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Old 8th May 2005, 09:04 PM   #1
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Default Beyond SET and ParaFeed.... Complementary Current Triode

Having participated in some efforts recently to DC compensate the SET transformer, so the air gap could be eliminated, I have worked up some new circuits that are analogs to SET design.

These provide the same single triode output tube gain, but provide either twice the power, or even 4 times the power (see next post), while preserving zero DC flux in the output transformer. This means that an ordinary P-P type transformer can be used, which will provide better bandwidth (ie. better Bass, and the option of NFB if wanted), and the cost and weight are less with more power output.

This uses the technique of complementary current to drive the other winding of a P-P transformer. Your first impression will be that this is just some sneaky version of P-P, but a closer look is required. The pentode is used as a programmable current source. With no plate feedback effect ( as with a triode output ), its high impedance output has no say or control of the output voltage. The triode is calling the shots here.

See figure. The constant current source in the "tail" guarantees that the currents in each half of the circuit add up to a constant. This means that for every change in current the triode commands to one winding, an equal but opposite change in current occurs in the other pentode/ transformer half. Since the other winding section is inverted in phase with respect to the output, this just means an exact doubling of the current change as far as the output is concerned. So the output current changes will simply be doubled by this exact duplication. Like having double the gm and wattage rating of the triode.

(using P = I squared R might lead one to think this would quadruple the output power. But, since the triode now sees a higher impedance, 2x, due to the pentode's assistance, we would use .7 times the turns on each half of the xfmr to get the same impedance back...., so power ends up being just doubled. Not bad though, twice the power output from your favorite triode. )

The pentode grid bias gets adjusted for equal DC idle current in the xfmr halves, so no DC is present flux wise in the xfmr. The 1 Ohm resistors are for setting bias. The pentode could be replaced with an N channel Mosfet, since it is just acting as a programmable current source. (You will notice that the pentode, or Mosfet, are operating in grounded grid mode, so the same current goes in as goes out, so no sonic effects on the signal) The triode's cathode sees 1/gm of the pentode (or Mosfet) as a cathode resistance to ground, so the Mosfet will reduce this effect dramatically.

One can still use a partial cathode feedback winding, or distributed loading, for the triode if desired, just put the winding in series with the triode's cathode to CCS connection point.

Don
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Old 8th May 2005, 09:59 PM   #2
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Default alternate circuit

Here is an alternative circuit that accomplishes complementary current triode operation for a single winding type primary transformer.

See next figure. This uses a setup that looks like it's halfway between Mu follower and SRPP. The LEDs provide a constant DC voltage reference, C1 holding a constant voltage during operation.

With a constant voltage effectively across R1 and R2, the IR drops must sum to a constant. R1 and R2 are nominally equal in value, so the triode current and Mosfet current must therefore sum to a constant. (R1 and R2 are relatively small value resistors here, being used as current sense resistors.) This is just what we want, complementary currents. The triode controls the output and the Mosfet acts as a programmable complementary current source. (could use a pentode in place of the Mosfet too) Notice that a Mu follower provides a near constant current load, and the SRPP provides a voltage follower output, but here we use a varying current source on top.

In reality, the Mosfet (or pentode) needs some gate to source (grid to cathode) drive signal for operation, so R2 gets bumped up in value a little to provide this. R2 would be adjusted under large signal conditions into a dummy load to get zero DC current thru the transformer. This would be done using a milliammeter in the xfmr primary to ground connection and tweaking R2 during large sinewave signal (drive to triode) operation for DC null.

The previous circuit, in the last post, has an advantage over this circuit in that its instantaneous current draw on the B+ supply is constant. (all circuits here are class A, which at least have constant average current draw).

(couldn't get the figure accepted, even though its only 8.4 KB. I will try to post it separately in the next post.)

Don
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Old 8th May 2005, 10:13 PM   #3
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figure 2 hopefully

no luck, let me try figure 3 instead

no luck again

Well, figure 3 uses the figure 2 design on each end of the transformer primary. This is like what is called H bridge operation.
However, the triode drive is kept on only one side of the xfmr, the other side gets a pentode on the bottom with an AC grounded grid (like in fig. 1), and both sides have their cathodes connected to a CCS like in the first figure.

I'll have to try re-scanning the figures I guess.

Don
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Old 8th May 2005, 10:26 PM   #4
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figure 2 maybe
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Old 8th May 2005, 10:31 PM   #5
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figure 3 finally

This design provides 4x the power of the triode in normal SET mode. It also has constant instantaneous current draw on the power supplies like figure 1, so should be very easy on power supply filtering requirements.

Also, the triode sees 4x the load impedance due to the assistance of all the complementary currents, so the primary winding can have its turns ratio cut in half for the same Zo load as in SET mode. (of course, the actual # turns will be QUITE a bit lower yet than even 1/2 that on the SET xfmr, since we have no air gap in this xfmr, fewer turns required to get same primary inductance)

Don
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Old 8th May 2005, 10:57 PM   #6
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Oops, minor error. The primary turns ratio for figure 3 already gets cut in half automatically, since the AC ground point in the primary winding moves to the center. So no actual change in total primary turns is needed versus the figure 2 design.

Don
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Old 8th May 2005, 11:28 PM   #7
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Hi Don,

These are interesting circuits, and remind me of John Broskies clever doodling at tubecad.com.

They are clearly all designed to remove or cancel standing DC current in the transformer. Admirable, but a bit complicated. Why not just use parafeed? It gives excellent results, and uses an audio optimized choke which is expensive, but it's much simpler and should remove the 'sonic overlay' of silicon which your mosfet technique would doubtless add......

How about an attempt to rid Class AB of the nasty switching artefacts? Or a tube VA followed by a Class AB output stage?

Cheers,

Hugh
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Old 9th May 2005, 01:22 AM   #8
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Default pros and cons

Hi Hugh,

Well, the original start of the DC compensation thread was motivated, I think, to find something simpler or cheaper than parafeed. The figure 1 ( CCT1 ) schematic is reasonably simple I think. The choke for parafeed is expensive and heavy and also parafeed has some limitations due to the capacitor in the circuit. Aside from the usual paranoia over capacitors, the main concern is resonances between the inductors (2 of them actually, including the xfmr) and the C, which complicates design and may cause limitations to bandwidth, especially if NFB is contemplated.

CCT1 not only avoids all this, but doubles the power output from the triode. A lot more triodes would be useable for CCT operation than SET, with the doubled power. The current source makes for an accurate complementer for the current signal by simple algebra, or Kirchoffs Laws. The use of a cascode Mosfet or transistor at the top of the CCS tail can remove any variable junction capacitance issues. (of course, one could use a tube CCS instead) The pentode or Mosfet side are operated in grounded grid where the same current comes out as goes in, so has to be pretty transparent. So I feel pretty confident on the sound quality of CCT1.

CCT2 and CCT3 (figures 2 and 3) on the other hand are a little more risque on transparency. I will have to listen to them to see how they perform. But getting 4x power out of your favorite triode has to be a strong attraction.


On the Class AB and switching spikes during crossover problem, see here:
http://www.diyaudio.com/forums/showt...499#post404499
The Hawksford Error Correction idea was developed for exactly this reason (although for SS designs originally) and is well regarded in the SS community. No reason it can't be used for tube designs too.

Don
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Old 9th May 2005, 03:06 AM   #9
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"Tube VA followed by a Class AB output stage"

Using the CCT2 circuit as a driver for a class AB SS output stage should work pretty good. Complementary drive currents would preserve the triode sound yet produce symmetrical drive for the N and P channels. Would want Hawksford EC in the SS output stage to clean it up. Get that SET sound with 300 Watt gusto.

Another interesting approach are Hybrid output stages. More than just class AB SS, these use SS and Tube in the output stage itself.
http://www.diyaudio.com/forums/showt...103#post595103
(Easier to drive the tubes too.)

Don
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Old 9th May 2005, 03:16 AM   #10
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Fig. 1:
Does LTP mean anything to you? It's PP. Surely you've seen the cheapa$$ amp schems which use just one driver tube for PP output with a common cathode resistor or CCS to drive the other? Same thing, just better cathode balance, but unbalanced devices so you'll get lots more 2nd H.

Since the dynamic resistance of a pentode plate is towards infinity, a single one at idle opposite the triode is true balanced-DC SET. Let me put it this way: if there is some delta I in the pentode plate, it will affect the effective load resistance the triode sees; thus, it must be either helping or fighting the triode. Zero dI means zero interaction means lone triode.

Fig. 2:
Score one for the SS guys: all a mu stage was ever designed to do was drive the output with something stiffer than the triode, which gets the red carpet and velvet rope. I'm assuming your resistor divider is designed to share the load somewhat, so this isn't strictly true; however, the FET is in the signal path regardless, doing its share of the oomph. If you don't count SS as an amplification device, then yes it is tubed; however, if you are a hardlining tubophile, this schematic will be burned in effigy.

Fig. 3:
Same deal, twice as bad: H bridge is *NEVER* the answer*. You get double the voltage drop in any direction. Note that output impedance is *in series*, so Zo is DOUBLE a single stage.
CT transformers were invented to solve this problem by allowing both tubes to sit on the ground while bumping away with only one series voltage drop at any given time. (Half bridge is certainly feasable, but lacking P-type tubes, you need a coupling transformer, and that just gets ugly.)

(*Well okay, to be fair: full bridge is useful when limited supply voltage is a design requirement; half bridge requires capacitor coupling or a split supply. Still, it's only practical where low voltage drops are attainable, as with good solid modern SS.)

Technically, if the bias is set properly, the SS will disappear (as far as voltage drop, being it saturates in the 1-5V range), leaving only the tubes' (30-200V) voltage drop as the limiting factor. Now it will look like half bridge, which looks like PP with the CT arranged out.

...Thanks for the thoughts, but I'm afraid you'll have to continue looking for that ever-elusive *original* tube circuit.

Tim
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