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PostPosted: May 22nd, 2018, 11:28 pm 
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I want to add screen regulation to an amp I built a few years ago. I was going to use zeners but then I discovered the LR8. Any comments on my proposed design below? Will it work? Will it blow? Will it give me decent load/line regulation? Most circuits I've seen online use an NPN (TIP50) instead of a MOSFET, but MOSFETs seem ideal for this application since they require so little gate current. My input voltage will be 430V and I'm shooting for a Vout of 325V.

U1 will be an LR8, obviously. M1 is will be an IRFB11N50APBF.

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PostPosted: May 23rd, 2018, 7:05 pm 
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:character-oldtimer: Looks like it'll work, but you might want a little more load on the LR8, the <8mA through R1 is pretty close to the minimum 5mA spec. Also, you really need to regulate either 1: B+ & screen, or 2: neither, but not one or the other....

Roscoe


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PostPosted: May 24th, 2018, 9:46 am 
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Thanks, Roscoe. The LR8 datasheet I was looking at had a 0.5mA min output current so I think that will be ok.

This is the first I've heard about regulating B+ and the screens, not just the screens. I've had a lot of people recommend regulating the screens to me, but no one has mentioned regulating B+ (which I believe refers to the plate voltage, right?). Is there no advantage to regulating just the screens? Regulating both would be a pretty big change to my amp and I'm not sure I'm up for that. Should I just leave it the way it is? Here is the full schematic: https://www.dropbox.com/s/qe1a8vvzyq7x1d0/ST-70%20clone%20schematic.pdf?dl=0. I was planning on replacing the box that says "regulated screens option" with the circuit above.


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PostPosted: May 24th, 2018, 9:53 am 
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justinis wrote:
Thanks, Roscoe. The LR8 datasheet I was looking at had a 0.5mA min output current so I think that will be ok.


Oops. With the 407K R1 it'll be just less than 0.8mA, so you may still want to add more load.... Either a smaller R1 (and adjusted R3 to get the voltage where you want it) or a 100K resistor in parallel with C2 ought to do the job. Keep in mind that 100K at 325V is just over 1W.

justinis wrote:
This is the first I've heard about regulating B+ and the screens, not just the screens. I've had a lot of people recommend regulating the screens to me, but no one has mentioned regulating B+ (which I believe refers to the plate voltage, right?). Is there no advantage to regulating just the screens? Regulating both would be a pretty big change to my amp and I'm not sure I'm up for that. Should I just leave it the way it is? Here is the full schematic: https://www.dropbox.com/s/qe1a8vvzyq7x1d0/ST-70%20clone%20schematic.pdf?dl=0. I was planning on replacing the box that says "regulated screens option" with the circuit above.


If you only regulate one, then the difference between B+ and screen voltage changes with line voltage. I'll let one of the guys who routinely builds with pentodes comment further...

Roscoe


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PostPosted: May 24th, 2018, 10:22 am 
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Interesting... I see your point. The screen voltage will always be a certain value thanks to the regulator's internal reference, while the plate voltage will fluctuate. I don't really understand the negative effects of this, but it does seem odd.

Right now I use a resistor divider to set the gate voltage of a MOSFET that drops the difference between the plates and screens voltages. Maybe if I made this resistor divider draw more current than the screens do, I could get the best of both worlds? The screen voltage and plate voltage would still move together, but the screen voltage wouldn't fluctuate with screen current. What do you think? Obviously I would need to work out how much power would be wasted and how hi-watt the resistors would have to be.


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PostPosted: May 24th, 2018, 11:36 am 
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Roscoe Primrose wrote:
justinis wrote:
Thanks, Roscoe. The LR8 datasheet I was looking at had a 0.5mA min output current so I think that will be ok.


Oops. With the 407K R1 it'll be just less than 0.8mA, so you may still want to add more load.... Either a smaller R1 (and adjusted R3 to get the voltage where you want it) or a 100K resistor in parallel with C2 ought to do the job. Keep in mind that 100K at 325V is just over 1W.

justinis wrote:
This is the first I've heard about regulating B+ and the screens, not just the screens. I've had a lot of people recommend regulating the screens to me, but no one has mentioned regulating B+ (which I believe refers to the plate voltage, right?). Is there no advantage to regulating just the screens? Regulating both would be a pretty big change to my amp and I'm not sure I'm up for that. Should I just leave it the way it is? Here is the full schematic: https://www.dropbox.com/s/qe1a8vvzyq7x1d0/ST-70%20clone%20schematic.pdf?dl=0. I was planning on replacing the box that says "regulated screens option" with the circuit above.




If you only regulate one, then the difference between B+ and screen voltage changes with line voltage. I'll let one of the guys who routinely builds with pentodes comment further...

Roscoe


Mr. Wrong here,

But I'd always heard that regulating the screens, not B+ is adequate, since cathode current is dependent on screen voltage to a large degree. It is necessary to NOT regulate the bias supply if fixed bias is used (why is fixed bias adjustable??? Crazy terminology). As B+ changes vs. line voltage, so does unregulated bias voltage. The tracking of those two minimizes cathode current variations due to line voltage variation.

A look at a typical set of plate curves shows current variation vs B+ and vs G2 variation.

Hope this helps


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PostPosted: May 24th, 2018, 1:51 pm 
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Hmm... that makes sense. I am using 'fixed' bias with an unregulated supply. So that's one for and one against regulated screens (without regulating plates)... not sure how to proceed... :crazy:


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PostPosted: May 24th, 2018, 3:55 pm 
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As those wiser than me have instructed, and has worked for me (YMMV)

1) Regulate G2 (screens)
2) Do not regulate bias supply unless you regulate B+
3) not necessary to regulate B+ with a pentode (in pentode mode)

Is that better?

Perhaps others will chime in.......

Stuart


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PostPosted: May 24th, 2018, 4:06 pm 
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Oh yes I understood what you meant, I was just hoping Roscoe would chime in as to whether he agreed with your analysis. Makes perfect sense to me but I am no expert on these things (which is why I come here!). But it sounds like you have it on good authority and evidence, so that should be good enough for me.

Out of curiosity, do you think there's a sonic advantage to regulating everything (B+, G2, bias)?


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PostPosted: May 24th, 2018, 4:14 pm 
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justinis wrote:
Oh yes I understood what you meant, I was just hoping Roscoe would chime in as to whether he agreed with your analysis. Makes perfect sense to me but I am no expert on these things (which is why I come here!). But it sounds like you have it on good authority and evidence, so that should be good enough for me.

Out of curiosity, do you think there's a sonic advantage to regulating everything (B+, G2, bias)?


In a single ended Class A amp, shunt reg/current source, probably yes. PP Class AB, I think other circuit issues will swamp any differences there. But please, try. It may be worth the effort.

I do know that a separate supply (transformer, rectifier, etc.) for the input stages, leaving the output section its own supply is a definite and very worthwhile improvement.

Stuart


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