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Any Tube Guys Here ?

Started by Unknown June 14, 2014
Tube based PWM at about 150 kc. (hey, it's tubes so kc is right...)

https://dl.dropboxusercontent.com/u/29948706/tube1.jpg

The reason I depict R1 as going to the plate is because it technically must=
. In this app the load might be on the ngative side. That is, like in a tot=
em pole arraingement.=20

It's obvious what a screen grid does kinda, but I am not so sure. I am not =
sure exactly how to run this. I want maximum current and the ability to acc=
omplish PWM at 150 kHz. I know the tubes will not be cheap of course. the r=
eason for tubes is that I want about 5 kV output. I lso know this is insane=
, but so what.=20

Z1 keeps the thing from melting in the absence of excitation of course. I d=
on't need to pull a Sony with this type of components for a damn open 15K r=
esistor way in the other side of the thing. that much is fine acually, what=
 I don't know is what to do with the screen grid.=20

If I just connect the screen to the plate that is like a triode right ? If =
I lower the screen voltage from there what happens ? It has something to do=
 with the gain. I don't want to have to pump these things with more voltage=
 than they put out. I am already going to find T1 because it may have to ta=
ke the full output voltage.=20

This is how the rest of it looks :

https://dl.dropboxusercontent.com/u/29948706/tube2.jpg

As you can see, to make a sceeen supply separate makes for a few problems. =
I do not intend to go through what it takes to make for example this :

https://dl.dropboxusercontent.com/u/29948706/6as7_2a.bmp

One of them in stereo would best be done with like three power transformers=
. Count me out.=20

Now they still do make tubes right ? ofr audio purposes for one, but that w=
ould be no godd for this app because no output tube could handle 5 kV. tisi=
s in the realm of a TV horizontal output. Actually more like maybe the tube=
s used in linears for CB radios. Well not legally, they take ten meter ham =
jobs and retune them a bit. (that part for our overseas folk who might not =
know what "breaker one nine" means)

Anyway, the exact purpose of this thing is not important. I need to know ho=
w to most efficiently use the tube. Figure a pentod power tube like maybe a=
 6JE6 (??) or something like that. 6MJ6 maybe ?=20

At that point, bias the screen like I did, or tie it to the cathode, anode,=
 no node or what ? Of course I want it to conduct like all hell to keep it =
more efficient. At the same time I need it to switch fast and I don't need =
any strange ions floating around in there. (or whatever)

One other question, related but...well, the plate voltage will not be pulle=
d negative from the screen voltage will it ? Or will it ?=20

Thanks. Ideas ? Anything but calling me a lunatic. I get enough of that fro=
m the locals...
>"I am already going to find T1 because it may "
Screwed up an edit there. The issue is it will have to be a high potted part.
jurb6006@gmail.com Wrote in message:
> Tube based PWM at about 150 kc. (hey, it's tubes so kc is right...) > > https://dl.dropboxusercontent.com/u/29948706/tube1.jpg > > The reason I depict R1 as going to the plate is because it technically must. In this app the load might be on the ngative side. That is, like in a totem pole arraingement. > > It's obvious what a screen grid does kinda, but I am not so sure. I am not sure exactly how to run this. I want maximum current and the ability to accomplish PWM at 150 kHz. I know the tubes will not be cheap of course. the reason for tubes is that I want about 5 kV output. I lso know this is insane, but so what. > > Z1 keeps the thing from melting in the absence of excitation of course. I don't need to pull a Sony with this type of components for a damn open 15K resistor way in the other side of the thing. that much is fine acually, what I don't know is what to do with the screen grid. > > If I just connect the screen to the plate that is like a triode right ? If I lower the screen voltage from there what happens ? It has something to do with the gain. I don't want to have to pump these things with more voltage than they put out. I am already going to find T1 because it may have to take the full output voltage. > > This is how the rest of it looks : > > https://dl.dropboxusercontent.com/u/29948706/tube2.jpg > > As you can see, to make a sceeen supply separate makes for a few problems. I do not intend to go through what it takes to make for example this : > > https://dl.dropboxusercontent.com/u/29948706/6as7_2a.bmp
I don't think wiring the screens like that is a good idea. It looks like you've got positive feedback from the tube output to the screen - I'd expect it to oscillate. I think you may need bypass capacitors across R1 and R4. Also, there will need to be some feedback at DC from the output to the PWM stage - the output is unlikely to sit at exactly 0 volts all by itself. You'll also have to worry about the heater to cathode breakdown voltage of the bottom and particularly the top tube - the top tube isn't going to like it when its cathode is bouncing from 0 to some kV and its heater is sitting at 6.3 volts...
> One of them in stereo would best be done with like three power transformers. Count me out. > > Now they still do make tubes right ? ofr audio purposes for one, but that would be no godd for this app because no output tube could handle 5 kV. tisis in the realm of a TV horizontal output. Actually more like maybe the tubes used in linears for CB radios. Well not legally, they take ten meter ham jobs and retune them a bit. (that part for our overseas folk who might not know what "breaker one nine" means)
> Anyway, the exact purpose of this thing is not important. I need to know how to most efficiently use the tube. Figure a pentod power tube like maybe a 6JE6 (??) or something like that. 6MJ6 maybe ?
> At that point, bias the screen like I did, or tie it to the cathode, anode, no node or what ? Of course I want it to conduct like all hell to keep it more > > One other question, related but...well, the plate voltage will not be pulled negative from the screen voltage will it ? Or will it ?
The bottom tube's plate voltage will go below the screen voltage if the screen is bypassed. That's OK, that's how a pentode is supposed to work.
> Thanks. Ideas ? Anything but calling me a lunatic. I get enough of that from the locals... >
--
I've dabbled before:
http://webpages.charter.net/dawill/tmoranwms/Elec_Compound2.html

Concept, but not built.  Needs more work anyway:
http://seventransistorlabs.com/Images/Borg.gif

The biggest downside to pentodes in a switching converter is, they don't 
conduct in reverse like MOSFETs.  Think SCRs with voltage input and no 
latching.  This makes constant-current inverters much more attractive than 
constant-voltage.  (Assuming you can generate the constant current in the 
first place -- since we're already talking tubes, a massive choke, say 10H 
200mA, would be an appropriate approximation.)

Recieving tubes no longer hold any records on current or voltage 
capability: high voltage MOSFETs of substantial ratings are available as 
cheaply.
http://www.digikey.com/product-detail/en/IXTF1N450/IXTF1N450-ND/3737637
A few 2500V devices cascoded will give far better ratings.

Note that sweep tubes are only rated for peak voltages under hard cutoff 
(Vg1 = -200V or whatever).  I don't know what happens under forward bias 
at that voltage, but the inevitable ion bombardment probably isn't 
friendly to the cathode.

The other big downside to a pentode (or tetrode) is, cathode current is 
controlled firstly by grid voltage, and secondly by "virtual plate" 
voltage.  On a triode, this is just plate plate.  On a tetrode+, the 
screen grid (and, by even weaker effect again, the plate proper as well, 
hence the large but still finite plate resistance) serves this purpose. 
Of note... cathode current, in a tetrode+, is barely determined by plate 
voltage at all, whether positive or negative.  So, what happens when the 
plate voltage drops into the saturation region?  All that current that 
used to be flowing into the plate gets sucked right up by the screen grid. 
A few miliseconds later, said screen grid evaporates in a flash.  Umm, 
yeah.

There is no simple analogy of a screen grid among semiconductors.  Due to 
the positive backwards-transconductance (change in plate voltage --> 
change in screen current), static positive feedback can be implemented 
with a single tube, no transformer (or spooky effects like secondary 
emission in true tetrodes) required.

The most obvious "duh" solution, which I used in the two above examples, 
is simply limiting screen power with a resistor, and bypassing it with a 
cap to maintain reasonable AC performance.  This isn't bad for a single 
ended example with positive saturation voltage, but probably isn't going 
to work as well with reverse voltage (attempting to turn the tube on, 
while the plate is negative, will simply discharge the screen bypass cap, 
then when plate voltage reverses again, screen voltage will be too low and 
the tube will simply 'wheeze' and not saturate properly).

One can envision various approaches, like minimal bypass, or none at all 
(downside: screen voltage swings around, so Miller is back with a 
vengance), or various sorts of passive (diode?) or active (zener / gas 
tube / bootstrap tube / etc.?) clamping strategies.  All of which cost 
more tubes (or high voltage semiconductors).

Oh, and for a totem pole / bridge, you only have "N-vacuum" devices to 
choose from, so all voltages (grid and screen) must be bootstrapped or 
isolated.

Which isn't too bad, I suspect.  A floating +/-150V supply, referenced to 
the cathode, could provide plenty of screen bias (positive, with current 
limiting / clamping / whatever as chosen) and grid bias (negative, with 
transformer coupling, gain and drive).  The screen power would be 
substantial (watts), but nothing extreme as gate\\\\grid drives go.  The 
heater, by the way, also needs to be cathode referenced (give or take only 
200V), which suggests rather more like 30W than 3W of isolated power. 
Just toss more windings on the DC-DC...  (On the upside, the heat can be 
RF, no need to rectify and filter it.)

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

<jurb6006@gmail.com> wrote in message 
news:dbdc7dee-b02f-44f3-ac93-fb87bdf4918c@googlegroups.com...
Tube based PWM at about 150 kc. (hey, it's tubes so kc is right...)

https://dl.dropboxusercontent.com/u/29948706/tube1.jpg

The reason I depict R1 as going to the plate is because it technically 
must. In this app the load might be on the ngative side. That is, like in 
a totem pole arraingement.

It's obvious what a screen grid does kinda, but I am not so sure. I am not 
sure exactly how to run this. I want maximum current and the ability to 
accomplish PWM at 150 kHz. I know the tubes will not be cheap of course. 
the reason for tubes is that I want about 5 kV output. I lso know this is 
insane, but so what.

Z1 keeps the thing from melting in the absence of excitation of course. I 
don't need to pull a Sony with this type of components for a damn open 15K 
resistor way in the other side of the thing. that much is fine acually, 
what I don't know is what to do with the screen grid.

If I just connect the screen to the plate that is like a triode right ? If 
I lower the screen voltage from there what happens ? It has something to 
do with the gain. I don't want to have to pump these things with more 
voltage than they put out. I am already going to find T1 because it may 
have to take the full output voltage.

This is how the rest of it looks :

https://dl.dropboxusercontent.com/u/29948706/tube2.jpg

As you can see, to make a sceeen supply separate makes for a few problems. 
I do not intend to go through what it takes to make for example this :

https://dl.dropboxusercontent.com/u/29948706/6as7_2a.bmp

One of them in stereo would best be done with like three power 
transformers. Count me out.

Now they still do make tubes right ? ofr audio purposes for one, but that 
would be no godd for this app because no output tube could handle 5 kV. 
tisis in the realm of a TV horizontal output. Actually more like maybe the 
tubes used in linears for CB radios. Well not legally, they take ten meter 
ham jobs and retune them a bit. (that part for our overseas folk who might 
not know what "breaker one nine" means)

Anyway, the exact purpose of this thing is not important. I need to know 
how to most efficiently use the tube. Figure a pentod power tube like 
maybe a 6JE6 (??) or something like that. 6MJ6 maybe ?

At that point, bias the screen like I did, or tie it to the cathode, 
anode, no node or what ? Of course I want it to conduct like all hell to 
keep it more efficient. At the same time I need it to switch fast and I 
don't need any strange ions floating around in there. (or whatever)

One other question, related but...well, the plate voltage will not be 
pulled negative from the screen voltage will it ? Or will it ?

Thanks. Ideas ? Anything but calling me a lunatic. I get enough of that 
from the locals... 


On Fri, 13 Jun 2014 20:22:21 -0700 (PDT), jurb6006@gmail.com wrote:

>Tube based PWM at about 150 kc. (hey, it's tubes so kc is right...)
One good source for tube stages is the amateur radio literature from a few decades ago, like the ARRL Handbook. There are plenty of tube output stages to up to a few kW in the HF range. Some of the most popular tube types might still be available.
>https://dl.dropboxusercontent.com/u/29948706/tube1.jpg > >The reason I depict R1 as going to the plate is because it technically must. In this app the load might be on the ngative side. >That is, like in a totem pole arraingement.
For many high power tubes, the maximum screen grid voltage is much less than the maximum anode voltage, so that explains the R1/R2 voltage divider. When the tube is heavily conducting, the anode voltage can be quite low and if the screen is powered from a high voltage fixed source, the screen would become the most positive electrode, sucking all the electrons and melting the screen. By powering the screen indirectly from the anode, the screen can't become more positive than the anode.
>It's obvious what a screen grid does kinda, but I am not so sure. >I am not sure exactly how to run this. >I want maximum current and the ability to accomplish PWM at 150 kHz. >I know the tubes will not be cheap of course. the reason for tubes is that I want about 5 kV output.
How is that 5 kV measured, is it just B+ or the difference between B+ and B- or a peak voltage across and inductive load during current switch off ?
>I lso know this is insane, but so what. > >Z1 keeps the thing from melting in the absence of excitation of course. I don't need to pull a Sony with this type of components for a damn open 15K resistor way in the other side of the thing. that much is fine acually, what I don't know is what to do with the screen grid. > >If I just connect the screen to the plate that is like a triode right ? If I lower the screen voltage from there what happens ? It has something to do with the gain. I don't want to have to pump these things with more voltage than they put out. I am already going to find T1 because it may have to take the full output voltage. > >This is how the rest of it looks : > >https://dl.dropboxusercontent.com/u/29948706/tube2.jpg > >As you can see, to make a sceeen supply separate makes for a few problems.
High voltage/high power tubes are often directly heated, so you are going to need separate heater winding anyway. I do not understand why you want to do totem pole output with two tubes, wouldn't a normal push-pull approach be simpler ? Of course you will need an output transformer, but since the frequency appears to be fixed (instead of spanning a decade in most amateur radio linear amplifiers) and reasonably high (150 kHz) to keep the core size at a reasonable level. With selecting a modern suitable core material, the size and losses can be further reduced. With separate primary and secondary windings, you can chose the transformation ratio freely, so if you need 5 kV peak-to-peak, the tubes doesn't have to run at several kV, you might even be able to use transistors :-)
>I do not intend to go through what it takes to make for example this : > >https://dl.dropboxusercontent.com/u/29948706/6as7_2a.bmp
What tubes are used ? I have seen a similar circuit with six triodes feeding a 16 ohm loudspeaker directly.
On Sat, 14 Jun 2014 03:10:36 -0500, Tim Williams wrote:

> I've dabbled before: > http://webpages.charter.net/dawill/tmoranwms/Elec_Compound2.html
Beautiful! joe [snip>
> Tim
On Sat, 14 Jun 2014 03:10:36 -0500, "Tim Williams"
<tmoranwms@charter.net> wrote:

>I've dabbled before: >http://webpages.charter.net/dawill/tmoranwms/Elec_Compound2.html > >Concept, but not built. Needs more work anyway: >http://seventransistorlabs.com/Images/Borg.gif
If I understand correctly, the output transformer runs on audio frequencies and the 10 mH choke and the two 10 nF capacitors filter out the PWM on the primary side ? Audio transformers going down to 20 Hz are quite big. Wouldn't it be better to run the PWM through a small 20 kHz+ transformer and then do the filtering on the secondary side ? An elliptic filter with at least 20 dB notch at the PWM frequency should be enough to protect any tweeters (which usually can handle only a few watts).
<upsidedown@downunder.com>

> > Audio transformers going down to 20 Hz are quite big. Wouldn't it be > better to run the PWM through a small 20 kHz+ transformer and then do > the filtering on the secondary side ?
** Oh NOOOO, not this stupidity AGAIN !!! PWM signals are *wide band*, only needing a LP filter to recover the original modulation. .... Phil
On Sat, 14 Jun 2014 19:47:58 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

> ><upsidedown@downunder.com> > >> >> Audio transformers going down to 20 Hz are quite big. Wouldn't it be >> better to run the PWM through a small 20 kHz+ transformer and then do >> the filtering on the secondary side ? > >** Oh NOOOO, not this stupidity AGAIN !!! > >PWM signals are *wide band*, only needing a LP filter to recover the >original modulation.
What are you complaining about ? A variable pulse width with constant OFF time (variable cycle rate and frequency) ? Or perhaps some fixed frequency application, in which the edge starts at the same time, but the pulse width (and inversely the OFF time) varies ? Sure, a 50%/50% duty cycle PWM square wave have a lots of odd harmonics, but the transformer doesn't have to reproduce the higher harmonics that accurately. Some transformer prefiltering might be even useful for converting a square wave to something looking like a sawtooth to avoid some problems with transformer stray capacitances. I assume you are reacting against "elliptic filter" that you just clipped, it is just a low pass filter with a notch in the transition region. Assuming you need 20 dB attenuation on the switching frequency to protect the tweeters (and avoid harming your dog) and want -3 dB point at 20 kHz, For a first order RC or LR filter with 6 dB/octave, you need more than 3 octaves, so the switching frequency needs to be over 200 kHz. For a second order LC filter with -12 dB/octave, nearly two octaves are needed and the switching frequency above 60 kHz would be needed. A first order LR filter (R being the load) with an LC notch at the PWM frequency would handle the nominal switching frequency, while the LP part would handle the 3rd and 5th harmonics quite well even with such gentle slope. However, if you need -1 dB or 0.25 dB at 20 Hz, this requires even higher switching frequencies or sharper filters.
<upsidedown@downunder.com>
> "Phil Allison" >> >>> >>> Audio transformers going down to 20 Hz are quite big. Wouldn't it be >>> better to run the PWM through a small 20 kHz+ transformer and then do >>> the filtering on the secondary side ? >> >>** Oh NOOOO, not this stupidity AGAIN !!! >> >>PWM signals are *wide band*, only needing a LP filter to recover the >>original modulation. > > What are you complaining about ?
** Fucking idiots like you all over usenet, for one.
> Sure, a 50%/50% duty cycle PWM square wave ...
** There is no such animal - you cretinous ass. You are making a MASSIVE blunder. Wake up or piss the hell off. .... Phil