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Old 19th November 2012, 03:56 PM   #31
roger57 is offline roger57  Canada
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Quote:
Originally Posted by SY View Post
Just a quick rule of thumb- the DN2540 cascodes don't really work well until you have 10mA or more of current through them. Bipolar cascodes are much better in this regard. Also, make sure you have sufficient voltage across the CCS so it doesn't run out of compliance on signal swings (most important in plate circuits). That is, you want the voltage to accommodate any expected swing plus leave at least ~20 more volts across the CCS.
Hi SY,

Thank you for taking the time to respond. Since my grids are DC drive from the front end @ 95V, I have sufficient cathode voltage to keep the CCS happy. As for the 10mA, in it's current state, bringing up the current from it's present 9.5mA, or even 10.5mA doesn't seem to displease the 6SN7's, in spite what I've read about optimal current at 8mA.

I'm not stuck on using FETS; having no expereince with CCS to this point they seem to be the favorite. However, I will happily take another run at a bipolar, again using an MJ340 and a BC550C. My first try resulted in less than stellar performance, I'm not sure if it was related to using 2 LED's in the BC550 reference? This time I'll go with a single but higher voltage (as Salas has done) and see how that works out.

Again, I believe that the stage has become more sensitive to drive so this could be part of the problem. I'm getting 2Vp higher than before without touching anything else, so driving the 6L6GC grids harder could result in additional thirds being generated in the push-pull. I'm working on optimizing all front end values to minimize 3rd order distortion in the LTP.

Regards,
Gary
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Old 19th November 2012, 06:54 PM   #32
roger57 is offline roger57  Canada
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Default Ratio of Harmonics?

Hope someone can answer this question.

At what ratio of second to third harmonic, does the second harmonic begin to "mask" the dissonance of the third? If it depends on how far below the third is; what would that be?

Thanks,
Gary
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Old 19th November 2012, 08:12 PM   #33
Salas is online now Salas  Greece
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Only from small experience from own builds, nothing scientific here, when with 10 to 20% less 3rd than 2nd and minimum rest of odds if any, they were usually pleasing builds.
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Old 19th November 2012, 08:16 PM   #34
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8mA is a minimum for the 6SN7s- they'll work fine at 10mA as long as you watch plate dissipation.
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Old 19th November 2012, 08:31 PM   #35
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Quote:
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8mA is a minimum for the 6SN7s- they'll work fine at 10mA as long as you watch plate dissipation.
Guys,

Thanks again for your responses, very helpful. I'll make the front end changes tonight on my amplifier and report back on what I hear. I suspect it will make all the difference I am expecting at this point with the present regulators.

Gary
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Old 1st December 2012, 04:25 PM   #36
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Default No changes "tonight" !!

Gents,

I've been so busy at my full-time job I haven't had time to implement the circuit yet. It will also be part of the regulators change-out to the SSHV2 at the same time, so it will be a week or so yet.

Feedback is coming...

Gary
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Old 16th December 2012, 06:44 PM   #37
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2 SSHV2 regulators have been installed; 1 for the driver supply (375V) which requires about 20mA; current adjusted to 40mA. It''s working fine.

The other is the screen supply (350V) which requires about 60mA at full output; adjusted to 80mA.
Unfortunately, it looks like the SSHV2 doesn't like the load the screens are presenting. Looks like it was oscillating, and caused some issues with the cathode bias, where the cathodes voltages on the tubes went anywhere from 14V up to 25V. (They were all previously adjusted at 19V). Finally, the SSHV2 output dropped to 300V but maintained current. I disconnected it and ran it again just on the load and it seems to be OK. (350V, 80mA incl. 20mA shunt current) I tested screen current draw on my last regulator, it's not constant, requiring a few mA at idle and about 60mA at full load.

Would the SSHV2 have some defining parameter that would make it difficult to drive a finicky load such as screens? I'm using 100 ohm screen resistors right at the pins. The linear regulator I was using before (the Maida) didn't have any of these issues.

Gary
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Old 16th December 2012, 09:12 PM   #38
Salas is online now Salas  Greece
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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.
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Old 16th December 2012, 10:15 PM   #39
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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.
Hi Salas,

Find attached the LT Spice model.
Another odd thing; the heat sink I have it attached to is quite big and more than adequate (bigger than the picture I posted before). Input voltage the same. Yet, the temperature of the device is around 70C when running. I'm monitoring current and voltage, both are running at spec. It did not get this warm with just the 5.85K resistive load.
Something odd going on...perhaps oscillations. I'm going to scope it and see what might be there.

Gary
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Old 16th December 2012, 10:19 PM   #40
Salas is online now Salas  Greece
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Scope could confirm, if the CAT rating of your probe at x10 is safe. But screen is dynamic and spares 60-70mA reg CCS at low swing. So that could explain heat too. Do you have 1uF 400V capacitor to replace the now 0.33uF so we can increase the termination to see what happens?
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