The AM Forum

Hi. Modulator tube line-up is 6SJ7, 6SN7, 6L6. The 6L6 provides about 6W of power to the finals, which are a pair of KT88's.  These are wired as triode connection, class B,  zero bias.  Audio drive is applied (via an interstage transformer) to g2, and g2 is connected to g1 via a 15K resistor.

Is this circuit arrangement perfectly satisfactory?

I feel like changing the circuit and wiring the KT88's as tetrodes, having some vague notion that is might be better for audio quality.  I was even thinking maybe the line-up should exclude the 6V6 and be replaced by a splitter based say on another 6SN7.

Your comments much appreciated. Thanks.

single ended 6l6 transformer drive to p.p. kt88 .... I am guessing the generated audio is clean up to a point and then gets dirty before desired power level is made ....is this what is going on?

I spent several months this year in the decision process trying to decide how to best drive tetrode modders ... if you already have the grid bias and screen voltage and can get output power in class ab1, that's the easiest but perhaps not the most efficient way ... there is great appeal to the enhanced drive method in terms of no screen/bias supply but you have to pay the piper with audio drive power

after looking at several methods using transformers and analyzing the enhanced p.p. drive method one fact became obvious ... each tube at peak positive drive voltage is drawing current and the input impedance drops to a few kilo Ohms .... this places a large constraint on the driver if low distortion audio is the goal ... a proper analysis is called for here but most hams don't want to do this so here is a quick and dirty approximation to tie a distortion level to a driver required impedance .... if the load is several kilo ohms the the driver source impedance should be no more than 10% of that or sevral 100's of Ohms .... this is why most class b modder drivers use low mu triodes like 2A3 etc because their plate resistance is in the 700 ohm range .... this is why a 6080 makes a good replacement for 2A3 in this service if you have extra drive voltage

a cathode follower to directly drive the screen/control grid is called for here ....a discussion with Reed W2CQH pointed out the need for a high transconductance tube such as 6bq5/el34 to serve as a cathode follower which would drop the output Z below 100 ohms and further reduce distortion

most folks want to use negative fb to reduce distortion and it can work but can also give phase shift headaches as you deal with transformer imperfections ....I hope this helps ....there is much more to this and I hope others will share ....gl ...73 ... John

The reason I post is a concern as to audio quality. I've never used the AM mode so far, but intend to. In other words I have not run the modulator and established audio quality.

In the 1950's and probably earlier, it was fairly common to use triode connection for a tetrode and wire for class B, zero bias.

I am rebuilding and I'm wondering whether today, for audio quality reasons, class B, zero bias would be discarded in favour of running tetrodes, as tetrodes, in class AB1 or AB2 employing fixed or automatic bias.

If audio quality is perfectly satisfactory with class B, zero bias, then perhaps I should keep the arrangement. But, I just don't know.

I'm a real beginner BTW in all this. :c)

another beginner here, who's been looking into the following

vol. 16, #2, and
vol. 10, #4 in http://n4trb.com/AmateurRadio/RCA_Ham_Tips/rca_ham_tips.htm

(I'm building the vol. 10, #4 modulator). There's also a bunch of other
issues on that web page that discuss other circuits and some design-related
theory.

BTW, my basic starting position is that I have all the parts for wiring the modulator deck as it was.

I'm rebuilding, and the question is, should I rebuild exactly as was, or change from triode connection, to tetrode. In the interests of better audio quality.

The KT88, 6L6, EL34 etc tubes are NOT "zero bias" tubes.

Wired as triodes, with the grid driven they must have negative bias on the grids to bring the current down to the proper quiescent current for class B or more likely AB. Cathode bias (self bias) can work, but is not recommended usually for these tubes.

Typical tubes that are in fact "zero bias" tubes are 811, 805 and 838. Even these may need some cathode or grid bias depending on the plate voltage used, etc...

                _-_-bear

PS. assuming you have the bias properly set, there will not be a noticeable difference in audio quality when used in a modulator. The "pentode/tetrode" arrangement with proper screen bias will yield more power, which is desireable. I prefer class B2 or AB2 in addition...

Id just drive the KT-88's in AB1 with a 6SN7 and be done with it as nobody will be able to hear any distortion as long as the preamp/driver is clean. Worry about FB later but odds are that it wont be needed on the real world bands. If you need more audio use 6550's or the new KT-120's.

Dont get bogged down in the perfect audio scenario, thats strictly for the audiophools and most with them is myth anyway.

Carl

BFS,

I am not sure what impedance you are referring to? The input impedance of the grid of something like an 811? Since that tube will draw grid current, the driver only needs to be able to supply the requisite watt(s)... if you are driving the grid of the output tubes from a follower, almost anything will work fine. Like a 6SN7 for example...

Otoh, if ur driving the mod iron with a cathode, that is a different beast entirely, but somehow I don't think this is what ur referring to??

The 6080 and 6C33C are low gain Vreg tubes with super low plate Z, the plate Z on a 2A3 is substantially higher.

ZFA,

That is a nice classic modulator. Personally I wouldn't build one that way if I was going for a modern design. I'd get rid of the interstage transformer, unless I happened to have a high class one just laying about. I'd go with a form of cathode follower drive.

The only real issue with cathode follower drive is to make sure that there is enough swing available from the follower (therefore the tube that drives the follower) so with the gain of the output tube we get it to drive to clipping.

Imo the input stage of the old Ham Tips modulator is ok, but there are some better ways of doing the speech amp section that yield better phase splitting, but IF ur using an IT that really doesn't matter at all... but that RCA design will definitely work! :D

                   _-_-

Well, there is no bias to set. The circuit is very nearly the same as Fig. 9.35 on page 9.21 of the RSGB Communication handbook, 4th Edn.  Only difference is that cathodes were joined together then to ground via a meter marked "AUDIO WATTS".

P.S. I might be misunderstanding your quote here. :c)

The actual interstage transformer is made by Chicago Transformer. It is stamped on the bottom: 7533-F.  Does anyone recognise this IT and can anyone please supply the specs?

I guess you are saying that class B, zero bias arrangement is not inherently a problem - audio quality wise. But, is the KT88 a suitable tube to be driven class B, zero bias? That is feeding g2 with a 15K resistor to g1?

Originally the modulator would have used 807's. For some reason, production went to KT88's.

hi Bear ... the interesting thing about the enhanced drive method is that most of the tubes from 6v6 to 813 require about the same power level  :  3 to 5 watts  .... this tends to indicate roughly the the same drive requirements
and impedances ... any source impedance in a driver to a varying impedance load (such as this case) will distort the resultant waveform due to voltage drop

as to cathode followers not all tubes are as good as some .... from Radiotron hb the output impedance of a cathode follower is approximately the inverse of the transconductance (ex 6aq5 has 3300 micromhos which yields an output Z of 300 ohms - this does not count impedance of cathode path to ground ) the 6bq5 has a transconductance of 13000 micromhos and shows the preference of pentodes in this service

to other commentors ..... I do NOT agree that distortion reduction is a waste of time
 

Just to answer my own quote: In the books that I have, the authors quite happily show class B, zero bias circuits using 807 or KT88.

There is no sense of a problem with audio quality, as long as there is sufficient drive power from a low impedance source.

But here we are several decades later and I wonder is there any real advantage to discard the class B, zero bias arrangement using the triode connection? Possibly not from an audio quality perspective if the drive arrangement is sufficient.  Which I presume would be the case in my professionally designed circuit. Not sure about any other perspective.

Would you use a pair of 807's today for the finals?

Sorry, afaik there is no way to NOT provide a bias on the grid of the 807, KT88, or other tubes of this type. Cathode bias puts a negative bias on the grid by "raising" the cathode above ground via the voltage drop across a resistor, still negative bias.

Please scan and post the schematic you are referring to? Afraid I don't have an RSGB book handy! :D (a pdf of one would be nice, though!)


BFS, sure in theory, but in practice there is no appreciable distortion added assuming the tube driving has sufficient watts available to drive the grid fully... that implies that the Z of the driver is low enough right there. :D

One of the problems with adding loop feedback to flatten response and/or "reduce distortion" comes when you clip the circuit. Feedback has a nasty habit of making clipping look very nasty, whereas ZFB (zero loop feedback) circuits tend to clip much more calmly and sweetly, with few if any nasties on the waveform... Ymmv, of course.

                _-_-bear

thanks for the info. Yes, I have an IT (UTC S-something which I now forget)...some time ago I bought
a modulator discussed in AMFONE from Bob w4rfm and have been experimenting with it...and when I
saw the ham tips article I decided to rebuild it according to that schematic (I am going for an antique
design ;D with plenty of anode caps !).

The triode-connected configuration will give more output than AB1, and probably even more than AB2.  The difficulty with the zero-bias triode connection is the aforementioned driver problem.  A screen grid tube like the 6L6 makes a very poor class B driver unless negative feedback is used.  With feedback, the 6L6 makes almost as good a driver as a 2A3.  The feedback should go round the 6L6 only, and be accomplished with R-C networks, so that there is no transformer in the feedback loop other than possibly the driver transformer. That way you avoid the phase shift problems inherent to transformers.  The  6L6's driver transformer is not really in the feedback loop, since the secondary winding is effectively in series  with the source of feedback voltage, which is not "transformed" between the primary and secondary windings of the driver transformer. The driver secondary acts more as a passive component in series with the circuit.

The real weak link in the class-B modulator chain is the class-B driver transformer, the one that goes between the driver stage and the class-B modulator stage. There should be extremely tight  coupling (very low leakage inductance) between windings.  It is difficult to construct such a nearly-ideal transformer, so they tend to be expensive and hard to find, especially for single-ended circuits.  The cathode follower route may be a better way to go if a good driver transformer cannot be had.

I wouldn't classify a UTC S (Special) series transformer in that category.  You need something more like the LS (Linear Standard) series.

Here is a link to a 6as7 (6080) speech amp described in RCA ham tips.

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0704.pdf

I does use an interstage transformer before the 6as7. Probably this could be eliminated by using a tube phase splitter.

The 6as7 is used in place of a pair of 6b4s (or 2a3s). Lots cheaper these days, but in the 1947 article they talk about the advantages of the 6as7 over the 6b4 -- and back then the cost difference wasn't an issue.

In my situation, when the designer chose the circuit (G8VB) he has focused on efficiency and power output. So, he chose class B, zero bias, triode connection for the 807's/KT88's. I get that much.

Audio quality must have been "acceptable" at that time

OTOH, the 6L6 driver appears to have no feedback arrangement. The driver transformer in my case is a Chicago Transformer 7533-F. I don't know how good this transformer is.

So, it seems that if class B, zero bias is kept, there needs to be some additional feedback circuitry around the 6L6. That is my present understanding.

Agreed.


Interesting article from RCA Ham Tips.  The RCA book says the maximum plate voltage is 250v.  Tung-Sol says 275.  They won't quite take the +300 plate voltage of a 2A3, but will generate more plate swing voltage.

I once ran my quad of 2A3s at +400 volts @ 30 MA per tube, instead of the recommended +300 volts @ 40 MA per tube.  I could get more grid swing voltage at the same plate dissipation, but the tubes didn't last long.  I later got a driver transformer  with a better turns ratio and was able to lower the voltage to +300 and run them at specs.  I have had that same quad now for over a decade, and last time I tested them they were still perfectly good.  You can't always get away with running a tube beyond its published ratings.

FQA11N90 1 ohm RDS on

The way this is going, the answer to the question, is class B, zero bias arrangement satisfactory, is yes. But, it would be an advantage, or an improvement could be forthcoming if the 6L6 was wired for some feedback.

I myself cannot work out the feedback stuff. Well, not at the present time.

I'll put up a copy on the WWW of the modulator schematic and post a link to it. And folks can say how the driver stage to the KT88's can be improved.

Ok, let me be the first to admit to extreme brain density...

Here is a data sheet for a 6L6G by GE:

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf

Note the class A p-p suggested values?
In Class A, not B, there is suggested 250 B+ and ~-18v negative bias voltage.
Similarly they show a single tube in Class A triode connected at also 250vdc and a similar bias voltage...

So, how does one get less quiescent plate current with similar plate voltages and NO BIAS on the grid?? Seems to me that gets us more plate current, not less??

What am I missing here?

                        _-_-bear

 Bear, you are spot on, but reread the original post. The modulators (KT88) were to be run class B driving each tube screen, and then tie G1 to screen through a resistor. So without any audio drive, G1 volts = 0, G2 volts = 0, and the static plate current is just barely above zero. This is similar to what Gonset does on the G76 with 6DQ6 modulators. Also recall that handbook modulator with 12AU7 cathode followers driving a pair of 7C5's? No bias supply, or screen supply needed; great for AM mobile where battery draw was a big concern.

Jim
WD5JKO

Oh - driving BOTH the screen and grid tied together?

Triode connection is usually the screen tied to the Plate via a resistor.

If you tie the screen and grid together it is more or less screen drive... but ok.
I doubt that is terribly linear, as screen driven tubes are very non-linear... but it would be interesting to read about it in more detail. :)

Edit: wonder what the gain looks like with this connection??

              _-_-bear

http://www.pmillett.com/tubebooks/Books/orr_radio.pdf

I believe that that is commonly done. See page 662 of Orr's 1959 Radio Handbook (see the above link)

He shows 813 and 803 modulators that are driven with the screen and g1 tied together.

Also see the previous page where he shows the g1 resistor recommended with the lower power beam power tubes (e.g., 6l6, 807)

Hi

here's a schematic From Altec Amplifier ,using 811 triode as Finals,maybe it can be changed with 807 or KT 88 triode connected.
Used it with a modulation transformer with a feed back winding.

Gito

That is the Altec 1570B.
I have two examples of that amp.
Very nice, especially considering how small the output iron is...

A good way to drive the output tubes, fwiw. Standard connection for "triode strapped" 6W6 tubes which are like 6V6 but lower power, iirc.

Of course it does not have enough gain to be a speech amp, it takes standard "line level" input, you'd need to add another stage in front to use it as a modulator design.

As far as feedback, I'd take feedback before what would be mod iron, IF you needed it for some reason. The circuit is pretty good already, and if you use high quality mod iron it will be pretty good without feedback, imo. There are ways to use resistive dividers to get feedback from the secondary without having to find iron with a separate "feedback winding".

                  _-_-bear

1. shows how to use neg feedback on a tetrode driver for class B modulators. In the push pull circuit, the method is similar to that used in some BC rigs, taking the end plate signal and sending it back to an early stage. These also avoid pitfalls of tying G2 to plate which usually results in low power output. In the examples the G2 is run at the same potential as the plate but that is not a rule. The point is that G2 is run from a DC source and has no signal on it. The resistors shown in the single ended amp therefore connect as a simple voltage divider across the plate winding and have nothing to do really with G2.

2. is a graph comparing tolerance to varying loads of the amp architectures in attachment 1 compared to a 2A3 push pull amp, and also a straight amp with no feedback.

3. is a complete driver for class B load

4. is the high level class B modulator. It shows 813's but the KT88, 807, almost any tetrode or beam tube will do. If the tube is a pentode, the suppressor may be connected to G2 or to ground (probably best although there is a diagram using an 803 otherwise), and if it is a beam tube, the beam electrodes should be connected to ground. It also shows a different way of driving the class B tubes, using a cathode follower instead of by the plate and delivering lower power than if the circuit were taking power from the plate, but either way will work as long as the driver makes enough power. The circuit calls for 20W dissipation on the Class-A 807! Notice the drive is to the 813 is only 65v RMS, 74V peak. That is because it does not take much to get high signal current when both the screen and control grids are driven positive.

My advice to the OP is to use a 6L6 or better to drive the class B stage, and not have to run full tilt dissipation on it. Or if possible change over to a push pull circuit with teo 6L6's. But only some experimenting will tell.

Bear,
       Tying G2 to plate is usually used for low power class A Operation. Whth this configuration you have pretty much full Ep on the screen. You usually need some kind of biass to keep the tube from melting down.

G1 and G2 tied together and fed, usually creates a class B type situation. With little or no positive biass on the screen the tube usually sets there quietly waiting for something to do. This arrangement just about always needs to have the grids pushed into grid current to produce full current flow through the tube. (henceforth a big triode). Therefore this is usually used for higher powered class B operation. (little or no negative biass, a lot of grid current, and a much lower Z input)

Just think of a grounded grid leanyour with a tetrode and both grids tied to ground. With that circuit arrangement, you have basically created a class B triode amplifier.

Tying the screen and control grids together forms a high mu triode.  Tying the screen to plate forms a low mu triode.

The low mu connection with a 6L6 makes a usable substitute for the 6B4G, although the plate resistance is not as low as with the latter.

The high mu connection (with some tubes with a resistor in series with the  control grid) makes a good zero-bias class-B triode.

Lovely! :D

Of course I am wondering why, if this works as advertised it is not very popular with audio applications. Eg. making a 6L6 (etc.) into a 2A3 would be a neat trick, even if it tried to sit in Class B (positive bias is always an option?).

               _-_-bear

Would you positive-bias a 2A3/6A3/6B4G in audio applications?

no reason to unless low b+ is all that is available .... in class a2 ....grid current

  Patrick Turner of Turner Audio thinks the 6CM5/EL36 or 6FW5 (no plate cap) makes a superior triode when connected as such to better the 2A3/6B4 series tubes:

http://www.turneraudio.com.au/2323-triode-integrated-6cm5.htm

"6CM5 in triode gives µ = 4, Ra = 600 ohms, gm  =  6.6mA/V.
snip
 learnt one of the best kept secrets about the 6CM5 :-
Although its a beam tetrode it will sound very good as a triode.
When I first built it and compared it to a pair of Quad II
amps I had repaired, the 6CM5 seemed to sound better. The 6CM5 was meant for line output in tv sets, and capable of as much cathode current as a KT88. But anode dissipation is limited to around 18 watts in a class A or AB situation.  It could even be used as a single ended triode with about 4k anode load for about 5 watts with Ea = 375V and Ia = 50mA, and methinks it will out perform 2A3!"

Another thread about 6CM5 that shows grid 2 drive, cathode drive, etc. Also talks about 807 G-G in audio:

http://home.alphalink.com.au/~cambie/EL36.htm

Jim
WD5JKO

It's true, many of the older sweep beam tetrodes make very good high-mu triodes when you tie the screen to grid via a 20K or so resistor, and drive the screen. Some, such as selected variant of the 6BQ6GA/6CU6, which are rated as 14W, will handle 28-30W plate dissipation EACH, with NO problems, and when strapped as a low-mu triode, the 6CU6 is a VERY close match to a 300B!

That Turner guy is interesting reading. 

http://www.turneraudio.com.au/about-turner-audio.html

His personal life just keeps going on and on - I couldn't stop.  He has an interesting opinion about our lady-friends

Patrick is a good fellow.
I've been online with him in various groups for a very long time now.
First class guy. Sorry to read of his medical problems... just sent him an email
after reading that. :(

                     _-_-bear

Wow !  Ok, well - I've been solving the same problem over here for the past week or so.... driving, in this case, a pair of 807s in triode connected mode, and I have FINALLY and successfully done it - with high fidelity, and have learned quite a bit in the process.  Here are the results of this work (and it was a lot of work!!!):

1) The 807 (and probably many other beam power tubes or tetrodes) CANNOT be connected in a straight triode configuration.  Period.  You *must* run the grids at different potentials.  The screen voltage must be higher.  DO NOT use a resistor to accomplish this.  Doing so is folly.  The grid is a non-linear load, and introducing a linear element (a resistor) in series with the control grid (a non-linear load) will result in serious distortion.

I operate, with the 807s, using a 60 volt difference between the screen grid (at 55 volts) and the control grid (at -5 volts).  This difference is accomplished using 3 20 volt zeners in series - with a 100 ohm resistor in series with the zener string to protect it - all shunted by a 220uF capacitor (this is a non-critical value - 100uF will probably work just as well).  The driver is connected directly to the screen, and the control grid is connected to the screen and driver through the zener-resistor-capacitor network described above.

2) Do everything you can to GET THE TRANSFORMERS OUT OF THE DRIVER.  This is relatively straight-forward to accomplish.  Use anything other than a transformer - use a good cathode follower (with a triode); use a MOSFET (which is what I do).  Just don't put a transformer in there.  Bad!

Transformers are linear resistive elements, albiet not large resistances. They are imperfect in their ability to transer energy.  So, when the grid(s) go from an infinite impedance (negative voltage) to some finite resistance (being driven positively), there is a significant change in the load as seen by the transformer & driver, and this will result in a slope change on the driving signal (due to voltage drop) and this means DISTORTION.  Negative feedback will help, but nothing will compare to a source or cathode follower driver.

Also, transformers introduce phase shift - and this in turn limits your ability to put negative feedback around the whole modulator (and include the modulation transformer in the loop).

3) Put negative feedback around the driver, and put an overall loop of feedback around the whole modulator.

4) Results:  The 807 modulator is part of a Viking II.  Using the STOCK VIKING II MODULATION IRON and the 807s connected in the QIX triode configuration (using the zener-resistor-capacitor network, detailed above), I am able to achieve the so-called "class E modulation pattern" (otherwise known as "shark fins") with a transformer coupled modulator.  

No, it is not as good as a class E rig, but it's pretty darn good.  The mod iron limits the positive peaks to less than about 125%, and of course the low frequencies start to come apart below 40 cycles, but still, that's not bad - considering it's a transformer and not a very good one at that.

Hope this information is useful!

Regards,

Steve

http://homepage.ntlworld.com/richards_internet/RADIO/zerobiasmodulator.html (http://homepage.ntlworld.com/richards_internet/RADIO/zerobiasmodulator.html)

The link shows a modulator circuit that drives a pair of KT88's. I think originally the manufacturers employed 807's. In fact, I'm minded to use 807's myself.  The circuit is wired for class B, zero bias. The modulator is to modulate an 813 running at 150W input.

Given that the PSU is probably designed for class B, zero bias (PSU needs decent regulation I believe) it seems that one could do worse than try to keep the original class of operation and zero bias arrangement.

However, even though the circuit as is will provide satisfactory performance, I'm completely rebuilding, so could afford to make improvements.

Improvements mentioned revolve around the drive arrangement to the finals, that is around the 6L6 driver. Specifically, arranging for some feedback. Other improvements seem to be ditch the driver transformer. Even employing push-pull 6L6's, but perhaps that is not necessary.

Anyhow, the circuit shown is my starting position, but no doubt improvements can be made to the drive part of the circuit.

I don't know what the value of C13 was.

I think I have got C1 and C4 value correct.

I've got 300V to play with for the speech amplifier and 6L6 driver. Apparantly 750V for the finals.

Wow, that circuit has enough stuff wrong to write a book about it  ;D  Will it produce audio?  No doubt!  Will it sound good?  Well, beauty is in the ears of the beholder, but I don't think so  ::)  If you're going to use tubes, just use a cathode follower.  In the modulator itself, that resistor between the screen and the grid has to go.  Use the zener-resistor-capacitor network I talked about in an earlier post.  Drive it with something low impedance, and it will sound great!

Another thing - a pair of 807s is not really sufficient to modulate an 813, unless the 813 is running at really low power (about 120 watts input or less).  You did mention the 813 is running at 150 watts input - even that is pushing it a bit with 807s, but it might work.

If you run the 813 at ICAS ratings (2000 volts at 200mA), you will need something a whole lot bigger than 807s.  There are a number of triodes that can be safely used at 2000V which would make good modulators for an 813.  I'm assuming and suggesting a common power supply for the modulator and final - always the best way to go!

Regards,

Steve

What I'm trying to do is employ the transformers, so I'll be limited by them. I'm not that bothered about getting the maximum power out from the 813. There was a "kind of" variac arrangement giving an 813 plate voltage of 750V /1000V /1250V /1500V. It was suggested that plate voltage be set to 1000V at 185mA, which gave the legal limit of 150W input to the PA (150mA plate - 35mA screen).

So, the 813 was supposed to be run at 150W input to the plate. And of course the PSU is a function of that as well as the modulator arrangement. As I mention the set had KT88's installed, not 807's. But, I was wondering whether I could revert to 807's. They are cheaply available as NOS.

I was given a circuit with the set, and when I dismantled the set, it came out the same schematic, so I think I've drawn the circuit as was.

R17 & R18 are quite sizeable resistors.

R19 & R20 (there but not indicated as R19 & R20) are the two 15K resistors connecting g1 and g2 on the KT88's.

Interesting circuit ?? I would also make many changes to it. by the component values (r-c coupling of lower stages) it looks line it will produce some very thin "ear bleeding" audio. 100 Pf coupling caps are just too small to produce any real audio fidelity. It looks to have a little rudimentary feedback wrapped around the 6L6. With a little component value tweaking, It might be able to sound somewhat ok.

The 6SJ7 input / mic amp is prolly too much gain and may cause a hum pickup problem. But with 100pF coupling caps the other stages prolly wont pass it. Ib triode connected class B service with around 750v you can beat the snot out of a pair of 807s for about 120w of audio. Which would be enough for 200-240w DC input to the final. (look at any basic 807 / 6146 boat anchor) But............
Instead of beating the hell out of a pair of 807s, a pair of 809s or even better 811As would be loafing along at that power level.

If you are using tubes designed to run AB, why not run them that way?? If you want to run class B, why not use tubes designed to run that way?? 809s are getting a little scarce, but chinese 811As are cheap and easy to get.

A pair of 807s in AB1 driven by a phase splitter will be capable of making 100w without much problems. 811As will work just fine at 750v, they'll be just loafing along. Why reinvent the wheel when someone has allready done it for you?  ???  ???

I used a very similar circuit with my very first attempt at a class B modulator.  The best I could ever do was space shuttle audio.  A transformer might work if you can find a good one designed for class B driver service, but the only ones I have ever seen were made for a push-pull driver.  The "ham radio quality" units don't make the  grade.  I use a UTC LS-series driver transformer in one of my homebrew rigs, and an RCA broadcast transmitter driver in the other. Acceptable quality class B driver transformers are difficult to design and build, and therefore very expensive.

The zener network sounds like the way to go.  Back when I built mine zeners weren't available yet, or were speciality items, so I used the resistor (20k IIRC, from data in the ARRL handbook tube  charts). 813s will work with the grids tied directly together.

Many slopbucket linears have been described over the years using tetrodes and pentodes with all grids grounded directly, which means the control and screen grids are tied together and operate at the same voltage. One that comes to mind is described in the 1957 Handbook, using 4 1625s in parallel.  You had to modify the tubes by going into the base and moving the lead to the suppressor grid/deflection plate from the cathode pin.  A commercial version of the 1625 amp was put on the market using 837 pentodes, which bring out the suppressor grid to a separate pin, instead of 1625s. I wonder if many G-G amps using screen grid tubes wouldn't be a lot  cleaner if the zener network were incorporated in those as well.  But apparently some tubes operate well with both grids at equal potential while others require less voltage on the control grid. The problem is how to determine which tube requires which, other than trial-and-error, since data for zero-bias triode operation is usually not given in the manufacturers' tube charts.

Steve, where does the negative control grid voltage come from?  You mentioned in the previous post using the network with 60volts worth of zeners, so that 55 volts on the screen results in -5 volts on the control grid.  But isn't some kind of DC bias supply needed as a "keep-alive" circuit to maintain the 60v across the zeners, so that the control grid follows the screen? Another thing, I had always assumed that the purpose of the resistor was to reduce the amplitude of the audio signal applied to the control grid, compared to the screen, and that a tapped secondary similar to the tapped primary on the "ultra linear" circuit would work better, although I never tried it.  But with the zener, the control grid voltage follows the screen exactly with the same amplitude audio voltage, but with the control grid DC biased 60v negative relative to the screen.  So you must have used a DC bias supply somewhere in the circuit to maintain this potential difference and to cause a constant -60 volts to remain across the zeners. Zener action requires some minimum DC current to flow before the reverse potential appears across the diode.   Maybe a schematic would clear things up.


According the Handbook  chart, the triode connected 807 circuit is supposed to be capable of delivering 120 watts. Allowing for transformer losses you should still be able to get more than 100 watts of audio, which would be plenty to modulate 150 watts input, with a little positive peak head room to boot.  IIRC, the RCA manual lists the maximum ICAS audio output from a pair of 807s in class AB2 as 120 watts.

I have a question: 400W PEP is my limit and that is power measured out at the antenna terminals. What is 400W PEP in terms of average DC power input to the plate?

(Could be measuring between apples and oranges here). Not terribly important issue.

But anyway, the old set was made to run 150W into the plate, and that measure was the legal limit then.  So, I've basically got a transformer to supply 150W to the plate.  And for some reason an 813 was chosen as the PA.

The actual transformer is 1100V 0V 1100V @ 150mA.

Well, I've been wondering whether to use the regular connection (tetrode) with 807's.  I think at some point the thought was, with a little adjustment in the driver (feedback) or whatever, class B, zero bias was going to be okay. I was also thinking that perhaps my modulator PU is configured / designed for the class B setup, and I'm keeping and employing the tubes in the modulator.

It looks like my modulator circuit is non too brilliant, so should one do a wholesale rebuild using "proven" circuitry. I guess there could be an argument for that.

Probably the tubes I have are okay, it's just the circuit as is, is non too good.

What to do, seems still up in the air.

Steve,
Cool idea with the zener offset makes sense as the control grid pulls current. Then you just need a pulldown to -5 volts.
Any current through the isolation resistor would change gain. The zener forces a constant voltage offset. Tom and I found you have to hold the phase shift through the driver as low as possible. We had to increase the coupling cap values and change resistor values to hole a low phase shift low over a broad frequency range. I think we ended up around 30 total degree phase shift out to around 10 KHz. Then we could apply feedback from the mod transformer secondary and live with the phase shift introduced by the iron. I think Tom is running around 10 dB feedback. Many driver circuits I have seen have a nasty phase shift over frequency which may prevent shark fin audio because the high end has a drastic phase shift.
When Tom got it right he was able to reverse the audio phase for better positive peaks.

What exactly is a "shark fin" waveform?

Something the Class E crowd came up with to compete with the smugness of Flex Radio owners.

Tom Vu came up with shark fin audio in 1991 as we tested his first PDM rig.
Back then he thought is was some weird distortion

I can't say when the term "shark fin" actually came about.  I can say that we used to call it "Christmas Trees pointing to the (left or right)".

All low distortion, low phase shift, wide frequency response transmitters are capable of this pattern, and it is not an easy pattern to reproduce.  I first noticed it during the early '70s when I built my first PWM transmitter when I was in college.

Anyway, that's what it is.  It's just one of many measures of a good modulator.  Many modified rice boxes, flex radios, class E rigs, tube PWM rigs, tube grid modulated rigs, etc. can reproduce this pattern, which is typical of the male voice.

how about this:

This amp is a fixed screen bias design, with the driver providing output from the cathode, using a resistor to ground.

It is not the same as was talked about before which has the drive on the screen, and a dropping resistor to keep current down on the grid... in essence driving both the grid and the screen... making it a triode...

Nice amp though.

                _-_-bear

I heard W5PYT use the term shark-fin back in the 1950's.

da QIXmon wrote:


Not sure what ur thinking about Steve, but transformers are not linear nor really resistive? They have hysteresis for starters... resistors don't. Then as you say they have phase shift...

maybe you meant to write "non-linear"?? :D

                 _-_-bear

Wow HUZ I thought you were born in the 60s

Hatched.

These were on old recordings.

That's a lot better design.  That .02 into the 470k may be a little small, and some of the other coupling caps may be a little small (I didn't go through the whole thing with a fine tooth comb), but it is far superior to the other circuit, hands down !

AB2 won't give quite as much power as triode connected, but it should be sufficient.

The modulator output should be de-rated to account for positive peaks greater than 100%.  The extra power required is rather significant as you get up there...  But I'd use this circuit over the other one any day!

So the UK has adopted that PEP rubbish too?

400w pep output is basically 100w carrier output power at 100% modulation. Which is your basic 100w table top boatanchor. (DX-100, Viking 1 or 2, Heath Apache, etc)

As far as the DC input goes to get that output, it is strictly dependant on what your plate efficiency is. (It usually winds up somewhere around 50%+ so you would need somewhere around 100w of audio for 100% modulation.

The general rule of thumb has always been whatever your carrier output is, you should have about the same (or a little more for good luck) in modulating audio for plate modulated class C AM service.

That has also been shown as a concept in one of the handbooks (editors& engineers, ARRL, or RSGB) with 750V on the plates making 120W ICAS. I hate that I missed the old 1950's RSGB handbook I used to check out of the school library in my youth. I found a copy, but passed it up, was tired. regrettably.

I've now added the modulator power supply schematic:

http://homepage.ntlworld.com/richards_internet/RADIO/zerobiasmodulator.html (http://homepage.ntlworld.com/richards_internet/RADIO/zerobiasmodulator.html)

The thing is I'm comming from a certain angle. Which is, that I'm trying to utilise the components to hand from an old AM set.  I'm trying not to have to buy new transformers, or tubes - if I can help it. But, I realise taking that tack, my AM rig might not be the best. But hopefully it will provide pretty good audio.

So, what do we have? I have one transformer available for the modulator. It supplied two  HT voltages. Apparantly 300V for the speech amplifier and driver, and 750V for the finals, which were KT88's in class B, zero bias arrangement. The modulator is touted as a 100W modulator modulating an 813, that was to have 150W into the plate (1000V x 150mA). The UK limit was 150W DC to the PA final. That might be the same as todays 400W PEP limit we have in the UK.

Of course the PSU is simple, there is no separate supply for screen grids to the finals, or independent bias supply.

Given T1, the transformer, it suggested to me that perhaps class B zero bias should be kept. Trying as I am to utilise T1 and if I can help it avoid buying any more "iron". I guess though T1 may not preclude changing from a triode connection (hi mu) to a tetrode connection, but is that sensibe trying to utilise T1 only. (BTW, I really should I think use 807's instead of KT88's).

So, what to do, revolves a lot around T1. And perhaps, but I'm not sure, that suggests keeping class B, zero bias.

But, the thought was, if class B, zero bias is kept, does it produce decent enough audio, or can it be made to produce good audio. I think it's been said that I should not keep those 15K resistors between g1 and g2, but employ zener diodes.

You see, someone might say, if I want to base my modulator exlusively on T1, my best bet is to stick to class B zero bias, but improve the drive circuit and the g1 g2 connection.

Of course, I don't think I'd have much choice but to keep the interstage transfomer. A point of weakness.

But, the question would be, could you actually make a better modulator using tetrode connection - using T1? I could get rid of the interstage transformer then.

If you could make a better modulator class B, zero bias would be abandoned, but at the moment it's not determined whether a better modulator would result using 807's in say AB1 or AB2 - employing T1 only.

It's a bit involved, but I think you get the picture. :c)

R:

   I can certainly appreciate the concept of designing from the junkbox. With some creativity you can make some pretty interesting circuits based upon what you have. If you had more funds, and access to vintage parts you might do different.

  Your circuit as last posted make me reflect on the similarity to your modulator to the Gonset G76 transceiver. In the Gonset we use a 6CM6 (similar to a 6AQ5) to drive via a transformer the screen grids of a pair of 6DQ6 modulator tubes. What you have is essentially a scaled up version of the same thing. The one big difference is the the sweep tubes (mostly) don't need a lot of swing on G2 to make power like a 807/KT88. So here they ground the control grid G1, and only drive the screens (G2). This works extremely well, and efficient. This gets around the 15K resistors from G2 to G1, and therefore the 'Kink' in the transfer curve.

   With the Gonset the limitation is clean audio drive. Like your 6L6, the Gonset drive has NFB to the prior stage with a tapped resistor off the driver tube plate. With the Gonset, the distortion was seen in the G2 drive signal. So how good is it by just cleaning up the drive? I had to do three things to the 6CM6 audio driver. The items below use component numbers from your schematic:

1.) Take the audio pre-driver plate load R11, and increase the % NFB. Look at your circuit, take R14 (top), clip it, and pull down to R15 (bottom).  This puts R14 & R15 in parallel, and essentially adds 6DB more NFB. This will also drop your gain.

2.) Increase C10 cap to ~ 250 uf

3.) Look at lowering R16 to 270 ohms (so long as interstage tranny can take more primary current).

In your case do you have any sweep tubes lying around? Using a pair of 6DQ5's, or a quad of 6DQ6's would be ideal, and not need that 15K resistor. Just ground G1, and drive audio to the screens ONLY. Works FB.

After doing similar changes to the Gonset, using a dual trace scope, looking at the G2 mod drive (ch1), and the same tube plate (with AC 1000X compensated HV probe), the signals were identical except for the 180 degree inversion. So it seems that driving the sweep tubes this way, very good linearity is achievable. The tubes will pass on the driver distortion, so minimizing the driver distortion was the main objective.

On your power supply, I think you will need more capacitance for HT1. Go to at least 25 mfd. For HT2, that 400 ohm resistor might be better to be a 10H or so choke. Also a single 5R4 will be overloaded doing both modulator and RF. Maybe use a pair of them, or solid state the HT1 supply.

Just some ideas from an OM in Texas..

Jim
Wd5JKO

That pretty well tells the shark fin story. Shark Fins are a visual observation using a male voice and scope, showing how faithful the extreme low end is.  The area below 70hz or so, extending down to DC, is what appears to make the difference in seeing the fins or not.

For example, in the real whirl, when I ran my 4X1 rig using a backwards audio transformer and 8 ohm amp driver, I had to run my phase in polarity #1. There were no signs of shark fins. In polarity #2, the positive peaks were being swallowed by the negative peaks – terrible waveform. But it was acceptable overall in phase #1.

When I switched to the GFZ MOSFET audio driver for the 4X1 modulators, there was not much difference in the polarities as far as raw levels - polaritiy #1 and plarity #2 were close in performance, but not quite.  #2 was actually better now.  It wasn’t as obvious which one was the proper phase – EXCEPT when using a peak reading wattmeter or looking for the shark fins. The shark fin phase produced a certain low end audio resonance that was not there before. The optimum phase became PHASE #2! It seemed phase #2 was the best polarity for my voice all along, but I could not use my 4X1 rig to pull it off due to its lack of true DEEP low end fidelity.

Technically, the difference between the old transformer driver and the new MOSFET amp was the ability to sweep an audio tone down below about ~50hz SUPER cleanly, without much phase shift. The limitation was now the modulation transformer, that’s it.

In contrast, my class E rig easily sweeps down to 1 HZ. The shark fin peaks are tremendous and the proper #2 polarity choice is even more obvious than the 4X1 rig with the MOSFET driver and mod transformer.

Bottom line is the lower your freq response sweeps CLEANLY towards  0 HZ, the more obvious the shark fins are, EVEN if your voice does not contain much energy down there. I realize there’s no such thing as “sub-harmonics,” at least that's what I've read, but something is going on that takes advantage of these low frequencies when they are available. It might even be low frequency "intermod" generated by the human voicebox, (if that makes any sense)  but whatever it is, it sure sounds mellow to the ear.

BTW, when PYT referred to shark fins, I think he meant the terribly distorted audio you see when a rig is running poorly. Anyone who has built a rig probably has seen this waveform when first fired up and levels are all screwed up…  ;D

T

Thanks for some further insight. Yes, I see the similarity with the Gonset G76.

Well, since I am not wanting to re-use the KT88's, choice of tube finals is up for suggestion.

I think what you are saying is for driving the screen, I'd really want a sweep tube - like 6DQ6's.

I have no tubes for the modulator saving the KT88's which I don't want to use.

Generally, I know I can use class B, zero bias with a pair of 807's.

Assuming modification of the feedback circuit is "a given" irrespective of what circuit is used eventually, I think you are of the opinion that driving g2 grids, will be an advantage (audio quality wise) over class B zero bias with 807's. (Perhaps only really applies if 6L6 circuit kept).

I think maybe that driving g2 there is less load on the driver? Is that the advantage? And not having g1 connected to g2 via a resistor?

Microphone would probably be a Shure 444 I have kicking around.

Looking at the original circuit and the fact that I'm trying to utilise T1, I want someone to come in and say something like "Absolutely, definitely, 100% - ditch class B, zero bias and do...." (Or the opposite).

I wish things were so simple. :c)

Well, since you axed... I know you want to build it with tubes, but since you're still in the design phase, it's not too late to build up the ultimate hi-fi driver for your modulators:

http://amfone.net/Amforum/index.php?topic=25599.0

I was in the same spot as you about a year ago and opted to go with a MOSFET audio driver - and glad I did. The modulator tubes and final tubes are enuff glow to satisfy my fancy... ;D

T

What I will likely do is a first build with tubes, to "employ" the parts.

Then continue my homebrew endeavours, with ever more sophisticated designs. :c)

As to what WD5JKO brought to the story, I did find this comment by Jim:


"Many different pieces of ham radio gear from the 1950's to 1960's use
the technique you mention. This is particularly true for mobile tube type AM
transmitters, where size, weight, efficiency, and overall complexity are all
constrained. One such transmitter is the Gonset G-76 AM transceiver. For
push pull audio modulators, they use a pair of 6DQ6 sweep tubes driven from
a separate CT. winding on the audio output transformer. The audio output
tube is a 6CM6 which drives a speaker on receive, and acts as a audio driver
during transmit. Those 6DQ6's easily put out 50 watts of audio this way. Why
did they do this:

* The modulator tubes need NO screen DC voltage
* Efficiency is very important. RF stages run class C, and Audio modulator
stage is zero bias class B.
* The modulator tubes need NO grid 1 bias
* The audio driver and transformer were already there. It just needed some
relay switching, and a additional transformer winding to work.

It appears to me that treating a Beam Power tube this way results in high mu
triode. The result is a class B circuit capable of serious power, good
efficiency, and reasonably clean so long as the audio driver can effectively
deal with a varying load impedance.

For kicks, the RCA receiving tube manual (1973) shows the plate curves for
6L6GC where G1=0, and G2 is the input variable. Those curves sure look
linear to my eyes!"

So, taking the lead or circuit suggestion by JKO, I'd be sticking with class B, zero bias, but a better arrangement? I mean tying g1 to ground.

I know what you mean. In the end I suspect the only tube in my best AM rig will be the 813 or similar output tube. And you will look at it through a window. :c)

   I forgot I wrote some of that stuff. Keep in mind that with sweep tubes G2 driven (only), the peak audio level will be 100v or less to each Grid (g2) to ground. Going to a 6L6 variant like 807, or many others, the peak audio level will be several hundred volts (possibly 300-400v). The sweep tubes offer the advantage here unless you really want to build an elaborate driver circuit. Getting high drive amplitude at low distortion is somewhat tricky.

   So keeping a circuit as you have drawn will lessen the drive voltage swing need, but will put a 'kink' in the transfer response as G1 draws grid current, and then the 15K resistors drop voltage in response to the grid current. Steve QIX expertly pointed that out earlier. You could 'counter' this with global NFB, but with two transformers in there, stability will be a big issue due to phase shift.

    If you want to retain the KT-88 tubes, use them in tetrode mode with HT2 on the screens, HT1 on the plate, and make a simple bias (C-) supply to set the DC operating point of each tube to about 30ma per tube class Ab1 or Ab2.

  You have a lot of varied, but excellent input here representing different circuit configurations.

   I suppose your going to build it, and hope it works. You can also look at this as a 'work in progress' where you build, learn, tinker some more, and hopefully have a ton of fun along the way. After all thats what its all about isn't it? :-)

All the best,
Jim
WD5JKO

A 6DQ5 with G1 at zero saturates with about 60 volts peak on the screen

http://amfone.net/Amforum/index.php?action=dlattach;topic=25569.0;attach=22374 (http://amfone.net/Amforum/index.php?action=dlattach;topic=25569.0;attach=22374)

I was looking at the above circuit in the STC application note.

When I saw it, I thought, oh, all those power supplies to make.

But, I do wonder whether the 300v supply from my T1 that I've got, will supply the pre-amplifier, phase splitter driver, and 807 g2 grids.

Because the 6L6 would go, and the available current would go elsewhere.

The 750V also that I have, would supply the plates.

Some adjustments here and there and maybe it could work.

I would just have to make a bias supply.

Well, several ideas presented I can work on.

Steve, your post really grabs me. :c)  A voice in my head says, investigate the "QIX triode configuration". :c)

I'm going to experiment and see what I get using class B, zero bias, and seek to put into operation all the stuff mentioned: better feedback, cathode follower buffer, the "QIX triode connection", and whatever helps alleviate crossover distortion. I'll get back whenever, and post how things went.

Thanks all for contributions. Rich