, 4th or higher order harmonic to the fundamental! Merlin was just plain mistaken when he claimed that a second harmonic is introduced. There is no harmonic introduced. The fundamental is merely slightly quieter on half of the waveform; it's not shifted and no harmonic is added. Take a look at the image attached. Do ANY of these 2nd, 3rd or 4th harmonic output waveforms occur with a mismatched PI? NO. That's because the PI doesn't add harmonics through mismatched plate voltages; rather, it just makes the 1st order fundamental slightly asymmetric.
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