Class A modulation

[Barrie]

Hello:

I've read, in some old articles on modulation, that class A modulation has less inherent distortion than class B.  However, none of these articles explain how to establish the parameters for class A operation.

Some of the tube characteristic sheets have information for both A and B operation, but most do not.

I built a class A modulator 15 or so years ago using a pair of 304TLs.  The Eimac data sheet included all of the necessary information.

Now, I want to build a modulator using a pair of VT-127As, for which there is no information relative to audio service.

Since these tubes are somewhat similar to the 100TL, I have a starting point.  However, the Eimac data sheet for the 100TL has information for class B audio only.

I've Googled 'til I'm blue in the face, but I'm not finding information relative to establishing class A service.

Any hints?

TNX, & 73, Barrie, W7ALW

[WBear2GCR]

Barrie,

To go Class A with "Class B" tubes, the usual receipe is to drop the plate voltage, increase the bias to turn the tube on, and be sure that you can keep the dissipation within max rating limits. you get a whole lot less power from the tube than in Class B or AB. It's sitting there at half power when doing nothing...

Radiotron Designer's Handbook (downloadable these days) explains it in detail, FYI.

Why you would want to build a Class A modulator, is unclear to me. You lose a significant amount of power, and the improvement in distortion is very little and likely not discernable on-th-air. It might make sense for a hi-fi broadcast station, and it does make sense for home hi-fi, but for amateur service beyond relatively lower power rigs, I'm thinking that it doesn't buy you anything at all.

If distortion is your concern, then a slight class A bias up into class AB, run in AB2 and perhaps optimize with feedback and you'll likely be very very clean. Otoh, if you're not using very high quality iron, like LS series or some broadcast iron you'd probably never pass any of the presumed benefit to the signal anyhow...

                       _-_-WBear2GCR

[flintstone mop]

Hi Barry
For our application of Ham Radio and good quality audio even the Hi-Fi folks and the extnded low and high end audio, class B is perfect. Class A would require a LOT of audio power that can be easily obtained by going to class B using the same tubes. Distortion over a radio link would be undetectable!!! AM B'cast stations used class B in the audio stages and they were required by the FCC to have not more than 1% distortion...........not sure on that figure.

Fred

[Ed - N3LHB]

According to the handbook, the same tubes will provide 12 times the power in class B then in class A. Mo power is better

[w4bfs]

class A modulators have their place as well ... everything said about class B being more efficient and making more power is correct; however, will require a push pull mod xfmr ... as far as I know these are not in current production except for small speciality companies which implies a high price / unit .... check  Stu's (AB2EZ) work in a parallel thread about using torroids (with caveats) ... a class A modulator will allow use of just an inductor in Heising modulation and efficiency can be improved by use of a bias shift circuit to reduce waste heat ... so many ways to go, I have found at least a dozen each with advantages and disadvantages ... expect to spend a while going thru the iterations ...73 ... John

[Barrie]

Thanks for all the replies, guys.

I'm afraid I've made a big mistake.  I said Class A, when what I meant AB1.

I think that would make considerable difference?

73, Barrie, W7ALW

[Bacon, WA3WDR]

Some tubes, such as 805, 811A, 572B, 3-500Z, etc, want zero bias in push-pull class B audio service, and only produce significant plate current if the control grid goes positive.  These tubes make nice audio because they draw grid current across the entire conduction cycle of the signal, and in push-pull one tube grid is always conducting, except at dead zero where there would be no current anyway.  This makes the grids act like a fairly constant resistive load.  So the presence of grid current doesn't always mean what it does with higher mu tubes that need significant negative bias to cut off the plate current.

With non-zero bias tubes, like 810, 813, 807, 4-400, etc, AB1 produces less distortion than AB2, because the control grid is usually below the cathode voltage, and the control grid draws essentially zero current until it draws a sudden jolt of current when its voltage goes above the cathode, and this always causes some nonlinearity in the driver and in the output tubes to some extent.  But AB2 is used because that effect is relatively small, and AB1 produces less peak current than AB2, so therefore you get more power output with AB2.  You can push parameters by raising plate voltage and/or raising screen voltage if the tube has a screen grid, but you get less power out of a given set of tubes in AB1 than you would get in AB2.

See how much plate current each of the output tubes give you with zero control grid volts and maybe 125V on the plate.  Measure this quickly, because the tube will be dissipating some power.  With this as the peak plate curent, figure what B+ you want to run.  You can figure your peak output will be about (B+ - 125V) * (peak current - single tube resting current), and the RMS output (with two tubes) will be half of that.  You can figure the required plate-to-plate impedance will be 4 * ((B+ - 125V)/(peak current - one tube resting curent) (amps)).

So if your tube gives you 400 mA at 125V with zero grid bias, and you will be using 1000V B+, and your resting plate current is 80 mA (two tubes) then (1000 - 125) * (.400 - .04) = 315W peak or 157.5W RMS for two tubes.  The plate to plate impedance will be 4 * ((1000-125)/(.400 - .04)) = 9722 ohms.  Higher plate to plate load impedance will cause lower plate current and less maximum output.  Lower plate to plate load impedance may give a little more output, but lower efficiency, so there will be higher plate dissipation.

Some tubes can't pull much current down at 125V plate with zero grid volts, so you may have to fudge with the 125 figure, but this should get you in the ballpark.  If you can get the tube characteristic curves, you can get a feel for the minimum plate voltage for reasonable plate current at zero grid volts.

Also, you should consider tube dissipation.  This is tricky, because speech is not a sine wave.  With a sine wave, the dissipation will be about 2/3 of the output power at maximum RMS output.  With speech, the dissipation might be more like 1/4 to 1/3 of the maximum RMS output.  In the example above, plate dissipation will be about 52.5W per tube at full RMS output, and maybe 26 to 35W per tube for speech at full peak output.  If plate dissipation is too high, then you must increase the plate to plate load impedance or reduce B+, etc.

[WBear2GCR]

Barrie,

Yep, Class AB1 is very different than Class A.

It has to do with for how much of an input signal cycle each of the two output tubes (working in push-pull) are conducting - or "on", and how much they are "on" (conducting) when resting (doing nothing, no input signal).

Class B refers to a condition where the tubes are each just "cut off" - or not conducting much at all, if anything. In effect, they are set just on the threshold of starting to conduct. That means that when a positive going input signal is applied, only one of the two tubes will start to conduct - the other one sits around waiting its turn when the signal turns negative.

In AB, both tubes are set so that they conduct, just a bit, sitting around (how much they conduct is why it is AB, not B and not A). Now when a positive going signal is applied both tubes will conduct - one more and the other less. But only for a part of the input signal cycle where each tube is biased on slightly. After that, one is cut off, and the other continues to conduct. So, when the one is cut off and the other is on, that is the same as Class B. Thus, class AB.

In class A, both tubes are on "1/2 way" when resting.

What AB1 refers to is the condition where the tube is not drawing any grid current.
What AB2 refers to is the condition where the tube draws grid current.
Usually the tube only draws grid current when driven "on" (conducting heavily) very hard, and then only if the drive circuit is designed to be able to deliver grid current.

So there is A2, B2 and AB2 possible, as well as A1, AB1, and AB2.
Circuits that can deliver grid current also produce more output power for the same operating conditions of the output tubes, and include transformer coupled and DC coupled driver circuits.

Does this explain enough?

 

                 _-_-WBear2GCR