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Old 16th December 2012, 09:23 PM   #41
Salas is offline Salas  Greece
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*I miss pentode2 symbol to run the .asc but I see the diagram. R28,29 100 Ohm for real, yes?
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Old 16th December 2012, 09:31 PM   #42
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
Yes there will be situations that it can be at phase odds. Especially when anode regulation is there too. Do both regulators draw from same voltage source? Can you show the power tube stage circuit detail? Maybe we can work around that by tweaking the screen reg a bit.
I missed your question. The plate supply comes in around 430V and feeds both regulators.
I scoped the screen supply, and nothing odd there; just 10mV of noise. Where the trouble starts is when I apply a signal to the input; the cathode voltages go wild. I already trashed 1 tube so I don't want to apply an input signal until I have this figured out.
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Old 16th December 2012, 09:36 PM   #43
Salas is offline Salas  Greece
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So you scoped the screen pin with the reg on and its DC line when no signal goes to the tube?
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Old 16th December 2012, 09:39 PM   #44
roger57 is offline roger57  Canada
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*I miss pentode2 symbol to run the .asc but I see the diagram. R28,29 100 Ohm for real, yes?
Salas,

Here's corrected model with the installed screen resistors (100 ohms) Also find the model for the "pentode 2"
Ignore the frequency response; the transformer is modeled without real parasitic values. The model was to verify overall correctness, voltage values and distortion plots.
Attached Files
File Type: asc 6L6GC_Full_Amp_Simulation_As_Built_Plus_CCS_DN2540x2.asc (13.2 KB, 14 views)
File Type: zip pentode2.zip (543 Bytes, 10 views)
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Old 16th December 2012, 09:40 PM   #45
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
So you scoped the screen pin with the reg on and its DC line when no signal goes to the tube?
Sorry, we're a little out of sync. I just uploaded good files.
Yes, that's right, I scoped with 350V DC at screen resistor, with no signal in and there's no funny stuff.
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Old 16th December 2012, 09:56 PM   #46
Salas is offline Salas  Greece
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Now it deploys well but I will have to add models since you got an include text I would naturally miss. So, 4 screens in parallel from a shunt reg that does not do funnies but when signal goes to tubes they go biasing all over the place...Hmm...The change from Maida is now the voltage source to screens has 25 times the closed loop bandwidth and much lower Zsource in high band too. It could have been the reg facing some load with capacitive characteristics that chokes its phase margin but that would have manifested immediately on idle. The tubes don't like the shunt reg's voltage source characteristics I am afraid when they are driven with signal. In your shoes I would up the screen resistors to highest permissible, change the 0.33uF on the reg to 1uF and give it a try with any spare low cost or bit worn out tubes in case some screen goes poof. If it would still don't go well, I would forget the idea of using this shunt reg in that amp for screens, and save the extra SSHV2s for other compatible applications in the future.
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Old 16th December 2012, 10:05 PM   #47
roger57 is offline roger57  Canada
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Originally Posted by Salas View Post
Now it deploys well but I will have to add models since you got an include text I would naturally miss. So, 4 screens in parallel from a shunt reg that does not do funnies but when signal goes to tubes they go biasing all over the place...Hmm...The change from Maida is now the voltage source to screens has 25 times the closed loop bandwidth and much lower Zsource in high band too. It could have been the reg facing some load with capacitive characteristics that chokes its phase margin but that would have manifested immediately on idle. The tubes don't like the shunt reg's voltage source characteristics I am afraid when they are driven with signal. In your shoes I would up the screen resistors to highest permissible, change the 0.33uF on the reg to 1uF and give it a try with any spare low cost or bit worn out tubes in case some screen goes poof. If it would still don't go well, I would forget the idea of using this shunt reg in that amp for screens, and save the extra SSHV2s for other compatible applications in the future.
Salas,
Sorry, I missed your post asking about the 1uf/400V capacitor. I do have one.
I have measured the screen before and it "idles" with no signal in around 4 or 5mA. At full voltage in, around 1.8V, I measured 3V across a 50 ohm resistor and it dropped 3V (60mA) This is RMS; the model's predicted value is
about 70mA.

However, I've bottom mounted both power devices so it will take me awhile to take it apart to change it out. While it's apart, I might used a more thermally conductive heat pad on the IRF840 so it's spot temperature doesn't get so high.

Either way...I need to remove the regulator to change the cap or to just go back to the Maida linear regulator. However, what do you think are the chances of success with 470 ohm resistors, and the 1uF capacitor? Changing all the screen resistors will be a pain. I know that they will ultimately affect the sound, so I wouldn't want to go more than 470 ohm.

Attached is the tube model lib from Norman Koren's archive.

Gary
Attached Files
File Type: zip Tube.zip (4.1 KB, 4 views)
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Old 16th December 2012, 10:18 PM   #48
Salas is offline Salas  Greece
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Gary, with 470 Ohms there is a chance the screens will get some extra damping enough to make them happy with their new DC voltage source, but its only a hypothesis to test. Just not to retreat on first no go by the tubes. Can't predict. You could just piggyback the cap for testing on the existing 0.33u for 1.33u total. So not to dismantle now. But its your choice testing. Maybe no cigar.
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Old 16th December 2012, 10:51 PM   #49
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
Gary, with 470 Ohms there is a chance the screens will get some extra damping enough to make them happy with their new DC voltage source, but its only a hypothesis to test. Just not to retreat on first no go by the tubes. Can't predict. You could just piggyback the cap for testing on the existing 0.33u for 1.33u total. So not to dismantle now. But its your choice testing. Maybe no cigar.
OK...I realize it's a difficult thing to know, given the nature of the circuit, layout and other variables. However, I'm first going to use the old Maida screen regulator in lieu of the SSHV2 to ensure nothing else is amiss. If that works OK in the circuit alongside the SSHV2 feeding the driver/splitter circuit, I will then change the cap and grid resistors and try again the SSHV2 on the screen supply. One thing I should have mentioned is that when I had the Maida regs installed, I was using a 100uF Jensen cap at the output (linear regs like this anyhow) and I'm sure this capacitance had a lot to do with the stability of the linear regulator. As we know, it doesn't do much for the overall performance though.

I do have one further question. I am using a common point ground post for every single ground connection in this amplifier. That is, every ground goes to this point. I have the FORCE & SENSE 0's of the SSHV2's tied to this point.
I did not tie anything to the input ground of the SSHV2 PCB since it is already common with the FORCE 0 line. Should this cause any problem? I think when using this grounding method it is the correct way to do it, unless there is an issue with the remote sensing. It doesn't appear to affect the operation but maybe there's more to it?

Last edited by roger57; 16th December 2012 at 10:55 PM.
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Old 16th December 2012, 11:10 PM   #50
Salas is offline Salas  Greece
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Sense and force look for the nearest return point that the consumption node refers to. If you have a sub star for the 4 cathode resistors, take those lines there, and tie input separately where the PSU that feeds the reg returns.
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