Regulated Screen Supplies?

[jbefumo]

I've been experimenting with this high-voltage/low-current IC for a few months now and it seems to work quite well, without a lot of extraneous parts:


This is the datasheet:  http://www.farnell.com/datasheets/1849628.pdf


I originally sought it out for my single-ended KT-120 build, where I needed to couple a 400V plate voltage with 225V screen, and was afraid a big enough resistor would fluctuate more than I wanted.


Since then I've experimented with it in a few other applications, and it seems to work well.


Just wondering if anyone else has tried anything like this, or, more importantly, if anyone foresees any 'gotchas' that I have not anticipated.


Thanks,


Joe

[HotBluePlates]

I originally sought it out for my single-ended KT-120 build, where I needed to couple a 400V plate voltage with 225V screen, and was afraid a big enough resistor would fluctuate more than I wanted.
...
Just wondering ... if anyone foresees any 'gotchas' that I have not anticipated.

Are you using cathode bias?

The value of highly-stable screen voltage is not just stability with regard to its absolute value, but stability of screen-to-cathode voltage, as that is what the value to which the tube responds.

This isn't a problem if you're using fixed bias with the cathode tied to ground. But if the cathode voltage is not super-steady due to series resistance to ground, then the regulated screen voltage won't result in super-steady screen-to-cathode voltage.

[silverfox]

So is this a form of Ultra Linear? My thinking is the screen taps on an Ultra Linear transformer serve to regulate the screen voltage.

silverfox.

[jjasilli]

I think it's a sort of variation on the general ultra-linear concept.  In UL screen voltage is held at a certain specific fraction or portion of plate voltage.  It's not regulation per se, because voltage may sag with larger current draw.  However I think most UL trannies are quite robust so that sag is minimal. 


As Hotblue points out there's also the issue of the relation to cathode voltage.


I think screen resistors fall into the same general concept.  Except that at idle the screen draws so little current that the screen resistors may have no noticeable effect on voltage drop from plate vs. screen B+ supply.  But with signal input the screens draw current which causes a voltage drop across the screen resistors to the screens.  Of course, this is more dynamic than actual regulation.


Diodes or active regulation may be used.  There was a thread on this some time back. 

[DummyLoad]

with a max I of 30mA and max Pdiss of 2.5W, that device would be much more useful for preamp power.


for use as g2 regulation of a power stage, use it as a V ref. for a series pass device that can handle the current and power dissipation required.


--pete

[PRR]

> this high-voltage/low-current IC

Supertex LR8. Mouser:
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyLR8K4-G

In the old days, we'd just use a MOSFET.

20mA may be enough for one 6550's screen.

What I am thinking is Phantom Power in tube microphone preamps. (However the 10mA max demand is more than many mike preamps complete.)

Might also be handy for one TL072 in a tube amp.

[silverfox]

At the expense of turning a guitar amp into a Hi Fi amp, and I suppose there's a place for that; however much of the tone of RR and Metal depends on the changing parameters of the amp as it is over driven, starved for power or otherwise maligned. Having said that I got to wondering last night if an Op Amp circuit with a Mosfet output would solve the screen regulation problem. A sort of London Power for screens, (maybe that's what PRR is referring to). To complicate things, and this is really a question also, how about a divider network taken from the same source that also regulated the phase inverter voltage like the HiWatt amp, (that circuit uses a tube).

The block diagram below is from the Data Sheet, make the Pass Element a Mosfet,

silverfox.

[kagliostro]

Actually I don't remember how, but I'm sure that the voltage controlled using a mosphet can be bondet to the flutuation of B+

Franco

[HotBluePlates]

So is this a form of Ultra Linear? My thinking is the screen taps on an Ultra Linear transformer serve to regulate the screen voltage.

Ultralinear is a form of operation midway between true pentode (fixed screen V, plate V varies with signal current through load) and triode mode (screen tied to plate, screen V varies exactly-as plate V varies, reduced output & distortion).

Because the screen voltage is derived from a tap on the OT winding in Ultralinear, the screen voltage changes with signal current but not as much as the plate voltage. And because the plate current creates the voltage drop across OT primary impedance, a portion of which is applied to the screen, some people analyze the topology as negative feedback from plate to screen. End result is power output between pentode and triode mode, and possibly lower distortion than either.

So you'll see that if the screen is locked to a regulated voltage (or an unchanging screen-to-cathode voltage), then you're solidly in pentode mode of operation.

[Bluemeany]

Hi Kagliostro.

Just use the standard VVR circuit that is around. That should perform exactly as mentioned. It won't "regulate" in a traditional sense but will fluctuate with the B+ keeping the anode and screen supplies roughly the same voltage apart.

[HotBluePlates]

... I got to wondering last night if an Op Amp circuit with a Mosfet output would solve the screen regulation problem. ...

Is there a "screen regulation problem"? Could someone explain why they think they need a tightly-regulated screen?

On the other side of the coin, a few years back I proposed to Tubenit that he try using a "too-big" screen resistor as a way to add sag to an amp that was otherwise stiff sounding. By "too-big" I mean 2-4kΩ on a 6V6 or 6L6 where you'd normally find a 470Ω in a Fender amp (if any screen resistor at all). The goal here was purposeful "de-regulation" of the screen voltage as a way to add sag, with the side effect of slightly educing power output.

What I'm getting at is to get folks thinking about why the screen would need regulation, what you gain from it, etc.

Note also thee's not really a need to worry about keeping the screen so-many-volts below the plate voltage. Every time you play a pentode/beam power tube amp, the screen spends half of the time at a voltage higher than the momentary plate voltage...

[kagliostro]

... I got to wondering last night if an Op Amp circuit with a Mosfet output would solve the screen regulation problem

I think one of the problems is to define what is to be intended for screen regulation problem

I intend it like the necessity to have screens at a very low voltage if compared to B+, like when we want use as power tube a tube like

EL504 used very often in the Davoli Krundaal amps (as example) or the russian GU50 tubes and we dodn't have under hand a PT

with an adequate voltage winding, to me this is the screen regulation problem (regulation problem appears if I try to use a resistor

to drop a large part of B+ to reach levels that sometime can be much less than 50% of B+)

Instead, if we intend the screen regulation problem as a problem due to the stability of the screen voltage this is not a problem to be

considered in guitar amps, as HotBluePlates say, we can have benefits from a (controlled in range) flutuation on screens voltages

HiFi is a different thing and there I don't know if it will be preferable to have a rock voltage on screens


Franco

[HotBluePlates]

I think one of the problems is to define what is to be intended for screen regulation problem

I intend it like the necessity to have screens at a very low voltage if compared to B+, like when we want use as power tube a tube like EL504 used very often in the Davoli Krundaal amps (as example) or the russian GU50 tubes and we don't have under hand a PT with an adequate voltage winding, to me this is the screen regulation problem (regulation problem appears if I try to use a resistor to drop a large part of B+ to reach levels that sometime can be much less than 50% of B+) ...

I would not consider that a regulation problem but a supply voltage availability problem.

Yes, if you try to drop half the B+ across a resistor, and take no other precaution, the screen voltage will sag with applied signal. That's why the old books suggest that if you use resistors to derive a low screen voltage, that the screen's voltage is taken from a tap on a voltage divider passing much more current than what the screen will draw (so that screen current variation doesn't upset the screen voltage).

Or you use a separate transformer winding & rectifier/filter to derive a separate, lower screen voltage.

Or you use a tap on the B+ winding at a suitable, lower voltage than B+ to generate your lower screen voltage.

With the exception of the voltage divider, these approaches answer the original need of a reduced screen voltage without creating an artificial problem of poor regulation through use of a poor choice in how the screen voltage is derived.

The original poster needed a low screen voltage for his single-ended build; maybe a regulated supply was a good way to derive the lowered screen voltage, or maybe a different PT or powe supply circuit would have been a better choice. I really don't know the details, and can't say for sure, but regulation doesn't appear to be the key ingredient and I wanted folks to think about that.

[jbefumo]

Oh, damn! Yeah -- was originally going to use fixed bias, but then went for cathode bias.  Will have to think about this. Maybe go back in and add a fixed bias supply...

I originally sought it out for my single-ended KT-120 build, where I needed to couple a 400V plate voltage with 225V screen, and was afraid a big enough resistor would fluctuate more than I wanted.
...
Just wondering ... if anyone foresees any 'gotchas' that I have not anticipated.

Are you using cathode bias?

The value of highly-stable screen voltage is not just stability with regard to its absolute value, but stability of screen-to-cathode voltage, as that is what the value to which the tube responds.

This isn't a problem if you're using fixed bias with the cathode tied to ground. But if the cathode voltage is not super-steady due to series resistance to ground, then the regulated screen voltage won't result in super-steady screen-to-cathode voltage.

[jbefumo]

Hmm -- and my OT does have an UL tap, and I could easily use a Pentode/UL switch -- truth of the matter is, I stumbled across that regulator, thought it looked cool, and just had to find something to try it on .... guess that's what prototypes are for.  Have actually been thinking that it's probably extra complication for no good reason, and a potential point of failure down the road....  Technology for its own sake ...

So is this a form of Ultra Linear? My thinking is the screen taps on an Ultra Linear transformer serve to regulate the screen voltage.

silverfox.

[jbefumo]

The truth of the matter is that I've only just come to fully grok the operation of a triode, and the pentode concepts, while generally understood, still has some fuzzy edges in my mind.  Somewhere along the line I got the idea that the grid state was particularly critical in single-ended designs (can't recall where from, or if indeed it's a valid point), but I do love the single-ended tone, and was worried about frying that big, beautiful KT120, and that is what got me started in the direction of regulating/controlling the grid voltage.  Somehow, once I got that into my head, the idea of U/L was just swept off the table ...Why not use a jackhammer for thumbtacks?

Hmm -- and my OT does have an UL tap, and I could easily use a Pentode/UL switch -- truth of the matter is, I stumbled across that regulator, thought it looked cool, and just had to find something to try it on .... guess that's what prototypes are for.  Have actually been thinking that it's probably extra complication for no good reason, and a potential point of failure down the road....  Technology for its own sake ...

So is this a form of Ultra Linear? My thinking is the screen taps on an Ultra Linear transformer serve to regulate the screen voltage.

silverfox.

[jbefumo]

Huh!  amazing how the simplest ideas are the ones I fail to consider.  And the PT I'm using has a bunch of taps, too -- just need to think through it carefully, 'cause none of them have a CT, and I'd hate to fry it with inept grounding ...

I think one of the problems is to define what is to be intended for screen regulation problem

I intend it like the necessity to have screens at a very low voltage if compared to B+, like when we want use as power tube a tube like EL504 used very often in the Davoli Krundaal amps (as example) or the russian GU50 tubes and we don't have under hand a PT with an adequate voltage winding, to me this is the screen regulation problem (regulation problem appears if I try to use a resistor to drop a large part of B+ to reach levels that sometime can be much less than 50% of B+) ...

I would not consider that a regulation problem but a supply voltage availability problem.

Yes, if you try to drop half the B+ across a resistor, and take no other precaution, the screen voltage will sag with applied signal. That's why the old books suggest that if you use resistors to derive a low screen voltage, that the screen's voltage is taken from a tap on a voltage divider passing much more current than what the screen will draw (so that screen current variation doesn't upset the screen voltage).

Or you use a separate transformer winding & rectifier/filter to derive a separate, lower screen voltage.

Or you use a tap on the B+ winding at a suitable, lower voltage than B+ to generate your lower screen voltage.

With the exception of the voltage divider, these approaches answer the original need of a reduced screen voltage without creating an artificial problem of poor regulation through use of a poor choice in how the screen voltage is derived.

The original poster needed a low screen voltage for his single-ended build; maybe a regulated supply was a good way to derive the lowered screen voltage, or maybe a different PT or powe supply circuit would have been a better choice. I really don't know the details, and can't say for sure, but regulation doesn't appear to be the key ingredient and I wanted folks to think about that.

[jbefumo]

Well, a belated THANK YOU for all these great ideas! Not sure why I never got notification of the posts, so glad I checked in to see what was going on. 


Since the original build, I've refined the circuit board layout significantly, so was planning on rebuilding it anyway.  the tone of the amp is really nice (see my "Princeton on Steroids" topic), so I think I'm headed in a good direction.  I LOVE the sound of that paralleled octal preamp tube, and the single-ended KT120 is really nice.  I think fixed bias with some NFB may make it a bit more solid.  I'm thinking that the U/L tap may be the course of least resistance (no pun intended) ... or maybe course of least impedance...


Should probably clarify that while I'm definitely not looking for a 'high fi' sound, I do lean toward aiming for the best possible clean tone and headroom, since, as someone else observed somewhere, "there's no such thing as a clean pedal..."


I really do appreciate all the great insightful suggestions and comments!


Joe

I think one of the problems is to define what is to be intended for screen regulation problem

I intend it like the necessity to have screens at a very low voltage if compared to B+, like when we want use as power tube a tube like EL504 used very often in the Davoli Krundaal amps (as example) or the russian GU50 tubes and we don't have under hand a PT with an adequate voltage winding, to me this is the screen regulation problem (regulation problem appears if I try to use a resistor to drop a large part of B+ to reach levels that sometime can be much less than 50% of B+) ...

I would not consider that a regulation problem but a supply voltage availability problem.

Yes, if you try to drop half the B+ across a resistor, and take no other precaution, the screen voltage will sag with applied signal. That's why the old books suggest that if you use resistors to derive a low screen voltage, that the screen's voltage is taken from a tap on a voltage divider passing much more current than what the screen will draw (so that screen current variation doesn't upset the screen voltage).

Or you use a separate transformer winding & rectifier/filter to derive a separate, lower screen voltage.

Or you use a tap on the B+ winding at a suitable, lower voltage than B+ to generate your lower screen voltage.

With the exception of the voltage divider, these approaches answer the original need of a reduced screen voltage without creating an artificial problem of poor regulation through use of a poor choice in how the screen voltage is derived.

The original poster needed a low screen voltage for his single-ended build; maybe a regulated supply was a good way to derive the lowered screen voltage, or maybe a different PT or powe supply circuit would have been a better choice. I really don't know the details, and can't say for sure, but regulation doesn't appear to be the key ingredient and I wanted folks to think about that.

[HotBluePlates]

I saw your amp's schematic in another thread. Now it completely makes sense why you'd use a regulator to derive your screen voltage (because it's much lower than your B+/plate voltage).

I'll say this: if you're happy with how the amp is performing, you're done! Enjoy!

If you are fighting for every last milli-Watt from the amp, then you can lurch down the road of worrying about the impact of changing screen-to-cathode voltage. A cathode bypass cap will take care of some of it, but it is normal for the cathode voltage to shift in a cathode biased amp due to the effect of distortion. Under distortion, the top half of the wave is not exactly the same size/shape as the bottom half, and this creates a net offset to the voltage across the cathode resistor, and the bypass cap charges to this bias offset.

Outside of gross distortion, this is probably not a big bias-shift and certainly not a big change of screen-to-cathode voltage ("big" meaning enough to constrain plate current significantly). So know it's there, but maybe not worry too much.

[jbefumo]

Thanks -- glad I wasn't a complete idiot .... ;^)


Sometimes I tend to go off in a direction, and completely forget why I started down that path ...


My original idea was to build a tweed Deluxe, but single-ended -- like a Deluxe/Champ hybrid, with the tone/feel of the champ, but the power of the Deluxe.  I had in my mind the 22-Watt figure (which, if memory serves, corresponds to the output of a blackface Deluxe, not a tweed, but not certain).  I initially started out planning to use a KT88, and as I was trying to learn the ins and outs as I was spending money, grossly mis-specified my PT and OT, so I had way too much plate voltage, and the screen voltage was going to be an issue.  Then, I obtained the KT12o (which I'm coming to like very much in its own right), and since the datasheet had specs for 'typical application' that included 400V plate, 225V grid, and 20W output, I just shot for that.


As you can see from my schematic, I'm using the OT I originally figured as 5k into 8-ohm as 2.5k into 4-ohm, and that's the reason for that particular 'design decision'.


Once again, I do like the tone, but I might like it to be a bit tighter, and am thinking that fixed bias may be a benefit.  The Princeton chassis is already a bit tight, so eliminating the regulation circuit, perhaps by using the U/L tap, will free up enough room for the bias circuitry.


btw ... this is the PT I'm using ... is attached, so if anyone has any suggestions, cautions, warnings about using more than one of those taps simultaneously, THAT would be most helpful!  I hate that aroma of frying transformer ....

I saw your amp's schematic in another thread. Now it completely makes sense why you'd use a regulator to derive your screen voltage (because it's much lower than your B+/plate voltage).

I'll say this: if you're happy with how the amp is performing, you're done! Enjoy!

If you are fighting for every last milli-Watt from the amp, then you can lurch down the road of worrying about the impact of changing screen-to-cathode voltage. A cathode bypass cap will take care of some of it, but it is normal for the cathode voltage to shift in a cathode biased amp due to the effect of distortion. Under distortion, the top half of the wave is not exactly the same size/shape as the bottom half, and this creates a net offset to the voltage across the cathode resistor, and the bypass cap charges to this bias offset.

Outside of gross distortion, this is probably not a big bias-shift and certainly not a big change of screen-to-cathode voltage ("big" meaning enough to constrain plate current significantly). So know it's there, but maybe not worry too much.

[PRR]

> the PT I'm using ...warnings about using more than one of those taps simultaneously

Search the forum with that part number.

It has been discussed several times. You can not get two high-current supplies at different voltages from it. It is a production foul-up. It was supposed to be 345V AC CT with lower options, but the CT didn't get Centered, and everything else is fouled.

It IS an excellent part for ONE supply voltage, with a four-diode bridge rectifier. Except the "optimum" output is the full winding, 690V AC or 970V DC!! Taking half the full winding is like taking half your money's worth. Weber does sell it off-price, but not half-price; and you still have to lug-around the capacity you don't use.

[jbefumo]

> this high-voltage/low-current IC

Yeah, based on everything I'm learning as a result of these replies, I'm going to get rid of that and work with the many taps available from the PT on this single-ended amp.  As it stands, yes -- I'm just powering the screen of that one KT120 at the moment. I'll have to retrace my steps and try to remember where I got the idea that this was something essential for single-ended running at high B+ -- but never considered using it for a P-P amp.



Supertex LR8. Mouser:
http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyLR8K4-G

In the old days, we'd just use a MOSFET.

20mA may be enough for one 6550's screen.

What I am thinking is Phantom Power in tube microphone preamps. (However the 10mA max demand is more than many mike preamps complete.)

Might also be handy for one TL072 in a tube amp.

[jbefumo]

Thanks! I suspect you probably saved me from the expense of a new transformer and a smokey room. 

> the PT I'm using ...warnings about using more than one of those taps simultaneously

Search the forum with that part number.

It has been discussed several times. You can not get two high-current supplies at different voltages from it. It is a production foul-up. It was supposed to be 345V AC CT with lower options, but the CT didn't get Centered, and everything else is fouled.

It IS an excellent part for ONE supply voltage, with a four-diode bridge rectifier. Except the "optimum" output is the full winding, 690V AC or 970V DC!! Taking half the full winding is like taking half your money's worth. Weber does sell it off-price, but not half-price; and you still have to lug-around the capacity you don't use.

[jbefumo]

On the other hand, the many taps WERE really useful until I found exactly the right combination to give me the B+ I wanted under load.


Edcor makes a "Tinker Box' transformer with a bunch of center-tapped outputs that looks like it would be a real useful bench tool.


Joe

Thanks! I suspect you probably saved me from the expense of a new transformer and a smokey room. 

> the PT I'm using ...warnings about using more than one of those taps simultaneously

Search the forum with that part number.

It has been discussed several times. You can not get two high-current supplies at different voltages from it. It is a production foul-up. It was supposed to be 345V AC CT with lower options, but the CT didn't get Centered, and everything else is fouled.

It IS an excellent part for ONE supply voltage, with a four-diode bridge rectifier. Except the "optimum" output is the full winding, 690V AC or 970V DC!! Taking half the full winding is like taking half your money's worth. Weber does sell it off-price, but not half-price; and you still have to lug-around the capacity you don't use.