Relationship between screen voltages and plate voltages in power tubes

[tubeswell]

Is HBP still around? Haven't seen one of his posts for a wee while. (Seeing as how this 8-year old thread's been resurrected 'n'all)

[sluckey]

He was last logged on on 4/23 but hasn't posted since last June. I didn't realize it had been that long. Don't know what's up?

[Williamblake]

This is a great thread and please excuse my probably ignorant question but assuming you had a power supply with two independent secondary windings, one for the plate and one for the screen, would it make sense to connect the screen supply to the cathode instead of 0V to have screen voltage as stable a possible? Assuming you want as much alternating DC-current in the cathode resistor as possible  and no alternating screen voltage.

[silverfox]

So that's how Ultra-Linear works.


silverfox.

[shooter]

that's how Ultra-Linear works.

You forgot, they make magic MOJO also 

[Ritchie200]

that's how Ultra-Linear works.

You forgot, they make magic MOJO also 

YES!

Jim

[shooter]

aside;

My gigging step-Son is a fender amp kid  , he wound up with a Marshall 18W clone and called last night all geeked about the sound n wanted to know why the difference.  I explained about QUALITY IRON (and the rest)   

NOW he wants my old amps he test drove but wasn't quite happy with since they didn't sound fendery enough 

[Colas LeGrippa]

Very interesting but why bother trying to lower the screens B+ if  the guitarists dislike it ?
The UL tap is not popular because the B+ for the sreens is too low, not before it is taken at a point near 40% of the opt winding. Guitarists like me don't care about replacing the tubes one or twice a year. They care about the tone.
If the tone would be a lot creamier and better and extraordinary with a much lower screen B+ amplifiers  builders would drop UL trannies in their builds or modify the B+ supply accordingly.
In many of my builds, the screens voltages is higher than the plates. It doesn't affect the tone drastically to feed them with 5 or 10 volts over or under. Tubes life is more affected by bias than by supply, Imho.


Jack

[HotBluePlates]

... assuming you had a power supply with two independent secondary windings, one for the plate and one for the screen, would it make sense to connect the screen supply to the cathode instead of 0V to have screen voltage as stable a possible? ...

I think it would matter only in cases where you have a large cathode load/resistance, and can't use a bypass cap for some reason.  An example would be the old McIntosh amps of the 50's because half of the tube's load is between cathode & ground, and they had to come up with some clever tricks to avoid the cathode-to-screen voltage varying by a hundred volts or more with signal.


"As stable as possible" brings up situations where you have to remind yourself to keep perspective.  If you really mean "as possible" then you pay "all the money" and skip power supplies in favor of massive battery banks to provide the d.c. you want.   

Most likely, you have a push-pull output section so your opposing tubes can share a cathode resistor (and have closer to net-zero current change), or you've taken the advice from the tube data sheet to use individual cathode resistors, but heavily bypass each one.

So that's how Ultra-Linear works. ...

Very interesting but why bother trying to lower the screens B+ if  the guitarists dislike it ?
The UL tap is not popular because the B+ for the sreens is too low ...

Huh??  How did UL enter the discussion here?

[silverfox]

Huh??  How did UL enter the discussion here?

I thought the goal is to control the screen voltage off set. That's what UL taps do I think.

Now I'm wondering if a Mosfet in the 15ish watt variety would serve as a sort of follower and regulate the screen voltage at an off set? Sort of a tube regulator thing. Just an idea. Hi Watt does it with the PI; DL504, Kinda.

Nice to hear from you again.

silverfox.

[pdf64]

UL is a very different thing to having a separate dropped HT supply for the screen grids.
UL acts on a tube’s dynamic conditions, effectively changing it into a different, triode-pentode hybrid tube type. A completely different set of characteristics and load lines apply to tubes used in UL.
Whereas the separate, lower screen grid supply thing just limits max plate current (and puts much less stress on the screen grids), the tube still operates in pentode mode.

[Williamblake]

Thanks for explaining all this.

[silverfox]

UL is a very different thing to having a separate dropped HT supply for the screen grids.
UL acts on a tube’s dynamic conditions, effectively changing it into a different, triode-pentode hybrid tube type. A completely different set of characteristics and load lines apply to tubes used in UL.
Whereas the separate, lower screen grid supply thing just limits max plate current (and puts much less stress on the screen grids), the tube still operates in pentode mode.

Okay- So could a Mosfet be used as a sort of follower-regulator, for the screens instead of a resistor that introduced sag? Of course it would cost a little more. Could the Mosfet be employed to act like a UL tap?


silverfox.

[pdf64]

I guess that might be theoretically possible but there’s massive voltage on a power tube plate, so a super tough device would be needed, along with some non trivial circuitry.
But mainly what would be the point? A tube amp needs an OT anyway, so if UL is desired then just get an OT with the taps.

[PRR]

> could a Mosfet

Can't boost the screen *above* the B+, as UL does.

[Ritchie200]

Very interesting but why bother trying to lower the screens B+ if  the guitarists dislike it ?
The UL tap is not popular because the B+ for the sreens is too low, not before it is taken at a point near 40% of the opt winding.
Jack

Colas,
I think you may be misapplying the 40% UL screen tap rating. Screens can be whatever you want in a UL application. Think of the 40% tap as the ratio between the triode/pentode operation, not a percentage of screen voltage reduction. Remember, if you connect screens to the center tap you have classic pentode operation. Screens connected to plate and we have triode operation.  We are cheating connecting to the 40% tap (or other % of tube manufacturer suggested tap) and getting the best of both worlds resulting in less distortion and a more "linear" operation.  There are actually quite a few "boutique" builders  using UL designs these days.  They tend to be very pedal friendly and also hi-gain friendly as they give you what you plug in to it and are not necessarily part of the distortion characteristics.  This allows for the same audio result over the power/decibel capacity of the amp.


Jim UL fan!

[jjasilli]

Well, UL has a specific, technical meaning: the %age of tap being "optimum" for the tube type is UL.  Yes, we can tap @ any %age we want; but this is no longer UL per se.  This is because UL not only provides a certain screen voltage; but also does so in a way that provides a feedback effect which significantly reduces "distortion", while providing nearly full pentode power.  If you deviate from that optimum point you are no longer ultralinear.  For more info, and the optimum %age for various tube types, see:  https://en.wikipedia.org/wiki/Ultra-linear

[Ritchie200]

Yeah, I wasn't real clear there....  I think(?) Colas was assuming the 40% tap meant a 40% reduction of screen voltage that he thinks is already low in a UL application.  That is simply not the case (as I tried to allude to).  For example, a bone stock Major has 630V on the plates and 629.9V on the screens at idle.  Ok, maybe 628V but you get the picture.  The tap and UL design has nothing to do with lower screen voltages.  That is what I meant with "Screens can be whatever you want".  Sorry, I was not real clear.


Jim still a UL Fan!

[jjasilli]

UL is a very different thing to having a separate dropped HT supply for the screen grids.
UL acts on a tube’s dynamic conditions, effectively changing it into a different, triode-pentode hybrid tube type. A completely different set of characteristics and load lines apply to tubes used in UL.
Whereas the separate, lower screen grid supply thing just limits max plate current (and puts much less stress on the screen grids), the tube still operates in pentode mode.

Okay- So could a Mosfet be used as a sort of follower-regulator, for the screens instead of a resistor that introduced sag? Of course it would cost a little more. Could the Mosfet be employed to act like a UL tap?

silverfox.

A number of ways to drop screen voltage have been discussed on this Forum and on the net, including mosfet circuits.  I think the most satisfactory ways are UL, or a separate secondary screen winding.  Other ways: diodes; zeners; attenuating series R; voltage dividers; mosfet or other regulator circuits.  I think the problem is that (unless the plate supply is also regulated) these things tend to get the screens' voltage & current out of whack with the rest of the tube, under signal conditions.

[pdf64]

...The beauty of UL is that it solves the voltage problem at the source of the B+ supply, and doesn't require filtering (resistor, choke, cap) to the screen...

Just to pick up on the above, I think UL doesn't really solve a voltage problem so much as a dissipation problem. By tying g2 to dynamically (partway) track the plate voltage, it avoids the high g2 dissipation condition of regular pentode operation, ie when at high signal level peaks, the plate swings down ~100V whilst g2 is being held up near its HT node.
I don't see that it's correct to think that in UL, g2 doesn't require filtering; rather that it's stuck with the degree of ripple at the OT CT node. It's still a grid and will have some amplification of the Vac it's presented with. It's a complex scenario and there may be some degree of cancellation due to the inherent local feedback mechanism. But to avoid high signal levels being modulated by HT ripple, it's best to supply the OT CT with a well smoothed HT, eg via a CLC Pi filter, as per the Sunn Model T https://el34world.com/charts/Schematics/files/Sunn/Sunn_model_t.pdf

[jjasilli]

Plate & screen supplies are generally spoken of in terms of supply voltage.  Yes, reduced supply voltage causes a reduction in dissipation per Ohm's Law (including the Power Formula).  This is a necessary implication, unless current draw were to somehow increase commensurately with reduced voltage, which it does not.  So, this can be expressed in terms of any of the 3 components of Ohm's Law : watts (dissipation), voltage, or current across the impedance of the tube element.


I see no reason why a filter cap circuit could not be added to the screen UL supply.  But this is not done, presumably because it isn't needed.

[pdf64]

Plate & screen supplies are generally spoken of in terms of supply voltage.  Yes, reduced supply voltage causes a reduction in dissipation per Ohm's Law (including the Power Formula).  This is a necessary implication, unless current draw were to somehow increase commensurately with reduced voltage, which it does not.  So, this can be expressed in terms of any of the 3 components of Ohm's Law : watts (dissipation), voltage, or current across the impedance of the tube element.


I see no reason why a filter cap circuit could not be added to the screen UL supply.  But this is not done, presumably because it isn't needed.

Not sure why a reduced supply voltage is mentioned (presumably in regard of UL operation)? As UL is often used to apply a higher voltage on to  g2 than regular pentode mode, the tube info supporting that, possibly due to the dynamic dissipation reduction with UL mentioned just.
https://tubedata.altanatubes.com.br/sheets/127/6/6550.pdf

As I see it, the use of UL taps precludes any additional HT filtering for the g2 supply; how could it be done If it was feasible surely the Model T etc would have that, rather than use 2 monster chokes to accommodate the HT current requirement of the whole amp?
I've read about very complex OTs that somehow have completely separate UL windings for g2, but those are surely a special case, outside of UL implementation as it is generally understood?

[shooter]

I've read about very complex OTs

If you've ever looked at the complex mathematics for solving complex, dynamic frequencies, normal sane folk will just plug in the ear buds n fire up a fat one 

[pdf64]

Haha, I found EM theory modules the most difficult, worse even than laplace. Whatever, any competency I had in those topics, useful or not, is now long gone.
A redeeming feature of buying an OT is that someone else has dealt with the horrendous math involved in their design, and the tedium of their construction.
Leaving the customer free to do something more suited to them, whether that’s drilling a turretboard or sitting back as described 

[HotBluePlates]

Huh??  How did UL enter the discussion here?

I thought the goal is to control the screen voltage off set. That's what UL taps do I think. ...

UL is a very different thing to having a separate dropped HT supply for the screen grids.
UL acts on a tube’s dynamic conditions, effectively changing it into a different, triode-pentode hybrid tube type. A completely different set of characteristics and load lines apply to tubes used in UL.
Whereas the separate, lower screen grid supply thing just limits max plate current (and puts much less stress on the screen grids), the tube still operates in pentode mode.

Okay- So could a Mosfet be used as a sort of follower-regulator, for the screens instead of a resistor that introduced sag? ... Could the Mosfet be employed to act like a UL tap? ...

Maybe the other tangents made it obvious UL doesn't do what you think it does.

At idle, the screen is at the same (or higher) voltage as the plate in distributed-loading (UL to you & me).  But when signal is applied, it bounces all over the place just like the plate voltage.  The screen voltage goes lower and higher, but because of the tapping along the winding the screen's voltage changes less than the plate's voltage.

If you're wanting a "stable screen voltage" than UL, or mimicking UL, isn't the way to go.

[jjasilli]

https://www.ampbooks.com/mobile/amp-technology/ultralinear/   (especially for Ritchie200)



https://www.diyaudio.com/forums/tubes-valves/189803-ultralinear-operation.html

[Baptiste]

Very interesting topic, and funny to see it on top, I was just wondering if it was okay to set Vg2 higher than Va on purpose.

From what I read here, it seems to make no harm for small voltage differences (<10V), but what about 30 or 40V ? And what would be the practical limit ?

I need to double check, but I was looking at loadlines for a 6ak6 SE, and felt I would need to set Va around 140v, Vg2 at 180v to get the loadline lower than the knee. At the recommended point of operation, it goes right through it...

[PRR]

> At the recommended point of operation, it goes right through it...

That's usually near optimum. Why are you different?

FWIW, RCA had a super oscilloscope that ran 6L6 at Vg2=300V and Vp near 140V.

[Baptiste]

> At the recommended point of operation, it goes right through it...

That's usually near optimum. Why are you different?

Well, I thought the recommended way for guitar was noticeably lower than knee. It's the case on a champ for example, and I'm pretty sure I read this on Merlin Blencowe's book, or on his website. It was not quantified though.

Regarding the anode and g2 voltages relationship, given your example of the oscilloscope, I guess this is a non issue for the tube :-)

[PRR]

> I thought the recommended way for guitar was noticeably lower than knee.

Maybe. It may reduce grid-blocking. OTOH a higher impedance gets you near the same place. On the third hand you may not hit your power goal without exceeding some rating.

[Baptiste]

> I thought the recommended way for guitar was noticeably lower than knee.

Maybe. It may reduce grid-blocking.

From what I remember, it is to get more compression, thanks to the inrush current on G2 you get at very low anode voltages. It creates sag on Vg2, lowers the whole plate characteristics, and at lower Vg2, the unchanged loadline more or less goes through the knee...
I'll try to find a link to make sure I'm not totally off.

OTOH a higher impedance gets you near the same place.

Sure, but it also increases the max anode voltage swing. It's generally no big deal I suppose, but the 6ak6 I was looking at, it specifically has a reputation for not handling very well its maximum voltage ratings. That's why I was pondering this question.

[HotBluePlates]

Very interesting topic, and funny to see it on top, I was just wondering if it was okay to set Vg2 higher than Va on purpose.

From what I read here, it seems to make no harm for small voltage differences (<10V), but what about 30 or 40V ? And what would be the practical limit ? ...

"Practical limit" depends on the tube, and the voltage applied to the screen.  The bottom graph on page 6 of this 6L6GC data sheet shows you want to keep at least 25-150v on the plate, depending on screen voltage.  To avoid high screen current:

• Plate should be at least 25v when 50v is on G2 (25v difference)
• Plate should be at least 50v when 150v is on G2 (100v difference)
• Plate should be at least 75v when 250v is on G2 (175v difference)
• Plate should be at least 150v when 350v is on G2 (200v difference)

But 6L6 has aligned grids and tries to encourage cathode current to fly past the screen & towards the plate.

... I was looking at loadlines for a 6ak6 SE, and felt I would need to set Va around 140v, Vg2 at 180v to get the loadline lower than the knee. At the recommended point of operation, it goes right through it...

I thought the recommended way for guitar was noticeably lower than knee.

From what I remember, it is to get more compression, thanks to the inrush current on G2 you get at very low anode voltages. It creates sag on Vg2, lowers the whole plate characteristics, and at lower Vg2, the unchanged loadline more or less goes through the knee...

If you're seeking maximum output power, going through the knee is the way to go; it's how RCA advises you to design a pentode/beam power stage right in the front-matter of their tube manuals.

From there, just be aware of the tradeoffs.
   - If you use a lower-impedance load to rotate the loadline above the knee you reduce odd-harmonic distortion and lower output power, while steering clear of screen current rises.

   - If you use a higher-impedance load to rotate the loadline below the knee, you increase odd-harmonic distortion (grid lines bunch together at both peak plate current and at minimum plate current) and lower output power.  Screen current may rise in the area of peak plate current (which is minimum plate voltage), so screen resistors might be advisable in which case you also have the opportunity for causing compression (because screen resistor voltage-drop will dynamically lower screen voltage, which lowers possible plate current, which reduces output power).

The right balance is whatever you decide delivers the performance you want, while not destroying the tube.